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LIGO (Laser Interferometric Gravitational-wave Observatory)

Interferometer    LIGO Facility    Detection Status    Facility Status    References

On February 11, 2016, the LIGO (Laser Interferometric Gravitational-wave Observatory) Collaboration announced the detection of gravitational waves. For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the Earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos. 1)

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

The gravitational waves were detected on September 14, 2015 at 09:51 UTC by both of the twin LIGO detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. Each LIGO detector is 4 km long, the observatories are separated by a distance of 3,002 km. As a gravitational wave passes through a detector, it distorts spacetime such that one arm lengthens, and the other shortens. By comparing the disturbances at the two detectors, the scientists can confirm the direct detection of a gravitational wave. - The LIGO Observatories are funded by the NSF (National Science Foundation), and were conceived, built, and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors. 2)

Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. About 3 times the mass of the sun was converted into gravitational waves in a fraction of a second—with a peak power output about 50 times that of the whole visible universe. By looking at the time of arrival of the signals—the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford—scientists can say that the source was located in the Southern Hemisphere.

According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole, converting a portion of the combined black holes’ mass to energy, according to Einstein’s formula E=mc2. This energy is emitted as a final strong burst of gravitational waves. It is these gravitational waves that LIGO has observed.

The new LIGO discovery is the first observation of gravitational waves themselves, made by measuring the tiny disturbances the waves make to space and time as they pass through the Earth.

The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed—and the discovery of gravitational waves during its first observation run. The US National Science Foundation leads in financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the STFC (Science and Technology Facilities Council) of the UK, and Australia (Australian Research Council) also have made significant commitments to the project. Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University, and the University of Wisconsin- Milwaukee. Several universities designed, built, and tested key components for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Florida, Stanford University, Columbia University of the City of New York, and Louisiana State University.

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom, and the University of the Balearic Islands in Spain.

LIGO Interferometer

LIGO is a national US facility for gravitational-wave research providing opportunities for the broader scientific community to participate in detector development, observation, and data analysis. The design and construction of LIGO was carried out by a team of scientists, engineers, and staff at the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT), and collaborators from over 80 scientific institutions world-wide that are members of the LSC (LIGO Scientific Collaboration).

LIGO is the world's largest gravitational wave observatory and a marvel of engineering. Comprising two enormous laser interferometers located thousands of kilometers apart, LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves. 3)

LIGO's interferometers are the largest ever built. With arms 4 km long, they are 360 times larger than the one used in the Michelson-Morley experiment (which had arms 11 m long).


Figure 1: Basic Michelson interferometer with Fabry Perot cavities. Mirrors placed near the beam splitter keep the laser contained within the arms. This increases the distance traveled by the beams, greatly improving LIGO's sensitivity to changes in arm length like those caused by gravitational waves (image credit: Caltech, MIT)

This is particularly important in the search for gravitational waves because the longer the arms of an interferometer, the farther the laser travels, and the more sensitive the instrument becomes. Attempting to measure a change in arm length 1,000 times smaller than a proton means that LIGO has to be more sensitive than any scientific instrument ever built, so the longer the better. But there are obvious limitations to how long one can build an interferometer. Even with arms 4 km long, if LIGO's interferometers were basic Michelsons they would still not be long enough to detect gravitational waves...and yet they are. How is this possible?

This dilemma was fixed by adding something called "Fabry Perot cavities" to the basic Michelson design. The Figure 1 shows how a basic Michelson interferometer is modified to include Fabry Perot cavities. It is created by adding mirrors near the beam splitter that continually reflect parts of each laser beam back and forth within the 4 km long arms about 280 times before they are merged together again.

With Fabry Perot cavities, LIGO's interferometer arms are effectively 1120 km long, making them 144,000 times bigger than Michelson's original instrument! This bit of 'mirror magic' greatly increases LIGO's sensitivity and makes it capable of detecting changes in arm-length thousands of times smaller than a proton, all while keeping the physical size of the interferometer manageable.

Those familiar with telescopes will recognize this effect. Increasing a telescope's focal length doesn't just increase the magnification of any given eyepiece, it also magnifies the tiniest vibrations making them visible in the eyepiece; the longer the focal length, the smaller the vibration you see in the eyepiece. In a telescope, these vibrations are unwelcome, but LIGO is designed to feel them. And at effectively 1120 km long, LIGO's arms can readily magnify the smallest conceivable vibrations enough that they are measurable.

Power Boosted Laser: Length isn't the only design factor important to LIGO's sensitivity; laser power is too. While increasing length increases the interferometer's sensitivity to vibrations, increasing laser power improves the interferometer's resolution. The more photons that merge at the beam splitter, the sharper the resulting interference pattern becomes, making it 'easier' to recognize a gravitational wave signature.

But there's a problem here too. For LIGO to operate at full sensitivity, its laser has to shine at 750 kW, but LIGO's laser enters the interferometer at most at 200 W. And just as it is impossible to build a 1120 km-long interferometer, building a 750 kW laser is also a practical impossibility. So how does LIGO boost the power of its laser 3750 times without actually using more power?


Figure 2: Basic Michelson Interferometer with Fabry Perot cavities and Power Recycling mirror. LIGO's interferometers use multiple power recycling mirrors, but for simplicity only one is shown in the diagram (image credit: Caltech, MIT)

More mirrors! Specifically, "power recycling" mirrors placed between the laser source and the beam splitter. Like the beam splitter itself, the power recycling mirror is only partly reflective (a 'one-way mirror'). Figure 2 shows schematically where such a mirror is located.

In a power recycling mirror, light from the laser first passes through the mirror to reach the beam splitter where it is split and directed down the arms of the interferometer. The instrument is aligned so well that nearly all of the reflected laser light from the arms follows a path back to the recycling mirrors rather than to the photodetector. Laser light coming from the ends of the arms is thereby reflected back into the interferometer (hence 'recycling') where those photons add to the ones just entering; more photons equals more power. This process greatly boosts the power of the beam without needing to generate a 750 kW beam at the outset.

The boost in power generated by this recycling process enhances the interference pattern that results when the two beams are superimposed after their long journey through the interferometer. Since we expect to see particular interference patterns when a gravitational wave passes by, the more prominent the pattern, the easier it is for us to recognize and confirm that we have, in fact, detected gravitational waves.

LIGO Technology (including updates)

• March 20, 2020: Scientists have developed a new type of deformable mirror that could increase the sensitivity of ground-based gravitational wave detectors. "In addition to improving today's gravitational wave detectors, these new mirrors will also be useful for increasing sensitivity in next generation detectors and allow detection of new sources of gravitational waves," says Huy Tuong Cao. — Cao is a research team leader at the University of Adelaide node of the Australian Center of Excellence for Gravitational Waves Discovery (OzGrav ) and a member of the University's School of Physical Sciences. 4)


Figure 3: The illustration shows the cross-section of a thermal bimorph mirror and its constituents. Controlling the temperature of the mirror changes the curvature of the reflected wavefront. Overlaid on the cross-section is the simulated radial stress, showing a concentration of stress at the boundary of the two layers, where the adhesive holds the structure together (image credit: Huy Tuong Cao)

- Detectors such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO ) are set to benefit from the new technology.

- Advanced LIGO measures faint ripples in space time called gravitational waves, which are caused by distant events such as collisions between black holes or neutron stars.

- Deformable mirrors, which are used to shape and control laser light, have a surface made of tiny mirrors that can each be moved, or actuated, to change the overall shape of the mirror.

- Published in The Optical Society's journal Applied Optics , Cao and colleagues have, for the first time, made a deformable mirror based on the bimetallic effect in which a temperature change is used to achieve mechanical displacement. 5)

- "Our new mirror provides a large actuation range with great precision," says Cao. "The simplicity of the design means it can turn commercially available optics into a deformable mirror without any complicated or expensive equipment. This makes it useful for any system where precise control of beam shape is crucial."

- The new technology was conceived by Cao and Aidan Brooks of LIGO as part of a visitor program between the University of Adelaide and LIGO Laboratory, funded by the Australian Research Council and National Science Foundation.

Building a better mirror

- Ground-based gravitational wave detectors use laser light traveling back and forth down an interferometer's two arms to monitor the distance between mirrors at each arm's end. Gravitational waves cause a slight but detectable variation in the distance between the mirrors.

- Detecting this tiny change requires extremely precise laser beam steering and shaping, which is accomplished with a deformable mirror.

- "We are reaching a point where the precision needed to improve the sensitivity of gravitational wave detectors is beyond what can be accomplished with the fabrication techniques used to make deformable mirrors," says Cao.

- Most deformable mirrors use thin mirrors to induce large amount of actuation, but these thin mirrors can produce undesirable scattering because they are hard to polish. The researchers designed a new type of deformable mirror using the bimetallic effect by attaching a piece of metal to a glass mirror. When the two are heated together the metal expands more than the glass, causing the mirror to bend.

- The new design not only creates a large amount of precise actuation but is also compact and requires minimum modifications to existing systems. Both the fused silica mirrors and aluminum plates used to create the deformable mirror are commercially available. To attach the two layers, the researchers carefully selected a bonding adhesive that would maximize actuation.

- "Importantly, the new design has fewer optical surfaces for the laser beam to travel through,” Cao says. "This reduces light loss caused by scattering or absorption of coatings."

Precision characterization

- Creating a highly precise mirror requires precision characterization techniques. The researchers developed and built a highly sensitive Hartmann wave front sensor to measure how the mirror's deformations changed the shape of laser light.

- "This sensor was crucial to our experiment and is also used in gravitational detectors to measure minute changes in the core optics of the interferometer," Cao says. "We used it to characterize the performance of our mirrors and found that the mirrors were highly stable and have a very linear response to changes in temperature."

- The tests also showed that the adhesive is the main limiting factor for the mirrors' actuation range.

- The researchers are currently working to overcome the limitation caused by the adhesive and will perform more tests to verify compatibility before incorporating the mirrors into Advanced LIGO.

• Designing instruments like LIGO's interferometers, capable of measuring a distance on the order of 10-19 m required inventing and refining innovative technology. Most of LIGO’s most impressive technology resides in its seismic isolation systems (which remove unwanted vibrations), vacuum systems (to make sure the laser light is kept pure), optics components (to preserve laser light and laser power), and computing infrastructure (to handle the mindboggling amount of data that LIGO collects). These systems are like LIGO's internal organs. If any one fails, the whole instrument suffers (Ref. 3).

While each of these components is a feat of engineering in itself, without working seamlessly together, LIGO, as a single multifaceted instrument could never achieve its scientific goals. A basic overview of each of LIGO's critical systems is provided.

Seismic Isolation: LIGO’s greatest strength is also its greatest weakness. Since LIGO is designed to sense the smallest conceivable motions of mirrors caused by the passage of a fleeting gravitational wave, it is also extremely sensitive to all vibrations near (such as trucks driving on nearby roads) and far (earthquakes on the other side of the world). Without taking extraordinary measures, any number of Earthly vibrations could move LIGO’s primary mirrors (LIGO scientists call them “test masses”) enough to hide a gravitational wave signal. Isolating LIGO from as much environmental vibration as possible is the linchpin in LIGO’s quest to feel gravitational waves. To that end, LIGO uses multiple means of eliminating vibration falling into two broad categories: “active” and “passive” damping systems.

Active Damping: The first line of defense against unwanted vibration is LIGO’s “active” damping system. The Internal Seismic Isolation (ISI) system consists of devices that sense ground movements and then deliberately perform counter movements to eliminate them, keeping the instrument motion-free.

Obviously, the local environment is always creating vibrations of one form or another. LIGO’s ISI system contains sensors designed to feel different frequencies caused by different environmental vibrations. These sensors work side-by side and send their signals to a computer that combines the effects of all of these motions and then generates a net counter-motion to cancel all of the vibrations simultaneously. It is very similar to how noise-canceling headphones work!

Passive Damping: LIGO’s passive damping system holds the all-important mirrors perfectly still through a 4-stage pendulum called a "quad". In the quad, LIGO’s test masses (its mirrors) are suspended at the end of four pendulums by 0.4 mm thick fused-silica (glass) fibers. The "Main Chain" side faces the laser beam, while the "Reaction mass" side helps to keep the test mass steady from noise not associated with sources from space. This configuration absorbs any movement not completely canceled out by the active (ISI) system. The sheer mass of the suspension components (each mirror weighs 40 kg) also helps to prevent motion of the mirrors thanks to the Law of Inertia.

Working together, these active and passive vibration damping systems ensure that LIGO's lasers and mirrors are isolated from as much external noise and vibration as is physically possible.

Vacuum: LIGO contains one of the largest and purest sustained vacuums on Earth. In volume, it is surpassed only by the Large Hadron Collider in Switzerland. The atmospheric pressure inside LIGO's vacuum tubes is one-trillionth that of air pressure at sea level. LIGO needs such a good vacuum for two reasons:

1) Air – or even just a few molecules of air – can create noise which masks the tiny changes in distance between mirrors we seek to detect. One way this can happen is due to Brownian motion, or the fact that everything with a temperature above absolute zero is moving with heat energy. Molecules of air hitting the mirrors can cause them to move, masking the gravitational waves. Another way residual air in the laser light path causes problems is similar to the shimmering one sees over a hot road – air has an ‘index of refraction’, and like a glass lens, can change the path of light. Even just a few molecules passing through the laser light beam can affect the apparent distance between the mirrors, once again masking the subtle effects of a passing gravitational wave.

2) The second critical reason for operating in a vacuum is to eliminate the chances that dust will drift into the path of the laser, or worse, onto a mirror causing some of the light to scatter (i.e., be reflected in some random direction away from its path).

Without operating in such a high quality vacuum, LIGO’s lasers would be absorbed and deflected enough to create unwanted interference patterns that might be mistaken for or drown out a signal from a gravitational wave. Losing any laser photons could cripple LIGO’s ability to detect gravitational waves.

Creating such a large volume of empty space on Earth was no easy task. Many techniques were used to remove all the air and other molecules from LIGO’s vacuum tubes:

• The tubes were heated to between 150ºC and 170ºC for 30 days to drive out residual gas molecules.

• Turbo-pump vacuums (like little jet engines that create suction instead of thrust) sucked out the bulk of the air contained in the tubes

• Ion pumps then extract individual remaining gas molecules by electrically charging them and then attracting them away with opposite charge, like a magnet. Since the metal inside the vacuum chamber is always emitting some gaseous molecules ("outgassing"), these pumps operate continuously in order to maintain the pristine vacuum inside the tubes.

It took 40 days (1100 hours) to remove all 10,000 m3 of air and other residual gases from each of LIGO’s vacuum tubes to reach an air pressure one-trillionth that at sea level.

Optics System: LIGO's optics system consists of lasers, a series of mirrors, and a photodetector (a device that measures varying light levels). In order to measure a movement thousands of times smaller than a proton, LIGO's optical components must operate harmoniously and with unprecedented precision. It all begins with the main laser.

LASER: We all encounter lasers daily in laser pointers, in cat toys, or in the barcode scanners at the grocery store. Because of their omnipresence most of us tend to take them for granted without really knowing how they work. If you're one of those people, you are not alone! If you want to know how they work, Cambridge University’s “Naked Science Scrapbook” video, “How do lasers work?” provides a fun, easy-to-understand explanation. Once you grasp the basic principles, understanding LIGO's laser is a snap!

The first thing to understand is that the word, "laser" is actually an acronym for “Light Amplification by the Stimulated Emission of Radiation”. This means that "laser" refers to a process more than a thing, but most of us now use the term ubiquitously to refer to the device that generates the laser beam, or the beam itself. This might be splitting hairs, but when it comes to understanding LIGO's laser, it's an important distinction.

The heart of LIGO is its 200 W laser beam. But you might be surprised to know that the beam doesn't start out at 200 W. It actually takes four steps to amplify its power and refine its wavelength to a level of precision never before seen in a laser of this kind.

The very first glimmer of light that ultimately becomes LIGO's powerful laser emerges from a laser diode, which uses electricity to generate an 808 nanometer (nm) near-infrared beam of about 4 W. This is the same kind of device that a typical laser pointer uses. While 4 W doesn't seem like a lot, the laser in the average laser pointer shines at less than 5 mW (milliWatt). So LIGO's 4 W beam is 800 times more powerful than that laser pointer you use to entertain your pet!

The second step in boosting LIGO's laser up to 200 W occurs when the 4 W beam enters a device called a Non-Planar Ring Oscillator (NPRO). The NPRO consists of a remarkably small boat-shaped crystal about the size of a pinky fingernail! The 4 W beam bounces around inside this crystal and stimulates the emission (the S and E in LASER) of a 2 W beam with a wavelength of 1064 nm (in the invisible infrared part of the spectrum).

Step three in LIGO's laser amplification occurs when the now 2 W beam enters another amplifying device that boosts the 1064 nm beam from 2 to 35 W. Getting from 35 W to 200 W requires a different kind of device, however. So the 35 W beam is sent through a device called a High Powered Oscillator (HPO), which performs further amplification and refinement, and generates the 200 W beam of pristine "lased" light. This is the beam that ultimately enters LIGO's interferometer.

This multi-stage amplified laser is required for LIGO because of its need to continually produce a pristine single wavelength of light. In fact, LIGO's laser is the most stable ever made to produce light at this wavelength. This stability is one of several factors critical for LIGO's ability to detect gravitational waves.

Mirrors: LIGO’s mirrors are the highest quality available, both in material and shape. Made of very pure fused silica glass, they absorb just one in 3-million photons that hit them. This is important because it means that most of the laser light is reflected. Reflecting most of the light means that the mirrors are not prone to heating. Too much heat from the laser could alter the mirror shapes enough that they degrade the quality of the laser light. Any degradation would hamper LIGO's ability to distinguish a gravitational wave from environmental noise. Lastly, the highly reflective surface preserves laser power — again, the more power, the better LIGO's resolution.

The mirrors also refocus the laser, keeping the beam traveling coherently, meaning that it doesn't spread out as it travels throughout its multiple reflections before encountering the photodetector.

Finally, the mirrors were polished so precisely that the difference between the theoretical design (the perfect mirror shape as designed on a computer) and the actual polished mirror surface is measured in atoms! This is critical because, with all the reflections it goes through, each laser in each arm travels about 1120 km before being merged with its partner and reflected one last time to the photodetector. Maintaining the stability and purity of the laser light is one of LIGO's biggest challenges.

Computation and Data Collection: Computers are required both to run the LIGO instruments and to process the data that it collects.

When it is in 'observing' mode, LIGO generates terabytes (1000's of gigabytes) of data every day. All of this information must be transferred to a network of supercomputers for storage and archiving. Such supercomputers are located at each of the observatories, at Caltech, at MIT, and at various other institutions. Once the data is secured, scientists can use customized computer programs to scour the data for gravitational waves.

The amount of data LIGO collects is as incomprehensively large as gravitational wave signals are small. LIGO's archive already holds the equivalent over over 1-million DVDs of data and will add the equivalent of about 178-thousand DVDs each year to its archive. In actual numerical terms, the data archive at Caltech holds over 4.5 Petabytes (PB, or 4.5 x 1015bytes) of data, and will grow at a rate of about 0.8 PB (800 terabytes) per year. What's a petabyte? If you wanted to count up to a petabyte by counting one byte per second, it would take you 35.7 million years to reach one petabyte!

Storing information is one thing; processing it is another. Processing and analyzing all of LIGO's data requires a vast computing infrastructure. For LIGO's first observing run in 2015, the LIGO Lab will provide 35 MSU (million service units) worth of computing cycles/time. This is equivalent to running a modern 4-core laptop computer for 1,000 years! The amount of computing time is expected to grow by a factor of 10 to around 400 MSU by the time LIGO has completed its third observing run.

LIGO Facility

LIGO's original instrument, a largely 'proof of concept' model dubbed "Initial LIGO", engaged in "science observations" from 2002 to 2010. No detections were made in that time, but enormous strides in detector engineering were achieved as a result of what was learned during that initial run. 2010 marked the end of the Initial LIGO project, and as planned, between 2010 and 2014, both interferometers were completely overhauled to incorporate much more sophisticated engineering.

This "Advanced LIGO" project successfully improved the capabilities of the detectors, and within days of turning on the new and improved instruments, LIGO made its first detection of gravitational waves, generated by a pair of colliding black holes some 1.3 billion light years away. Since that historic day, LIGO's engineers have continued to improve the detectors' sensitivities. The success of these improvements is evidenced by the many more gravitational wave detections that have since been made. Ultimately, with continued refinement and upgrading, Advanced LIGO's detectors will achieve a sensitivity 10 times greater than Initial LIGO, bringing 1000 times more galaxies into LIGO's observational range.


Figure 4: Photo of the LIGO facility on Livingston, Louisiana (image credit: LIGO)

Light is—or has been up until now—the only way to study objects in the universe (actually the entire electromagnetic spectrum). This includes everything from the Moon, all the way out to the most distant objects ever observed. Astronomers and astrophysicists use observatories that can see in not only visible light, but in all other parts of the electromagnetic spectrum, to study objects in the universe. And we’ve learned an awful lot. But things will change with this announcement. 6) 7)

Gravitational waves are a new way to study notoriously difficult things to observe like black holes and neutron stars. Black holes emit no light at all, and their characteristics and properties are inferred from cause and effect relationships with objects near them. But the detection of gravitational waves holds the promise of answering questions about black holes, neutron stars, and even the early days of our universe, including the Big Bang.

It’s almost impossible to overstate the magnitude of this discovery. Once we understand how to better detect and observe gravitational waves, we may come to a whole new understanding of the universe, and we may look back on this day as truly ground-breaking and revolutionary.

Some background:

Experiments to detect gravitational waves began with Weber and his resonant mass detectors in the 1960s, followed by an international network of cryogenic resonant detectors. Interferometric detectors were first suggested in the early 1960s and the 1970s (Ref. 2). A study of the noise and performance of such detectors, and further concepts to improve them, led to proposals for long-baseline broadband laser interferometers with the potential for significantly increased sensitivity.

The existence of gravitational waves was first demonstrated in the 1970s and 80s by Joseph Taylor, Jr., and colleagues. Taylor and Russell Hulse discovered in 1974 a binary system composed of a pulsar in orbit around a neutron star. Taylor and Joel M. Weisberg in 1982 found that the orbit of the pulsar was slowly shrinking over time because of the release of energy in the form of gravitational waves. For discovering the pulsar and showing that it would make possible this particular gravitational wave measurement, Hulse and Taylor were awarded the Nobel Prize in Physics in 1993.

• October 3, 2017: The LIGO Laboratory, comprising LIGO Hanford, LIGO Livingston, Caltech, and MIT are excited to announce that LIGO’s three longest-standing and greatest champions have been awarded the 2017 Nobel Prize in Physics: Barry Barish and Kip Thorne of Caltech and Rainer Weiss of MIT. The LIGO Scientific Collaboration is absolutely delighted to congratulate Rainer Weiss, Barry Barish, and Kip Thorne on winning the 2017 Nobel Prize in Physics. Weiss and Thorne are two of the founders of the LIGO project. Barish was the Principal Investigator of LIGO from 1994 to 2005, during the period of its construction and initial operation. 8)


Figure 5: Image credit: LIGO/Caltech/MIT

Decades in the Making: While from the outside, it may seem surprising that this Nobel Prize was awarded a scant 2 years after the discovery of gravitational waves (often, Nobel Prizes are awarded many years after discoveries), for the three laureates, it actually comes at the culmination of decades of effort. LIGO may have only recently detected gravitational waves, but its journey to doing so began nearly 45 years ago.

The very idea for LIGO came to Rainer Weiss in the early 1970’s when, as associate professor of physics at MIT, he had to find a way to explain gravitational waves (a prediction of general relativity) to his students. In an interview with MIT news writer, Jennifer Chu, Weiss recalled his revelation:

“That was my quandary at the time, and that’s when the invention was made. I said, ‘What’s the simplest thing I can think of to show these students that you could detect the influence of a gravitational wave? ... The obvious thing to me was, let’s take freely floating masses in space and measure the time it takes light to travel between them. The presence of a gravitational wave would change that time. [Later] knowing what you could do with lasers, I worked it out: Could you actually detect gravitational waves this way? And I came to the conclusion that yes, you could detect gravitational waves ....”

Sometime later, in 1972, Weiss carefully thought through and wrote down his idea, subsequently publishing it as a paper titled, "Electromagnetically Coupled Broadband Gravitational Antenna" . In this paper, Weiss described in great detail, the design and promise of using laser interferometry to detect gravitational waves. Within its 22 pages, the paper laid out the blueprint for the Laser Interferometer Gravitational-Wave Observatory (at the time, Weiss called it an antenna.) — And with that, LIGO was born (at least on paper).

Today, no one disputes the fact that LIGO owes its very existence to Rainer Weiss.

Transforming LIGO from concept to reality, however, would take another 20 years. There is no way that one man, even the enigmatic Rai Weiss, could get LIGO built by himself. So early on in those first two decades of navigating a circuitous path, trying to keep the idea of a radical scientific experiment alive, Rai found an ideal partner. In 1975, a fateful meeting between Weiss and Kip Thorne of Caltech would set into motion the development of one of the most complicated and risky scientific experiments ever conceived. Thorne, already a highly respected, accomplished, and influential theoretical physicist (with expertise in gravitational waves since the late 1960s), commanded a level of respect among peers, colleagues, research groups, and funding agencies that was unequalled. Kip’s contributions in setting the astrophysics goals were central to the design of the Observatories and the first instruments, and his presence lent a high level of validity and credence to the idea.

As much as Rai is responsible for conceiving of LIGO, Kip Thorne is equally responsible for convincing countless others of LIGO’s potential for success. As a result of his interactions with Weiss, Thorne convinced Caltech to create a gravitational-wave research group. The group would be led by Ron Drever, from Glasgow University (Drever, considered a co-founder of LIGO, would go on to co-invent the Pound–Drever–Hall technique for laser stabilization, which was critical to LIGO’s ability to detect gravitational waves). Sadly, Drever passed away earlier this year, but not before he learned of LIGO’s success.

Thus was formed the LIGO triumvirate. With Rai’s vision and original idea, Thorne’s brilliant theoretical physics mind, Drever’s brilliant engineering, and all of their remarkable capacities to champion the effort, nothing would stand in the way of LIGO’s evolution from concept to reality.

In 1989, Weiss and Thorne, along with Ron Drever, Fred Raab, and Robbie Vogt, submitted a proposal for LIGO to the U.S. National Science Foundation (NSF) which has enthusiastically supported this effort from the start. The proposal included Rai’s original design for the instrument, updated to include engineering and technology innovations that had occurred in the intervening 17 years. To their credit, and at great risk, the NSF approved the proposal. In 1994 construction began on the twin LIGO detectors in Hanford, Washington and Livingston, Louisiana.

That’s when Barry Barish joined the trio. Barish became LIGO Principal Investigator in 1994, helping the growing cadre move from a small intensely focused group cooking up basic ideas to a large and broad team that could actually deliver the Observatories and hardware. Barry came with knowledge of how to build a project that would succeed, but more importantly, he came with vision and an incredible ability to strategize and make the science happen. His insights led not only to construction of the Observatories and the initial detectors, but also to the creation of the LIGO Scientific Collaboration and to the successful proposal to build Advanced LIGO—the instrument that would finally make the first detection. Barry was the perfect complement to Kip and Rai to bring LIGO to success.

The rest is history: Another 21 years would pass before the efforts of Rai Weiss, Kip Thorne, and Barry Barish finally paid off. On September 14, 2015, LIGO detected the gossamer flutters of spacetime created by the merging of two massive black holes some 1.3 billion light years away. Nearly half-a-century after its conception, LIGO had fulfilled its destiny.

So, while this Nobel Prize is being awarded barely two years after LIGO’s historic detection, it acknowledges 45 years of effort: from conception, through design, planning, testing and prototyping, through decades of research and engineering, invention and innovation, advances in computing, lasers, and optics, and especially the championing and advocating of three remarkable men: Barry Barish, Kip Thorne, and Rai Weiss. There can be no doubt; their recognition by the Nobel Committee is well earned. — Congratulations to you all!

• By the early 2000s, a set of initial detectors was completed, including TAMA 300 in Japan, GEO600 in Hannover, Germany, the LIGO (Laser Interferometer Gravitational-Wave Observatory) in the United States, and Virgo in Italy. Combinations of these detectors made joint observations from 2002 through 2011, setting upper limits on a variety of gravitational-wave sources while evolving into a global network. In 2015, Advanced LIGO became the first of a significantly more sensitive network of advanced detectors to begin observations.

Status of the LIGO-Virgo Network gravity-wave detection

• January 6, 2021: Warning: Neutron Star Collision Imminent. On 17 August 2017, an alert went out roughly 40 minutes after the LIGO observatory detected gravitational waves from a pair of colliding neutron stars. This alert sent telescopes worldwide slewing rapidly in an all-hands-on-deck effort to image the fireworks show accompanying the merger. 9)

- But what if that alert had gone out before the collision?

When Stars Collide

- When neutron stars merge, they are expected to produce both a gravitational-wave signal from their inspiral and a host of spectrum-spanning electromagnetic signatures from before, during, and after the collision.

- The event captured in August 2017, known as GW170817, is one of just two binary neutron star mergers we’ve observed with LIGO and its European sister observatory Virgo so far. But these collisions are likely to become a common detection in the future, particularly as LIGO and Virgo continue to upgrade and approach their design sensitivity.


Figure 6: Artist’s impression of the electromagnetic signal from the merger of two neutron stars. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

Reducing the Lag

- In the case of GW170817, interferometer glitches and data transfer issues prevented an alert from going out until 40 minutes after the merger. By the time follow-up telescopes had been advised where to search for the collision, the relevant region of the sky was below the horizon; the first manual follow-up couldn’t be conducted until nearly 8 hours after the merger.

- To capture a neutron star merger without this delay, we’ll clearly need reduce the alert lag time — but could LIGO send out alerts even before the neutron stars collide? A study led by Surabhi Sachdev (The Pennsylvania State University) has recently demonstrated that a LIGO data analysis pipeline will be capable of providing advance warning for some future mergers. 10)

Early Warning

- Sachdev and collaborators analyze the performance of GstLAL, an early warning gravitational-wave detection pipeline for LIGO/Virgo that looks for signals of neutron star binaries approaching merger.


Figure 7: Cumulative distributions of the sky localizations of injected binary neutron star merger signals recovered by the authors’ pipeline. Results show that at least one event per year will be detected before merger and localized to within 100 deg2 (Sachdev et al. 2020)

- By injecting merger signals into a simulated dataset, the authors show that the pipeline is able to recover many of these signals 10–60 seconds before the merger occurs. These early detections are made possible when mergers happen nearby, so that a large signal-to-noise ratio can accumulate as the neutron stars inspiral in their last few moments collision.

A Well-Notified Future

- Sachdev and collaborators predict that when LIGO and Virgo reach their design sensitivity, nearly 50% of total detectable mergers will be spotted at least 10 seconds before merger — a total that amounts to perhaps 6 to 60 events per year, depending on the neutron star merger rate.

- With rapid localization and quick relay times for alerts, this early-warning system could provide follow-up telescopes with the opportunity to capture neutron-star mergers in real time, as they happen. Such observations would provide insight into what magnetic conditions are like around the neutron stars, how heavy elements are synthesized, and whether binary neutron stars are the source of fast radio bursts.

• December 25, 2020: Our collaborations report new constraints from two searches for continuous gravitational waves emitted by neutron stars. In a first paper, the LIGO and Virgo collaborations present results from an all-sky search for continuous gravitational waves from unknown neutron stars in binary systems. In a second paper, which is the first using the full data set from the third observing run (O3), our colleagues from the KAGRA (Kamioka Gravitational Wave Detector) collaboration have now officially joined forces with LIGO and Virgo scientists in looking for potential signals from the enigmatic X-ray pulsar J0537-6910. This second analysis also relied on precise timing data from NASA's X-ray telescope NICER on board the International Space Station. Read more about both works in the science summaries here and here. 11)

• November 23, 2020: The LIGO Laboratory congratulates Siddharth Soni of Louisiana State University for winning this year’s LIGO Laboratory Award for Excellence in Detector Characterization and Calibration for his outstanding work to reduce transient noise due to stray light in the Advanced LIGO detector at LIGO Livingston Observatory during the Observing Run O3. 12)


Figure 8: Siddharth Soni (Louisiana State University) is the winner of the 2020 Award for Excellence in Detector Characterization and Calibration (Photo: S. Soni)

- Soni identified, characterized, and mitigated multiple sources of stray light at the LIGO Livingston Observatory. Through his research, he discovered new light scattering paths and characterized new methods to mitigate these newly identified noise sources. Soni’s work directly reduced one of the most challenging sources of LIGO transient noise, one associated with multiple retracted LIGO-Virgo candidate event alerts, and allowed the confident detection of subsequent distant, weak gravitational wave signals.

- Soni’s work is a major success for the greater LIGO Detector Characterization effort. Identifying noise transients, linking them to particular interferometer conditions, and eliminating the source of the noise is one of the most effective paths to improved detector sensitivity and greater astrophysical discovery potential for LIGO.

- Soni will receive a $1000 prize and will present an invited seminar at one of the LIGO Laboratory sites (LIGO-Hanford, LIGO-Livingston, Caltech, or MIT) to share his achievements with LIGO Laboratory members. Soni will receive an award certificate remotely during the LIGO-Virgo Collaboration meeting in March 2021.

• November 3, 2020: The LIGO Scientific Collaboration is delighted to congratulate our LSC (LIGO Scientific Collaboration) OzGrav colleagues David Blair, David McLelland, Susan Scott and Peter Veitch — recipients of the 2020 Prime Minister's Prize for Science, Australia’s most prestigious award for outstanding achievements in scientific research. Their award is in recognition of the recipients’ many years of dedicated work that led to the discovery of gravitational waves, and which continues to yield new discoveries and scientific advances for Australia. 13)


Figure 9: Winners of the 2020 Australian Prime Minister's Prize for Science. Clockwise from the top-left are David Blair, David McLelland, Peter Veitch, and Susan Scott (photo credit: Australian Government)

• October 28, 2020: After several months of thorough analysis, the LIGO Scientific Collaboration and the Virgo Collaboration have released an updated catalog of gravitational wave detections, GWTC-2. The catalog contains 50 gravitational wave detections, including 39 new signals from black-hole or neutron-star collisions detected in just the first half of the LIGO/Virgo third observing run, O3a, which ran from April 1 to Oct 1, 2019. O3a's contributions to the catalog more than tripled the number of confirmed detections (the first two observing runs combined yielded 11 detections, which were included in the first catalog, GWTC-1). The new set includes some of the most interesting systems we have seen so far, and enables qualitatively new studies of astrophysical populations and fundamental physics. 14)


Figure 10: The second catalog of gravitational wave events "GWTC-2", was published today. Since September 2015, LIGO/Virgo have detected 50 gravitational waves, including 39 new ones just from the first half of O3. This graphic illustrates the current total number and masses of LIGO/Virgo black hole and neutron star merger events (in blue) compared with previously known black holes (in purple). Mergers are indicated by arrows connecting two progenitor objects with a final merged object of higher mass (image credit: LIGO-Virgo / Northwestern U / Frank Elavsky & Aaron Geller)

- The sharp increase in the number of detections was made possible by significant improvements to the instruments with respect to previous observation periods. These included increased laser power, improved mirrors and, remarkably, the use of quantum squeezing technology. All together, these enhancements resulted in a ~60% improvement in the range to which signals can be detected over the range achieved in the previous observing run, O2.

- The detectors were also able to operate without interruption for longer periods of time than in the past, increasing opportunities to capture passing gravitational wave signals. We use a statistic called a "duty cycle" to describe these times. Virgo's duty cycle was 76%, meaning it was operating for 76% of the time between April 1 and October 1. LIGO Hanford and Livingston detectors achieved duty cycles of 71% and 76% respectively. All three were operating simultaneously for 44.5% of the time, while for nearly 82% of time, there were two detectors operating in unison. These are remarkable feats given the complexity of the instruments and how vulnerable they are to environmental and seismic conditions.


Figure 11: Cumulative count of gravitational wave events detected by LIGO/Virgo, split by observing run. O1 yielded 3 detections, O2 yielded 8 detections, and just the first half of O3 yielded 39 detections! Collectively, the 2.5 observing runs represented here covered nearly 600 days since Sept. 2015, though some days within runs, no detectors were operating due to maintenance or environmental factors (image credit:LVC (LIGO/Virgo Collaboration)

- With all of these new signals, we can begin to better understand the populations of black holes and neutron stars that exist in the greater universe. By analyzing the entire population of binary black hole mergers simultaneously, we can maximize the astrophysical information we extract. We infer that the distribution of black hole masses does not follow a simple power-law distribution. Measuring the deviations from this power law will enable us to learn about the formation of these black holes, be they from stellar deaths or previous collisions of smaller black holes. Considering the entire population together, we can also study difficult-to-measure properties such as black hole spin. In this catalog, we find that some merging black holes have spins that are misaligned with their orbital angular momentum, raising questions about how they formed. This discovery and further observations will allow us to probe the regimes in which these binaries formed.

- We can also use the many signals in the updated catalog to put Einstein’s theory of gravity (general relativity) to the test in more and better ways than before. This was done by comparing the data against predictions from the theory, and constraining possible deviations. The results from multiple signals were combined using new statistical methods to obtain the tightest constraints so far on the properties of gravity in the strong, highly-dynamical regime of black hole mergers. With the new catalog, LIGO and Virgo were also able to directly study the properties of the remnant objects produced during the mergers: by measuring the vibrations of these objects, and by ruling out potential echoes after the main signals, LIGO and Virgo confirmed that the remnants behaved as we expect from black holes in Einstein’s theory.

- The 39 new detections reported in the new catalog correspond to only the first six months of LIGO and Virgo’s third observing run. Results from the remaining five months of O3 are currently being analyzed. In the meantime, the LIGO and Virgo instruments are undergoing upgrades to prepare for the fourth observing run (planned to begin in mid-2022), which will also include the KAGRA detector in Japan. More exciting discoveries are on the horizon!

• September 2, 2020: A lopsided merger of two black holes may have an oddball origin story, according to a new study by researchers at MIT and elsewhere. 15) 16)

- The merger was first detected on April 12, 2019 as a gravitational wave that arrived at the detectors of both LIGO (the Laser Interferometer Gravitational-wave Observatory), and its Italian counterpart, Virgo. Scientists labeled the signal as GW190412 and determined that it emanated from a clash between two David-and-Goliath black holes, one three times more massive than the other. The signal marked the first detection of a merger between two black holes of very different sizes.

- Now the new study, published today in the journal Physical Review Letters, shows that this lopsided merger may have originated through a very different process compared to how most mergers, or binaries, are thought to form. 17)

- It’s likely that the more massive of the two black holes was itself a product of a prior merger between two parent black holes. The Goliath that spun out of that first collision may have then ricocheted around a densely packed “nuclear cluster” before merging with the second, smaller black hole — a raucous event that sent gravitational waves rippling across space.

- GW190412 may then be a second generation, or “hierarchical” merger, standing apart from other first-generation mergers that LIGO and Virgo have so far detected.

- “This event is an oddball the universe has thrown at us — it was something we didn’t see coming,” says study coauthor Salvatore Vitale, an assistant professor of physics at MIT and a LIGO member. “But nothing happens just once in the universe. And something like this, though rare, we will see again, and we’ll be able to say more about the universe.”

- Vitale’s coauthors are Davide Gerosa of the University of Birmingham and Emanuele Berti of Johns Hopkins University.


Figure 12: A lopsided merger of two black holes may have unusual origins, based on a reanalysis of LIGO data (image credit: MIT News)

A struggle to explain

- There are two main ways in which black hole mergers are thought to form. The first is known as a common envelope process, where two neighboring stars, after billions of years, explode to form two neighboring black holes that eventually share a common envelope, or disk of gas. After another few billion years, the black holes spiral in and merge.

- “You can think of this like a couple being together all their lives,” Vitale says. “This process is suspected to happen in the disc of galaxies like our own.”

- The other common path by which black hole mergers form is via dynamical interactions. Imagine, in place of a monogamous environment, a galactic rave, where thousands of black holes are crammed into a small, dense region of the universe. When two black holes start to partner up, a third may knock the couple apart in a dynamical interaction that can repeat many times over, before a pair of black holes finally merges.

- In both the common envelope process and the dynamical interaction scenario, the merging black holes should have roughly the same mass, unlike the lopsided mass ratio of GW190412. They should also have relatively no spin, whereas GW190412 has a surprisingly high spin.

- “The bottom line is, both these scenarios, which people traditionally think are ideal nurseries for black hole binaries in the universe, struggle to explain the mass ratio and spin of this event,” Vitale says

Black hole tracker

- In their new paper, the researchers used two models to show that it is very unlikely that GW190412 came from either a common envelope process or a dynamical interaction.

- They first modeled the evolution of a typical galaxy using STAR TRACK, a simulation that tracks galaxies over billions of years, starting with the coalescing of gas and proceeding to the way stars take shape and explode, and then collapse into black holes that eventually merge. The second model simulates random, dynamical encounters in globular clusters — dense concentrations of stars around most galaxies.

- The team ran both simulations multiple times, tuning the parameters and studying the properties of the black hole mergers that emerged. For those mergers that formed through a common envelope process, a merger like GW190412 was very rare, cropping up only after a few million events. Dynamical interactions were slightly more likely to produce such an event, after a few thousand mergers.

- However, GW190412 was detected by LIGO and Virgo after only 50 other detections, suggesting that it likely arose through some other process.

- “No matter what we do, we cannot easily produce this event in these more common formation channels,” Vitale says.

- The process of hierarchical merging may better explain the GW190412’s lopsided mass and its high spin. If one black hole was a product of a previous pairing of two parent black holes of similar mass, it would itself be more massive than either parent, and later significantly overshadow its first-generation partner, creating a high mass ratio in the final merger.

- A hierarchical process could also generate a merger with a high spin: The parent black holes, in their chaotic merging, would spin up the resulting black hole, which would then carry this spin into its own ultimate collision.

- “You do the math, and it turns out the leftover black hole would have a spin which is very close to the total spin of this merger,” Vitale explains.

No escape

- If GW190412 indeed formed through hierarchical merging, Vitale says the event could also shed light on the environment in which it formed. The team found that if the larger of the two black holes formed from a previous collision, that collision likely generated a huge amount of energy that not only spun out a new black hole, but kicked it across some distance.

- “If it’s kicked too hard, it would just leave the cluster and go into the empty interstellar medium, and not be able to merge again,” Vitale says.

- If the object was able to merge again (in this case, to produce GW190412), it would mean the kick that it received was not enough to escape the stellar cluster in which it formed. If GW190412 indeed is a product of hierarchical merging, the team calculated that it would have occurred in an environment with an escape velocity higher than 150 km/s. For perspective, the escape velocity of most globular clusters is about 50 km/s.

- This means that whatever environment GW190412 arose from had an immense gravitational pull, and the team believes that such an environment could have been either the disk of gas around a supermassive black hole, or a “nuclear cluster” — an incredibly dense region of the universe, packed with tens of millions of stars.

- “This merger must have come from an unusual place,” Vitale says. “As LIGO and Virgo continue to make new detections, we can use these discoveries to learn new things about the universe.”

- This research was funded, in part by the U.S. National Science Foundation and MIT’s Solomon Buchsbaum Research Fund.

• August 19, 2020: When LIGO, the Laser Interferometer Gravitational-Wave Observatory, and its European counterpart, Virgo, detect a gravitational ripple from space, a public alert is sent out. That alert lets researchers know with a decently high confidence that this ripple was probably caused by an exceptional cosmic event, such as the collision of neutron stars or the merging of black holes, somewhere in the universe. 18)

- Then starts the scramble. A pair of researchers is assigned to the incoming event, analyzing the data to get a preliminary location in the sky whence the ripple emanated. Telescopes are pointed in that direction, more data is amassed, and the pair of researchers conducts further followup studies to try to determine what kind of event caused the wave.

- “I often think of it as if we’re in a dark forest and listening to the ground,” says Eva Huang, a third-year Department of Physics graduate student in Assistant Professor Salvatore Vitale’s lab in the MIT Kavli Institute for Astrophysics and Space Research (MKI). “From the footsteps, we’re trying to guess what kind of animal is passing by.”

- The LIGO-Virgo Collaboration keeps a rotation system to determine which researchers get to investigate the latest detection. Sylvia Biscoveanu, a second-year graduate student also in Vitale’s lab, was next on the list when LIGO suspended its third observational run due to Covid-19. If a cosmic event happens in the universe and there’s no one there to detect it, did it even happen?

Data analysis in isolation

- When MIT similarly scaled back on-campus research in mid-March due to the coronavirus pandemic, the LIGO team at MKI adapted quickly to the new work-from-home normal. “Our work is physically less dependent on being at MIT,” says Vitale, who is also a member of the LIGO Scientific Collaboration. “Still, there are consequences.”

- For Biscoveanu, working from home has entailed being at her computer for at least eight hours a day. “In terms of actually being able to do my research, I haven’t suffered,” she says. What has suffered is her ability to exchange ideas with other members of the LIGO group at MIT. “I had just moved to a bigger office with a bunch of graduate students, and we were really looking forward to being able to talk to each other and ask questions regularly,” says Biscoveanu. “I definitely don’t get as much of that at home.”

- Mentorship also looks different when everyone is at home. Vitale has always had an open-door policy with his graduate students. “I do weekly meetings with my students, but on top of that I had close-to-daily interactions with them,” he says. Unless his door was closed, Vitale says, his students could come in and talk anytime. That immediate connection, he has found, is hard to replicate in the digital world.

- “The thing I tell my students is that we don’t work in a hut where everyone is making their own project and then it’s done,” says Vitale. “Research is more than the sum of its parts.” One advantage of working in a group is the ability to turn to a colleague to discuss a paper you just read, a problem you’re facing, or a crazy idea you had the night before. That’s harder to do when everyone is stuck in their own hut.

- “Now you have to go in the chat room or arrange a teleconference if you want to ask a question,” says Ken Ng, a third-year graduate student in the Vitale group. Ng uses gravitational waves to study particle physics, with his work focusing on axions, a proposed elementary particle that is orders of magnitude smaller than the tiniest particle observed. Telecons and Slack, he has found, can be particularly inefficient when you’re trying to quickly sketch out an idea. “I’m actually thinking of buying a white board,” he says.

Space never stops

- When the third observation run was suspended a month before it was supposed to end, it had collected 56 gravitational wave candidates. In comparison, the first two runs combined amassed a total of 11 candidates. So even though fresh data isn’t arriving in the lab, the work hasn’t ceased, and LIGO scientists are scrutinizing the data from home. “If the pandemic had happened a few months before, we could have missed half the data,” says Ng, looking on the positive side.

- Compared to the other members of the lab, Ng is no pandemic rookie. When the Covid-19 pandemic struck, he thought, “Again?” Ng, who is from Hong Kong, faced the SARS outbreak in 2002 and considers himself the pandemic veteran of the group. That experience has kept him from panicking these days. “I know the importance of social distancing and mask-wearing,” he explains.

- Still, for some in the group, social distancing has led to less productivity and feelings of guilt. “I sometimes feel that, because my work is less impacted, I cannot allow myself to feel frustrated,” says Huang. Her work — analyzing LIGO data to decipher the cosmic events responsible for detected waves — can be done at home, unlike researchers who need to be physically in-lab. Throughout the pandemic, Huang has worked hard to combat the feeling that she needs to earn permission to be self-compassionate. “I can be, and need to be, kind to myself during this time.

- All are looking forward to the day when they can come back to campus. Partly, Ng confesses, for the free food. But mostly to continue studying gravitational waves in the same space. “I miss being able to chat randomly when people are in an office,” he says.

- Vitale acknowledges that there have been some benefits of working from home. “This has obliged everyone to think a bit harder about how to express what we want to say,” he says. Still, like his students, he also can’t wait to leave his hut and get back to campus. “I think for all of us, it will also just be nice to be back at the office and re-establish a clear separation between our living and our working spaces, that right now are collapsed in the same entity.”

• June 23, 2020: When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive die, they explode in supernovas and leave behind dense, dead remnants of stars called neutron stars. For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. The question remained: does anything lie in this so-called mass gap? 19)


Figure 13: In August of 2019, the LIGO-Virgo gravitational-wave network witnessed the merger of a black hole with 23 times the mass of our sun and a mystery object 2.6 times the mass of the sun. Scientists do not know if the mystery object was a neutron star or black hole, but either way it set a record as being either the heaviest known neutron star or the lightest known black hole (image credit: LIGO/Caltech/MIT/R. Hurt (IPAC))

- Now, in a new study from the National Science Foundation's (NSF's) Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo detector in Europe, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap. The object was found on August 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo. A paper about the detection is being published today, June 23, in The Astrophysical Journal Letters.

- "Discoveries such as this are puzzles, and force us to scratch our heads a bit," says David Reitze, executive director of the LIGO Laboratory at Caltech. "Have we observed the most massive neutron star or the lightest mass black hole? Discerning the true nature of this 'mass gap' object will require more observations, but those observations will undoubtedly shine new light on a part of the universe that has previously been inaccessible to us."

- "This is going to change how scientists talk about neutron stars and black holes," says co-author Patrick Brady, a professor at the University of Wisconsin, Milwaukee, and the LIGO Scientific Collaboration spokesperson. "The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell."

- The cosmic merger described in the study, an event dubbed GW190814, resulted in a final black hole about 25 times the mass of the sun (some of the merged mass was converted to a blast of energy in the form of gravitational waves). The newly formed black hole lies about 800 million light-years away from Earth.


Figure 14: This graphic shows the masses for black holes detected through electromagnetic observations (purple), the black holes measured by gravitational-wave observations (blue), the neutron stars measured with electromagnetic observations (yellow), and the neutron stars detected through gravitational waves (orange). GW190814 is highlighted in the middle of the graphic as the merger of a black hole and a mystery object around 2.6 times the mass of the sun (image credit: LIGO-Virgo/ Frank Elavsky & Aaron Geller (Northwestern))

- Before the two objects merged, their masses differed by a factor of 9, making this the most extreme mass ratio known for a gravitational-wave event. Another recently reported LIGO-Virgo event, called GW190412, occurred between two black holes with a mass ratio of about 4:1.

- "It's a challenge for current theoretical models to form merging pairs of compact objects with such a large mass ratio in which the low-mass partner resides in the mass gap. This discovery implies these events occur much more often than we predicted, making this a really intriguing low-mass object," says co-author Vicky Kalogera, a professor at Northwestern University. "The mystery object may be a neutron star merging with a black hole, an exciting possibility expected theoretically but not yet confirmed observationally. However, at 2.6 times the mass of our sun, it exceeds modern predictions for the maximum mass of neutron stars, and may instead be the lightest black hole ever detected."

- When the LIGO and Virgo scientists spotted this merger, they immediately sent out an alert to the astronomical community. Dozens of ground- and space-based telescopes followed up in search of light waves generated in the event, but none picked up any signals. So far, such light counterparts to gravitational-wave signals have been seen only once, in an event called GW170817. That event, discovered by the LIGO–Virgo network in August of 2017, involved a fiery collision between two neutron stars that was subsequently witnessed by dozens of telescopes on Earth and in space. Neutron-star collisions are messy affairs with matter flung outward in all directions and are thus expected to shine with light. Conversely, black hole mergers, in most circumstances, are thought not to produce light.

- According to the LIGO and Virgo scientists, there are a few possible reasons why the August 2019 event was not seen by light-based telescopes. First, this event was six times farther away than the merger observed in 2017, making it harder for instruments to pick up any light signals. Secondly, if the collision involved two black holes, it likely would have not shone with any light. Thirdly, if one object was in fact a neutron star, its 9-fold-more-massive black-hole partner might have swallowed it whole; a neutron star consumed whole by a black hole would not give off any light.

- "I think of Pac-Man eating a little dot," says Kalogera. "When the masses are highly asymmetric, the smaller neutron star can be eaten in one bite."

- How will researchers ever know if the mystery object was a neutron star or a black hole? Future observations with LIGO, Virgo, and possibly other telescopes may catch similar events that would help reveal whether additional objects exist in the mass gap.

- "This is the first glimpse of what could be a whole new population of compact binary objects," says Charlie Hoy, a member of the LIGO Scientific Collaboration and a graduate student at Cardiff University. "What is really exciting is that this is just the start. As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the universe."

- "The mass gap has been an interesting puzzle for decades, and now we've detected an object that fits just inside it," says Pedro Marronetti, program director for gravitational physics at the NSF. "That cannot be explained without defying our understanding of extremely dense matter or what we know about the evolution of stars. This observation is yet another example of the transformative potential of the field of gravitational-wave astronomy, which brings novel insights with every new detection."

• May 22, 2020: Gravitational-wave researchers at the University of Birmingham have developed a new model that promises to yield fresh insights into the structure and composition of neutron stars. 20)


Figure 15: The results from a numerical relativity simulation of two merging neutron stars similar to GW170817 (image credit: University of Birmingham)

- The model shows that vibrations, or oscillations, inside the stars can be directly measured from the gravitational-wave signal alone. This is because neutron stars will become deformed under the influence of tidal forces, causing them to oscillate at characteristic frequencies, and these encode unique information about the star in the gravitational-wave signal.

- This makes asteroseismology - the study of stellar oscillations - with gravitational waves from colliding neutron stars a promising new tool to probe the elusive nature of extremely dense nuclear matter.

- Neutron stars are the ultradense remnants of collapsed massive stars. They have been observed in the thousands in the electromagnetic spectrum and yet little is known about their nature. Unique information can be gleaned through measuring the gravitational waves emitted when two neutron stars meet and form a binary system. First predicted by Albert Einstein, these ripples in spacetime were first detected by the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) in 2015.

- By utilizing the gravitational wave signal to measure the oscillations of the neutron stars, researchers will be able to discover new insights into the interior of these stars. The study is published in Nature Communications. 21)

- Dr Geraint Pratten, of the University of Birmingham's Gravitational Wave Institute, is lead author of the study. He explained: "As the two stars spiral around each other, their shapes become distorted by the gravitational force exerted by their companion. This becomes more and more pronounced and leaves a unique imprint in the gravitational wave signal.

- "The tidal forces acting on the neutron stars excite oscillations inside the star giving us insight into their internal structure. By measuring these oscillations from the gravitational-wave signal, we can extract information about the fundamental nature and composition of these mysterious objects that would otherwise be inaccessible."

- The model developed by the team enables the frequency of these oscillations to be determined directly from gravitational-wave measurements for the first time. The researchers used their model on the first observed gravitational-wave signal from a binary neutron star merger - GW170817.

- Co-lead author, Dr Patricia Schmidt, added: "Almost three years after the first gravitational-waves from a binary neutron star were observed, we are still finding new ways to extract more information about them from the signals. The more information we can gather by developing ever more sophisticated theoretical models, the closer we will get to revealing the true nature of neutron stars."

- Next generation gravitational wave observatories planned for the 2030s, will be capable of detecting far more binary neutron stars and observing them in much greater detail than is currently possible. The model produced by the Birmingham team will make a significant contribution to this science.

- "The information from this initial event was limited as there was quite a lot of background noise that made the signal difficult to isolate," says Dr Pratten. "With more sophisticated instruments we can measure the frequencies of these oscillations much more precisely and this should start to yield some really interesting insights."

• April 20, 2020: The expectations of the gravitational-wave research community have been fulfilled: gravitational-wave discoveries are now part of their daily work as they have identified in the past observing run, O3, new gravitational-wave candidates about once a week. But now, the researchers have published a remarkable signal unlike any of those seen before: GW190412 is the first observation of a binary black hole merger where the two black holes have distinctly different masses of about 8 and 30 times that of our Sun. This not only has allowed more precise measurements of the system’s astrophysical properties, but it has also enabled the LIGO/Virgo scientists to verify a so far untested prediction of Einstein’s theory of general relativity. 22)


Figure 16: Binary black hole merger where the two black holes have distinctly different masses of about 8 and 30 times that of our Sun [image credit: N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project]

- “For the very first time we have ‘heard’ in GW190412 the unmistakable gravitational-wave hum of a higher harmonic, similar to overtones of musical instruments,” explains Frank Ohme, leader of the Independent Max Planck Research Group “Binary Merger Observations and Numerical Relativity” at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover. “In systems with unequal masses like GW190412 – our first observation of this type – these overtones in the gravitational-wave signal are much louder than in our usual observations. This is why we couldn’t hear them before, but in GW190412, we finally can.” This observation once again confirms Einstein’s theory of general relativity, which predicts the existence of these higher harmonics, i.e. gravitational waves at two or three times the fundamental frequency observed so far.

- “The black holes at the heart of GW190412 have 8 and 30 times the mass of our Sun, respectively. This is the first binary black-hole system we have observed for which the difference between the masses of the two black holes is so large!” says Roberto Cotesta, a PhD student in the “Astrophysical and Cosmological Relativity” division at the AEI in Potsdam. “This big mass difference means that we can more precisely measure several properties of the system: its distance to us, the angle we look at it, and how fast the heavy black hole spins around its axis.”

Figure 17: Numerical simulation of two black holes that inspiral and merge, emitting gravitational waves. One black hole is 3.5x more massive than the other and spins, which makes the orbit precess. The simulated gravitational wave signal is consistent with the observation made by the LIGO and Virgo gravitational wave detectors on April 12th, 2019 (GW190412), [video credit: N. Fischer, H. Pfeiffer, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetimes project]

Details on the visualization:
* The „apparent horizon“ of the black holes in the simulation are shown in black. At 1:09 the simulation finds an enveloping apparent horizon that signals the two black holes have merged.
* The gravitational radiation is translated to colors around the black holes. The colors transition from blue, representing weak radiation, to red, representing strong radiation. Specifically, the coloring represents the real part of the gravitational wave strain with its inverse radial scaling removed for visualization. The strain is computed from the simulation’s extrapolated waveform, which is shown at the bottom of the screen.

A Signal Like None Before

- GW190412 was observed by both LIGO detectors and the Virgo detector on 12th of April 2019, early during the detectors’ third observation run O3. Analyses reveal that the merger happened at a distance of 1.9 to 2.9 billion light-years from Earth. The new unequal mass system is a unique discovery, since all binaries observed previously by the LIGO and Virgo detectors consisted of two roughly similar masses.

- Unequal masses imprint themselves on the observed gravitational-wave signal, which in turn allow scientists to more precisely measure certain astrophysical properties of the system. The presence of higher harmonics makes it possible to break an ambiguity between the distance to the system and the angle we look at its orbital plane; therefore these properties can be measured with higher precision than in equal-mass systems without higher harmonics.

- “During O1 and O2, we have observed the tip of the iceberg of the binary population composed of stellar-mass black holes,” says Alessandra Buonanno, director of the “Astrophysical and Cosmological Relativity” division at the AEI in Potsdam and College Park professor at the University of Maryland. “Thanks to the improved sensitivity, GW190412 has begun to reveal us a more diverse, submerged population, characterized by mass asymmetry as large as 4 and black holes spinning at about 40% the possible maximum value allowed by general relativity,” she adds.

- AEI researchers contributed to detecting and analyzing GW190412. They have provided accurate models of the gravitational waves from coalescing black holes that included, for the first time, both the precession of the black-holes’ spins and multipole moments beyond the dominant quadrupole. Those features imprinted in the waveform were crucial to extract unique information about the source’s properties and carry out tests of general relativity. The high-performance computer clusters “Minerva” and “Hypatia” at AEI Potsdam and “Holodeck” at AEI Hannover contributed significantly to the analysis of the signal.

Testing Einstein’s theory

- LIGO/Virgo scientists also used GW190412 to look for deviations of the signals from what Einstein’s general theory of relativity predicts. Even though the signal has properties unlike all others found so far, the researchers could find no significant departure from the general-relativistic predictions.

An Improved International Network of Detectors Using Squeezed Light

- This discovery is the second reported from the third observation run (O3) of the international gravitational-wave detector network. Scientists at the three large detectors have made several technological upgrades to the instruments.

- “During O3, squeezed light was used to enhance the sensitivity of LIGO and Virgo. This technique of carefully tuning the quantum-mechanical properties of the laser light was pioneered at the German-British detector GEO600,” explains Karsten Danzmann, director at the AEI Hannover and director of the Institute for Gravitational Physics at Leibniz University Hannover. “The AEI is leading the world-wide efforts to maximize the degree of squeezing, which has already improved the sensitivity of the GEO600 detector by a factor of two. Our advances in this technology will benefit all future gravitational-wave detectors.”

2 done, 54 on the to-do list

- The detector network has issued alerts for 56 possible gravitational-wave events (candidates) in O3 (April 1, 2019 to March 27, 2020 with an interruption for upgrades and commissioning in October 2019). Out of these 56, one other confirmed signal, GW190425, has already been published. LIGO and Virgo scientists are examining all remaining 54 candidates and will publish all those for which detailed follow-up analyses confirm their astrophysical origin.

- The observation of GW190412 means that similar systems are probably not as rare as predicted by some models. Therefore, with additional gravitational-wave observations and growing event catalogues in the future, more such signals are to be expected. Each of them could help astronomers better understand how black holes and their binary systems are formed, and shed new light on the fundamental physics of space-time. 23)

• January 6, 2020: A pair of neutron stars has been found with an unusually high mass. On April 25, 2019, the LIGO Livingston Observatory picked up what appeared to be gravitational ripples from a collision of two neutron stars. LIGO Livingston is part of a gravitational-wave network that includes LIGO (the Laser Interferometer Gravitational-wave Observatory), funded by the National Science Foundation (NSF) and the European Virgo detector. Now, a new study confirms that this event was indeed likely the result of a merger of two neutron stars. This would be only the second time this type of event ever been observed in gravitational waves. 24)

- The first such observation, which took place in August of 2017, made history for being the first time that both gravitational waves and light were detected from the same cosmic event. The April 25 merger, by contrast, did not result in any light being detected. However, through an analysis of the gravitational-wave data alone, researchers have learned that the collision produced an object with an unusually high mass.

- "From conventional observations with light, we already knew of 17 binary neutron star systems in our own galaxy and we have estimated the masses of these stars," says Ben Farr, a LIGO team member based at the University of Oregon. "What's surprising is that the combined mass of this binary is much higher than what was expected."

- "We have detected a second event consistent with a binary neutron star system and this is an important confirmation of the August 2017 event that marked an exciting new beginning for multi-messenger astronomy two years ago," says Virgo spokesperson Jo van den Brand, a professor at Maastricht University, Nikhef, and VU University Amsterdam in the Netherlands. Multi-messenger astronomy occurs when different types of signals, such as those based on gravitational waves and light, are witnessed simultaneously.

- The study, ('GW190425: Observation of a Compact Binary Coalescence with Total Mass~3.4Mo') submitted to The Astrophysical Journal Letters, is authored by an international team comprised of the LIGO Scientific Collaboration and the Virgo Collaboration, the latter of which is associated with the Virgo gravitational-wave detector in Italy. The results were presented at a press briefing today, January 6, at the 235th meeting of the American Astronomical Society, held in Honolulu, Hawaii.

- Neutron stars are the remnants of dying stars that undergo catastrophic explosions as they collapse at the end of their lives. When two neutron stars spiral together, they undergo a violent merger that sends gravitational shudders through the fabric of space and time.

- LIGO became the first observatory to directly detect gravitational waves in 2015; in that instance, the waves were generated by the fierce collision of two black holes. Since then, LIGO and Virgo have registered dozens of additional candidate black hole mergers.

- The August 17 neutron star merger was witnessed by both LIGO detectors, one in Livingston, Louisiana, and one in Hanford, Washington, together with a host of light-based telescopes around the world (neutron star collisions produce light, while black hole collisions are generally thought not to do so). This merger was not clearly visible in the Virgo data, but that fact provided key information that ultimately pinpointed the event's location in the sky.

- The April 2019 event was first identified in data from the LIGO Livingston detector alone. The LIGO Hanford detector was temporarily offline. The event took place more than 500 million light-years from Earth, and so was too faint to be detected with Virgo's current sensitivity. However, using the Livingston data, combined with information derived from Virgo's lack of a detection, the team narrowed the location of the event to a patch of sky more than 8,200 square degrees in size, representing about 20 percent of the sky. For comparison, the August 2017 event was narrowed to a region of just 16 square degrees, or 0.04 percent of the sky.

- "This is our first published event for a single-observatory detection," says Caltech's Anamaria Effler (BS '06), a scientist who works at LIGO Livingston. "But Virgo made a valuable contribution. We used information about its non-detection to tell us roughly where the signal must have originated from."


Figure 18: Artist's rendition of a binary neutron star merger (image credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet)

Figure 19: Simulation of the binary neutron star coalescence GW190425. This movie shows a numerical simulation representing the binary neutron star coalescence and merger which resulted in the detected gravitational-wave event GW190425. The two neutron stars shown here have properties consistent with the detection made by the Advanced LIGO/Virgo detectors [video credit: Numerical Relativity Simulation: T. Dietrich (Nikhef), Wolfgang Tichy (Florida Atlantic University) and the CoRe-collaboration Scientific Visualization: T. Dietrich (Nikhef), S. Ossokine, and A. Buonanno (Max Planck Institute for Gravitational Physics)] 25)

- The LIGO data reveal that the combined mass of the merged bodies is about 3.4 times that of the mass of our sun. Previously detected neutron star collisions produced final masses of no more than 2.9 times that of the sun. One possibility for the unusually high mass is that the collision took place not between two neutron stars but between a neutron star and a black hole, since black holes are heavier than neutron stars. But if this were the case, the black hole would have to be exceptionally small for its class. Instead, the scientists believe it is much more likely that LIGO witnessed a shattering of two neutron stars.

- "What we know from the data are the masses, and the individual masses most likely correspond to neutron stars. However, as a binary neutron star system, the total mass is much higher than any of the other known galactic neutron star binaries," says Surabhi Sachdev (MS '17, PhD '19), a LIGO team member based at Penn State. "And this could have interesting implications for how the pair originally formed."

- Neutron star pairs are thought to form in two possible ways. They might form from binary systems of massive stars that each end their lives as neutron stars, or they might arise when two separately formed neutron stars come together within a dense stellar environment. The LIGO data for the April 25 event do not indicate which of these scenarios is more likely, but they do suggest that more data and new models are needed to explain the merger's unexpectedly high mass.

• September 9, 2019: The final chapter of the historic detection of the powerful merger of two neutron stars in 2017 officially has been written. After the extremely bright burst finally faded to black, an international team led by Northwestern University painstakingly constructed its afterglow — the last bit of the famed event’s life cycle. 26)


Figure 20: An artistic rendering of two neutron stars merging (image credit: NSF/LIGO/Sonoma State/A. Simonnet)

- Not only is the resulting image the deepest picture of the neutron star collision’s afterglow to date, it also reveals secrets about the origins of the merger, the jet it created and the nature of shorter gamma ray bursts.

- “This is the deepest exposure we have ever taken of this event in visible light,” said Northwestern’s Wen-fai Fong, who led the research. “The deeper the image, the more information we can obtain.”

- The study will be published this month in The Astrophysical Journal Letters. Fong is an assistant professor of physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences and a member of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics), an endowed research center at Northwestern focused on advancing studies with an emphasis on interdisciplinary connections.

- Many scientists consider the 2017 neutron-star merger, dubbed GW170817, as LIGO’s (Laser Interferometer Gravitational-Wave Observatory) most important discovery to date. It was the first time that astrophysicists captured two neutron stars colliding. Detected in both gravitational waves and electromagnetic light, it also was the first-ever multi-messenger observation between these two forms of radiation.

- The light from GW170817 was detected, partly, because it was nearby, making it very bright and relatively easy to find. When the neutron stars collided, they emitted a kilo nova — light 1,000 times brighter than a classical nova, resulting from the formation of heavy elements after the merger. But it was exactly this brightness that made its afterglow — formed from a jet travelling near light-speed, pummeling the surrounding environment — so difficult to measure.

- “For us to see the afterglow, the kilonova had to move out of the way,” Fong said. “Surely enough, about 100 days after the merger, the kilonova had faded into oblivion, and the afterglow took over. The afterglow was so faint, however, leaving it to the most sensitive telescopes to capture it.”

Hubble to the rescue

- Starting in December 2017, NASA’s Hubble Space Telescope detected the visible light afterglow from the merger and revisited the merger’s location 10 more times over the course of a year and a half.


Figure 21: The box indicates where the now-faded afterglow was located (image credit: Northwestern University)

- At the end of March 2019, Fong’s team used the Hubble to obtain the final image and the deepest observation to date. Over the course of seven-and-a-half hours, the telescope recorded an image of the sky from where the neutron-star collision occurred. The resulting image showed — 584 days after the neutron-star merger — that the visible light emanating from the merger was finally gone.

- Next, Fong’s team needed to remove the brightness of the surrounding galaxy, in order to isolate the event’s extremely faint afterglow.

- “To accurately measure the light from the afterglow, you have to take all the other light away,” said Peter Blanchard, a postdoctoral fellow in CIERA and the study’s second author. “The biggest culprit is light contamination from the galaxy, which is extremely complicated in structure.”

- Fong, Blanchard and their collaborators approached the challenge by using all 10 images, in which the kilonova was gone and the afterglow remained as well as the final, deep Hubble image without traces of the collision. The team overlaid their deep Hubble image on each of the 10 afterglow images. Then, using an algorithm, they meticulously subtracted — pixel by pixel — all light from the Hubble image from the earlier afterglow images.

- The result: a final time-series of images, showing the faint afterglow without light contamination from the background galaxy. Completely aligned with model predictions, it is the most accurate imaging time-series of GW170817’s visible-light afterglow produced to date.

- “The brightness evolution perfectly matches our theoretical models of jets,” Fong said. “It also agrees perfectly with what the radio and X-rays are telling us.”

Illuminating information

- With the Hubble’s deep space image, Fong and her collaborators gleaned new insights about GW170817’s home galaxy. Perhaps most striking, they noticed that the area around the merger was not densely populated with star clusters.

- “Previous studies have suggested that neutron star pairs can form and merge within the dense environment of a globular cluster,” Fong said. “Our observations show that’s definitely not the case for this neutron star merger.”

- According to the new image, Fong also believes that distant, cosmic explosions known as short gamma ray bursts are actually neutron star mergers — just viewed from a different angle. Both produce relativistic jets, which are like a fire hose of material that travels near the speed of light. Astrophysicists typically see jets from gamma ray bursts when they are aimed directly, like staring directly into the fire hose. But GW170817 was viewed from a 30-degree angle, which had never before been done in the optical wavelength.

- “GW170817 is the first time we have been able to see the jet ‘off-axis,’” Fong said. “The new time-series indicates that the main difference between GW170817 and distant short gamma-ray bursts is the viewing angle.”

- The study was primarily supported by the National Science Foundation (award numbers AST-1814782 and AST-1909358) and NASA (award numbers HST-GO-15606.001-A and SAO-G09-20058A). 27)

• On April 25, 2019, NFS's (National Science Foundation's) LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based Virgo detector registered gravitational waves from what appears likely to be a crash between two neutron stars—the dense remnants of massive stars that previously exploded. One day later, on April 26, the LIGO-Virgo network spotted another candidate source with a potentially interesting twist: it may in fact have resulted from the collision of a neutron star and black hole, an event never before witnessed. 28) 29)

- "The universe is keeping us on our toes," says Patrick Brady, spokesperson for the LIGO Scientific Collaboration and a professor of physics at the University of Wisconsin-Milwaukee. "We're especially curious about the April 26 candidate. Unfortunately, the signal is rather weak. It's like listening to somebody whisper a word in a busy café; it can be difficult to make out the word or even to be sure that the person whispered at all. It will take some time to reach a conclusion about this candidate."

- "NSF's LIGO, in collaboration with Virgo, has opened up the universe to future generations of scientists," says NSF Director France Cordova. "Once again, we have witnessed the remarkable phenomenon of a neutron star merger, followed up closely by another possible merger of collapsed stars. With these new discoveries, we see the LIGO-Virgo collaborations realizing their potential of regularly producing discoveries that were once impossible. The data from these discoveries, and others sure to follow, will help the scientific community revolutionize our understanding of the invisible universe."

- The discoveries come just weeks after LIGO and Virgo turned back on. The twin detectors of LIGO—one in Washington and one in Louisiana—along with Virgo, located at the European Gravitational Observatory (EGO) in Italy, resumed operations April 1, after undergoing a series of upgrades to increase their sensitivities to gravitational waves—ripples in space and time. Each detector now surveys larger volumes of the universe than before, searching for extreme events such as smash-ups between black holes and neutron stars.

- "Joining human forces and instruments across the LIGO and Virgo collaborations has been once again the recipe of an incomparable scientific month, and the current observing run will comprise 11 more months," says Giovanni Prodi, the Virgo Data Analysis Coordinator, at the University of Trento and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. "The Virgo detector works with the highest stability, covering the sky 90 percent of the time with useful data. This is helping in pointing to the sources, both when the network is in full operation and at times when only one of the LIGO detectors is operating. We have a lot of groundbreaking research work ahead."

- In addition to the two new candidates involving neutron stars, the LIGO-Virgo network has, in this latest run, spotted three likely black hole mergers. In total, since making history with the first-ever direct detection of gravitational waves in 2015, the network has spotted evidence for two neutron star mergers; 13 black hole mergers; and one possible black hole-neutron star merger.

- When two black holes collide, they warp the fabric of space and time, producing gravitational waves. When two neutron stars collide, they not only send out gravitational waves but also light. That means telescopes sensitive to light waves across the electromagnetic spectrum can witness these fiery impacts together with LIGO and Virgo. One such event occurred in August 2017: LIGO and Virgo initially spotted a neutron star merger in gravitational waves and then, in the days and months that followed, about 70 telescopes on the ground and in space witnessed the explosive aftermath in light waves, including everything from gamma rays to optical light to radio waves.

- In the case of the two recent neutron star candidates, telescopes around the world once again raced to track the sources and pick up the light expected to arise from these mergers. Hundreds of astronomers eagerly pointed telescopes at patches of sky suspected to house the signal sources. However, at this time, neither of the sources has been pinpointed.

- "The search for explosive counterparts of the gravitational-wave signal is challenging due to the amount of sky that must be covered and the rapid changes in brightness that are expected," says Brady. "The rate of neutron star merger candidates being found with LIGO and Virgo will give more opportunities to search for the explosions over the next year."


Figure 22: How to catch a gravitational wave. The world’s first captured gravitational waves were created in a violent collision between two black holes, 1.3 billion light-years away. When these waves passed the Earth, 1.3 billion years later, they had weakened considerably: the disturbance in spacetime that LIGO measured was thousands of times smaller than an atomic nucleus (image credit: LIGO)

- The April 25 neutron star smash-up, dubbed S190425z, is estimated to have occurred about 500 million light-years away from Earth. Only one of the twin LIGO facilities picked up its signal along with Virgo (LIGO Livingston witnessed the event but LIGO Hanford was offline). Because only two of the three detectors registered the signal, estimates of the location in the sky from which it originated were not precise, leaving astronomers to survey nearly one-quarter of the sky for the source.

- The possible April 26 neutron star-black hole collision (referred to as S190426c) is estimated to have taken place roughly 1.2 billion light-years away. It was seen by all three LIGO-Virgo facilities, which helped better narrow its location to regions covering about 1,100 square degrees, or about 3 percent of the total sky.

- "The latest LIGO-Virgo observing run is proving to be the most exciting one so far," says David H. Reitze of Caltech, Executive Director of LIGO. "We're already seeing hints of the first observation of a black hole swallowing a neutron star. If it holds up, this would be a trifecta for LIGO and Virgo—in three years, we'll have observed every type of black hole and neutron star collision. But we've learned that claims of detections require a tremendous amount of painstaking work—checking and rechecking—so we'll have to see where the data takes us."

- LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at

- The Virgo Collaboration is currently composed of approximately 350 scientists, engineers, and technicians from about 70 institutes from Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration members can be found at More information is available on the Virgo website at

• April 26, 2019: In the few weeks since LIGO kicked off its third observing run, it’s also already detected three potential black hole collisions and a neutron star merger, bringing its total lifetime gravitational wave haul to 14. 30)

- It took astronomers a century to make the first-ever gravitational wave detection, confirming a core prediction of Albert Einstein’s theory of general relativity. But this month, the floodgates have opened.

- On Friday (26 April), scientists with the LIGO announced they’ve likely detected a second gravitational wave event in as many days. Detectors at three locations around the world caught the arrival of a probable ripple in space-time around 11:20 a.m. E.T. (~15:20 GMT). It followed right on the heels of a gravitational wave detection Thursday that sent astronomers racing to observe the event with their telescopes.

- In all, it’s the fifth gravitational wave detection this month. And the influx has astronomers excited about kickstarting the era of multi-messenger astronomy, where scientists can combine gravitational wave data with observations from conventional telescopes to gain new insights into extreme cosmic events like colliding black holes and neutron stars.

- Scientists suspect Thursday’s event marked the second-ever gravitational wave detection of two colliding neutron stars, the collapsed cores left behind when giant stars go supernova. The merger would have likely spawned a new black hole. Astronomers spent Thursday searching for any signs of the collision on the sky. They’re less certain about the celestial event that led to today’s detection: There’s about a one in seven chance that it was a false alarm caused by earthly vibrations. Its signal is right at the threshold of what LIGO can pick out.

- If this latest signal does turn out to be real cosmic collision, though, scientists say that there’s a chance it may be the hallmark of a never-before-seen event: the collision of a neutron star and a black hole. But odds still favor it as a third neutron star merger.

• April 26, 2019: For just the second time, physicists working on the LIGO (Laser Interferometer Gravitational-Wave Observatory) and at Virgo have caught the gravitational waves of two neutron stars colliding to likely form a black hole. 31) 32)

- The ripples in space-time traveled some 500 million light-years and reached the detectors at LIGO, as well as its Italian sister observatory, Virgo, at around 4 a.m. E.T. Thursday (~8:00 GMT), 25 April. Team members say there’s a more than 99 percent chance that the gravitational waves were created from a binary neutron star merger.

Shot at a Kilonova

- In the moments after the event, a notice went out alerting astronomers around the world to turn their telescopes to the heavens in hopes of catching light from the explosion, called a kilonova. Kilonovae are 1,000 times brighter than normal novae, and they create huge amounts of heavy elements, like gold and platinum. That brightness makes it easy for astronomers to find these events in the night sky — provided they’ve been given a heads-up and location from LIGO first.

- LIGO’s twin L-shaped observatories — one in Washington state and one in Louisiana — work by shooting a laser beam down the long legs of their “L.” Their experimental setup is precise enough that even the minimal disturbance caused by a passing gravitational wave is enough to trigger a slight change in the laser’s appearance. It made the first ever detection of gravitational waves in 2016. Then it followed up by detecting merging neutron stars in 2017.

- Scientists use any slight delays between when signals reach the detectors to help them better triangulate where the waves originated in the sky. But one of LIGO’s twin detectors was offline Thursday when the gravitational wave reached Earth, making it hard for astronomers to triangulate exactly where the signal was coming from. That sent astronomers racing to image as many galaxies as they could across a region covering one-quarter of the sky.

- And instead of finding one potential binary neutron star merger, astronomers turned up at least two different candidates. Now the question is which, if any, are related to the gravitational wave that LIGO saw. Sorting that out will require more observations, which were already happening around the world as darkness fell.

- “I would assume that every observatory in the world is observing this now,” says astronomer Josh Simon of the Carnegie Observatories. “These two candidates (they’ve) found are relatively close to the equator, so they can be seen from both the Northern and Southern Hemisphere.”

- Simon also says that, as of Thursday afternoon in the United States, telescopes in Europe and elsewhere should be gathering spectra on these objects. His fellow astronomers at the Carnegie Observatories turned their telescopes at Chile’s Las Campanas Observatory to the event Thursday night.


Figure 23: An artist’s illustration of two colliding neutron stars (image credit: NASA/Swift/Dana Berry)

• March 26, 2019: The National Science Foundation's LIGO (Laser Interferometer Gravitational-Wave Observatory) is set to resume its hunt for gravitational waves—ripples in space and time—on April 1, after receiving a series of upgrades to its lasers, mirrors, and other components. LIGO—which consists of twin detectors located in Washington and Louisiana—now has a combined increase in sensitivity of about 40 percent over its last run, which means that it can survey an even larger volume of space than before for powerful, wave-making events, such as the collisions of black holes. 33)

- Joining the search will be Virgo, the European-based gravitational-wave detector, located at the European Gravitational Observatory (EGO) in Italy, which has almost doubled its sensitivity since its last run and is also starting up April 1.

- "For this third observational run, we achieved significantly greater improvements to the detectors' sensitivity than we did for the last run," says Peter Fritschel, LIGO's chief detector scientist at MIT. "And with LIGO and Virgo observing together for the next year, we will surely detect many more gravitational waves from the types of sources we've seen so far. We're eager to see new events too, such as a merger of a black hole and a neutron star."

- In 2015, after LIGO began observing for the first time in an upgraded program called Advanced LIGO, it soon made history by making the first direct detection of gravitational waves. The ripples traveled to Earth from a pair of colliding black holes located 1.3 billion light-years away. For this discovery, three of LIGO's key players—Caltech's Barry C. Barish, the Ronald and Maxine Linde Professor of Physics, Emeritus, and Kip S. Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus, along with MIT's Rainer Weiss, professor of physics, emeritus—were awarded the 2017 Nobel Prize in Physics.

- Since then, the LIGO-Virgo detector network has uncovered nine additional black hole mergers and one explosive smashup of two neutron stars. That event, dubbed GW170817, generated not just gravitational waves but light, which was observed by dozens of telescopes in space and on the ground.

- "With our three detectors now operational at a significantly improved sensitivity, the global LIGO-Virgo detector network will allow more precise triangulation of the sources of gravitational waves," says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration. "This will be an important step toward our quest for multi-messenger astronomy."


Figure 24: Current operating facilities in the global network include the twin LIGO detectors—in Hanford, Washington, and Livingston, Louisiana— Virgo in Italy and GEO600 in Germany (image credit: LIGO-Virgo-GEO600 collaboration)

- Now, with the start of the next joint LIGO-Virgo run, the observatories are poised to detect an even greater number of black hole mergers and other extreme events, such as additional neutron-neutron star mergers or a yet-to-be-seen black hole-neutron star merger. One of the metrics the team uses for measuring increases in sensitivity is to calculate how far out they can detect neutron-neutron star mergers. In the next run, LIGO will be able to see those events out to an average of 550 million light-years away, or more than 190 million light-years farther out than before.

- A key to achieving this sensitivity involves lasers. Each LIGO installation consists of two long arms that form an L shaped interferometer. Laser beams are shot from the corner of the "L" and bounced off mirrors before traveling back down the arms and recombining. When gravitational waves pass by, they stretch and squeeze space itself, making imperceptibly tiny changes to the distance the laser beams travel and thereby affecting how they recombine. For this next run, the laser power has been doubled to more precisely measure these distance changes, thereby increasing the detectors’ sensitivity to gravitational waves.

- Other upgrades were made to LIGO's mirrors at both locations, with a total of five of eight mirrors being swapped out for better-performing versions. "We had to break the fibers holding the mirrors and very carefully take out the optics and replace them," says Calum Torrie, LIGO's mechanical-optical engineering head at Caltech. "It was an enormous engineering undertaking."

- This next run also includes upgrades designed to reduce levels of quantum noise. Quantum noise occurs due to random fluctuations of photons, which can lead to uncertainty in the measurements and can mask faint gravitational-wave signals. By employing a technique called "squeezing," initially developed for gravitational-wave detectors at the Australian National University, and matured and routinely used since 2010 at the GEO600 detector, researchers can shift the uncertainty in the photons around, making their amplitudes less certain and their phases, or timing, more certain. The timing of photons is what is crucial for LIGO's ability to detect gravitational waves.

- Torrie says that the LIGO team has spent months commissioning all of these new systems, making sure everything is aligned and working correctly. "One of the things that is satisfying to us engineers is knowing that all of our upgrades mean that LIGO can now see farther into space to find the most extreme events in our universe."

• February 27, 2019: LIGO and Virgo are pleased to announce that the strain data from the O2 observing run have been released. These data are now available through the Gravitational Wave Open Science Center ( 34)

- The O2 observing run began on November 30, 2016 and ended on August 25, 2017. The release includes over 150 days of data from each of the two LIGO observatories, as well as 20 days of data from Virgo, making this the largest data set of "advanced" gravitational wave detectors to date. Observations in O2 include seven binary black hole mergers, as well as the first binary neutron star merger observed in gravitational waves, all recently published with the GWTC-1 catalog. The LIGO Scientific Collaboration and Virgo Collaboration have published a number of papers based on these data; please see the LIGO Scientific Collaboration web pages for a list of these papers, and several more will be appearing soon. Along with the strain data, the release contains detailed documentation and links to open source software tools.

- O2 is the second observing run of Advanced LIGO, and the first observing run of Advanced Virgo, which joined O2 on August 1st, 2017. Data from Advanced LIGO's first observing run (O1) are already available online, and have been used in a number of scientific publications, text books, artistic projects, and classroom activities. As with previous data releases, the O2 data set should be useful for both scientific investigations and educational activities.


Figure 25: Time-frequency maps and reconstructed signal waveforms for the ten BBH (Binary Body Hole) events (image credit: LIGO Scientific Collaboration and the Virgo Collaboration)

• February 21, 2019: An international research team including astronomers from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has combined radio telescopes from five continents to prove the existence of a narrow stream of material, a so-called jet, emerging from the only gravitational wave event involving two neutron stars observed so far. With its high sensitivity and excellent performance, the 100 m radio telescope in Effelsberg played an important role in the observations. 35)

- The results are published in this week’s issue of „Science“. 36)

- In August 2017, two neutron stars were observed colliding, producing gravitational waves that were detected by the American LIGO and European Virgo detectors. Neutron stars are ultra-dense stars, roughly the same mass as the Sun, but similar in size to a city like Cologne. This event is the first and only one of this type that has been observed so far, and it happened in a galaxy 130 million light years away from Earth, in the constellation of Hydra.

- Astronomers observed the event and the subsequent evolution across the entire electromagnetic spectrum, from gamma-rays, X-rays to visible light and radio waves. Two hundred days after the merger, observations combining radio telescopes in Europe, Africa, Asia, Oceania, and North America proved the existence of a jet emerging from this violent collision. These findings are now published in the scientific journal Science by an international team of astronomers, led by Giancarlo Ghirlanda from the Italian National Institute for Astrophysics (INAF).

- This neutron star merger represented the first case where it was possible to associate a detection of gravitational waves to an object emitting light. The event has confirmed scientific theories that have been under discussion for tens of years, and the association of neutron star mergers with one of the most powerful explosions in the Universe: gamma-ray bursts. After the merger, a huge amount of material was expelled into space, forming a shell around the object. Astronomers have been tracing its evolution at different wavelengths. However, there were still some remaining questions concerning this event that could not be clarified by any previous observations.

- "We expected part of the material to be ejected through a collimated jet, but it was unclear whether this material could successfully pierce through the surrounding shell." explains Ghirlanda. “There were two competing scenarios: In one case, the jet cannot break through the shell, instead generating an expanding bubble around the object. In the other, the jet is successful in penetrating the shell and then propagates further into space”, expands Tiziana Venturi (INAF). Only the acquisition of very sensitive radio images with very high resolution would discard one scenario or the other. This required the use of a technique known as very long baseline interferometry (VLBI) that allows astronomers to combine radio telescopes all around the Earth.

- The authors of this publication conducted global observations in the direction of the merger on 12 March 2018 using thirty-three radio telescopes from the European VLBI Network (that connects telescopes from Spain, the United Kingdom, The Netherlands, Germany, Italy, Sweden, Poland, Latvia, South Africa, Russia, and China), e-MERLIN in the UK, the Australian Long Baseline Array in Australia and New Zealand, and the Very Long Baseline Array in the USA.


Figure 26: Artist’s impression of the jet of material launched after the merger of the two neutron stars (image credit: JIVE, Katharina Immer)

- „Our 100 m radio telescope in Effelsberg participated in the observations and was a key element, due to its high sensitivity and excellent performance“, says Carolina Casadio, a member of the research team from the Max Planck Institute for Radio Astronomy (MPIfR).

- The data from all telescopes were sent to JIVE (Joint Institute for VLBI in Europe), The Netherlands, where the most advanced processing techniques were used to produce an image with a resolution comparable to resolving a person on the surface of the Moon. In the same analogy, the expanding bubble would appear with an apparent size equivalent to a truck on the Moon, whereas a successful jet would be detected as a much more compact object.

- "Comparing the theoretical images with the real ones, we find that only a jet could appear sufficiently compact to be compatible with the observed size.", explains Om Sharan Salafia from INAF in Italy. The team determined that this jet contained as much energy as produced by all the stars in our Galaxy during one year. “And all that energy was contained in a size smaller than one light year.” says Zsolt Paragi, also from JIVE.

- ”Within Europe we utilize the RadioNet consortium for an efficient use of our members’ radio telescopes. The observations described here combine radio observatories all over Europe and world wide. They require a well-coordinated effort of the collaborating observatories and institutions to achieve such exciting results”, explains Anton Zensus, Director at MPIfR and coordinator of the RadioNet consortium.

- In the coming years, many more of these neutron star binary mergers will be discovered. “The obtained results also suggest that more than 10% of all these mergers should exhibit a successful jet.”, explains Benito Marcote from JIVE. "These types of observations will allow us to unveil the processes that take place during and after some of the most powerful events in the Universe.", concludes Sándor Frey from the Konkoly Observatory in Hungary.


Figure 27: Image of the source obtained from the combination of thirty-three radio telescopes from five continents. The source can be seen in the center of the image as a red spot (false color image made entirely for illustration), image credit: Giancarlo Ghirlanda/Science

• December 4, 2018: Scientists with the LIGO and Virgo gravitational wave observatories report four new sets of these ripples in spacetime. Those additions bring the total count to 11, the researchers say in a study published December 3 at, marking major progress since the first gravitational wave detection in 2015 (SN: 3/5/16, p. 6). 37)

- All but one of the 11 sets of waves were stirred up in violent collisions of two black holes. The one remaining detection, reported in October 2017, instead came from the smashup of two stellar corpses called neutron stars (SN: 11/11/17, p. 6).

• December 4, 2018: Researchers from the University of Portsmouth (UoP) have made vital contributions to the observations of four new gravitational waves, which were announced this weekend (1 December). 38)

- The new results are from the National Science Foundation’s LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based VIRGO gravitational-wave detector. The results were announced at the Gravitational Wave Physics and Astronomy Workshop in College Park, Maryland, USA.

- Three years ago LIGO made the first observation of a binary black hole merger. Today, there have been observations of 11 gravitational-wave signals (10 stellar-mass binary black hole mergers and one merger of two neutron stars, which are the dense, spherical remains of stellar explosions).

- These observations are revolutionizing our understanding of the processes by which high mass stars (10 – 100 times as heavy as our sun) are formed, how they evolve, and the method by which black holes are produced.

- The new events are known as GW170729, GW170809, GW170818, and GW170823, in reference to the dates they were detected. All of the events are included in a new catalogue, also released Saturday, with some of the events breaking records.

- Researchers from the newly-formed Gravitational-Wave Physics Group in the University’s ICG (Institute of Cosmology and Gravitation) in the University of Portsmouth have played a significant role in the observation of the first 11 gravitational-wave events.

- Dr Laura Nuttall, a senior lecturer in the ICG, made the initial observation of GW170729. This event, detected in the second observing run on July 29 2017, is the most massive and distant gravitational-wave source ever observed. Any theoretical work trying to understand the mechanisms by which black holes form, now must allow for black holes as massive as this one to be produced.

- Dr Andrew Lundgren, a reader at the ICG, was one of the main developers of the noise subtraction scheme, which was necessary to increase the sensitivity of the LIGO observatories to be able to confirm that GW170729, GW170809 and GW170818 were genuine gravitational-wave signals. Without this work, we would only be talking about eight observed gravitational-wave signals today. 39)

- Dr Ian Harry, a senior lecturer at the ICG, is one of the two main developers of the PyCBC algorithm, which is responsible for searching for merging black holes and neutron stars in Advanced LIGO and Advanced Virgo data. This code made the first observation of many of the 11 gravitational-wave signals seen so far.

- Dr Harry said: “I’m happy to see the vital contributions our researchers have made to the observation of the first 11 gravitational-wave events. Gravitational-wave observations offer us a way to observe astrophysical sources that have never been seen before, including the collision of two black holes. These observations allow us to begin to understand the processes by which black holes are produced and explore the environments in which they are formed.

- “However, this is only the beginning of gravitational-wave astronomy, and as our observatories become more sensitive we expect to observe hundreds of sources in the coming years. My personal hope is that we observe something truly unexpected in the next years, which would help us to better understand the Universe that we live in. Gravitational-wave astronomy is one of the fastest growing fields in astronomy and collaboration between the new Gravitational-Wave Physics Group and existing ICG researchers offers tremendous possibilities for future world-leading research.”


Figure 28: These observations allow us to begin to understand the processes by which black holes are produced and explore the environments in which they are formed (image credit: UoP Team)

• August 17, 2018: Today LIGO commemorates the one-year anniversary of its most important discovery to-date: The detection of a merging pair of neutron stars, aka a BNS (Binary Neutron Star) merger. 40)

After a 130 million year journey, the gravitational waves generated by these exotic stars arrived at LIGO’s Hanford and Livingston detectors in the United States, and the Virgo detector in Italy on 17 August 2017. Dubbed a ‘kilonova’ (a term coined in 2010 in a paper wherein it was theorized that a pair of merging neutron stars would emit light about 1000 times brighter than a classical nova), the detection also led to a massive explosion of multimessenger astronomy results gathered by astronomers from all around the globe. LIGO announced the discovery to the world with papers published on 16 October 2017.

How important was this detection? Well, on the day of the announcement, 84 scientific papers were published about it.

Today, an internet search for “GW170817” will yield over 110,000 results, all related to this one event that captured the world scientific community’s attention (incidentally, a search for “GW150914”, LIGO’s first detection, yields a mere 80,000 hits).

Why all the Excitement? Up until that day in August 2017, LIGO’s detections had all been gravitational waves caused by merging black holes. While there’s no doubt that those discoveries have been monumental, the scope and magnitude of this discovery would prove unprecedented. The LIGO and Virgo detection has become probably the most widely studied astronomical event in human history. Within days, this object was being examined by nearly one-third of the world’s electromagnetic (EM) astronomers. The fact that EM astronomers were able to observe the phenomenon alongside GW astronomers is what truly elevated this event to history-making levels. LIGO’s previous detections of merging black holes did not result in such widespread study because, by their very nature, black holes are believed not to emit electromagnetic waves (i.e., light of any wavelength). No amount of searching by astronomers using telescopes designed to observe EM radiation has revealed anything. Only gravitational wave observatories like LIGO and Virgo can ‘observe’ black holes colliding.

Neutron stars are different, however. Unlike black holes, neutron stars are made up of actual matter, including copious amounts of neutrons (hence their moniker), and when you accelerate or slam matter together you get electromagnetic radiation (again, not something one expects to detect from colliding black holes).


Figure 29: Basic anatomy of a neutron star. By Robert Schulze [CC BY-SA 3.0 (]

But like black holes, neutron stars are massive and compact enough to generate gravitational waves when they collide. This combination of properties (material composition and density) means that colliding neutron stars can emit both gravitational waves AND electromagnetic radiation: “light” in all its forms from gamma rays to radio waves. In fact, it was a burst of gamma rays, arriving 1.7 seconds after the gravitational waves, which alerted the broader astronomical community to something truly extraordinary and led to an unprecedented global effort to study the phenomenon.

• February 22, 2018: The National Science Foundation funded Advanced LIGO Documentary Project released a new video looking at information coming from first multi-messenger detection of colliding neutron stars. On August 8, 2017, LIGO joined forces with Virgo and over 70 astronomical observatories to look at a neutron star merger through gravitational waves and electromagnetic waves (light). This video explores the significance of that event. 41)

Figure 30: LIGO: A Discovery that Shook the World. This is the third video in Advanced LIGO Documentary Project's eight-part series on LIGO's historic discovery of gravitational waves and the birth of the new age of gravitational wave astronomy. In August 2017, LIGO and its Italian partner, VIRGO, made a discovery as important as its historic first detection of gravitational waves in 2015. They detected gravitational waves from two colliding neutron stars, which ejected a spectacular gamma ray burst that was seen by seven space-based telescopes and dozens of astronomical observatories on earth. It was the long dreamed-of marriage of gravitational wave astronomy with conventional astronomy, and the results were spectacular (video credit: Advanced LIGO Documentary Project)

• December 11, 2017: Physics World announced that the first multi-messenger detection of a neutron star merger was 2017's breakthrough of the year. On August 17, 2017 LIGO detected a gravitational wave that was expected to come from a neutron star merger. Around 2 seconds later a Gamma Ray Burst occurred and was detected by the Fermi Gamma-ray Space Telescope. Together with LIGO, Virgo and Fermi's information astronomers were able to piece together approximately where in the sky the neutron star merger occurred. Telescopes around the world pointed there scopes at the spot and soon identified precisely where the neutron stars had collided. In the coming months more than 70 telescopes observed multiple frequencies of electromagnetic radiation coming from the neutron star merger, yielding a treasure trove of information about the kilonova that occurred when the two neutron stars collided. Over 50 collaborations, including LIGO participated in this venture. For more on this detection check out our news article, or the press release. Last year Physics World awarded LIGO with the 2016 breakthrough of the year, due to the detection it's detection of gravitational waves. 42)

• October 16, 2017: Another LIGO gravitational wave detection has spawned an explosion of new science across the global astronomical community. On August 17, 2017, the two LIGO instruments (funded by the National Science Foundation) and its sister facility, Virgo, near Pisa, Italy, sensed tell-tale signs of the remnant cores of two massive stars spiraling toward and then smashing into each other some 130 million light years away. The objects were quickly identified as neutron stars, the collapsed cores of stars that were once much more massive than our Sun. They are called “neutron stars” because their matter is so densely packed it is composed primarily of neutrons. One such star containing as much matter as our Sun would be just 10 to 15 km in diameter, and a teaspoon of its material would weigh about one-billion tons on Earth. Using the signals received in LIGO’s detectors, the masses of the neutron stars were determined to 1.1 to 1.6 times as massive as our Sun. 43)

- LIGO Hanford Observatory (LHO) Head, Michael Landry explained what LIGO saw when it made this landmark discovery: “LIGO and Virgo detected 100 seconds of gravitational waves as these two neutron stars spiraled together in a massive and fiery collision,” he said. “In a sprawling follow-up campaign involving about one-quarter of the world’s professional astronomers, observatories in space and on the Earth have detected radiation in all wavelengths from gamma rays to radio waves. But the LIGO and Virgo detectors were absolutely essential in identifying and pinpointing the event in the sky, allowing this campaign to proceed”, Landry added.

- This discovery adds a new way of learning about the universe through “multi-messenger astronomy”, where data from traditional telescopes, neutrino detectors, and now gravitational wave observatories are shared and compared to glean even deeper insights into the nature of the universe.

- This historic detection came just three days after another historic detection, LIGO’s fourth, which was also detected by the Virgo interferometer in Pisa Italy, making it the first detection by Virgo, and the first three-detector observation of a gravitational wave. Reveling after that event, LIGO scientists were astonished to learn of yet another detection, this one completely different from anything LIGO had seen before.

- Salvatore (Salvo) Vitale, assistant professor of physics at MIT, was attending a conference in Amsterdam along with other LIGO scientists, when he first got word of this second detection in 3 days. The first alert he received included a ‘false alarm rate’ (FAR), a measure of how likely it is that the event was not real. In this case, the FAR was reported as 3 x 10-12, which is, according to Vitale, “ridiculously low!”

- How ridiculously low? This figure suggests that the chance that some random but nearly identical bits of ‘noise’ that happened to look like gravitational waves appeared in the instruments at essentially the same time was less than 1 in 80,000 years.

- Two minutes after that first alert, the first scan of the event, automatically generated from the Hanford data, was distributed, and it was distinctly different from anything LIGO had seen before. Signals of black hole mergers last just fractions of a second. This signal lasted well in excess of 30 seconds (in the end, it was shown to have lasted nearly 2 minutes, 500 times longer than black hole mergers). This was a clear indicator that the objects that created the signal were much less massive than black holes. To Vitale and everyone else, the unique properties of the signal could mean only one thing: LIGO had caught its first gravitational wave from merging neutron stars.

- This was in itself a surprise, as Vitale explained. “I saw the omega scan from Hanford, and saw that there was a clear chirp signal, which I remember thinking is ridiculous, because we never thought we’d see anything in an omega scan from a binary neutron star merger .... But this [one] was so loud that we saw it too!”


Figure 31: Top: Thirty-seconds worth of binary neutron star inspiral as it appeared in the LIGO detectors. The entire signal lasted 100 seconds. Bottom: LIGO's first black hole merger detection. The duration of the "chirp" was just 0.2 seconds; 500 times shorter than the signal generated by the neutron stars (image credit: Caltech/MIT/LIGO Lab)

- At the same time that all this was happening, LIGO scientists were alerted to another remarkable astronomical event, which occurred within 2 seconds of LIGO’s detection. The Fermi gamma ray space telescope had recorded a "short" gamma ray burst (sGRB) just 1.7 seconds after the arrival of the gravitational waves.

- Gamma ray bursts are seen quite frequently, but what causes them has remained a mystery. Knowing that neutron star mergers were expected to generate electromagnetic radiation, likely of very high energy, excitement among LIGO scientists began to grow as it became more and more plausible that the first electromagnetic counterpart to a gravitational wave (GW) had been observed. The time of arrival of the sGRB and GW signals was especially telling, and important to validating the relationship between them.

- Vitale explained, “You want the gamma ray burst to come after the gravitational waves because first you have to smash the objects together, then the material is warmed up, and then you get the radiation. So you would expect to see the gravitational waves first.”

- As the pieces began to fall into place, the magnitude of LIGO’s detection became all the more weighty.

- “Then it was, like .... ‘Okay. Oookay .... let’s take a chair .... and sit down .....” said Vitale, laughing as he recalled his feelings at that moment.

- The only way to confirm a correlation between the GRB and the GW, however, would be to find the source object on the sky; but there was a problem. At that point, only the LIGO Hanford data had been processed and distributed; without the Livingston data, no such localization of a source would be possible.

- Matt Evans (Assistant Professor of Physics at MIT) recalls the flurry of communication he was receiving in those early moments.

- “There was this hubbub by Salvo talking about a signal at LHO that looked like a binary neutron star coincident with the Fermi alert. But there hadn’t been anything from Livingston, so there was a moment of doubt of the validity of the signal.”

- The missing data from Livingston was puzzling. Reed Essick (Postdoctoral Fellow, UChicago Kavli Institute for Cosmological Physics) explained: "On the search side, everything looked good, and a sanity check of the detectors told us that LLO (LIGO Livingston Observatory) was in science-mode. So why didn’t the event ‘trigger’ in Livingston?”

- Essick decided to check Livingston data for ‘glitches’, random bits of loud, sudden noise that can drown out other signals in the detectors. Running an algorithm designed specifically for this task, Essick saw that a glitch had in fact occurred at LLO at the same time that the signal appeared in the Hanford interferometer. Sifting through the files manually, Essick found the data stamped with the time of the glitch (and the detection), and there it was.

- This was why LLO didn’t automatically send out a trigger alert. The glitch caused LLO’s computers to disregard, or ‘veto’ that part of the data stream. Looking at it, it’s no wonder! At first glance, it looked ugly. However, Evans explained that it really wasn’t as bad as it seemed.

- “The glitch looks really terrible on the scan. But the truth is, it’s large in amplitude and short in time, so it wouldn’t ruin our ability to do any science on it.”

- Evans added, “Glitches happen every few hours, so the probability of one landing on top of a signal is very low. Nevertheless, people have been working on this sort of possibility for a while, so we were prepared.”

- Most remarkably, despite the size of the glitch, the gravitational wave signal itself was still clearly visible (see image of Figure 32). Seeing that was a moment that Vitale remembered vividly.

- “It was a mix of happiness, tension, and disbelief. We saw that beautiful image of the chirp going through the glitch and coming out the other side. And at that point, it was pretty incredible.”

- In a mathematical equivalent of a game of Operation, at least three teams of people began working on separating the glitch from the signal. As with everything LIGO-related, even cleaning a fraction of a second of data required a group effort! Ultimately, the work paid off. After a few hours, the glitch had been cleanly removed, and some extraordinary science was about to begin.

Spreading the Word

- In anticipation of this kind of event, over the years, LIGO had signed agreements with 90 astronomical observatories around the world to hunt for signs of electromagnetic (EM) radiation escaping from a gravitational wave event; LIGO would share sky-coordinates with its partners, who would then start searching. Until 17 August 2017, no one had found any such counterparts, but the lure of being among the first ones to detect some familiar radiation from a gravitational wave event has kept LIGO’s astronomy partners engaged for over 10 years. This event, combined with a coincident gamma ray detection, represented the best chance yet for astronomers to find something, but to do so, LIGO had to tell them where to look. To that end, LLO data (cleaned of the glitch) were combined with Hanford and Virgo data, and a sky-map narrowing down the possible location of the source of the gravitational waves was generated. The GW map was then merged with Fermi’s GRB map and another region calculated using the INTEGRAL gamma ray space telescope. The result was nothing short of amazing.


Figure 32: The glitch that prevented LIGO Livingston's automated system from distributing the signal from the merging neutron stars. Despite the glitch, the curved GW signal is clearly visible. It looked bad at first, but only 10ms of data needed to be cleaned up (image credit: Caltech/MIT/LIGO Lab)

- The location of the source of the gravitational waves as predicted by LIGO-Virgo data sat beautifully inside the regions of the sky estimated to contain the source of the gamma ray burst as determined by Fermi and INTEGRAL. The resulting search area was small enough that within 12 hours LIGO’s optical astronomy partners had successfully tracked down and imaged a residual fireball at the edge of a galaxy (NGC4993) some 130 million light years distant–most of that time was spent waiting for dusk in Chile, where the first observations could be made. The long-awaited discovery of an EM counterpart to a gravitational-wave detection had been confirmed! Furthermore, the answer to another long-sought-after question was solved: Astronomers could now say with certainty, that at least one source of short gamma ray bursts in the universe is merging neutron stars.


Figure 33: The skymap created by LIGO-Virgo (green) showing the possible location of the source of gravitational waves, compared with regions containing the location of the gamma ray burst source from Fermi (purple) and INTEGRAL (grey). The inset shows the actual position of the galaxy (orange star) containing the "optical transient" that resulted from the merger of two neutron stars (image credit: NASA/ESO)


Figure 34: "The six happiest people in the world" on August 17, 2017. Salvatore Vitale (left) snapped this photo moments after first seeing the LLO scan. The tell-tale trace of gravitational waves generated by merging neutron stars was clearly visible. Clockwise from top: Chris van den Broeck, Jo van den Brand, Peter Pang, Ka Wa Tsang, and Michalis Agathos (image credit: Salvo Vitale)


Figure 35: The first image of an 'optical transient' resulting from the merger of two neutron stars, and the first image of an optical counterpart to a gravitational wave detection. The box at left shows the host galaxy NGC4993, 130 million light years distant, as it appeared in a Hubble Space Telescope image taken April 28, 2017. At right is the same galaxy imaged by the Swope Telescope just a few hours after the gravitational wave and gamma ray detections on August 17, 2017. The arrow points to the short-lived visible fireball that resulted from the merger of two neutron stars in that galaxy (image credit: Swope Supernova Survey via UC Santa Cruz)

- For LIGO, this optical observation was important for another reason. The distance to the galaxy, as determined by astronomers, was wholly consistent with LIGO’s estimated distance of the source of the gravitational waves. Thus, the astronomers provided a completely independent verification that LIGO’s methods for determining the distances to gravitational wave sources are sound.

- From that moment on, all eyes were, and continue to be (now two months after the initial detection), on the skies. LIGOs astronomy partners immediately began observing this object in every wavelength possible, from gamma rays to visible radiation to radio waves, as the remnant ‘object’ changes over time. To date, some 70 of LIGO’s optical astronomy partners have observed this extraordinary event.

- Today, few would disagree with the statement that the level of interest in and study of this event is unprecedented. Within 8 weeks of the detection, over 100 scientific papers describing the results of follow-up studies were written by scientists around the world. Dozens of these papers were published on Monday, October 16th alone, with many more certain to follow in the months and possibly years to come, making this the most broadly and intensely studied astronomical event in human history.

Deeper Meaning

Beyond the obvious scientific importance of this discovery, the importance of this event for the LIGO Laboratory and the wider collaboration goes much deeper. For many, this single detection represents the apex of careers, the culmination of decades of hard work and dedication to LIGO and gravitational wave science.

Dr. Fred Raab, Associate Director for Observatory Operations at the LIGO Hanford Observatory, shared what this latest discovery means to him: “After nearly 30 years of working toward this discovery, I knew that observing the last minute of a binary neutron star system would give unprecedented precision in its parameters. Yet I was unable to continue reading an early paper draft past where I first saw the number for the chirp mass, a key parameter of the system. I stared in wonder for minutes at that number, measured to 4 significant figures for a pair of stars more than 100 million light years from Earth.”

Raab continued, “This observation means that LIGO is transitioning now from studying extreme regions of space-time to extreme states of matter.”

Mike Zucker, LIGO Systems Scientist, had a similar reaction in those first days after the detection:

“I literally stayed awake for days after GW170817 watching the [astronomy notices] roll in, marveling at all the extraordinary implications as the revelations topped each other one by one. I’m just a detector mechanic, but I consider this to be the most significant achievement of my career.”

Janeen Romie, LLO’s Detector Engineering Group Lead, was in her office in Livingston talking to her husband on the phone when she got the first alert: "I noticed that it was a binary neutron star merger and I was like, ‘I’ve got to get off the phone! I’ve got to run down the hall! I’ve got to find out what’s going on!"

Unable to share the news with anyone outside of LIGO, the only thing Romie could do is run and talk to her colleagues.

"That’s why it was so funny for me,” she laughed, “I hung up on my husband!”

Matt Evans shared what he believes is most meaningful to the LIGO laboratory in light of this discovery: “This detection, and especially the triple binary black hole detection with Virgo are important because they demonstrate that we (LIGO) are not the only ones claiming to detect gravitational waves”, he said. “This event solidifies our position in astronomy, not just physics. Other projects around the world will benefit greatly. We really can do multi-messenger astronomy, and that is really meaningful and useful.”

This sentiment was echoed by Essick and Vitale: “What’s most important is represented by the ‘O’ in LIGO”, said Essick, referring to the fact that the “O” in LIGO stands for Observatory. “We’ve been selling the idea that we will detect binary neutron stars for decades, and now we’ve finally done it. We’ve delivered on that promise.”

Vitale would agree, “I think this event brings us a step closer to astronomy", he said. "The detection of these events’s not just ‘collecting stamps’ anymore. Now we can do lots of cool stuff.”

Salvatore's feelings ran a bit deeper still: “Those few days were among the most beautiful days of my life”, he said. “We kept receiving the circulars from the astronomers. They’d say, ‘oh, we have a GRB’, ‘oh, we have found an EM counterpart in optical’, ‘oh, we found the galaxy’, ‘we found the X-rays’, etc. It was ... incredible.”

He paused for a moment; then continued: “It’s also been sad”, he said. “I don’t know if I’ll ever live a moment like that again in my life.” - It's doubtful that anyone at LIGO will.

• October 16, 2017: LIGO’s latest gravitational wave detection has spawned an explosion of new science across the global astronomical community. On August 17, 2017, the two LIGO instruments (funded by the National Science Foundation) and its sister facility, Virgo, near Pisa, Italy, sensed tell-tale signs of the remnant cores of two massive stars spiraling toward and then smashing into each other some 130 million light years away. The objects were quickly identified as neutron stars, the collapsed cores of stars that were once much more massive than our Sun. They are called “neutron stars” because their matter is so densely packed it is composed primarily of neutrons. One such star containing as much matter as our Sun would be just 10 to 15 km in diameter, and a teaspoon of its material would weigh about one-billion tons on Earth. Using the signals received in LIGO’s detectors, the masses of the neutron stars were determined to 1.1 to 1.6 times as massive as our Sun. 44)

- LIGO Hanford Observatory (LHO) Head, Michael Landry explained what LIGO saw when it made this landmark discovery: “LIGO and Virgo detected 100 seconds of gravitational waves as these two neutron stars spiraled together in a massive and fiery collision,” he said. “In a sprawling follow-up campaign involving about one-quarter of the world’s professional astronomers, observatories in space and on the Earth have detected radiation in all wavelengths from gamma rays to radio waves. But the LIGO and Virgo detectors were absolutely essential in identifying and pinpointing the event in the sky, allowing this campaign to proceed”, Landry added.

- This discovery adds a new way of learning about the universe through “multi-messenger astronomy”, where data from traditional telescopes, neutrino detectors, and now gravitational wave observatories are shared and compared to glean even deeper insights into the nature of the universe.

When It Rains, It Pours

- This historic detection came just three days after another historic detection, LIGO’s fourth, which was also detected by the Virgo interferometer in Pisa Italy, making it the first detection by Virgo, and the first three-detector observation of a gravitational wave. Reveling after that event, LIGO scientists were astonished to learn of yet another detection, this one completely different from anything LIGO had seen before.

- Salvatore (Salvo) Vitale, assistant professor of physics at MIT, was attending a conference in Amsterdam along with other LIGO scientists, when he first got word of this second detection in 3 days. The first alert he received included a ‘false alarm rate’ (FAR), a measure of how likely it is that the event was not real. In this case, the FAR was reported as 3x10-12, which is, according to Vitale, “ridiculously low!”

- How ridiculously low? This figure suggests that the chance that some random but nearly identical bits of ‘noise’ that happened to look like a gravitational waves appeared in the instruments at essentially the same time was less than 1 in 80,000 years.

- Two minutes after that first alert, the first scan of the event, automatically generated from the Hanford data, was distributed, and it was distinctly different from anything LIGO had seen before. Signals of black hole mergers last just fractions of a second. This signal lasted well in excess of 30 seconds (in the end, it was shown to have lasted nearly 2 minutes, 500 times longer than black hole mergers). This was a clear indicator that the objects that created the signal were much less massive than black holes. To Vitale and everyone else, the unique properties of the signal could mean only one thing: LIGO had caught its first gravitational wave from merging neutron stars.

- This was in itself a surprise, as Vitale explained. “I saw the omega scan from Hanford, and saw that there was a clear chirp signal, which I remember thinking is ridiculous, because we never thought we’d see anything in an omega scan from a binary neutron star merger .... But this [one] was so loud that we saw it too!”

• October 16, 2017: For the first time ever, astronomers have observed both gravitational waves and light (electromagnetic radiation) from the same event, thanks to a global collaborative effort and the quick reactions of both ESO’s facilities and others around the world. 45)

- On 17 August 2017 the NSF's Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, working with the Virgo Interferometer in Italy, detected gravitational waves passing the Earth. This event, the fifth ever detected, was named GW170817. About two seconds later, two space observatories, NASA’s Fermi Gamma-ray Space Telescope and ESA’s INTErnational Gamma Ray Astrophysics Laboratory (INTEGRAL), detected a short gamma-ray burst from the same area of the sky.

- The LIGO–Virgo observatory network positioned the source within a large region of the southern sky, the size of several hundred full Moons and containing millions of stars [1]. As night fell in Chile many telescopes peered at this patch of sky, searching for new sources. These included ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA) and VLT Survey Telescope (VST) at the Paranal Observatory, the Italian Rapid Eye Mount (REM) telescope at ESO’s La Silla Observatory, the LCO 0.4-meter telescope at Las Cumbres Observatory, and the American DECam at Cerro Tololo Inter-American Observatory. The Swope 1-meter telescope was the first to announce a new point of light. It appeared very close to NGC 4993, a lenticular galaxy in the constellation of Hydra, and VISTA observations pinpointed this source at infrared wavelengths almost at the same time. As night marched west across the globe, the Hawaiian island telescopes Pan-STARRS and Subaru also picked it up and watched it evolve rapidly.

- “There are rare occasions when a scientist has the chance to witness a new era at its beginning,” said Elena Pian, astronomer with INAF, Italy, and lead author of one of the Nature papers. “This is one such time!”

- ESO launched one of the biggest ever “target of opportunity” observing campaigns and many ESO and ESO-partnered telescopes observed the object over the weeks following the detection [2]. ESO’s Very Large Telescope (VLT), New Technology Telescope (NTT), VST, the MPG/ESO 2.2-meter telescope, and the Atacama Large Millimeter/submillimeter Array (ALMA) [3] all observed the event and its after-effects over a wide range of wavelengths. About 70 observatories around the world also observed the event, including the NASA/ESA Hubble Space Telescope.

- Distance estimates from both the gravitational wave data and other observations agree that GW170817 was at the same distance as NGC 4993, about 130 million light-years from Earth. This makes the source both the closest gravitational wave event detected so far and also one of the closest gamma-ray burst sources ever seen [4].

- The ripples in spacetime known as gravitational waves are created by moving masses, but only the most intense, created by rapid changes in the speed of very massive objects, can currently be detected. One such event is the merging of neutron stars, the extremely dense, collapsed cores of high-mass stars left behind after supernovae [5]. These mergers have so far been the leading hypothesis to explain short gamma-ray bursts. An explosive event 1000 times brighter than a typical nova — known as a kilonova — is expected to follow this type of event.

- The almost simultaneous detections of both gravitational waves and gamma rays from GW170817 raised hopes that this object was indeed a long-sought kilonova and observations with ESO facilities have revealed properties remarkably close to theoretical predictions. Kilonovae were suggested more than 30 years ago but this marks the first confirmed observation.

- Following the merger of the two neutron stars, a burst of rapidly expanding radioactive heavy chemical elements left the kilonova, moving as fast as one-fifth of the speed of light. The color of the kilonova shifted from very blue to very red over the next few days, a faster change than that seen in any other observed stellar explosion.

- When the spectrum appeared on our screens I realized that this was the most unusual transient event I’d ever seen,” remarked Stephen Smartt, who led observations with ESO’s NTT as part of the extended Public ESO Spectroscopic Survey of Transient Objects (ePESSTO) observing program. “I had never seen anything like it. Our data, along with data from other groups, proved to everyone that this was not a supernova or a foreground variable star, but was something quite remarkable.”


Figure 36: This artist’s impression shows two tiny but very dense neutron stars at the point at which they merge and explode as a kilonova. Such a very rare event is expected to produce both gravitational waves and a short gamma-ray burst, both of which were observed on 17 August 2017 by LIGO–Virgo and Fermi/INTEGRAL respectively. Subsequent detailed observations with many ESO telescopes confirmed that this object, seen in the galaxy NGC 4993 about 130 million light-years from the Earth, is indeed a kilonova. Such objects are the main source of very heavy chemical elements, such as gold and platinum, in the Universe (image credit: ESO/L. Calçada/M. Kornmesser)

- Spectra from ePESSTO and the VLT’s X-shooter instrument suggest the presence of caesium and tellurium ejected from the merging neutron stars. These and other heavy elements, produced during the neutron star merger, would be blown into space by the subsequent kilonova. These observations pin down the formation of elements heavier than iron through nuclear reactions within high-density stellar objects, known as r-process nucleosynthesis, something which was only theorised before.

- The data we have so far are an amazingly close match to theory. It is a triumph for the theorists, a confirmation that the LIGO–VIRGO events are absolutely real, and an achievement for ESO to have gathered such an astonishing data set on the kilonova,” adds Stefano Covino, lead author of one of the Nature Astronomy papers.

- “ESO’s great strength is that it has a wide range of telescopes and instruments to tackle big and complex astronomical projects, and at short notice. We have entered a new era of multi-messenger astronomy!” concludes Andrew Levan, lead author of one of the papers.


[1] The LIGO–Virgo detection localised the source to an area on the sky of about 35 square degrees.

[2] The galaxy was only observable in the evening in August and then was too close to the Sun in the sky to be observed by September.

[3] On the VLT, observations were taken with: the X-shooter spectrograph located on Unit Telescope 2 (UT2); the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) and Nasmyth Adaptive Optics System (NAOS) – Near-Infrared Imager and Spectrograph (CONICA) (NACO) on Unit Telescope 1 (UT1); VIsible Multi-Object Spectrograph (VIMOS) and VLT Imager and Spectrometer for mid-Infrared (VISIR) located on Unit Telescope 3 (UT3); and the Multi Unit Spectroscopic Explorer (MUSE) and High Acuity Wide-field K-band Imager (HAWK-I) on Unit Telescope 4 (UT4). The VST observed using the OmegaCAM and VISTA observed with the VISTA InfraRed CAMera (VIRCAM). Through the ePESSTO programme, the NTT collected visible spectra with the ESO Faint Object Spectrograph and Camera 2 (EFOSC2) spectrograph and infrared spectra with the Son of ISAAC (SOFI) spectrograph. The MPG/ESO 2.2-meter telescope observed using the Gamma-Ray burst Optical/Near-infrared Detector (GROND) instrument.

[4] The comparatively small distance between Earth and the neutron star merger, 130 million light-years, made the observations possible, since merging neutron stars create weaker gravitational waves than merging black holes, which were the likely case of the first four gravitational wave detections.

[5] When neutron stars orbit one another in a binary system, they lose energy by emitting gravitational waves. They get closer together until, when they finally meet, some of the mass of the stellar remnants is converted into energy in a violent burst of gravitational waves, as described by Einstein’s famous equation E=mc2.

September 27, 2017: The Virgo Collaboration and the LIGO Scientific Collaboration have jointly observed the merger of two black holes. This is the fourth confirmed detection of a binary black hole merger, and the first detection made using a network of three interferometers. 46) 47)

The detected waves—observed on August 14th, 2017 at 10:30:43 UTC (6:30AM EDT) —were produced by a pair of black holes with 31 and 25 solar masses. They merged to produce a spinning black hole of 53 solar masses. Combining the signal from Virgo with the signal observed in the two LIGO observatories improved the sky localization of the source by over a factor of 10. 48)

The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run ‘O2’ , searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since 1 August 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share. 49) 50)

The detected gravitational waves—ripples in space and time—were emitted during the final moments of the merger of two black holes with masses about 31 and 25 times the mass of the sun and located about 1.8 billion light-years away. The newly produced spinning black hole has about 53 times the mass of our sun, which means that about 3 solar masses were converted into gravitational-wave energy during the coalescence.


Figure 37: The GWevent GW170814 observed by LIGO Hanford, LIGO Livingston, and Virgo. Times are shown from August 14, 2017, 10:30:43 UTC. Top row: SNR time series produced in low latency and used by the low-latency localization pipeline on August 14, 2017. The time series were produced by time shifting the best-match template from the online analysis and computing the integrated SNR at each point in time. The single-detector SNRs in Hanford, Livingston, and Virgo are 7.3, 13.7, and 4.4, respectively. Second row: Time-frequency representation of the strain data around the time of GW170814. Bottom row: Time-domain detector data (in color), and 90% confidence intervals for waveforms reconstructed from a morphology-independent wavelet analysis (light gray) and BBH (Binary Black Hole) models (image credit: LIGO and Virgo Collaboration)

The era of gravitational-wave (GW) astronomy began with the detection of binary black hole (BBH) mergers, by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, during the first of the Advanced Detector Observation Runs. 51) Three detections, GW150914, GW151226, and GW170104, and a lower significance candidate, LVT151012, have been announced so far. The Advanced Virgo detector joined the second observation run on August 1, 2017.

• June 15, 2016: A second gravitational wave source was detected by LIGO as reported on June 15, 2016. The LSC (LIGO Scientific Collaboration) and the Virgo Collaboration used data from the twin LIGO detectors — located in Livingston, Louisiana, and Hanford, Washington — to make the discovery, which is accepted for publication in the journal Physical Review Letters. 52) 53)

From the data of the gravitational wave event, named GW151226, the researchers concluded the second set of gravitational waves were produced during the final moments of the merger of two black holes that were 14 and 8 times the mass of the Sun, and the collision produced a single, more massive spinning black hole 21 times the mass of the Sun. In comparison, the black holes detected in September 2015 were 36 and 29 times the Sun’s mass, merging into a black hole of 62 solar masses.

The inferred component masses are consistent with values dynamically measured in X-ray binaries, but are obtained through the independent measurement process of gravitational- wave detection. Although it is challenging to constrain the spins of the initial black holes, we can conclude that at least one black hole had spin greater than 0.2. These recent detections in Advanced LIGO’s first observing period have revealed a population of binary black holes that heralds the opening of the field of gravitational-wave astronomy.

The merger occurred approximately 1.4 billion years ago. The detected signal comes from the last 27 orbits of the black holes before their merger. Based on the arrival time of the signals—the Livingston detector measured the waves 1.1 milliseconds before the Hanford detector—researchers can roughly determine the position of the source in the sky.

“GW151226 perfectly matches our theoretical predictions for how two black holes move around each other for several tens of orbits and ultimately merge,” said Alessandra Buonanno of UMD (University of Maryland). “Remarkably, we could also infer that at least one of the two black holes in the binary was spinning.”

“It is very significant that these black holes were much less massive than those observed in the first detection,” said Gabriela Gonzalez, LSC spokesperson and professor of physics and astronomy at Louisiana State University. “Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe.”

• June 1, 2017: Another gravitational wave has been detected by the LIGO (Laser Interferometer Gravitational-wave Observatory). An international team announced the detection today, while the event itself was detected on January 4, 2017. 54) 55)

The team, including engineers and scientists from Northwestern University in Illinois, published their results in the journal Physical Review Letters. 56)

Like the previous two detections, this one was created by the merging of two black holes. These two were different sizes from each other; one was about 31.2 solar masses, and the other was about 19.4 solar masses. The combined 50 solar mass event caused the third wave, which is named GW170104. The black holes were about 3 billion light years away.

LIGO is showing us that their is a population of binary black holes out there. “Our handful of detections so far is revealing an intriguing black hole population we did not know existed until now,” said Northwestern’s Vicky Kalogera, a senior astrophysicist with the LSC (LIGO Scientific Collaboration), which conducts research related to the twin LIGO detectors, located in the U.S.

“Now we have three pairs of black holes, each pair ending their death spiral dance over millions or billions of years in some of the most powerful explosions in the universe. In astronomy, we say with three objects of the same type you have a class. We have a population, and we can do analysis.”

This third finding strengthens the case for the existence of a new class of black holes: binary black holes that are locked in relationship with each other. It also shows that these objects can be larger than thought before LIGO detected them. “It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us.” – David Shoemaker, MIT.

“We have further confirmation of the existence of black holes that are heavier than 20 solar masses, objects we didn’t know existed before LIGO detected them,” said David Shoemaker of MIT, spokesperson for the LIGO Scientific Collaboration . “It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us.”

“With the third confirmed detection of gravitational waves from the collision of two black holes, LIGO is establishing itself as a powerful observatory for revealing the dark side of the universe,” said David Reitze of Caltech, executive director of the LIGO Laboratory and a Northwestern alumnus. “While LIGO is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars.”

• In February 2016, gravitational-wave astronomy is going international, as LIGO India (sometimes referred to as INDIGO) received the green light recently in the wake of the detection announcement. Set to begin science operations around 2019, the third LIGO detector will be constructed in India. This will give LIGO the ‘third vector’ it was initially envisioned with, allowing researchers to pin down the source direction in the sky. Other detectors are on the hunt as well, including VIRGO near Pisa, Italy, GEO600 in Germany, and KAGRA (Kamioka Gravitational Wave Detector), University of Tokyo, Japan. 57)

The LISA Pathfinder mission of ESA also started science operations in late February 2016. Launched on December 3rd, 2015 from Kourou, French Guiana, LISA Pathfinder won’t detect gravitational waves. It will, however, pave the way for a full-up spaceborne gravitational wave detector, eLISA (evolved Laser Interferometer Space Antenna), set to launch sometime in the 2030s.

On February 11, 2016, the LIGO Scientific Collaboration and Virgo Collaboration announced the first confirmed observation of gravitational waves from colliding black holes. The gravitational wave signals were observed by the LIGO's twin observatories on September 14, 2015. This confirms a key prediction of Einstein's theory of general relativity and provides the first direct evidence that black holes merge. 58)

• Already on 14 September 2015 at 09:50:45 UTC, the LIGO Hanford, WA, and Livingston, LA, observatories detected the coincident signal GW150914 shown in Figure 39. The initial detection was made by low-latency searches for generic gravitational-wave transients and was reported within three minutes of data acquisition. 59)

Figure 38: The First Observation of Gravitational Waves. On September 14, 2015, LIGO observed ripples in the fabric of spacetime. This video narrative tells the story of the science behind that important detection. (video credit: Caltech)


Figure 39: The gravitational-wave event GW150914 observed by the LIGO Hanford (H1, left column panels) and Livingston (L1, right column panels) detectors. Times are shown relative to September 14, 2015 at 09:50:45 UTC (image credit: LIGO consortium).

• February 11, 2016: For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos. 60)

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

The gravitational waves were detected on September 14, 2015 at 5:51 a.m. Eastern Daylight Time (09:51 UTC) by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. The LIGO Observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

Minimize LIGO continued

Status of the LIGO / Virgo Facilities

• April 1, 2019: A bit more than three years after the first, landmark detection of gravitational waves (GWs), the LIGO and Virgo laser interferometer GW observatories today kick off their third observation run. 61)

- Known in the GW community simply as “O3,” the year-long observation run will likely yield a bumper crop of new astronomical observations—the result of a 40 percent improvement in the already jaw-dropping sensitivity of the two LIGO facilities in the United States and a near-doubling of the sensitivity of the Virgo facility in Italy. The O3 period also could see the long-awaited on-streaming of the KAGRA GW observatory in Japan. And, in a new twist, the LIGO/Virgo Scientific Collaboration (LSC) will be making data about possible GW detections publicly available in near realtime.

Boosting laser power

- The O3 run will add to the impressive string of milestones achieved in the first two GW observation runs. These include the detection of gravitational waves from ten binary black-hole mergers, as well as from the collision of a pair of ultra-dense neutron stars. The latter detection—coordinated with observations from more traditional optical, X-ray and gamma-ray telescopes in a path-breaking example of “multi-messenger astronomy”—resulted in a breathtaking harvest of new scientific information.

- LSC scientists are confident that the LIGO and Virgo observatories will log observations at an even faster clip in O3, as a result of technical improvements implemented since the end of the last observation run, O2, in August 2017.

- The improvements include a doubling of the power of the facilities’ lasers, which in these observatories are amplified in two Fabry–Pérot cavities 3 to 4 km long that form the arms of a gigantic, L-shaped Michelson-interferometer. Also installed in the upgrade round were scattered-light suppressors, or “baffles,” designed to control stray light within the huge interferometers.

Hammering down noise

- In addition to laser power, other recent upgrades have centered on efforts to boost sensitivity by ferreting out and eliminating noise sources in a range of subsystems.

- At LIGO, this has included the huge engineering challenge of swapping out a number of the 40-kg mirrors, or test masses, exquisitely suspended at either end of the laser interferometer arms. As a passing gravitational wave ripples through spacetime, tiny movements in these hefty mirrors result in infinitesimal changes of the interferometer arms’ lengths, which are read as picowatt-scale power fluctuations at the dark port of the interferometer. The new, better-performing versions of the mirrors include improved coatings to diminish thermal noise.


Figure 40: Engineers Hugh Radkins and Betsy Weaver at work inside the vacuum system of the detector at LIGO Hanford Observatory, Washington, USA, during recent hardware upgrades. With the upgrades complete, the two U.S. LIGO facilities, in partnership with the Virgo facility in Italy, are now poised to begin third observation run (image credit: LIGO/Caltech/MIT/Jeff Kissel)

- At Virgo, meanwhile, the steel wires suspending the main mirrors have been replaced with fused-silica versions that quiet down vibrational noise and extend the facility’s ability to pick up low- and medium-frequency GWs. And during O3, both LIGO and Virgo will use a trick of quantum mechanics, the injection of a “squeezed” state of light at the photodetector, to narrow down the uncertainties in photon arrival times attributable to fluctuations in the quantum vacuum.

- These and other technical improvements were partly developed and matured at yet another facility, GEO 600, a smaller GW observatory in Europe that has served as a vital testbed for technologies to sharpen the observing power of the larger sites. GEO 600 will also participate in the O3 run.

Sampling more of the cosmos

- The recent sensitivity upgrades will enable the global GW network to sample a much-expanded slice of the cosmos for evidence of high-energy astronomical events. In the O3 run, for example, LIGO’s sensitivity in the wake of the recent upgrades should enable it to sniff out binary neutron-star mergers to a distance of 550 million light-years—more than 190 million light-years further than in O2.

- That, coupled with an eightfold expansion of the volume of space now visible to Virgo, could increase the rate of detection of binary black-hole collisions to anywhere from a few events per month to a few per week, and binary neutron-star mergers to between one per year and one per month. There’s also the possibility of picking up more exotic, previously inaccessible events, such as the merger of a black hole and a neutron star.

Instant access to data

- In another change, the public will have near-immediate access to this harvest of discoveries, through new software developed by LSC scientists. The software will be “able to send open public alerts within five minutes” after a GW detection, according to Sarah Antier, a postdoctoral research associate at the Université Paris Diderot, France.

- That will allow rapid public access to parameters such as type of signal, sky position and estimated distance for a given GW event. Those parameters, in turn, will let both professional and amateur astronomers looking at various slices of the electromagnetic spectrum quickly train their instruments on the right patch of sky to follow up on the GW observation.

• February 18, 2019: The National Science Foundation (NSF) is awarding Caltech and MIT $20.4 million to upgrade the Laser Interferometer Gravitational-wave Observatory (LIGO), an NSF-funded project that made history in 2015 after making the first direct detection of ripples in space and time, called gravitational waves. 62) 63)

- The investment is part of a joint international effort in collaboration with UK Research and Innovation and the Australian Research Council, which are contributing additional funds. While LIGO is scheduled to turn back on this spring, in its third run of the "Advanced LIGO" phase, the new funding will go toward "Advanced LIGO Plus."

- Advanced LIGO Plus is expected to commence operations in 2024 and to increase the volume of deep space the observatory can survey by as much as seven times.

- "I'm extremely excited about the future prospects that the Advanced LIGO Plus upgrade affords gravitational-wave astrophysics," said Caltech's David Reitze, executive director of LIGO.

- "With it we expect to detect gravitational waves from black hole mergers on a daily basis, greatly increasing our understanding of this dark sector of the universe. Gravitational-wave observations of neutron star collisions, now very rare, will become much more frequent, allowing us to more deeply probe the structure of their exotic interiors."

- Since LIGO's first detection of gravitational waves from the violent collision of two black holes, it has observed nine additional black hole mergers and one collision of two dense, dead stars called neutron stars.

- The neutron star merger gave off not just gravitational waves but light waves, detected by dozens of telescopes in space and on the ground. The observations confirmed that heavy elements in our universe, such as platinum and gold, are created in neutron star smashups like this one.

- "This award ensures that NSF's LIGO, which made the first historic detection of gravitational waves in 2015, will continue to lead in gravitational-wave science for the next decade," said Anne Kinney, assistant director for NSF's Mathematical and Physical Sciences Directorate, in a statement.

- "With improvements to the detectors - which include techniques from quantum mechanics that refine laser light and new mirror coating technology - the twin LIGO observatories will significantly increase the number and strength of their detections. Advanced LIGO Plus will reveal gravity at its strongest and matter at its densest in some of the most extreme environments in the cosmos. These detections may reveal secrets from inside supernovae and teach us about extreme physics from the first seconds after the universe's birth."

- Michael Zucker, the Advanced LIGO Plus leader and co-principal investigator, and a scientist at the LIGO Laboratory, operated by Caltech and MIT, said, "I'm thrilled that NSF, UK Research, and Innovation and the Australian Research Council are joining forces to make this key investment possible. Advanced LIGO has altered the course of astrophysics with 11 confirmed gravitational-wave events over the last three years. Advanced LIGO Plus can expand LIGO's horizons enough to capture this many events each week, and it will enable powerful new probes of extreme nuclear matter as well as Albert Einstein's general theory of relativity."


Figure 41: Aerial view of LIGO Hanford Observatory (image credit: LIGO)

• November 20, 2018: The LIGO Laboratory congratulates Derek Davis of Syracuse University and T.J. Massinger of Caltech for winning the first LIGO Laboratory Award for Excellence in Detector Characterization and Calibration. 64)

- The LIGO gravitational wave detectors have registered gravitational wave signals from multiple black hole mergers and the spectacular collision of two neutron stars since Advanced LIGO first began observing in 2015. Davis and Massinger’s work to reduce the noise present in LIGO detector data was key to making these discoveries possible by allowing searches to more easily distinguish the signatures of true astrophysical gravitational wave events in noisy detector data.

- By improving how deep in space the LIGO-Hanford detector could sense by up to 50%, at least three gravitational wave signals were confidently detected during Advanced LIGO's second observing run (O2) that would not have been otherwise. Their efforts are an outstanding example of the detector characterization work needed to lay the groundwork for future discoveries in gravitational wave astrophysics and multi-messenger astronomy.

- Davis and Massinger will share a $1000 prize and are invited to present colloquia at one of the the LIGO Laboratory sites (LIGO-Hanford, LIGO-Livingston, Caltech, or MIT) to share their achievements with LIGO Laboratory members. They will each receive an award certificate at the LIGO-Virgo Collaboration meeting in March 2019.

- Derek Davis is currently a Ph.D. student at Syracuse University. As a part of the LIGO Scientific Collaboration, they serve as the Event Validation Lead for the LIGO Detector Characterization group, leading follow-up investigations of candidate gravitational-wave detections.

- T.J. Massinger is currently a postdoctoral scholar at Caltech. He earned his PhD in 2016 from Syracuse University. Within the LIGO Detector Characterization group, he serves as instrument science lead and as a liaison to the compact binary coalescence data analysis working group.

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6) Evan Gough, ”Gravitational Waves Discovered: A New Window on the Universe,” Universe Today, Feb. 11, 2016, URL:

7) Markus Pössel, ”Gravitational waves and how they distort space,” Universe Today, Feb. 8, 2016, URL:

8) ”Nobel Prize awarded to LIGO Founders,” LIGO News Release, 3 October 2017, URL:

9) Susanna Kohler, ”Warning: Neutron Star Collision Imminent,” AAS Nova, 6 January 2021, URL:

10) Surabhi Sachdev, Ryan Magee, Chad Hanna, Kipp Cannon, Leo Singer, Javed Rana SK, Debnandini Mukherjee, Sarah Caudill, Chiwai Chan, Jolien D. E. Creighton, Becca Ewing, Heather Fong, Patrick Godwin, Rachael Huxford, Shasvath Kapadia, Alvin K. Y. Li, Rico Ka Lok Lo, Duncan Meacher, Cody Messick, Siddharth R. Mohite, Atsushi Nishizawa, Hiroaki Ohta, Alexander Pace, Amit Reza, B. S. Sathyaprakash, Minori Shikauchi, Divya Singh, Leo Tsukada, Daichi Tsuna, Takuya Tsutsui, and Koh Ueno, ”An Early-warning System for Electromagnetic Follow-up of Gravitational-wave Events,” The Astrophysical Journal Letters, Volume 905, Number 2, Published: 21 December 2020,

11) ”New constraints on continuous gravitational waves from neutron stars,” LIGO News, 25 December 2020, URL:

12) ”Winner of the 2020 Award for Excellence in Detector Characterization and Calibration Announced,” LIGO News Release, 23 November 2020, URL:

13) ”Prime Minister’s Prizes for Science 2020,” Australian Government, 3 Nov. 2020, URL:

14) Maximiliano Isi and Colm M. Talbot, ”LIGO and Virgo Announce New Detections in Updated Catalog,” LIGO, 28 October 2020, URL:

15) Jennifer Chu ,”An unexpected origin story for a lopsided black hole merger -Researchers suggest a novel process to explain the collision of a large black hole and a much smaller one,” MIT News Office, 2 September 2020, URL:

16) ”RIT (Rochester Institute of Technology) scientists contribute to the first discovery of an intermediate-mass black hole,” RIT, 9 September 2020, URL:

17) Davide Gerosa, Salvatore Vitale, and Emanuele Berti, ”Astrophysical Implications of GW190412 as a Remnant of a Previous Black-Hole Merger,” Physical Review Letters, Volume 125, 101103, Published: 2 September 2020,

18) Fernanda Ferreira, ”Are we still listening to space?,” MIT School of Science, 19 August 2020, URL:

19) Whitney Clavin, ”LIGO-Virgo Finds Mystery Object in "Mass Gap",” Caltech News, 23 June 2020, URL:

20) ”New gravitational-wave model can bring neutron stars into even sharper focus,” Space Daily, 22 May 2020, URL:

21) Geraint Pratten, Patricia Schmidt, Tanja Hinderer, ”Gravitational-Wave Asteroseismology with Fundamental Modes from Compact Binary Inspirals,” Nature Communications, Volume 11, Article No. 2553, Published: 21 May 2020,

22) ”A signal like none before — LIGO and Virgo detectors catch first gravitational wave from binary black hole merger with unequal masses,” AEI (Albert Einstein Institute) Hannover, 20 April 2020, URL:

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24) ”LIGO-Virgo Network Catches Another Neutron Star Collision,” Caltech, 6 January 2020, URL:

25) ”LIGO-Virgo gravitational wave network catches another neutron star collision,”, 7 January, 2020, URL:

26) Amanda Morris, ”Afterglow sheds light on the nature, origin of neutron star collisions - Researchers use Hubble to capture deepest optical image of first neutron star merger,” Northwestern University, 9 September 2019, URL:

27) Wen-fai Fong (Northwestern/CIERA), P. K. Blanchard, K. D. Alexander, J. Strader, R. Margutti, A. Hajela, V. A. Villar, Y. Wu, C. S. Ye, E. Berger, R. Chornock, D. Coppejans, P. S. Cowperthwaite, T. Eftekhari, D. Giannios, C. Guidorzi, A. Kathirgamaraju, T. Laskar, A. MacFadyen, B. D. Metzger, M. Nicholl, K. Paterson, G. Terreran, D. J. Sand, L. Sironi, P. K. G. Williams, X. Xie, J. Zrake, ”The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin,” The Astrophysical Journal Letters (accepted), 2019 DOI: 10.17909/t9-6qez-fw41

28) ”LIGO and Virgo detect neutron star smash-ups,” MIT News Office, 2 May 2019, URL:

29) ”LIGO and Virgo detect neutron star smash-ups,” news, 2 May 2019, URL:

30) ”LIGO spots two gravitational waves in just two days,” Astronomy, 26 April 2019, URL:

31) ”LIGO detects gravitational waves from another neutron star merger,” Astronomy, 26 April 2019, URL:

32) ”Ancient star-crash detection ushers new dawn for discovery,” ANU (Austalian National University), 3 May 2019, URL:

33) ”LIGO and Virgo Resume Search for Ripples in Space and Time,” LIGO Caltech, 26 March 2019, URL:

34) ”The O2 Data Set Is Now Available,” LIGO Caltech, 27 February 2019, URL:

35) ”(Re)solving the jet/cocoon riddle of a gravitational wave event —A global network of radio telescopes exposes the aftermath of a violent merger of neutron stars ,” Max Planck Institute for Radio Astronomy, 21 February 2019, URL:

36) G. Ghirlanda, O. S. Salafia, Z. Paragi, M. Giroletti, J. Yang, B. Marcote, J. Blanchard, I. Agudo, T. An, M. G. Bernardini, R. Beswick, M. Branchesi, S. Campana, C. Casadio, E. Chassande-Mottin, M. Colpi, S. Covino, P. D’Avanzo, V. D’Elia, S. Frey, M. Gawronski, G. Ghisellini, L. I. Gurvits, P. G. Jonker, H. J. van Langevelde, A. Melandri, J. Moldon, L. Nava, A. Perego, M. A. Perez-Torres, C. Reynolds, R. Salvaterra, G. Tagliaferri, T. Venturi, S. D. Vergani, M. Zhang,”Compact radio emission indicates a structured jet was produced by a binary neutron star merger,” Science Report, 21 February 2019: eaau8815, DOI: 10.1126/science.aau8815,

37) Emily Conover, ”Scientists’ collection of gravitational waves just got a lot bigger,” Science News, 4 December 2018, URL:

38) ”Researchers make vital contribution to new gravitational wave discoveries,” UoP News, 3 December 2018, URL:

39) J. C. Driggers, S. Vitale, A. P. Lundgren, M. Evans, K. Kawabe, S. E. Dwyer, K. Izumi, R. M. S. Schofield, A. Effler, D. Sigg, P. Fritschel, M. Drago, A. Nitz, B. P. Abbott, R. Abbott, T. D. Abbott, C. Adams, R. X Adhikari, V. B. Adya, A. Ananyeva, S. Appert, K. Arai, S. M. Aston, C. Austin, S. W. Ballmer, D. Barker, B. Barr, L. Barsotti, J. Bartlett, I. Bartos, J. C. Batch, A. S. Bell, J. Betzwieser, G. Billingsley, J. Birch, S. Biscans, C. D. Blair, R. M. Blair, R. Bork, A. F. Brooks, H. Cao, G. Ciani, F. Clara, S. J. Cooper, P. Corban, S. T. Countryman, P. B. Covas, M. J. Cowart, D. C. Coyne, A. Cumming, L. Cunningham, K. Danzmann, C. F. Da Silva Costa, E. J. Daw, D. DeBra, R. DeSalvo, K. L. Dooley, S. Doravari, T. B. Edo, T. Etzel, T. M. Evans, H. Fair, A. Fernandez-Galiana, E. C. Ferreira, R. P. Fisher, H. Fong, R. Frey, V. V. Frolov, P. Fulda, M. Fyffe, B. Gateley, J. A. Giaime, K. D. Giardina, E. Goetz, R. Goetz, S. Gras, C. Gray, H. Grote, K. E. Gushwa, E. K. Gustafson, R. Gustafson, E. D. Hall, G. Hammond, J. Hanks, J. Hanson, T. Hardwick, G. M. Harry, M. C. Heintze, A. W. Heptonstall, J. Hough, R. Jones, S. Kandhasamy, S. Karki, M. Kasprzack, S. Kaufer, R. Kennedy, N. Kijbunchoo, W. Kim, E. J. King, P. J. King, J. S. Kissel , W. Z. Korth, G. Kuehn, M. Landry, B. Lantz, M. Laxen, J. Liu, N. A. Lockerbie, M. Lormand, M. MacInnis, D. M. Macleod, S. Marka, Z. Marka, A. S. Markosyan, E. Maros, P. Marsh, I. W. Martin, D. V. Martynov, K. Mason, T. J. Massinger, F. Matichard, N. Mavalvala, R. McCarthy, D. E. McClelland, S. McCormick, L. McCuller, J. McIver, D. J. McManus, T. McRae, G. Mendell, E. L. Merilh, P. M. Meyers, R. Mittleman, K. Mogushi, D. Moraru, G. Moreno, C. M. Mow-Lowry, G. Mueller, N. Mukund, A. Mullavey, J. Munch, T. J. N. Nelson, P. Nguyen, L. K. Nuttall, J. Oberling, M. Oliver, P. Oppermann, Richard J. Oram, B. O'Reilly, D. J. Ottaway, H. Overmier, J. R. Palamos, W. Parker, A. Pele, S. Penn, C. J. Perez, M. Phelps, V. Pierro, I. M. Pinto, M. Pirello, M. Principe, L. G. Prokhorov, O. Puncken, V. Quetschke, E. A. Quintero, H. Radkins, P. Raffai, K. E. Ramirez, S. Reid, D. H. Reitze, N. A. Robertson, J. G. Rollins, V. J. Roma, C. L. Romel, J. H. Romie, M. P. Ross, S. Rowan, K. Ryan, T. Sadecki, E. J. Sanchez, L. E. Sanchez, V. Sandberg, R. L. Savage, D. Sellers, D. A. Shaddock, T. J. Shaffer, B. Shapiro, D. H. Shoemaker, B. J. J. Slagmolen, B. Smith, J. R. Smith, B. Sorazu, A. P. Spencer, K. A. Strain, D. B. Tanner, R. Taylor, M. Thomas, P. Thomas, K. A. Thorne, E. Thrane, K. Toland, C. I. Torrie, G. Traylor, M. Tse, D. Tuyenbayev, G. Vajente, G. Valdes, A. A. van Veggel, S. Vass, A. Vecchio, P. J. Veitch, K. Venkateswara, G. Venugopalan, T. Vo, C. Vorvick, M. Walker, R. L. Ward, J. Warner, B. Weaver, R. Weiss, P. Wessels, B. Willke, C. C. Wipf, J. Worden, H. Yamamoto, C. C. Yancey, Hang Yu, Haocun Yu, L. Zhang, M. E. Zucker, J. Zweizig, ”Improving astrophysical parameter estimation via offline noise subtraction for Advanced LIGO,” Astrophysics, Instrumentation and Methods for Astrophysics, 2018, URL:

40) ”LIGO Celebrates One-Year Anniversary of Historic Binary Neutron Star Merger Detection,” LIGO, News Release, 17 August 2018, URL:

41) ”New Video: LIGO: A Discovery that Shook the World,” LIGO Caltech, 22 February 2018, URL:

42) ”Physics World 2017 Breakthrough of the Year Award,” LIGO Caltech, 11 December 2017, URL:

43) Kimberly M. Burtnyk, ”LIGO Detection of Colliding Neutron Stars Spawns Global Effort to Study the Rare Event,” Caltech News Release, 16 October 2017, URL:

44) Kimberly M. Burtnyk, ”LIGO Detection of Colliding Neutron Stars Spawns Global Effort to Study the Rare Event,” LIGO Caltech News Release, 16 October 2017, URL:

45) ”ESO Telescopes Observe First Light from Gravitational Wave Source -Merging neutron stars scatter gold and platinum into space,” eso1733 — Science Release, 16 October 2017, URL:

46) ”LIGO and Virgo make the first joint detection of merging black holes,” LIGO, 27 September 2017, URL:

47) ”Gravitational waves from a binary black hole merger observed by LIGO and Virgo,” LIGO Press Release, 27 September 2017, URL:

48) B. P. Abbott, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley, C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. B. Adya, C. Affeldt, M. Afrough, B. Agarwal, M. Agathos, K. Agatsuma, N. Aggarwal, O. D. Aguiar, et al., ” GW170814: A Three-Detector Observation of Gravitational Waves from a Binary Black Hole Coalescence,” Physical Review Letters, Volume 119, 141101, Published: 6 October 2017, URL:

49) ”A very exciting LIGO-Virgo Observing run is drawing to a close August 25,” LSC News, 25 August 2017, URL:

50) ”LIGO's 2nd Observation Run Finishes,” LIGO Caltech, 25 August 2017, URL:

51) J. Aasi, B. P. Abbott, R. Abbott, T. Abbott, M. R. Abernathy, K. Ackley, C. Adams, T. Adams, P. Addesso, R. X. Adhikari, V. Adya, C. Affeldt, N. Aggarwal, O. D. Aguiar, A. Ain, P. Ajith, A. Alemic, B.Allen, et al., ”Advance LIGO- The LIGO Scientific Collaboration,” IOP Science, Classical and Quantum Gravity, Volume 32, Number 7, Published: 3 March 2015,, URL:

52) ”Gravitational Waves Detected from Second Pair of Colliding Black Holes,” University of Maryland, June 15, 2016, URL:

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54) Evan Gough, ”Third Gravitational Wave Event Detected,” Universe Today, June 1, 2017, URL:

55) Megan Fellman, ”LIGO detects gravitational waves for third time,” Northwestern University, June 1, 2017, URL:

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57) David Dickinson, ”The Future of Gravitational Wave Astronomy: Pulsar Webs, Space Interferometers and Everything,” Universe Today, Feb. 25, 2016, URL:

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60) ”Gravitational waves detected 100 years after Einstein's prediction,” LIGO News Release, 11 February 2016, URL:

61) ”LIGO and Virgo Open a New Hunting Season,” OSA (Optical Society of America), 1 April 2019, URL:

62) ”US-UK-Australia funding to improve global gravitational wave network,” Space Daily, 18 February 2019, URL:

63) ”LIGO Receives New Funding to Search for More Extreme Cosmic Events — Grants from the U.S., United Kingdom, and Australia will fund next-generation improvements to LIGO,” LIGO, 14 February, 2019, URL:

64) ”Winners of the 2018 Award for Excellence in Detector Characterization and Calibration Are Announced,” LIGO Caltech News Release, 20 November 2018, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

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