Astronomy and Telescopes
Black Hole and its Shadow - first direct visual evidence of a supermassive black hole
A long standing goal in astrophysics is to directly observe the immediate environment of a black hole with an angular resolution comparable to the event horizon. Such observations could lead to images of strong gravity effects that are expected near a black hole, and to the direct detection of dynamics near the black hole as matter orbits at near light speeds. This capability would open a new window on the study of general relativity in the strong field regime, accretion and outflow processes at the edge of a black hole, the existence of event horizons, and fundamental black hole physics. 1)
The EHT (Event Horizon Telescope) is an international collaboration that has formed to continue the steady long-term progress on improving the capability of VLBI (Very Long Baseline Interferometry) at short wavelengths in pursuit of this goal. This technique of linking radio dishes across the globe to create an Earth-sized interferometer, has been used to measure the size of the emission regions of the two supermassive black holes with the largest apparent event horizons: SgrA* (Sagittarius A*) at the center of the Milky Way and M87 (Messier 87) in the center of the Virgo A galaxy. In both cases, the sizes match that of the predicted silhouette caused by the extreme lensing of light by the black hole. Addition of key millimeter and submillimeter wavelength facilities at high altitude sites has now opened the possibility of imaging such features and sensing the dynamic evolution of black hole accretion. The EHT project includes theoretical and simulation studies that are framing questions rooted at the black hole boundary that may soon be answered through observations.
By linking together existing telescopes using novel systems, the EHT leverages considerable global investment to create a fundamentally new instrument with an angular resolving power that is the highest possible from the surface of the Earth. Over the coming years, the international EHT team will mount observing campaigns of increasing resolving power and sensitivity, aiming to bring black holes into focus.
Astronomers Capture First Image of a Black Hole
• 10 April 2019: The Event Horizon Telescope (EHT) — a planet-scale array of eight ground-based radio telescopes forged through international collaboration — was designed to capture images of a black hole. Today, in coordinated press conferences across the globe, EHT researchers reveal that they have succeeded, unveiling the first direct visual evidence of a supermassive black hole and its shadow. 2)
This breakthrough was announced today in a series of six papers published in a special issue of The Astrophysical Journal Letters. The image reveals the black hole at the center of Messier 87 , a massive galaxy in the nearby Virgo galaxy cluster. This black hole resides 55 million light-years from Earth and has a mass 6.5 billion times that of the Sun .
The EHT links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution . The EHT is the result of years of international collaboration, and offers scientists a new way to study the most extreme objects in the Universe predicted by Einstein's general relativity during the centennial year of the historic experiment that first confirmed the theory .
"We have taken the first picture of a black hole," said EHT project director Sheperd S. Doeleman of the Center for Astrophysics, Harvard & Smithsonian. "This is an extraordinary scientific feat accomplished by a team of more than 200 researchers."
Black holes are extraordinary cosmic objects with enormous masses but extremely compact sizes. The presence of these objects affects their environment in extreme ways, warping spacetime and super-heating any surrounding material.
"If immersed in a bright region, like a disc of glowing gas, we expect a black hole to create a dark region similar to a shadow — something predicted by Einstein's general relativity that we've never seen before, explained chair of the EHT Science Council Heino Falcke of Radboud University, the Netherlands. "This shadow, caused by the gravitational bending and capture of light by the event horizon, reveals a lot about the nature of these fascinating objects and allowed us to measure the enormous mass of M87's black hole."
Multiple calibration and imaging methods have revealed a ring-like structure with a dark central region — the black hole's shadow — that persisted over multiple independent EHT observations.
"Once we were sure we had imaged the shadow, we could compare our observations to extensive computer models that include the physics of warped space, superheated matter and strong magnetic fields. Many of the features of the observed image match our theoretical understanding surprisingly well," remarks Paul T. P. Ho, EHT Board member and Director of the East Asian Observatory . "This makes us confident about the interpretation of our observations, including our estimation of the black hole's mass."
Creating the EHT was a formidable challenge which required upgrading and connecting a worldwide network of eight pre-existing telescopes deployed at a variety of challenging high-altitude sites. These locations included volcanoes in Hawai`i and Mexico, mountains in Arizona and the Spanish Sierra Nevada, the Chilean Atacama Desert, and Antarctica.
The EHT observations use a technique called VLBI which synchronizes telescope facilities around the world and exploits the rotation of our planet to form one huge, Earth-size telescope observing at a wavelength of 1.3 mm. VLBI allows the EHT to achieve an angular resolution of 20 µas (micro-arcseconds) — enough to read a newspaper in New York from a sidewalk café in Paris .
The telescopes contributing to this result were ALMA (Atacama Large Millimeter/submillimeter Array), APEX (Atacama Pathfinder EXperiment), the IRAM (Institute for Radio Astronomy in the Millimeter Range) 30 m telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope . Petabytes of raw data from the telescopes were combined by highly specialised supercomputers hosted by the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory.
The construction of the EHT and the observations announced today represent the culmination of decades of observational, technical, and theoretical work. This example of global teamwork required close collaboration by researchers from around the world. Thirteen partner institutions worked together to create the EHT, using both pre-existing infrastructure and support from a variety of agencies. Key funding was provided by the US National Science Foundation (NSF), the EU's European Research Council (ERC), and funding agencies in East Asia.
"We have achieved something presumed to be impossible just a generation ago," concluded Doeleman. "Breakthroughs in technology, connections between the world's best radio observatories, and innovative algorithms all came together to open an entirely new window on black holes and the event horizon."
Note : The shadow of a black hole is the closest we can come to an image of the black hole itself, a completely dark object from which light cannot escape. The black hole's boundary — the event horizon from which the EHT takes its name — is around 2.5 times smaller than the shadow it casts and measures just under 40 billion km across.
Note : Supermassive black holes are relatively tiny astronomical objects — which has made them impossible to directly observe until now. As a black hole's size is proportional to its mass, the more massive a black hole, the larger the shadow. Thanks to its enormous mass and relative proximity, M87's black hole was predicted to be one of the largest viewable from Earth — making it a perfect target for the EHT.
Note : Although the telescopes are not physically connected, they are able to synchronize their recorded data with atomic clocks — hydrogen masers — which precisely time their observations. These observations were collected at a wavelength of 1.3 mm during a 2017 global campaign. Each telescope of the EHT produced enormous amounts of data — roughly 350 terabytes per day — which was stored on high-performance helium-filled hard drives. These data were flown to highly specialised supercomputers — known as correlators — at the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory to be combined. They were then painstakingly converted into an image using novel computational tools developed by the collaboration.
Note : 100 years ago, two expeditions set out for the island of Príncipe off the coast of Africa and Sobra in Brazil to observe the 1919 solar eclipse, with the goal of testing general relativity by seeing if starlight would be bent around the limb of the sun, as predicted by Einstein. In an echo of those observations, the EHT has sent team members to some of the world's highest and isolated radio facilities to once again test our understanding of gravity.
Note : The East Asian Observatory (EAO) partner on the EHT project represents the participation of many regions in Asia, including China, Japan, Korea, Taiwan, Vietnam, Thailand, Malaysia, India and Indonesia.
Note : Future EHT observations will see substantially increased sensitivity with the participation of the IRAM NOEMA Observatory, the Greenland Telescope and the Kitt Peak Telescope.
Note : ALMA is a partnership of the European Southern Observatory (ESO; Europe, representing its member states), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan, together with the National Research Council (Canada), the Ministry of Science and Technology (MOST; Taiwan), Academia Sinica Institute of Astronomy and Astrophysics (ASIAA; Taiwan), and Korea Astronomy and Space Science Institute (KASI; Republic of Korea), in cooperation with the Republic of Chile. APEX is operated by ESO, the 30-meter telescope is operated by IRAM (the IRAM Partner Organizations are MPG (Germany), CNRS (France) and IGN (Spain)), the James Clerk Maxwell Telescope is operated by the EAO, the Large Millimeter Telescope Alfonso Serrano is operated by INAOE and UMass, the Submillimeter Array is operated by SAO and ASIAA and the Submillimeter Telescope is operated by the Arizona Radio Observatory (ARO). The South Pole Telescope is operated by the University of Chicago with specialized EHT instrumentation provided by the University of Arizona.
Focus on the First Event Horizon Telescope Results
This research was presented in a series of six papers published today in a special issue of The Astrophysical Journal Letters, along with a Focus Issue: 3)
We report the first image of a black hole.
This Focus Issue shows ultra-high angular resolution images of radio emission from the supermassive black hole believed to lie at the heart of galaxy M87 (Figure 2). A defining feature of the images is an irregular but clear bright ring, whose size and shape agree closely with the expected lensed photon orbit of a 6.5 billion solar mass black hole. Soon after Einstein introduced general relativity, theorists derived the full analytic form of the photon orbit, and first simulated its lensed appearance in the 1970s. By the 2000s, it was possible to sketch the "shadow" formed in the image when synchrotron emission from an optically thin accretion flow is lensed in the black hole's gravity. During this time, observational evidence began to build for the existence of black holes at the centers of active galaxies, and in our own Milky Way. In particular, a steady progression in radio astronomy enabled very long baseline interferometry (VLBI) observations at ever-shorter wavelengths, targeting supermassive black holes with the largest apparent event horizons: M87, and Sgr A* in the Galactic Center. The compact sizes of these two sources were confirmed by studies at 1.3mm, first exploiting baselines that ran from Hawai'i to the mainland US, then with increased resolution on baselines to Spain and Chile.
Over the past decade, the EHT extended these first measurements of size to mount the more ambitious campaign of imaging the shadow itself. During 5-11 April 2017, the Event Horizon Telescope (EHT) observed M87 and calibrators on four separate days using an array that included eight radio telescopes at six geographic locations: Arizona (USA), Chile, Hawai'i (USA), Mexico, the South Pole, and Spain (Figure 2). Years of preparation (and an astonishing spate of planet-wide good weather) paid off with an extraordinary multi-petabyte yield of data. The results presented here, from observations through images to interpretation, issue from a team of instrument, algorithm, software, modeling, and theoretical experts, following a tremendous effort by a group of scientists that span all career stages, from undergraduates to senior members of the field. More than 200 members from 59 institutes in 20 countries and regions have devoted years to the effort, all unified by a common scientific vision.
The sequence of Letters in this issue provides the full scope of the project and the conclusions drawn to date. Paper II opens with a description of the EHT array, the technical developments that enabled precursor detections, and the full range of observations reported here. Through the deployment of novel instrumentation at existing facilities, the collaboration created a new telescope with unique capabilities for black hole imaging. Paper III details the observations, data processing, calibration algorithms, and rigorous validation protocols for the final data products used for analysis. Paper IV gives the full process and approach to image reconstruction. The final images emerged after a rigorous evaluation of traditional imaging algorithms and new techniques tailored to the EHT instrument — alongside many months of testing the imaging algorithms through the analysis of synthetic data sets. Paper V uses newly assembled libraries of general relativistic magnetohydrodynamic (GRMHD) simulations and advanced ray-tracing to analyze the images and data in the context of black hole accretion and jet-launching. Paper VI employs model fits, comparison of simulations to data, and feature extraction from images to derive formal estimates of the lensed emission ring size and shape, black hole mass, and constraints on the nature of the black hole and the space-time surrounding it. Paper I is a concise summary.
Our image of the shadow confines the mass of M87 to within its photon orbit, providing the strongest case for the existence of supermassive black holes. These observations are consistent with Doppler brightening of relativistically moving plasma close to the black hole lensed around the photon orbit. They strengthen the fundamental connection between active galactic nuclei and central engines powered by accreting black holes through an entirely new approach. In the coming years, the EHT Collaboration will extend efforts to include full polarimetry, mapping of magnetic fields on horizon scales, investigations of time variability, and increased resolution through shorter wavelength observations.
In short, this work signals the development of a new field of research in astronomy and physics as we zero in on precision images of black holes on horizon scales. The prospects for sharpening our focus even further are excellent.
The Event Horizon Telescope Collaboration et al., 2019, ApJL (The Astrophysical Journal Letter), Vol. 875, L1, Published: 10 April 2019
The Event Horizon Telescope Collaboration et al., 2019, ApJL Vol. 875, L2, Published: 10 April 2019
The Event Horizon Telescope Collaboration et al., 2019, ApJL Vol. 875, L3, Published: 10 April 2019
The Event Horizon Telescope Collaboration et al., 2019, ApJL Vol. 875, L4, Published: 10 April 2019
The Event Horizon Telescope Collaboration et al., 2019, ApJL, Vol. 875, L5, Published: 10 April 2019
The Event Horizon Telescope Collaboration et al., 2019, ApJL Vol. 875, L6, Published: 10 April 2019
Some context and background on Black Holes
Astronomers have finally glimpsed the blackness of a black hole. By stringing together a global network of radio telescopes, they have for the first time produced a picture of an event horizon — a black hole's perilous edge — against a backdrop of swirling light. 4)
"We have seen the gates of hell at the end of space and time," said astrophysicist Heino Falcke of Radboud University in Nijmegen, the Netherlands, at a press conference in Brussels. "What you're looking at is a ring of fire created by the deformation of space-time. Light goes around, and looks like a circle."
The images — of a glowing, ring-like structure — show the supermassive black hole at the center of the galaxy M87, which is around 16 megaparsecs (55 million light years) away and 6.5 billion times the mass of the Sun. They reveal, in greater detail than ever before, the event horizon — the surface beyond which gravity is so strong that nothing that crosses it, even light, can ever climb back out.
The highly anticipated results, comparable to recognizing a doughnut on the Moon's surface, were unveiled today by the Event Horizon Telescope (EHT) collaboration in seven simultaneous press conferences on four continents. The findings were also published in a suite of papers in Astrophysical Journal Letters on 10 April.
The image is a "tremendous accomplishment", says astrophysicist Roger Blandford at Stanford University in California, who was not involved with the work. "When I was a student, I never dreamt that anything like this would be possible," he says. "It is yet another confirmation of general relativity as the correct theory of strong gravity."
Figure 4: The first image of a black hole: A three minute guide. Astronomers from the Event Horizon Telescope Collaboration have taken the first ever image of a black hole - at the heart of the galaxy M87 (video credit: Nature, published 10 April 2019)
The image is a "tremendous accomplishment", says astrophysicist Roger Blandford at Stanford University in California, who was not involved with the work. "When I was a student, I never dreamt that anything like this would be possible," he says. "It is yet another confirmation of general relativity as the correct theory of strong gravity."
"I was so delighted," says Andrea Ghez, an astronomer at the University of California, Los Angeles. The images provide "clear evidence" of a ‘photon ring' around a black hole, she says.
Nearly a century ago, physicists first deduced that black holes should exist from Albert Einstein's general theory of relativity, but most of the evidence so far has been indirect. The EHT (Event Horizon Telescope) has now made a new, spectacular confirmation of those predictions.
The team observed two supermassive black holes — M87's and Sagittarius A*, the void at the Milky Way's center — over five nights in April 2017. They mustered enough resolution to capture the distant objects by linking up eight radio observatories across the globe — from Hawaii to the South Pole — and each collected more data than the Large Hadron Collider does in a year (see ‘Global effort'). The data set is likely to be the largest ever collected by a science experiment, and it took two years of work to produce the pictures.
After combining the observatories' data, the team started analysis in mid-2018. They quickly realized that they could get a first, clean picture from M87. "We focused all our attention on M87 when we saw our first results because we saw this is going to be awesome," says Falcke.
At the Brussels press conference, astrophysicist and collaboration member Monika Moscibrodzka, also at Radboud, said that the measurements so far are not precise enough to measure how fast the M87 hole spins — a crucial feature for a black hole. But it indicates the direction in which it's spinning, which is clockwise in the sky, she said. Further studies could also help researchers understand how the black hole produces its gigantic jets.
The teams will also now turn their attention to the Sagittarius A* data. Because Sagittarius A* is nearly 1,000 times smaller than the M87 black hole, matter orbited it many times during each observing session, producing a rapidly changing signal rather than a steady one, says Luciano Rezzolla, a theoretical astrophysicist at the Goethe University of Frankfurt in Germany and a member of the EHT team. That makes the data more complicated to interpret, but also potentially richer in information.
Event horizons are the defining feature of black holes. To a nearby observer, an event horizon should appear as a spherical surface shrouding its interiors from view. Because light can cross the surface only one way — inwards — the globe should look completely black (see ‘Power of the dark').
A black hole's event horizon should appear five times larger than it is, because the hole warps the surrounding space and bends the paths of light. The effect, discovered by physicist James Bardeen at the University of Washington in Seattle in 1973, is similar to the way that a spoon looks larger when dipped in a glass of water. Moreover, Bardeen showed that the black hole would cast an even larger ‘shadow'. This is because within a certain distance of the event horizon, most light rays bend so much that they effectively orbit the black hole.
To actually resolve details on the scale of the event horizon, radio astronomers calculated that they would need a telescope the size of Earth (a telescope's resolution is also proportional to its size). Fortunately, a technique called interferometry could help. It involves multiple telescopes, located far apart from one another and pointed at the same object simultaneously. Effectively, the telescopes work as if they were shards of one big dish.
Various teams around the world refined their techniques, and retrofitted some major observatories so that they could add them to a network. In particular, a group led by Shep Doeleman, now at Harvard University in Cambridge, Massachusetts, adapted the 10-meter South Pole Telescope and the US $1.4-billion ALMA (Atacama Large Millimeter/submillimeter Array) in Chile to do the work.
In 2014, Falcke, Doeleman and groups from around the world joined forces to form the EHT collaboration. They did their first Earth-spanning observation campaign in 2017. They observed Sagittarius A* and M87 during a two-week window in April when the locations of the observatories are most likely to get good weather simultaneously.
The raw data, which ran into petabytes, were collected on hard disks and travelled by air, sea and land to be compiled at the Max Planck Institute for Radio Astronomy in Bonn, Germany and the Massachusetts Institute of Technology's Haystack Observatory in Westford.
Last year, while the data were still being processed, Falcke told Nature that he expected the experiment to gather a wealth of information about the structure of the black holes, but not yet a pretty picture. At best, it would resemble "an ugly peanut", he said. "Or maybe, the first image will be just a few blots. It may not even resemble a peanut."
The EHT ran another observing campaign in 2018 — the analysis of those data is still in the works — but cancelled a planned observing campaign this year because of security issues near one of its most important sites, the 50 meter LMT (Large Millimeter Telescope) in Puebla, Mexico. They plan to continue to do observations once a year starting in 2020.
The collaboration is now looking for funding to establish a foothold in Africa, which would fill in a major gap in the network. The plan is to relocate a 15 meter dish — a decommissioned Swedish telescope — from Chile to the Gamsberg Table Mountain in Namibia. For now, the network has already secured two major additions: a dish in Greenland and an array in the French Alps.
An expanded EHT network could provide detail on what happens inside the voids — "how the world behaves inside black holes, and if it is as we expected it to be", says David Sánchez Argüelles, a physicist at the LMT.
"It was a great sense of relief to see this, but also surprise," says Doeleman of the results. "You know what I was really expecting to see? A blob. To see this ring is probably the best outcome that we could have had."
Chilean Senate Honors ALMA For 1st Image of Black Hole
18 April 2019: The Honorable Senate of Chile invited representatives from the ALMA observatory to its session held today, 17 April , in recognition of its role in obtaining the first image of a black hole published by the Event Horizon Telescope (EHT) last week. In addition to congratulating the observatory, the five scientists from the ALMA team who were directly involved in the process were recognized individually: Alejandro Sáez, Violette Impellizzeri, Hugo Messias, Rubén Herrero-Illana and Akihiko Hirota. In addition, Neil Nagar and Venkatessh Ramakrishnan from the Universidad de Concepción were also distinguished. 6)
After the successful revelation of the first image of a black hole on 10 April by the Event Horizon Telescope consortium, which had a global impact, the Honorable Senate of Chile decided to award a silver medal to the ALMA Observatory and to the seven scientists who participated from Chile as coauthors in the investigation. The ceremony was held in the Senate Hall.
"I thank all of the ALMA staff for their extraordinary efforts in enabling ALMA's vital participation in this very successful, iconic experiment to obtain the first image of a black hole" said ALMA Director, Sean Doguherty.
After the ceremony, the President of the Senate, Jaime Quintana, said to be "very proud that they have accepted the invitation to give them the Silver Medal of the Senate. A research team that has made a big contribution to Science with this great discovery. Furthermore, taking into account that this is what we have been talking about from the Senate in the last decade: to strengthen links between Science and Politics, an important matter, and a key objective of Congreso del Futuro".
The Giant Galaxy Around the Giant Black Hole
On April 10, 2019, the Event Horizon Telescope (EHT) unveiled the first-ever image of a black hole's event horizon, the area beyond which light cannot escape the immense gravity of the black hole. That giant black hole, with a mass of 6.5 billion Suns, is located in the elliptical galaxy Messier 87 (M87). EHT is an international collaboration whose support in the U.S. includes the National Science Foundation. 7)
This image from NASA's Spitzer Space Telescope shows the entire M87 galaxy in infrared light. The EHT image, by contrast, relied on light in radio wavelengths and showed the black hole's shadow against the backdrop of high-energy material around it.
Located about 55 million light-years from Earth, M87 has been a subject of astronomical study for more than 100 years and has been imaged by many NASA observatories, including the Hubble Space Telescope, the Chandra X-ray Observatory and NuSTAR. In 1918, astronomer Heber Curtis first noticed "a curious straight ray" extending from the galaxy's center. This bright jet of high-energy material, produced by a disk of material spinning rapidly around the black hole, is visible in multiple wavelengths of light, from radio waves through X-rays. When the particles in the jet impact the interstellar medium (the sparse material filling the space between stars in M87), they create a shockwave that radiates in infrared and radio wavelengths of light but not visible light. In the Spitzer image, the shockwave is more prominent than the jet itself.
The brighter jet, located to the right of the galaxy's center, is traveling almost directly toward Earth. Its brightness is amplified due to its high speed in our direction, but even more so because of what scientists call "relativistic effects," which arise because the material in the jet is traveling near the speed of light. The jet's trajectory is just slightly offset from our line of sight with respect to the galaxy, so we can still see some of the length of the jet. The shockwave begins around the point where the jet appears to curve down, highlighting the regions where the fast-moving particles are colliding with gas in the galaxy and slowing down.
The second jet, by contrast, is moving so rapidly away from us that the relativistic effects render it invisible at all wavelengths. But the shockwave it creates in the interstellar medium can still be seen here.
Located on the left side of the galaxy's center, the shockwave looks like an inverted letter "C." While not visible in optical images, the lobe can also be seen in radio waves, as in this image from the National Radio Astronomy Observatory's Very Large Array.
By combining observations in the infrared, radio waves, visible light, X-rays and extremely energetic gamma rays, scientists can study the physics of these powerful jets. Scientists are still striving for a solid theoretical understanding of how gas being pulled into black holes creates outflowing jets.
Infrared light at wavelengths of 3.6 and 4.5 µm are rendered in blue and green, showing the distribution of stars, while dust features that glow brightly at 8.0 µm are shown in red. The image was taken during Spitzer's initial "cold" mission.
The Jet Propulsion Laboratory in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena. Space operations are based at Lockheed Martin Space Systems in Littleton, Colorado. Data are archived at the Infrared Science Archive housed at IPAC (Infrared Processing and Analysis Center) at Caltech. Caltech manages JPL for NASA.
Black holes are among the most fascinating objects in the Universe. Enclosing huge amounts of matter in relatively small regions, these compact objects have enormous densities that give rise to some of the strongest gravitational fields in the cosmos, so strong that nothing can escape – not even light. 8)
This artistic impression shows two black holes that are spiralling towards each other and will eventually coalesce. A black hole merger was first detected in 2015 by LIGO, the Laser Interferometer Gravitational-Wave Observatory, which detected the gravitational waves – fluctuations in the fabric of spacetime – created by the giant collision.
Black holes and gravitational waves are both predictions of Albert Einstein's general relativity, which was presented in 1915 and remains to date the best theory to describe gravity across the Universe.
Karl Schwarzschild derived the equations for black holes in 1916, but they remained rather a theoretical curiosity for several decades, until X-ray observations performed with space telescopes could finally probe the highly energetic emission from matter in the vicinity of these extreme objects. The first ever image of a black hole's dark silhouette, cast against the light from matter in its immediate surrounding, was only captured recently by the Event Horizon Telescope and published just last month.
As for gravitational waves, it was Einstein himself who predicted their existence from his theory, also in 1916, but it would take another century to finally observe these fluctuations. Since 2015, the ground-based LIGO and Virgo observatories have assembled over a dozen detections, and gravitational-wave astronomy is a burgeoning new field of research.
But another of Einstein's predictions found observational proof much sooner: the gravitational bending of light, which was demonstrated only a few years after the theory had appeared, during a total eclipse of the Sun in 1919.
In the framework of general relativity, any object with mass bends the fabric of spacetime, deflecting the path of anything that passes nearby – including light. An artistic view of this distortion, also known as gravitational lensing, is depicted in this representation of two merging black holes.
One hundred years ago, astronomers set out to test general relativity, observing whether and by how much the mass of the Sun deflects the light of distant stars. This experiment could only be performed by obscuring the Sun's light to reveal the stars around it, something that is possible during a total solar eclipse.
On 29 May 1919, Sir Arthur Eddington observed the distant stars around the Sun during an eclipse from the island of Príncipe, in West Africa, while Andrew Crommelin performed similar observations in Sobral, in the north east of Brazil. Their results, presented six months later, indicated that stars observed near the solar disc during the eclipse were slightly displaced, with respect to their normal position in the sky, roughly by the amount predicted by Einstein's theory for the Sun's mass to have deflected them.
"Lights All Askew in the Heavens," headlined the New York Times in November 1919 to announce the triumph of Einstein's new theory. This inaugurated a century of exciting experiments investigating gravity on Earth and in space and proving general relativity more and more precisely.
Supermassive black holes, with masses ranging from millions to billions of Suns, sit at the core of most massive galaxies across the Universe. We don't know exactly how these huge, enormously dense objects took shape, nor what triggers a fraction of them to start devouring the surrounding matter at extremely intense rates, radiating copiously across the electromagnetic spectrum and turning their host galaxies into ‘active galactic nuclei'. 9)
Tackling these open questions in modern astrophysics is among the main goals of two future missions in ESA's space science program: Athena, the Advanced Telescope for High-ENergy Astrophysics, and LISA, the Laser Interferometer Space Antenna. Currently in the study phase, both missions are scheduled for launch in the early 2030s.
We have made giant leaps over the past hundred years, but there is still much for us to discover Athena, ESA's future X-ray observatory, will investigate in unprecedented detail the supermassive black holes that sit at the center of galaxies. LISA, another future ESA mission, will detect gravitational waves from orbit, looking for the low-frequency fluctuations that are released when two supermassive black holes merge and can only be detected from space.
If Athena and LISA could operate jointly for at least a few years, they could perform a unique experiment: observing the merger of supermassive black holes both in gravitational waves and X-rays, using an approach known as multi-messenger astronomy.
We have never observed such a merger before: we need LISA to detect gravitational waves at the onset of the merger and tell us where to look in the sky, then we need Athena to observe it at high precision in X-rays to see how the mighty collision affects the gas surrounding the black holes. We don't know what happens during such a cosmic clash so this experiment, much like the eclipse of 1919 that first proved Einstein's theory, is set to shake our understanding of gravity and the Universe.
On Earth, we deal with gravity every day. We feel it, we fight it, and – more importantly – we investigate it. Space agencies such as ESA routinely launch spacecraft against our planet's gravity, and sometimes these spacecraft borrow the gravity of Earth or other planets to reach interesting places in the Solar System. We study the gravity field of Earth from orbit, and fly experiments on parabolic flights, sounding rockets and the International Space Station to examine a variety of systems under different gravitational conditions. On the grandest scales, our space science missions explore how gravity affects planets, stars and galaxies across the cosmos and probe how matter behaves in the strong gravitational field created by some of the Universe's most extreme objects like black holes.
Figure 14: One hundred years ago this month, observations performed during a total solar eclipse proved for the first time the gravitational bending of light predicted by Albert Einstein's new theory of gravity, general relativity. In this video, Günther Hasinger, ESA Director of Science, reflects on this historic measurement that inaugurated a century of exciting experiments, investigating gravity on Earth and in space and proving general relativity in ever greater detail [video credit: ESA/CESAR (solar eclipse sequence); ESO/M. Kornmesser (black hole); Royal Astronomical Society (negative photo of the 1919 solar eclipse); ESA/Hubble, NASA (gravitationally lensed quasar); ESO/Gravity Consortium/L. Calçada (black hole simulation)] 10)
Einstein's general theory of relativity tested by star orbiting a black hole
July 26, 2019: More than 100 years after Albert Einstein published his iconic theory of general relativity, it is beginning to fray at the edges, said Andrea Ghez, UCLA professor of physics and astronomy. Now, in the most comprehensive test of general relativity near the monstrous black hole at the center of our galaxy, Ghez and her research team report July 25 in the journal Science that Einstein's theory of general relativity holds up. 11) 12)
"Einstein's right, at least for now," said Ghez, a co-lead author of the research. "We can absolutely rule out Newton's law of gravity. Our observations are consistent with Einstein's theory of general relativity. However, his theory is definitely showing vulnerability. It cannot fully explain gravity inside a black hole, and at some point we will need to move beyond Einstein's theory to a more comprehensive theory of gravity that explains what a black hole is."
Einstein's 1915 theory of general relativity holds that what we perceive as the force of gravity arises from the curvature of space and time. The scientist proposed that objects such as the sun and the Earth change this geometry. Einstein's theory is the best description of how gravity works, said Ghez, whose UCLA-led team of astronomers has made direct measurements of the phenomenon near a supermassive black hole — research Ghez describes as "extreme astrophysics."
Figure 15: Testing Einstein's theory of relativity near a black hole (video credit: UCLA, Published on 25 July 2019)
The laws of physics, including gravity, should be valid everywhere in the universe, said Ghez, who added that her research team is one of only two groups in the world to watch a star known as S0-2 make a complete orbit in three dimensions around the supermassive black hole at the center of the Milky Way. The full orbit takes 16 years, and the black hole's mass is about four million times that of the sun.
The researchers say their work is the most detailed study ever conducted into the supermassive black hole and Einstein's theory of general relativity.
The key data in the research were spectra that Ghez's team analyzed this April, May and September as her "favorite star" made its closest approach to the enormous black hole. Spectra, which Ghez described as the "rainbow of light" from stars, show the intensity of light and offer important information about the star from which the light travels. Spectra also show the composition of the star. These data were combined with measurements Ghez and her team have made over the last 24 years.
Spectra — collected at the W. M. Keck Observatory in Hawaii using a spectrograph built at UCLA by a team led by colleague James Larkin — provide the third dimension, revealing the star's motion at a level of precision not previously attained (images of the star the researchers took at the Keck Observatory provide the two other dimensions). Larkin's instrument takes light from a star and disperses it, similar to the way raindrops disperse light from the sun to create a rainbow, Ghez said.
"What's so special about S0-2 is we have its complete orbit in three dimensions," said Ghez, who holds the Lauren B. Leichtman and Arthur E. Levine Chair in Astrophysics. "That's what gives us the entry ticket into the tests of general relativity. We asked how gravity behaves near a supermassive black hole and whether Einstein's theory is telling us the full story. Seeing stars go through their complete orbit provides the first opportunity to test fundamental physics using the motions of these stars."
Figure 17: How a star's orbit helps us see a black hole (video credit: UCLA, Published on 25 July 2019)
Ghez's research team was able to see the co-mingling of space and time near the supermassive black hole. "In Newton's version of gravity, space and time are separate, and do not co-mingle; under Einstein, they get completely co-mingled near a black hole," she said.
"Making a measurement of such fundamental importance has required years of patient observing, enabled by state-of-the-art technology," said Richard Green, director of the National Science Foundation's division of astronomical sciences. For more than two decades, the division has supported Ghez, along with several of the technical elements critical to the research team's discovery.
Keck Observatory Director Hilton Lewis called Ghez "one of our most passionate and tenacious Keck users." "Her latest groundbreaking research," he said, "is the culmination of unwavering commitment over the past two decades to unlock the mysteries of the supermassive black hole at the center of our Milky Way galaxy."
The researchers studied photons — particles of light — as they traveled from S0-2 to Earth. S0-2 moves around the black hole at blistering speeds of more than 16 million miles per hour at its closest approach. Einstein had reported that in this region close to the black hole, photons have to do extra work. Their wavelength as they leave the star depends not only on how fast the star is moving, but also on how much energy the photons expend to escape the black hole's powerful gravitational field. Near a black hole, gravity is much stronger than on Earth.
Ghez was given the opportunity to present partial data last summer, but chose not to so that her team could thoroughly analyze the data first. "We're learning how gravity works. It's one of four fundamental forces and the one we have tested the least," she said. "There are many regions where we just haven't asked, how does gravity work here? It's easy to be overconfident and there are many ways to misinterpret the data, many ways that small errors can accumulate into significant mistakes, which is why we did not rush our analysis."
Ghez, a 2008 recipient of the MacArthur "Genius" Fellowship, studies more than 3,000 stars that orbit the supermassive black hole. Hundreds of them are young, she said, in a region where astronomers did not expect to see them.
It takes 26,000 years for the photons from S0-2 to reach Earth. "We're so excited, and have been preparing for years to make these measurements," said Ghez, who directs the UCLA Galactic Center Group. "For us, it's visceral, it's now — but it actually happened 26,000 years ago!"
This is the first of many tests of general relativity Ghez's research team will conduct on stars near the supermassive black hole. Among the stars that most interest her is S0-102, which has the shortest orbit, taking 11 1/2 years to complete a full orbit around the black hole. Most of the stars Ghez studies have orbits of much longer than a human lifespan.
Ghez's team took measurements about every four nights during crucial periods in 2018 using the Keck Observatory — which sits atop Hawaii's dormant Mauna Kea volcano and houses one of the world's largest and premier optical and infrared telescopes. Measurements are also taken with an optical-infrared telescope at Gemini Observatory and Subaru Telescope, also in Hawaii.
Andrea Ghez and her team have used these telescopes both on site in Hawaii and remotely from an observation room in UCLA's department of physics and astronomy.
Black Hole events and discovery status
- Supermassive black holes millions to billions of times the mass of our Sun lie at the heart of most galaxies, and astronomers are eager to know how these behemoths came to be. While they think most resulted from at least one merger between two smaller supermassive black holes, scientists lacked the observations that could give insight, since only one pair of supermassive black holes on the way to a merger had been found.
- A new study may change that: Researchers observing a supermassive black hole report signs that it has a closely orbiting companion. The enormous duo – called a binary – circle one another about every two years.
- If the team is correct, the diameter of the binary's orbit is 10 to 100 times smaller than the only other known supermassive binary, and the pair will merge in roughly 10,000 years. That might seem like a long time, but it would take a total of about 100 million years for black holes of this size to begin orbiting one another and finally come together. So this pair is more than 99% of the way to a collision.
- Joseph Lazio and Michele Vallisneri, at NASA's Jet Propulsion Laboratory in Southern California, provided insight into how supermassive black holes behave in a binary system and how to interpret the radio data.
- Evidence that this supermassive black hole may have a companion comes from observations by radio telescopes on Earth. Black holes don't emit light, but their gravity can gather disks of hot gas around them and eject some of that material into space. These jets can stretch for millions of light-years. A jet pointed toward Earth appears far brighter than a jet pointed away from Earth. Astronomers call supermassive black holes with jets oriented toward Earth blazars, and a blazar named PKS 2131-021 is at the heart of this recent paper.15)
- Located about 9 billion light-years from Earth, PKS 2131-021 is one of 1,800 blazars that a group of researchers at Caltech in Pasadena has been monitoring with the Owens Valley Radio Observatory in Northern California for 13 years as part of a general study of blazar behavior. But this particular blazar exhibits a strange behavior: Its brightness shows regular ups and downs as predictably as the ticking of a clock.
- Researchers now think this regular variation is the result of a second black hole tugging on the first as they orbit each other about every two years. Each of the two black holes in PKS 2131-021 is estimated to be a few hundred million times the mass of our Sun. To confirm the finding, scientists will try to detect gravitational waves – ripples in space – coming from the system. The first detection of gravitational waves from black hole binaries was announced in 2016.
- To confirm that the oscillations weren't random or the cause of a temporary effect around the black hole, the team had to look beyond the decade (2008 to 2019) of data from the Owens Valley Observatory. After learning that two other radio telescopes had also studied this system – the University of Michigan Radio Observatory (1980 to 2012) and the Haystack Observatory (1975 to 1983) – they dug into the additional data and found that it matched predictions for how the blazar's brightness should change over time.
- "This work is a testament to the importance of perseverance," said Lazio. "It took 45 years of radio observations to produce this result. Small teams, at different observatories across the country, took data week in and week out, month in and month out, to make this possible."
• 16 December 2021: How did supermassive black holes form? What is dark matter? In an alternative model for how the Universe came to be, as compared to the ‘textbook' history of the Universe, a team of astronomers propose that both of these cosmic mysteries could be explained by so-called ‘primordial black holes'. 16)
- Nico Cappelluti (University of Miami), Günther Hasinger (ESA Science Director) and Priyamvada Natarajan (Yale University), suggest that black holes existed since the beginning of the Universe and that these primordial black holes could themselves be the as-of-yet unexplained dark matter. The new study is accepted for publication in The Astrophysical Journal. 17)
- "Black holes of different sizes are still a mystery. We don't understand how supermassive black holes could have grown so huge in the relatively short time available since the Universe existed," explains Günther Hasinger.
- At the other end of the scale, there might also be very small black holes, as suggested by observations from ESA's Gaia, for example. If they exist, they are too small to have formed from dying stars.
- "Our study shows that without introducing new particles or new physics, we can solve mysteries of modern cosmology from the nature of dark matter itself to the origin of super-massive black holes," says Nico Cappelluti.
- If most of the black holes formed immediately after the Big Bang, they could have started merging in the early Universe, forming more and more massive black holes over time. ESA's future gravitational wave space observatory, LISA, might pick up the signals of those mergers if primordial black holes exist. Small black holes might simply be the primordial black holes that have not merged into larger ones yet.
- According to this model, the Universe would be filled with black holes all over. Stars would start to form around these clumps of ‘dark matter', creating solar systems and galaxies over billions of years. If the first stars indeed formed around primordial black holes, they would exist earlier in the Universe than is expected by the ‘standard' model.
- "Primordial black holes, if they do exist, could well be the seeds from which all black holes form, including the one at the centre of the Milky Way," says Priyamvada Natarajan.
- ESA's Euclid mission, which will probe the dark Universe in greater detail than ever before, could play a role in the quest to identify primordial black holes as dark matter candidates.
- The upcoming NASA/ESA/CSA James Webb Space Telescope, a cosmic time machine looking back over more than 13 billion years, will further shed light on this mystery.
- "If the first stars and galaxies already formed in the so-called ‘dark ages', Webb should be able to see evidence of them," adds Günther.
• 12 August 2021: The feeding patterns of black holes offer insight into their size, researchers report. A new study revealed that the flickering in the brightness observed in actively feeding supermassive black holes is related to their mass. 20)
- SMBHs (Supermassive Black Holes) are millions to billions of times more massive than the sun and usually reside at the center of massive galaxies. When dormant and not feeding on the gas and stars surrounding them, SMBHs emit very little light; the only way astronomers can detect them is through their gravitational influences on stars and gas in their vicinity. However, in the early universe, when SMBHs were rapidly growing, they were actively feeding – or accreting – materials at intensive rates and emitting an enormous amount of radiation – sometimes outshining the entire galaxy in which they reside, the researchers said.
- The new study, led by the University of Illinois Urbana-Champaign astronomy graduate student Colin Burke and professor Yue Shen, uncovered a definitive relationship between the mass of actively feeding SMBHs and the characteristic timescale in the light-flickering pattern. The findings are published in the journal Science. 21)
- The observed light from an accreting SMBH is not constant. Due to physical processes that are not yet understood, it displays a ubiquitous flickering over timescales ranging from hours to decades. "There have been many studies that explored possible relations of the observed flickering and the mass of the SMBH, but the results have been inconclusive and sometimes controversial," Burke said.
- The team compiled a large data set of actively feeding SMBHs to study the variability pattern of flickering. They identified a characteristic timescale, over which the pattern changes, that tightly correlates with the mass of the SMBH. The researchers then compared the results with accreting white dwarfs, the remnants of stars like our sun, and found that the same timescale-mass relation holds, even though white dwarfs are millions to billions times less massive than SMBHs.
- The light flickers are random fluctuations in a black hole's feeding process, the researchers said. Astronomers can quantify this flickering pattern by measuring the power of the variability as a function of timescales. For accreting SMBHs, the variability pattern changes from short timescales to long timescales. This transition of variability pattern happens at a characteristic timescale that is longer for more massive black holes.
- The team compared black hole feeding to our eating or drinking activity by equating this transition to a human belch. Babies frequently burp while drinking milk, while adults can hold in the burp for a more extended amount of time. Black holes kind of do the same thing while feeding, they said.
- "These results suggest that the processes driving the flickering during accretion are universal, whether the central object is a supermassive black hole or a mu"The firm establishment of a connection between the observed light flicker and fundamental properties of the accretor will certainly help us better understand accretion processes," said Yan-Fei Jiang, a researcher at the Flatiron Institute and study co-author.
- Astrophysical black holes come in a broad spectrum of mass and size. In between the population of stellar-mass black holes, which weigh less than several tens of times the mass of the sun, and SMBHs, there is a population of black holes called intermediate-mass black holes that weigh between about 100 and 100,000 times the mass of the sun.
- IMBHs (Intermediate Mass Black Holes) are expected to form in large numbers through the history of the universe, and they may provide the seeds necessary to grow into SMBHs later. However, observationally this population of IMBHs is surprisingly elusive. There is only one indisputably confirmed IMBH that weighs about 150 times the mass of the sun. But that IMBH was serendipitously discovered by the gravitational wave radiation from the coalescence of two less-massive black holes.
- "Now that there is a correlation between the flickering pattern and the mass of the central accreting object, we can use it to predict what the flickering signal from an IMBH might look like," Burke said.
- Astronomers worldwide are waiting for the official kickoff of an era of massive surveys that monitor the dynamic and variable sky. The Vera C. Rubin Observatory in Chile's Legacy Survey of Space and Time will survey the sky over a decade and collect light flickering data for billions of objects, starting in late 2023.
- "Mining the LSST (Large Synoptic Survey Telescope) data set to search for flickering patterns that are consistent with accreting IMBHs has the potential to discover and fully understand this long-sought mysterious population of black holes," said co-author Xin Liu, an astronomy professor at the U. of I (University of Illinois).
• 27 July 2021: Black holes, detected by their gravitational wave signal as they collide with other black holes, could be the product of much earlier parent collisions. 22)
- Such an event has only been hinted at so far, but scientists at the University of Birmingham in the UK, and Northwestern University in the US, believe we are getting close to tracking down the first of these so-called ‘hierarchical' black holes.
- In a review paper, published in Nature Astronomy, Dr Davide Gerosa, of the University of Birmingham, and Dr Maya Fishbach of Northwestern University, suggest that recent theoretical findings together with astrophysical modelling and recorded gravitational wave data will enable scientists to accurately interpret gravitational wave signals from these events. 23)
- Since the first gravitational wave was detected by the LIGO and Virgo detectors in September 2015, scientists have produced increasingly nuanced and sophisticated interpretations of these signals.
- There is now fervent activity to prove the existence of so-called ‘hierarchical mergers' although the detection of GW190521 in 2019 – the most massive black hole merger yet detected – is thought to be the most promising candidate so far.
- "We believe that most of the gravitational waves so far detected are the result of first generation black holes colliding," says Dr Gerosa. "But we think there's a good chance that others will contain the remnants of previous mergers. These events will have distinctive gravitational wave signatures suggesting higher masses, and an unusual spin caused by the parent collision."
- Understanding the characteristics of the environment in which such objects might be produced will also help narrow the search. This must be an environment with a large number of black holes, and one that is sufficiently dense to retain the black holes after they have merged, so they can go on and merge again.
- These could be, for example, nuclear star clusters, or accretion disks – containing a flow of gas, plasma and other particles – surrounding the compact regions at the center of galaxies.
- "The LIGO and Virgo collaboration has already discovered more than 50 gravitational wave events," says Dr Fishbach. "This will expand to thousands over the next few years, giving us so many more opportunities to discover and confirm unusual objects like hierarchical black holes in the universe."
• 20 July 2021: Black holes with masses equivalent to millions of suns do put a brake on the birth of new stars, say astronomers. Using machine learning and three state of the art simulations to back up results from a large sky survey, the researchers resolve a 20-year long debate on the formation of stars. Joanna Piotrowska, a PhD student at the University of Cambridge, will present the new work today (Tuesday 20 July) at the virtual National Astronomy Meeting (NAM 2021). 24)
- Star formation in galaxies has long been a focal point of astronomy research. Decades of successful observations and theoretical modelling resulted in our good understanding of how gas collapses to form new stars both in and beyond our own Milky Way. However, thanks to all-sky observing programs like the Sloan Digital Sky Survey (SDSS), astronomers realized that not all galaxies in the local Universe are actively star-forming - there exists an abundant population of "quiescent" objects which form stars at significantly lower rates.
- The question of what stops star formation in galaxies remains the biggest unknown in our understanding of galaxy evolution, debated over the past 20 years. Piotrowska and her team set up an experiment to find out what might be responsible.
- Using three state-of-the-art cosmological simulations – EAGLE, Illustris and IllustrisTNG – the astronomers investigated what we would expect to see in the real Universe as observed by the SDSS, when different physical processes were halting star formation in massive galaxies.
- The astronomers applied a machine learning algorithm to classify galaxies into star-forming and quiescent, asking which of three parameters: the mass of the supermassive black holes found at the center of galaxies (these monster objects have typically millions or even billions of times the mass of our Sun), the total mass of stars in the galaxy, or the mass of the dark matter halo around galaxies, best predicts how galaxies turn out.
- These parameters then enabled the team to work out which physical process: energy injection by supermassive black holes, supernova explosions or shock heating of gas in massive halos is responsible for forcing galaxies into semi-retirement.
- The new simulations predict the supermassive black hole mass as the most important factor in putting the brakes on star formation. Crucially, the simulation results match observations of the local Universe, adding weight to the researchers' findings.
- Piotrowska says: "It's really exciting to see how the simulations predict exactly what we see in the real Universe. Supermassive black holes – objects with masses equivalent to millions or even billions of Suns – really do have a big effect on their surroundings. These monster objects force their host galaxies into a kind of semi-retirement from star formation."
Figure 29: University of Cambridge PhD student Joanna Piotrowska explains how supermassive black holes shut down star formation and put galaxies into semi-retirement. She presented her work at the Royal Astronomical Society National Astronomy Meeting (NAM 2021), [video credit: Joanna Piotrowska, with music from Transition by StreamBeats https://www.streambeats.com/]
• 01 July 2021: There are certain rules that even the most extreme objects in the universe must obey. A central law for black holes predicts that the area of their event horizons — the boundary beyond which nothing can ever escape — should never shrink. This law is Hawking's area theorem, named after physicist Stephen Hawking, who derived the theorem in 1971. 25)
- Fifty years later, physicists at MIT and elsewhere have now confirmed Hawking's area theorem for the first time, using observations of gravitational waves. Their results appear today in Physical Review Letters. 26)
- In the study, the researchers take a closer look at GW150914, the first gravitational wave signal detected by the Laser Interferometer Gravitational-wave Observatory (LIGO), in 2015. The signal was a product of two inspiraling black holes that generated a new black hole, along with a huge amount of energy that rippled across space-time as gravitational waves.
- If Hawking's area theorem holds, then the horizon area of the new black hole should not be smaller than the total horizon area of its parent black holes. In the new study, the physicists reanalyzed the signal from GW150914 before and after the cosmic collision and found that indeed, the total event horizon area did not decrease after the merger — a result that they report with 95 percent confidence.
- Their findings mark the first direct observational confirmation of Hawking's area theorem, which has been proven mathematically but never observed in nature until now. The team plans to test future gravitational-wave signals to see if they might further confirm Hawking's theorem or be a sign of new, law-bending physics.
- "It is possible that there's a zoo of different compact objects, and while some of them are the black holes that follow Einstein and Hawking's laws, others may be slightly different beasts," says lead author Maximiliano Isi, a NASA Einstein Postdoctoral Fellow in MIT's Kavli Institute for Astrophysics and Space Research. "So, it's not like you do this test once and it's over. You do this once, and it's the beginning."
- Isi's co-authors on the paper are Will Farr of Stony Brook University and the Flatiron Institute's Center for Computational Astrophysics, Matthew Giesler of Cornell University, Mark Scheel of Caltech, and Saul Teukolsky of Cornell University and Caltech.
An age of insights
- In 1971, Stephen Hawking proposed the area theorem, which set off a series of fundamental insights about black hole mechanics. The theorem predicts that the total area of a black hole's event horizon — and all black holes in the universe, for that matter — should never decrease. The statement was a curious parallel of the second law of thermodynamics, which states that the entropy, or degree of disorder within an object, should also never decrease.
- The similarity between the two theories suggested that black holes could behave as thermal, heat-emitting objects — a confounding proposition, as black holes by their very nature were thought to never let energy escape, or radiate. Hawking eventually squared the two ideas in 1974, showing that black holes could have entropy and emit radiation over very long timescales if their quantum effects were taken into account. This phenomenon was dubbed "Hawking radiation" and remains one of the most fundamental revelations about black holes.
- "It all started with Hawking's realization that the total horizon area in black holes can never go down," Isi says. "The area law encapsulates a golden age in the '70s where all these insights were being produced."
- Hawking and others have since shown that the area theorem works out mathematically, but there had been no way to check it against nature until LIGO's first detection of gravitational waves.
- Hawking, on hearing of the result, quickly contacted LIGO co-founder Kip Thorne, the Feynman Professor of Theoretical Physics at Caltech. His question: Could the detection confirm the area theorem?
- At the time, researchers did not have the ability to pick out the necessary information within the signal, before and after the merger, to determine whether the final horizon area did not decrease, as Hawking's theorem would assume. It wasn't until several years later, and the development of a technique by Isi and his colleagues, when testing the area law became feasible.
Before and after
- In 2019, Isi and his colleagues developed a technique to extract the reverberations immediately following GW150914's peak — the moment when the two parent black holes collided to form a new black hole. The team used the technique to pick out specific frequencies, or tones of the otherwise noisy aftermath, that they could use to calculate the final black hole's mass and spin.
- A black hole's mass and spin are directly related to the area of its event horizon, and Thorne, recalling Hawking's query, approached them with a follow-up: Could they use the same technique to compare the signal before and after the merger, and confirm the area theorem?
- The researchers took on the challenge, and again split the GW150914 signal at its peak. They developed a model to analyze the signal before the peak, corresponding to the two inspiraling black holes, and to identify the mass and spin of both black holes before they merged. From these estimates, they calculated their total horizon areas — an estimate roughly equal to about 235,000 km2, or roughly nine times the area of Massachusetts.
- They then used their previous technique to extract the "ringdown," or reverberations of the newly formed black hole, from which they calculated its mass and spin, and ultimately its horizon area, which they found was equivalent to 367,000 km2 (approximately 13 times the Bay State's area).
- "The data show with overwhelming confidence that the horizon area increased after the merger, and that the area law is satisfied with very high probability," Isi says. "It was a relief that our result does agree with the paradigm that we expect, and does confirm our understanding of these complicated black hole mergers."
- The team plans to further test Hawking's area theorem, and other longstanding theories of black hole mechanics, using data from LIGO and Virgo, its counterpart in Italy.
- "It's encouraging that we can think in new, creative ways about gravitational-wave data, and reach questions we thought we couldn't before," Isi says. "We can keep teasing out pieces of information that speak directly to the pillars of what we think we understand. One day, this data may reveal something we didn't expect."
- This research was supported, in part, by NASA, the Simons Foundation, and the National Science Foundation.
• 26 May 2021: Astronomers continue to develop computer simulations to help future observatories better home in on black holes, the most elusive inhabitants of the universe. 27)
- Though black holes likely exist abundantly in the universe, they are notoriously hard to see. Scientists did not capture the first radio image of a black hole until 2019, and only about four dozen black hole mergers have been detected through their signature gravitational ripples since the first detection in 2015.
- That is not a lot of data to work with. So scientists look to black hole simulations to gain crucial insight that will help find more mergers with future missions. Some of these simulations, created by scientists like astrophysicist Scott Noble, track supermassive black hole binary systems. That is where two monster black holes like those found in the centers of galaxies orbit closely around each other until they eventually merge.
Figure 31: Gas glows brightly in this computer simulation of supermassive black holes only 40 orbits from merging. Models like this may eventually help scientists pinpoint real examples of these powerful binary systems (video credit: NASA's Goddard Space Flight Center)
- The simulations, created by computers working through sets of equations too complicated to solve by hand, illustrate how matter interacts in merger environments. Scientists can use what they learn about black hole mergers to identify some telltale characteristics that let them distinguish black hole mergers from stellar events. Astronomers can then look for these telltale signs and spot real-life black hole mergers.
- Noble, who works at NASA's Goddard Space Flight Center in Greenbelt, Maryland, said these binary systems emit gravitational waves and influence surrounding gases, leading to unique light shows detectable with conventional telescopes. This allows scientists to learn about different aspects of the same system. Multimessenger observations that combine different forms of light or gravitational waves could allow scientists to refine their models of black hole binary systems.
- "We've been relying on light to see everything out there," Noble said. "But not everything emits light, so the only way to directly ‘see' two black holes is through the gravitational waves they generate. Gravitational waves and the light from surrounding gas are independent ways of learning about the system, and the hope is that they will meet up at the same point."
- Binary black hole simulations can also help the Laser Interferometer Space Antenna (LISA) mission. This space-based gravitational wave observatory, led by the European Space Agency with significant contributions from NASA, is expected to launch in 2034. If simulations determine what electromagnetic characteristics distinguish a binary black hole system from other events, scientists could detect these systems before LISA flies, Noble said. These observations could then be confirmed through additional detections once LISA launches.
- That would allow scientists to verify that LISA is working, observe systems for a longer period before they merge, predict what is going to happen, and test those predictions.
- "We've never been able to do that before," Noble said. "That's really exciting."
- The simulations rely on code which describes how the density and pressure of plasma changes in strong-gravity regions near a single black hole or neutron star, Noble said. He modified the code to allow for two black holes to evolve.
- Noble is working with Goddard and university partners, including Bernard Kelly at the University of Maryland, Manuela Campanelli leading a team of researchers at the Rochester Institute of Technology, and Julian Krolik leading a Johns Hopkins University research team.
- Kelly creates simulations using a special approach called a moving puncture simulation.
- These simulations allow scientists to avoid representing a singularity inside the event horizon — the part of the black hole from which nothing can escape, Kelly said. Everything outside of that event horizon evolves, while the objects inside remain frozen from earlier in the simulation. This allows scientists to overlook the fact that they do not know what happens within an event horizon.
- To mimic real-life situations, where black holes accumulate accretion disks of gas, dust, and diffuse matter, scientists have to incorporate additional code to track how the ionized material interacts with magnetic fields.
- In 2018, the team published an analysis of a new simulation in The Astrophysical Journal that fully incorporated the physical effects of Einstein's general theory of relativity to show a merger's effects on the environment around it. The simulation established that the gas in binary black hole systems will glow predominantly in ultraviolet and X-ray light.
- Simulations also showed that accretion disks in these systems are not completely smooth. A dense clump forms orbiting the binary, and every time a black hole sweeps close, it pulls off matter from the clump. That collision heats up the matter, producing a bright signal and creating an observable fluctuation of light.
- In addition to improving their confidence in the accuracy of the simulations, Goddard astrophysicist Jeremy Schnittman said they also need to be able to apply the same simulation code to a single black hole or a binary and show the similarities and also the differences between the two systems.
- "The simulations are going to tell us what the systems should look like," Schnittman said. "LISA works more like a radio antenna as opposed to an optical telescope. We're going to hear something in the universe and get its basic direction, but nothing very precise. What we have to do is take other telescopes and look in that part of the sky, and the simulations are going to tell us what to look for to find a merging black hole."
- Kelly said LISA will be more sensitive to lower gravitational wave frequencies than the current ground-based gravitational wave observer, the Laser Interferometer Gravitational-Wave Observatory (LIGO). That means LISA will be able to sense smaller-mass binary systems much earlier and will likely detect merging systems in time to alert electromagnetic telescopes.
- For Schnittman, these simulations are key to understanding the real-life data LISA and other spacecraft collect. The case for models may be even stronger for binary black holes, Schnittman said, because the scientific community has little data.
- "We probably will never find a binary black hole with a telescope until we simulate them to the point we know exactly what we're looking for, because they're so far away, they're so tiny, you're going to see just one speck of light," Schnittman said. "We need to be able to look for that smoking gun."
• 15 April 2021: A pair of orbiting black holes millions of times the Sun's mass perform a hypnotic pas de deux in a new NASA visualization. The movie traces how the black holes distort and redirect light emanating from the maelstrom of hot gas – called an accretion disk – that surrounds each one. 28)
- Viewed from near the orbital plane, each accretion disk takes on a characteristic double-humped look. But as one passes in front of the other, the gravity of the foreground black hole transforms its partner into a rapidly changing sequence of arcs. These distortions play out as light from both disks navigates the tangled fabric of space and time near the black holes.
Figure 34: Explore how the extreme gravity of two orbiting supermassive black holes distorts our view. In this visualization, disks of bright, hot, churning gas encircle both black holes, shown in red and blue to better track the light source. The red disk orbits the larger black hole, which weighs 200 million times the mass of our Sun, while its smaller blue companion weighs half as much. Zooming into each black hole reveals multiple, increasingly warped images of its partner. Watch to learn more (video credit: NASA's Goddard Space Flight Center/Jeremy Schnittman and Brian P. Powell)
- "We're seeing two supermassive black holes, a larger one with 200 million solar masses and a smaller companion weighing half as much," said Jeremy Schnittman, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who created the visualization. "These are the kinds of black hole binary systems where we think both members could maintain accretion disks lasting millions of years."
- The accretion disks have different colors, red and blue, to make it easier to track the light sources, but the choice also reflects reality. Hotter gas gives off light closer to the blue end of the spectrum, and material orbiting smaller black holes experiences stronger gravitational effects that produce higher temperatures. For these masses, both accretion disks would actually emit most of their light in the UV, with the blue disk reaching a slightly higher temperature.
- Visualizations like this help scientists picture the fascinating consequences of extreme gravity's funhouse mirror. The new video doubles down on an earlier one Schnittman produced showing a solitary black hole from various angles.
- Seen nearly edgewise, the accretion disks look noticeably brighter on one side. Gravitational distortion alters the paths of light coming from different parts of the disks, producing the warped image. The rapid motion of gas near the black hole modifies the disk's luminosity through a phenomenon called Doppler boosting – an effect of Einstein's relativity theory that brightens the side rotating toward the viewer and dims the side spinning away.
- The visualization also shows a more subtle phenomenon called relativistic aberration. The black holes appear smaller as they approach the viewer and larger when moving away.
- These effects disappear when viewing the system from above, but new features emerge. Both black holes produce small images of their partners that circle around them each orbit. Looking closer, it's clear that these images are actually edge-on views. To produce them, light from the black holes must be redirected by 90 degrees, which means we're observing the black holes from two different perspectives – face on and edge on – at the same time.
- "A striking aspect of this new visualization is the self-similar nature of the images produced by gravitational lensing," Schnittman explained. "Zooming into each black hole reveals multiple, increasingly distorted images of its partner."
- Schnittman created the visualization by computing the path taken by light rays from the accretion disks as they made their way through the warped space-time around the black holes. On a modern desktop computer, the calculations needed to make the movie frames would have taken about a decade. So Schnittman teamed up with Goddard data scientist Brian P. Powell to use the Discover supercomputer at the NASA Center for Climate Simulation. Using just 2% of Discover's 129,000 processors, these computations took about a day.
- Astronomers expect that, in the not-too-distant future, they'll be able to detect gravitational waves – ripples in space-time – produced when two supermassive black holes in a system much like the one Schnittman depicted spiral together and merge.
• 10 April 2021: To celebrate two years since the EHT Collaboration, in which ALMA had a key role, released the first image of a Black Hole (Figure 1), we are happy to share five impressive things about these incredible objects. 29) 30)
1) Before knowing what black holes were, in 1784 geologist John Michell called them dark stars! The idea of black holes stems from Albert Einstein's theory of general relativity, which says that light is affected by gravity.
2) The first simulation of a black hole was a drawing of the accretion disk, made by hand, around a black hole, based on computer calculations by French astrophysicist Jean-Pierre Luminet in 1979.
3) Black holes are regions in space where gravity is extreme. Everything that comes too close is sucked in, and nothing can ever get out again. Even light, traveling at 300,000 kilometers per second, cannot escape the gravitational grip of a black hole!
4) Black holes cause huge jets of matter! – Most of the matter near the edge of a black hole ends up falling into it. However, some of the surrounding particles escape moments before capture and are propelled into space at great distances in jets.
5) The Event Horizon Telescope (EHT) collaboration, which produced the first-ever image of a black hole released in 2019, has a new view of the massive object at the Messier 87 (M87) center galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of a black hole.
- Impressive isn't it? We anticipate that next week, from April 12 to 16, NASA will organize its Black Hole Week, so you can continue to be surprised by these incredible objects, formerly called "dark stars."
• 10 November 2020: A new study lead by GSI scientists and international colleagues investigates black-hole formation in neutron star mergers. Computer simulations show that the properties of dense nuclear matter play a crucial role, which directly links the astrophysical merger event to heavy-ion collision experiments at GSI and FAIR (Facility for Antiproton and Ion Research). These properties will be studied more precisely at the future FAIR facility. The results have now been published in Physical Review Letters. With the award of the 2020 Nobel Prize in Physics for the theoretical description of black holes and for the discovery of a supermassive object at the center of our galaxy, the topic currently also receives a lot of attention. 31) 32)
- But under which conditions does a black hole actually form? This is the central question of a study lead by the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany, within an international collaboration. Using computer simulations, the scientists focus on a particular process to form black holes namely the merging of two neutron stars.
- Neutron stars consist of highly compressed dense matter. The mass of one and a half solar masses is squeezed to the size of just a few kilometers. This corresponds to similar or even higher densities than in the inner of atomic nuclei. If two neutron stars merge, the matter is additionally compressed during the collision. This brings the merger remnant on the brink to collapse to a black hole. Black holes are the most compact objects in the universe, even light cannot escape, so these objects cannot be observed directly.
- "The critical parameter is the total mass of the neutron stars. If it exceeds a certain threshold the collapse to a black hole is inevitable," summarizes Dr. Andreas Bauswein from the GSI theory department. However, the exact threshold mass depends on the properties of highly dense nuclear matter. In detail these properties of high-density matter are still not completely understood, which is why research labs like GSI collide atomic nuclei—like a neutron star merger but on a much smaller scale. In fact, the heavy-ion collisions lead to very similar conditions as mergers of neutron stars. Based on theoretical developments and physical heavy-ion experiments, it is possible to compute certain models of neutron star matter, so-call equations of state.
Figure 42: Using computer simulations, the scientists focus on a particular process to form black holes namely the merging of two neutron stars (simulation video of GSI)
- Employing numerous of these equations of state, the new study calculated the threshold mass for black-hole formation. If neutron star matter or nuclear matter, respectively, is easily compressible—if the equation of state is 'soft'—already the merger a relatively light neutron stars leads to the formation of a black hole. If nuclear matter is 'stiffer' and less compressible, the remnant is stabilized against the so-called gravitational collapse and a massive rotating neutron star remnant forms from the collision. Hence, the threshold mass for collapse itself informs about properties of high-density matter. The new study revealed furthermore that the threshold to collapse may even clarify whether during the collision nucleon dissolve into their constituents, the quarks.
- "We are very excited about this results because we expect that future observations can reveal the threshold mass," adds Professor Nikolaos Stergioulas of the department of physics of the Aristotle University Thessaloniki in Greece. Just a few years ago a neutron star merger was observed for the first time by measuring gravitational waves from the collision. Telescopes also found the electromagnetic counterpart and detected light from the merger event. If a black hole is directly formed during the collision, the optical emission of the merger is pretty dim. Thus, the observational data indicates if a black hole was created. At the same time the gravitational-wave signal carries information about the total mass of the system. The more massive the stars the stronger is the gravitational-wave signal, which thus allows determining the threshold mass.
- While gravitational-wave detectors and telescopes wait for the next neutron star mergers, the course is being set in Darmstadt for knowledge that is even more detailed. The new accelerator facility FAIR, currently under construction at GSI, will create conditions, which are even more similar to those in neutron star mergers. Finally, only the combination of astronomical observations, computer simulations and heavy-ion experiments can settle the questions about the fundamental building blocks of matter and their properties, and, by this, they will also clarify how the collapse to a black hole occurs.
• 13 October 2020: A team of gravitational-wave scientists led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Swinburne University of Technology, Hawthorn Victoria, Australia, reveal that when two black holes collide and merge, the remnant black hole ‘chirps' not once, but multiple times, emitting gravitational waves—intense ripples in the fabric space and time—that inform us about its shape. Today the study has been published in Communications Physics (from the prestigious Nature journal). 33) 34)
- Black holes are among the most fascinating objects in the universe. At their surface, known as the event horizon, gravity is so strong that not even light can escape. Usually, black holes are silent objects that swallow anything that falls too closely to them; however, when two black holes collide and merge, they produce one of the most catastrophic events in universe: In a fraction of a second, a highly deformed black hole forms and releases tremendous amounts of energy as it settles to its final state. This phenomenon gives astronomers a unique chance to observe rapidly changing black holes and explore gravity in its most extreme form.
- Although colliding black holes do not produce light, astronomers can observe the detected gravitational waves—ripples in the fabric of space and time—that bounce off them. Scientists speculate that, after a collision, the behavior of the remnant black hole is key to understanding gravity and should be encoded in the emitted gravitational waves.
- In the article published in Communications Physics (Nature), a team of scientists led by OzGrav alumnus Prof. Juan Calderón Bustillo—now ‘La Caixa Junior Leader - Marie Curie Fellow' at the Galician Institute for High Energy Physics (Santiago de Compostela, Spain)—has revealed how gravitational waves encode the shape of merging black holes as they settle to their final form.
- Graduate student and co-author Christopher Evans from the Georgia Institute of Technology (USA) says: ‘We performed simulations of black-hole collisions using supercomputers and then compared the rapidly changing shape of the remnant black hole to the gravitational waves it emits. We discovered that these signals are far more rich and complex than commonly thought, allowing us to learn more about the vastly changing shape of the final black hole'.
- The gravitational waves from colliding black holes are very simple signals known as ‘chirps'. As the two black holes approach each other, they emit a signal of increasing frequency and amplitude that indicates the speed and radius of the orbit. According to Prof. Calderón Bustillo, ‘the pitch and amplitude of the signal increases as the two black holes approach faster and faster. After the collision, the final remnant black hole emits a signal with a constant pitch and decaying amplitude—like the sound of a bell being struck'. This principle is consistent with all gravitational-wave observations so far, when studying the collision from the top.
- However, the study found something completely different happens if the collision is observed from the ‘equator' of the final black hole.
- 'When we observed black holes from their equator, we found that the final black hole emits a more complex signal, with a pitch that goes up and down a few times before it dies,' explains Prof. Calderón Bustillo. ‘In other words, the black hole actually chirps several times.'
- The team discovered that this is related to the shape of the final black hole, which acts like a kind of gravitational-wave lighthouse: ‘When the two original, ‘parent' black holes are of different sizes, the final black hole initially looks like a chestnut, with a cusp on one side and a wider, smoother back on the other,' says Bustillo. ‘It turns out that the black hole emits more intense gravitational waves through its most curved regions, which are those surrounding its cusp. This is because the remnant black hole is also spinning and its cusp and back repeatedly point to all observers, producing multiple chirps.'
- Co-author Prof. Pablo Laguna, former chair of the School of Physics at Georgia Tech and now Professor at University of Texas at Austin, pointed out ‘while a relation between the gravitational waves and the behavior of the final black hole has been long conjectured, our study provides the first explicit example of this kind of relation'.
• 06 October 2020: Roger Penrose, Reinhard Genzel, and Andrea Ghez are to be awarded the 2020 Nobel Prize in Physics for their theoretical and observational work on black holes, the Royal Swedish Academy of Sciences announced on Tuesday. Penrose will receive half the 10 million Swedish krona (roughly $1.1 million) prize; Ghez and Genzel will share the other half. 35)
- Penrose, of the University of Oxford, helped place the previously idealized concept of a black hole on sound theoretical footing in the 1960s by applying topology to general relativity and thus connecting the collapse of matter to the formation of a trapped surface and an inevitable singularity.
- Several decades later, Genzel (Max Planck Institute for Extraterrestrial Physics and the University of California, Berkeley) and Ghez (UCLA) each led a team that advanced the techniques of speckle imaging and adaptive optics to obviate atmospheric turbulence and analyze the motion of stars tightly orbiting Sagittarius A*, the radio source at the Milky Way's center. The researchers concluded that only a black hole, weighing in at about 4 million solar masses, could be responsible for the orbits they observed.
- Ghez is the fourth woman to be named a Nobel physics laureate, after Donna Strickland (2018), Maria Goeppert Mayer (1963), and Marie Curie (1903).
• October 6, 2020: The concept of a black hole—an object so massive that its gravity prevents light from escaping—emerged in pieces over the course of decades. Albert Einstein published his theory of gravity, the general theory of relativity, in 1915. It states that gravity arises when mass and energy warp the fabric of space and time, causing the trajectories of freely falling objects to curve like Earth's elliptical orbit around the Sun. Only 1 year later, German physicist Karl Schwarzschild worked out the shape of the pit in spacetime that a point mass would create and showed that it predicts an event horizon. That marks the edge of a sphere around the point mass from which light can still escape. 36)
- However, the whole notion that burned out stars could actually lead to these bizarre voids in space didn't arrive until 1939. That's when physicists J. Robert Oppenheimer and George Volkoff calculated that, if a neutron star grew too massive, it should collapse under its own weight to an infinitesimal point, leaving behind only its ultraintense gravitational field. Their work foreshadowed astrophysicists' current understanding of stellar-mass black holes, which form when sufficiently massive stars burn out and their cores collapse.
- Oppenheimer and his colleagues did not prove the imploding star had to form an event horizon. It was conceivable that the matter could somehow swirl away—or that the dead star's gravitational field might not stick around. In the 1960s, Penrose showed with extreme mathematical rigor that the formation of a black hole was essentially inevitable and that it would be indestructible, growing as it devoured more mass. "It didn't matter what you did, the horizon was always there," says Clifford Will, a general relativity expert at the University of Florida. "It wouldn't break apart, it would only grow."
- Will suggests the award could be considered a prize of sorts for Stephen Hawking, who died in 2018 and with whom Penrose collaborated. In fact, Penrose's key predictions are framed in the so-called Hawking Penrose theorems. Penrose notes that Hawking took his ideas regarding the formation of horizons around black holes and applied them to cosmology and the birth of the universe. "They were clearly advances on what I had done," Penrose says.
- In short, Penrose showed general relativity implied that black hole would be a real, stable astrophysical object, says Ulf Danielsson, a theoretical physicist at Uppsala University and a member of the Nobel physics committee. "Penrose laid a theoretical foundation so that we could say, ‘Yes, these objects exist, we can expect to find them if we go out and look for them.'"
- Since Penrose's advances, astronomers have found a wealth of evidence for black holes. They found stars orbiting invisible companions, and they could see superheated gases glowing hot as they disappeared into putative black holes. Gravitational wave detectors provided the clincher for such stellar size black holes, but not the galactic giants.
- The one at the center of the Milky Way, known as Sagittarius A* (Sgr A*), weighs millions of solar masses and is only 26,000 light-years away. But in addition to being black, it is quite small: Its event horizon would fit within Mercury's orbit. On top of that, the galactic center is cloaked from prying telescopes by gas and dust.
- By pushing observing techniques to their limits, the sparring teams of Ghez and Genzel carried out a very simple study: They mapped the progress of a single star as it orbited close to Sgr A* and showed, via simple Newtonian mechanics, that the object they were orbiting had to have a colossal mass. "With high school physics, you can get a long way to understanding that there must be something supermassive there that we can't see," says Selma de Mink, a theoretical astrophysicist at Harvard University.
- Their studies were enabled by infrared detectors. Wavelengths of about 2 micrometers proved to be a sweet spot: Those infrared photons could penetrate the haze and weren't too disturbed by turbulence in Earth's atmosphere. The infrared wavelengths were also small enough to locate stars relatively precisely.
- In the 1990s, Genzel and Ghez's groups both latched onto a single star, known as S2 or S0-2 by the two teams, which is the closest star to the galactic center yet detected. "Andrea and Reinhard have had a legendary competition over the years which has kept the field moving," says astrophysicist Heino Falcke of Radboud University. To get an accurate fix on S2, the teams needed the largest telescopes available: the four 8-meter telescopes of Europe's VLT (Very Large Telescope) in Genzel's case, and the twin 10-meter Keck telescopes for Ghez.
- In 2002, S2's elliptical orbit appeared to reach its closest point to Sgr A*. It came within 20 billion kilometers or 17 light-hours, and traveled at 5000 kilometers per second, 3% of the speed of light. The teams then had enough of an orbit to draw conclusions about the invisible object. They calculated it must weigh the equivalent of 4 million Suns and be a concentrated object: It could only be a black hole. "They proved through observation what Penrose had predicted with theory, that black holes actually do exist," says Gerry Gilmore of the University of Cambridge.
- The teams have continued to follow S2 through its first full orbit in 2008 and its second close approach in 2018. They have used those data to subject general relativity to ever more stringent tests. "They laid the foundations for supermassive black holes," Falcke says.
- As good as the S2 results were, researchers want even more direct evidence for the existence of SMBHs. And in 2019, the Event Horizon Telescope (EHT) succeeded in revealing the shadow of an even bigger monster at the center of M87, one of the Milky Way's neighboring galaxies. That black hole holds billions of solar masses. The EHT collaboration has tried to image Sgr A* but so far has been thwarted in presenting conclusive results.
- Ghez is just the fourth woman ever to win a Nobel Prize in Physics, and the second in the past 3 years. "That means a lot to me," de Mink says. In recent years, the Nobel science prizes have been criticized for their lack of diversity.
- At 55, Ghez is also a relatively young laureate. Penrose, 89, is among the oldest. But Penrose says he has no regrets about waiting so long to get the prize. "I know some people who got a Nobel too early, and it ruined their science," he says. "I think I'm about old enough."
• 26 August 2020: When a star passes too close to a supermassive black hole, tidal forces tear it apart, producing a bright flare of radiation as material from the star falls into the black hole. Astronomers study the light from these "tidal disruption events" (TDEs) for clues to the feeding behavior of the supermassive black holes lurking at the centers of galaxies. 37)
- New TDE observations led by astronomers at UC Santa Cruz now provide clear evidence that debris from the star forms a rotating disk, called an accretion disk, around the black hole. Theorists have been debating whether an accretion disk can form efficiently during a tidal disruption event, and the new findings, accepted for publication in the Astrophysical Journal and available online, should help resolve that question, said first author Tiara Hung, a postdoctoral researcher at UC Santa Cruz. 38)
- "In classical theory, the TDE flare is powered by an accretion disk, producing x-rays from the inner region where hot gas spirals into the black hole," Hung said. "But for most TDEs, we don't see x-rays—they mostly shine in the ultraviolet and optical wavelengths—so it was suggested that, instead of a disk, we're seeing emissions from the collision of stellar debris streams."
- Coauthors Enrico Ramirez-Ruiz, professor of astronomy and astrophysics at UCSC, and Jane Dai at the University of Hong Kong developed a theoretical model, published in 2018, that can explain why x-rays are usually not observed in TDEs despite the formation of an accretion disk. The new observations provide strong support for this model.
- "This is the first solid confirmation that accretion disks form in these events, even when we don't see x-rays," Ramirez-Ruiz said. "The region close to the black hole is obscured by an optically thick wind, so we don't see the x-ray emissions, but we do see optical light from an extended elliptical disk."
- The telltale evidence for an accretion disk comes from spectroscopic observations. Coauthor Ryan Foley, assistant professor of astronomy and astrophysics at UCSC, and his team began monitoring the TDE (named AT 2018hyz) after it was first detected in November 2018 by the All Sky Automated Survey for SuperNovae (ASAS-SN). Foley noticed an unusual spectrum while observing the TDE with the 3-meter Shane Telescope at UC's Lick Observatory on the night of January 1, 2019.
- "My jaw dropped, and I immediately knew this was going to be interesting," he said. "What stood out was the hydrogen line—the emission from hydrogen gas—which had a double-peaked profile that was unlike any other TDE we'd seen."
- Foley explained that the double peak in the spectrum results from the Doppler effect, which shifts the frequency of light emitted by a moving object. In an accretion disk spiraling around a black hole and viewed at an angle, some of the material will be moving toward the observer, so the light it emits will be shifted to a higher frequency, and some of the material will be moving away from the observer, its light shifted to a lower frequency.
- "It's the same effect that causes the sound of a car on a race track to shift from a high pitch as the car comes toward you to a lower pitch when it passes and starts moving away from you," Foley said. "If you're sitting in the bleachers, the cars on one turn are all moving toward you and the cars on the other turn are moving away from you. In an accretion disk, the gas is moving around the black hole in a similar way, and that's what gives the two peaks in the spectrum."
- The team continued to gather data over the next few months, observing the TDE with several telescopes as it evolved over time. Hung led a detailed analysis of the data, which indicates that disk formation took place relatively quickly, in a matter of weeks after the disruption of the star. The findings suggest that disk formation may be common among optically detected TDEs despite the rarity of double-peaked emission, which depends on factors such as the inclination of the disk relative to observers.
- "I think we got lucky with this one," Ramirez-Ruiz said. "Our simulations show that what we observe is very sensitive to the inclination. There is a preferred orientation to see these double-peak features, and a different orientation to see x-ray emissions."
- He noted that Hung's analysis of multi-wavelength follow-up observations, including photometric and spectroscopic data, provides unprecedented insights into these unusual events. "When we have spectra, we can learn a lot about the kinematics of the gas and get a much clearer understanding of the accretion process and what is powering the emissions," Ramirez-Ruiz said.
- In addition to Hung, Foley, Ramirez-Ruiz, and other members of the UCSC team, the coauthors of the paper also include scientists at the Niels Bohr Institute in Copenhagen (where Ramirez-Ruiz holds a Niels Bohr Professorship); University of Hong Kong; University of Melbourne, Australia; Carnegie Institution for Science; and Space Telescope Science Institute.
- Observations were obtained at Lick Observatory, the W. M. Keck Observatory, the Southern Astrophysical Research (SOAR) telescope, and the Swope Telescope at Las Campanas Observatory in Chile. This work was supported in part by the National Science Foundation, the Gordon and Betty Moore Foundation, the David and Lucile Packard Foundation, and the Heising-Simons Foundation.
• 30 June 2020: A North-American team of astronomers is collecting data from neutron stars across the Milky Way, weaving a galactic-sized web that will tingle when traversed by gravitational waves from the largest black holes in nature. Surprisingly, the success of this effort hinges on the robotic exploration of our own solar system. 39)
- This team of astronomers is united by NANOGrav (the North-American Nanohertz Observatory for Gravitational Waves) a National Science Foundation Physics Frontiers Center. The Green Bank Telescope (GBT) in Green Bank, West Virginia, is a fundamental tool in many NANOGrav projects, and collected data used in this research.
- "In the fall of 2016, looking at my computer screen felt like riding on a rollercoaster," says Stephen Taylor, a member of NANOGrav, then an astronomer at NASA's Jet Propulsion Laboratory, and now an assistant professor of physics and astronomy at Vanderbilt University. Just a few months earlier, LIGO had announced the historic detection of gravitational waves from merging stellar-sized black holes. Gravitational waves are propagating ripples in spacetime, predicted in Einstein's theory of gravity. Their 2015 detection opened new vistas on the most extreme objects in the universe. "And now our data showed early signs of waves from the gargantuan black-hole binaries at centers of galaxies—a discovery to rival LIGO's."
- Whereas LIGO and its French–Italian counterpart Virgo observe gravitational waves by monitoring the stretching and squeezing of 4-km-long laser interferometers, NANOGrav looks for changes in the shape of our entire galaxy, seeking signs of black holes up to a billion times more massive than LIGO's. To do so, NANOGrav has collected 15 years of data from galactic pulsars, neutron stars that emit radio pulses as they rotate. Their rotation is so regular that they can be used as geometric references: as passing gravitational waves stretch and squeeze the space between the pulsars and radio observatories on Earth, the waves can be identified as minuscule anomalies in sequence of pulses received at Earth.
- Fast forward to the January 2017 meeting of the American Astronomical Society in Texas. JPL astronomer Joe Simon felt very nervous about the NANOGrav presentation, which would announce the first hints of supermassive black holes in pulsar data. "Our collaboration was still frantically investigating unforeseen sources of error that may be impersonating gravitational waves. Only recently, our Australian colleagues had reported the total absence of these signals, to the point that they suggested astronomers rethink their notion of galactic centers. And yet there was definitely something in our data. But what?" A few weeks later, at a workshop in Colorado, JPL theoretical physicist Michele Vallisneri grinned broadly while sharing that "We too in NANOGrav have made a discovery. So far, we have found Jupiter!" What had happened? The story is told in a newly published ApJ article authored by Vallisneri, Taylor, Simon, and their NANOGrav colleagues. 40)
- To identify GWs unequivocally in pulsar signals, we need to listen from a place of absolute stillness at the true center of the solar system – a location known as the solar-system barycenter, which lies close to the surface of the sun. To find it we need to consider also the masses and positions of all planets, and even asteroids. Indeed, the barycenter is the location where the masses of all planets, moons, and asteroids balance out; it's the pivot of the solar system seesaw (Figure 48). When we observe pulsars with telescopes on Earth, the movement of our planet with respect to the barycenter creates modulations in the spacing of the radio pulses, just like an ambulance siren will alter pitch as it moves toward or away from us. It is only at the barycenter that all these artifacts disappear. "There is no way that we could build a radio telescope at the solar-system barycenter," says Simon, "but we use our knowledge of the masses and orbits of the planets, as measured from Earth and from spacecraft, to locate the barycenter and reconstruct the pulsar timings as they would have been from that privileged vantage point. The catch is that errors in the masses and orbits will translate to pulsar-timing artifacts that may well look like gravitational waves."
- "Nobody in our business had really worried about the accuracy of those orbits," continues Taylor. "JPL computes them for the astronomical community, and they are good enough to land a spacecraft on Mars within 100 feet of the intended target. But when we used slightly different versions of the JPL orbit tables to analyze the NANOGrav data, we came to divergent conclusions regarding the presence of GWs." While befuddling, says NANOGrav Physics Frontier Center co-director Maura McLaughlin (the Eberly distinguished professor of physics and astronomy at West Virginia University), this should be seen as a reason for pride in our collaboration: "We had not realized that our pulsar data had become so precise and abundant, and therefore so sensitive to tiny perturbations, that even 100-foot fluctuating error in the barycenter would be enough to throw us off."
- Vallisneri and the NANOGrav team members at JPL set to work to create a statistical treatment of orbit errors that could be folded into the NANOGrav analysis. They quickly concluded that the fault lay with Jupiter – or rather with our imperfect knowledge of its orbit. The stormy planet is by far the heaviest in the solar system, even if it is only a thousandth of the solar mass, and we have limited radio measurements (the most precise kind) of its orbit, because NASA's Galileo, which circled Jupiter between 1995 and 2003, failed to deploy its main antenna. To make things worse, Jupiter's orbital period is close to the timespan of NANOGrav's data, so errors in its orbit create most confusing artifacts for the analysis. By accounting for the possibility of these errors, the JPL team managed to reconcile the gravitational-wave results obtained with all recent orbit tables issued by JPL. Doing so, however, made the NANOGrav observations less sensitive to the waves, and watered down the hints of supermassive black holes.
- As NANOGrav continues to collect ever more abundant and precise pulsar timings, NANOGrav astronomers are confident that massive black holes will show up soon and unequivocally in the data. Even Jupiter seems to be lifting its curse: NASA's new Jupiter orbiter Juno is enabling much more faithful orbit estimates, even under NANOGrav's exacting standards. "The story of how we discovered Jupiter helps us stay modest, but it also emphasizes the oneness of astronomical research," concludes Vallisneri. "To probe extragalactic mysteries we had to rely on NASA's pioneering exploration of the solar system. We are so lucky to have colleagues and institutions that pursue both with equal impetus."
• 30 June 2020: The revolution in our understanding of the night sky and our place in the universe began when we transitioned from using the naked eye to a telescope in 1609. Four centuries later, scientists are experiencing a similar transition in their knowledge of black holes by searching for gravitational waves. 41)
- In the search for previously undetected black holes that are billions of times more massive than the sun, Stephen Taylor, assistant professor of physics and astronomy and former astronomer at NASA's Jet Propulsion Laboratory (JPL) together with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration has moved the field of research forward by finding the precise location – the center of gravity of our solar system – with which to measure the gravitational waves that signal the existence of these black holes.
- The potential presented by this advancement, co-authored by Taylor, was published in the journal The Astrophysical Journal in April 2020 (Ref. 40).
- Black holes are regions of pure gravity formed from extremely warped spacetime. Finding the most titanic black holes in the Universe that lurk at the heart of galaxies will help us understand how such galaxies (including our own) have grown and evolved over the billions of years since their formation. These black holes are also unrivaled laboratories for testing fundamental assumptions about physics.
- Gravitational waves are ripples in spacetime predicted by Einstein's general theory of relativity. When black holes orbit each other in pairs, they radiate gravitational waves that deform spacetime, stretching and squeezing space. Gravitational waves were first detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015, opening new vistas on the most extreme objects in the universe. Whereas LIGO observes relatively short gravitational waves by looking for changes in the shape of a 4-km long detector, NANOGrav, a National Science Foundation (NSF) Physics Frontiers Center, looks for changes in the shape of our entire galaxy.
- Taylor and his team are searching for changes to the arrival rate of regular flashes of radio waves from pulsars. These pulsars are rapidly spinning neutron stars, some going as fast as a kitchen blender. They also send out beams of radio waves, appearing like interstellar lighthouses when these beams sweep over Earth. Over 15 years of data have shown that these pulsars are extremely reliable in their pulse arrival rates, acting as outstanding galactic clocks. Any timing deviations that are correlated across lots of these pulsars could signal the influence of gravitational waves warping our galaxy.
- "Using the pulsars we observe across the Milky Way galaxy, we are trying to be like a spider sitting in stillness in the middle of her web," explains Taylor. "How well we understand the solar system barycenter is critical as we attempt to sense even the smallest tingle to the web." The solar system barycenter, its center of gravity, is the location where the masses of all planets, moons, and asteroids balance out.
- Where is the center of our web, the location of absolute stillness in our solar system? Not in the center of the sun as many might assume, rather it is closer to the surface of the star. This is due to Jupiter's mass and our imperfect knowledge of its orbit. It takes 12 years for Jupiter to orbit the sun, just shy of the 15 years that NANOGrav has been collecting data. JPL's Galileo probe (named for the famed scientist that used a telescope to observe the moons of Jupiter) studied Jupiter between 1995 and 2003, but experienced technical maladies that impacted the quality of the measurements taken during the mission.
- Identifying the center of the solar system's gravity has long been calculated with data from Doppler tracking to get an estimate of the location and trajectories of bodies orbiting the sun. "The catch is that errors in the masses and orbits will translate to pulsar-timing artifacts that may well look like gravitational waves," explains JPL astronomer and co-author Joe Simon.
- Taylor and his collaborators were finding that working with existing solar system models to analyze NANOGrav data gave inconsistent results. "We weren't detecting anything significant in our gravitational wave searches between solar system models, but we were getting large systematic differences in our calculations," notes JPL astronomer and the paper's lead author Michele Vallisneri. "Typically, more data delivers a more precise result, but there was always an offset in our calculations."
- The group decided to search for the center of gravity of the solar system at the same time as sleuthing for gravitational waves. The researchers got more robust answers to finding gravitational waves and were able to more accurately localize the center of the solar system's gravity to within 100 meters. To understand that scale, if the sun were the size of a football field, 100 meters would be the diameter of a strand of hair. "Our precise observation of pulsars scattered across the galaxy has localized ourselves in the cosmos better than we ever could before," said Taylor. "By finding gravitational waves this way, in addition to other experiments, we gain a more holistic overview of all different kinds of black holes in the Universe."
- As NANOGrav continues to collect ever more abundant and precise pulsar timing data, astronomers are confident that massive black holes will show up soon and unequivocally in the data.
- Taylor was partially supported by an appointment to the NASA Postdoctoral Program at JPL. The NANOGrav project receives support from the NSF Physics Frontier Center award #1430284 and this work was supported in part by NSF Grant PHYS-1066293 and by the hospitality of the Aspen Center for Physics. Data for this project were collected using the facilities of the Green Bank Observatory and the Arecibo Observatory.
- When two black holes spiral around each other and ultimately collide, they send out gravitational waves - ripples in space and time that can be detected with extremely sensitive instruments on Earth. Since black holes and black hole mergers are completely dark, these events are invisible to telescopes and other light-detecting instruments used by astronomers. However, theorists have come up with ideas about how a black hole merger could produce a light signal by causing nearby material to radiate.
- Now, scientists using Caltech's Zwicky Transient Facility (ZTF) located at Palomar Observatory near San Diego may have spotted what could be just such a scenario. If confirmed, it would be the first known light flare from a pair of colliding black holes.
- The merger was identified on May 21, 2019, by two gravitational wave detectors - the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector - in an event called GW190521g. That detection allowed the ZTF scientists to look for light signals from the location where the gravitational wave signal originated. These gravitational wave detectors have also spotted mergers between dense cosmic objects called neutron stars, and astronomers have identified light emissions from those collisions.
- The ZTF results are described in a new study published in the journal Physical Review Letters. The authors hypothesize that the two partner black holes, each several dozen times more massive than the Sun, were orbiting a third, supermassive black hole that is millions of times the mass of the Sun and surrounded by a disk of gas and other material. When the two smaller black holes merged, they formed a new, larger black hole that would have experienced a kick and shot off in a random direction. According to the new study, it may have plowed through the disk of gas, causing it to light up. 44)
- "This detection is extremely exciting," said Daniel Stern, coauthor of the new study and an astrophysicist at NASA's Jet Propulsion Laboratory in Southern California, which is a division of Caltech. "There's a lot we can learn about these two merging black holes and the environment they were in based on this signal that they sort of inadvertently created. So the detection by ZTF, coupled with what we can learn from the gravitational waves, opens up a new avenue to study both black hole mergers and these disks around supermassive black holes."
- The scientists attempted to get a more detailed look at the light of the supermassive black hole, called a spectrum, but by the time they looked, the flare had already faded. A spectrum would have offered more support for the idea that the flare came from merging black holes within the disk of the supermassive black hole. However, the researchers say they were able to largely rule out other possible causes for the observed flare, including a supernova or a tidal disruption event, which occurs when a black hole essentially eats a star (Ref. 43).
- What is more, the team says it is not likely that the flare came from the usual rumblings of the supermassive black hole, which regularly feeds off its surrounding disk. Using the Catalina Real-Time Transient Survey, led by Caltech, they were able to assess the behavior of the black hole over the past 15 years, and found that its activity was relatively normal until May of 2019, when it suddenly intensified.
- "Supermassive black holes like this one have flares all the time. They are not quiet objects, but the timing, size, and location of this flare was spectacular," says co-author Mansi Kasliwal (MS '07, PhD '11), an assistant professor of astronomy at Caltech. "The reason looking for flares like this is so important is that it helps enormously with astrophysics and cosmology questions. If we can do this again and detect light from the mergers of other black holes, then we can nail down the homes of these black holes and learn more about their origins."
- The newly formed black hole should cause another flare in the next few years. The process of merging gave the object a kick that should cause it to enter the supermassive black hole's disk again, producing another flash of light that ZTF should be able to see.
• 15 May 2020: Mergers between black holes and neutron stars in dense star clusters are quite unlike those that form in isolated regions where stars are few. Their associated features could be crucial to the study of gravitational waves and their source. Dr Manuel Arca Sedda of the Institute for Astronomical Computing at Heidelberg University came to this conclusion in a study that used computer simulations. The research may offer critical insights into the fusion of two massive stellar objects that astronomers observed in 2019. The findings were published in the journal "Communications Physics". 45) 46)
- Stars much more massive than our sun usually end their lives as a neutron star or black hole. Neutron stars emit regular pulses of radiation that allow their detection. In August 2017, for example, when the first double neutron star merger was observed, scientists all around the globe detected light from the explosion with their telescopes. Black holes, on the other hand, usually remain hidden because their gravitational attraction is so strong that even light cannot escape, making them invisible to electromagnetic detectors.
- If two black holes merge, the event may be invisible but is nonetheless detectable from ripples in space-time in the form of so-called gravitational waves. Certain detectors, like the LIGO (Laser Interferometer Gravitational Waves Observatory) in the USA, are able to detect these waves. The first successful direct observation was made in 2015. The signal was generated by the fusion of two black holes. But this event may not be the only source of gravitational waves, which could also come from the merger of two neutron stars or a black hole with a neutron star. Discovering the differences is one of the major challenges in observing these events, according to Dr Arca Sedda.
- In his study, the Heidelberg researcher analyzed the fusion of pairs of black holes and neutron stars. He used detailed computer simulations to study the interactions between a system made up of a star and a compact object, such as a black hole, and a third massive roaming object that is required for a fusion. The results indicate that such three-body interactions can in fact contribute to black hole-neutron star mergers in dense stellar regions like globular star clusters. "A special family of dynamic mergers that is distinctly different from mergers in isolated areas can be defined", explains Manuel Arca Sedda.
- The fusion of a black hole with a neutron star was first observed by gravitational wave observatories in August 2019. Yet optical observatories around the world were unable to locate an electromagnetic counterpart in the region from which the gravitational wave signal originated, suggesting that the black hole had completely devoured the neutron star without first destroying it. If confirmed, this could be the first observed black hole-neutron star merger detected in a dense stellar environment, as described by Dr Arca Sedda.
• 14 May 2020: Once you leave the majestic skies of Earth, the word "cloud" no longer means a white fluffy-looking structure that produces rain. Instead, clouds in the greater universe are clumpy areas of greater density than their surroundings. 47)
- Space telescopes have observed these cosmic clouds in the vicinity of supermassive black holes, those mysterious dense objects from which no light can escape, with masses equivalent to more than 100,000 Suns. There is a supermassive black hole in the center of nearly every galaxy, and it is called an "active galactic nucleus" (AGN) if it is gobbling up a lot of gas and dust from its surroundings. The brightest kind of AGN is called a "quasar." While the black hole itself cannot be seen, its vicinity shines extremely bright as matter gets torn apart close to its event horizon, its point of no return.
- But black holes aren't truly like vacuum cleaners; they don't just suck up everything that gets too close. While some material around a black hole will fall directly in, never to be seen again, some of the nearby gas will be flung outward, creating a shell that expands over thousands of years. That's because the area near the event horizon is extremely energetic; the high-energy radiation from fast-moving particles around the black hole can eject a significant amount of gas into the vastness of space.
- Scientists would expect that this outflow of gas would be smooth. Instead, it is clumpy, extending well beyond 1 parsec (3.3 light-years) from the black hole. Each cloud starts out small, but can expand to be more than 1 parsec wide — and could even cover the distance between Earth and the nearest star beyond the Sun, Proxima Centauri.
- Astrophysicist Daniel Proga at the University of Nevada, Las Vegas, likens these clumps to groups of cars waiting at a highway onramp with stoplights designed to regulate the influx of new traffic. "Every now and then you have a bunch of cars," he said.
- What explains these clumps in deep space? Proga and colleagues have a new computer model that presents a possible solution to this mystery, published in the Astrophysical Journal Letters, led by doctoral student Randall Dannen. Scientists show that extremely intense heat near the supermassive black hole can allow the gas to flow outward really fast, but in a way that can also lead to clump formation. If the gas accelerates too quickly, it will not cool off enough to form clumps. The computer model takes these factors into account and proposes a mechanism to make the gas travel far, but also clump. 48)
• 06 May 2020: ESO Instrument Finds Closest Black Hole to Earth. A team of astronomers from the European Southern Observatory (ESO) and other institutes has discovered a black hole lying just 1000 light-years from Earth. The black hole is closer to our Solar System than any other found to date and forms part of a triple system that can be seen with the naked eye. The team found evidence for the invisible object by tracking its two companion stars using the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile. They say this system could just be the tip of the iceberg, as many more similar black holes could be found in the future. 49)
- "We were totally surprised when we realized that this is the first stellar system with a black hole that can be seen with the unaided eye," says Petr Hadrava, Emeritus Scientist at the Academy of Sciences of the Czech Republic in Prague and co-author of the research. Located in the constellation of Telescopium, the system is so close to us that its stars can be viewed from the southern hemisphere on a dark, clear night without binoculars or a telescope. "This system contains the nearest black hole to Earth that we know of," says ESO scientist Thomas Rivinius, who led the study published today in Astronomy & Astrophysics. 50)
- Dietrich Baade, Emeritus Astronomer at ESO in Garching and co-author of the study, says: "The observations needed to determine the period of 40 days had to be spread over several months. This was only possible thanks to ESO's pioneering service-observing scheme under which observations are made by ESO staff on behalf of the scientists needing them."
- The hidden black hole in HR 6819 is one of the very first stellar-mass black holes found that do not interact violently with their environment and, therefore, appear truly black. But the team could spot its presence and calculate its mass by studying the orbit of the star in the inner pair. "An invisible object with a mass at least 4 times that of the Sun can only be a black hole," concludes Rivinius, who is based in Chile.
- Astronomers have spotted only a couple of dozen black holes in our galaxy to date, nearly all of which strongly interact with their environment and make their presence known by releasing powerful X-rays in this interaction. But scientists estimate that, over the Milky Way's lifetime, many more stars collapsed into black holes as they ended their lives. The discovery of a silent, invisible black hole in HR 6819 provides clues about where the many hidden black holes in the Milky Way might be. "There must be hundreds of millions of black holes out there, but we know about only very few. Knowing what to look for should put us in a better position to find them," says Rivinius. Baade adds that finding a black hole in a triple system so close by indicates that we are seeing just "the tip of an exciting iceberg."
- Already, astronomers believe their discovery could shine some light on a second system. "We realized that another system, called LB-1, may also be such a triple, though we'd need more observations to say for sure," says Marianne Heida, a postdoctoral fellow at ESO and co-author of the paper. "LB-1 is a bit further away from Earth but still pretty close in astronomical terms, so that means that probably many more of these systems exist. By finding and studying them we can learn a lot about the formation and evolution of those rare stars that begin their lives with more than about 8 times the mass of the Sun and end them in a supernova explosion that leaves behind a black hole."
- The discoveries of these triple systems with an inner pair and a distant star could also provide clues about the violent cosmic mergers that release gravitational waves powerful enough to be detected on Earth. Some astronomers believe that the mergers can happen in systems with a similar configuration to HR 6819 or LB-1, but where the inner pair is made up of two black holes or of a black hole and a neutron star. The distant outer object can gravitationally impact the inner pair in such a way that it triggers a merger and the release of gravitational waves. Although HR 6819 and LB-1 have only one black hole and no neutron stars, these systems could help scientists understand how stellar collisions can happen in triple star systems.
• 16 April 2020: Observations made with ESO's Very Large Telescope (VLT) have revealed for the first time that a star orbiting the supermassive black hole at the center of the Milky Way moves just as predicted by Einstein's general theory of relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This long-sought-after result was made possible by increasingly precise measurements over nearly 30 years, which have enabled scientists to unlock the mysteries of the behemoth lurking at the heart of our galaxy. 51)
- "Einstein's General Relativity predicts that bound orbits of one object around another are not closed, as in Newtonian Gravity, but precess forwards in the plane of motion. This famous effect — first seen in the orbit of the planet Mercury around the Sun — was the first evidence in favor of General Relativity. One hundred years later we have now detected the same effect in the motion of a star orbiting the compact radio source Sagittarius A* at the center of the Milky Way. This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun," says Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany and the architect of the 30-year-long program that led to this result.
- Located 26,000 light-years from the Sun, Sagittarius A* and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. One of these stars, S2, sweeps in towards the supermassive black hole to a closest distance less than 20 billion kilometers (one hundred and twenty times the distance between the Sun and Earth), making it one of the closest stars ever found in orbit around the massive giant. At its closest approach to the black hole, S2 is hurtling through space at almost three percent of the speed of light, completing an orbit once every 16 years. "After following the star in its orbit for over two and a half decades, our exquisite measurements robustly detect S2's Schwarzschild precession in its path around Sagittarius A*," says Stefan Gillessen of the MPE, who led the analysis of the measurements published today in the journal Astronomy & Astrophysics. 52)
- Most stars and planets have a non-circular orbit and therefore move closer to and further away from the object they are rotating around. S2's orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape. General Relativity provides a precise prediction of how much its orbit changes and the latest measurements from this research exactly match the theory. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.
- The study with ESO's VLT also helps scientists learn more about the vicinity of the supermassive black hole at the center of our galaxy. "Because the S2 measurements follow General Relativity so well, we can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*. This is of great interest for understanding the formation and evolution of supermassive black holes," say Guy Perrin and Karine Perraut, the French lead scientists of the project.
- This result is the culmination of 27 years of observations of the S2 star using, for the best part of this time, a fleet of instruments at ESO's VLT, located in the Atacama Desert in Chile. The number of data points marking the star's position and velocity attests to the thoroughness and accuracy of the new research: the team made over 330 measurements in total, using the GRAVITY, SINFONI and NACO instruments. Because S2 takes years to orbit the supermassive black hole, it was crucial to follow the star for close to three decades, to unravel the intricacies of its orbital movement.
- The research was conducted by an international team led by Frank Eisenhauer of the MPE with collaborators from France, Portugal, Germany and ESO. The team make up the GRAVITY collaboration, named after the instrument they developed for the VLT Interferometer, which combines the light of all four 8-meter VLT telescopes into a super-telescope (with a resolution equivalent to that of a telescope 130 meters in diameter). The same team reported in 2018 another effect predicted by General Relativity: they saw the light received from S2 being stretched to longer wavelengths as the star passed close to Sagittarius A*. "Our previous result has shown that the light emitted from the star experiences General Relativity. Now we have shown that the star itself senses the effects of General Relativity," says Paulo Garcia, a researcher at Portugal's Center for Astrophysics and Gravitation and one of the lead scientists of the GRAVITY project.
- With ESO's upcoming Extremely Large Telescope, the team believes that they would be able to see much fainter stars orbiting even closer to the supermassive black hole. "If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole," says Andreas Eckart from Cologne University, another of the lead scientists of the project. This would mean astronomers would be able to measure the two quantities, spin and mass, that characterize Sagittarius A* and define space and time around it. "That would be again a completely different level of testing relativity," says Eckart.
• 07 April 2020: One year ago, the Event Horizon Telescope (EHT) Collaboration published the first image of a black hole in the nearby radio galaxy M 87. Now the collaboration has extracted new information from the EHT data on the distant quasar 3C 279: they observed the finest detail ever seen in a jet produced by a supermassive black hole. New analyses, led by Jae-Young Kim from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, enabled the collaboration to trace the jet back to its launch point, close to where violently variable radiation from across the electromagnetic spectrum arises. 53)
- The EHT collaboration continues extracting information from the groundbreaking data collected in its global campaign in April 2017. One target of the observations was a galaxy 5 billion light-years away in the constellation Virgo that scientists classify as a quasar because an ultra-luminous source of energy at its center shines and flickers as gas falls into a giant black hole. The target, 3C 279, contains a black hole about one billion times more massive than our Sun. Twin fire-hose-like jets of plasma erupt from the black hole and disk system at velocities close to the speed of light: a consequence of the enormous forces unleashed as matter descends into the black hole's immense gravity.
- To capture the new image, the EHT uses a technique called very long baseline interferometry (VLBI), which synchronizes and links radio dishes around the world. By combining this network to form one huge virtual Earth-size telescope, the EHT is able to resolve objects as small as 20 micro-arcseconds (µas) on the sky — the equivalent of someone on Earth identifying an orange on the Moon. Data recorded at all the EHT sites around the world is transported to special supercomputers at MPIfR and at MIT's Haystack Observatory, where they are combined. The combined data set is then carefully calibrated and analyzed by team of experts, which then enables EHT scientists to produce images with the finest detail possible from the surface of the Earth.
- For 3C 279, the EHT can measure features finer than a light-year across, allowing astronomers to follow the jet down to the accretion disk and to see the jet and disk in action. The newly analyzed data show that the normally straight jet has an unexpected twisted shape at its base and, revealing features perpendicular to the jet that could be interpreted as the poles of the accretion disk where the jets are ejected. The fine details in the images change over consecutive days, possibly due to rotation of the accretion disk, and shredding and infall of material, phenomena expected from numerical simulations but never before observed.
- Jae-Young Kim, researcher at MPIfR and lead author of the paper, is enthusiastic and at the same time puzzled: "We knew that every time you open a new window to the Universe you can find something new. Here, where we expected to find the region where the jet forms by going to the sharpest image possible, we find a kind of perpendicular structure. This is like finding a very different shape by opening the smallest Matryoshka doll."
- Avery Broderick, an astrophysicist working at the Perimeter Institute, explains "For 3C 279, the combination of the transformative resolution of the EHT and new computational tools for interpreting its data have proved revelatory. What was a single radio 'core' is now resolved into two independent complexes. And they move — even on scales as small as light-months, the jet in 3C 279 is speeding toward us at more than 99.5% of light speed!"
- Because of this rapid motion, the jet in 3C 279 appears to move at about 20 times the speed of light. "This extraordinary optical illusion arises because the material is racing toward us, chasing down the very light it is emitting and making it appear to be moving faster than it is," clarifies Dom Pesce, a postdoctoral fellow at the Center for Astrophysics | Harvard & Smithsonian (CfA). The unexpected geometry suggests the presence of traveling shocks or instabilities in a bent, rotating jet, which might also explain emission at high energies such as gamma-rays.
- Anton Zensus, Director at the MPIfR and Chair of the EHT Collaboration Board, stresses the achievement as a global effort: "Last year we could present the first image of the shadow of a black hole. Now we see unexpected changes in the shape of the jet in 3C 279, and we are not done yet. As we told last year: this is just the beginning."
- "The EHT array is always improving," explains Shep Doeleman ot the CfA, EHT Founding Director. "These new quasar results demonstrate that the unique EHT capabilities can address a wide range of science questions, which will only grow as we continue to add new telescopes to the array. Our team is now working on a next-generation EHT array that will greatly sharpen the focus on black holes and allow us to make the first black hole movies."
- Opportunities to conduct EHT observing campaigns occur once a year in early Northern springtime, but the March/April 2020 campaign had to be cancelled in response to the CoViD-19 global outbreak. In announcing the cancellation Michael Hecht, astronomer from the MIT/Haystack Observatory and EHT Deputy Project Director, concluded that: "We will now devote our full concentration to completion of scientific publications from the 2017 data and dive into the analysis of data obtained with the enhanced EHT array in 2018. We are looking forward to observations with the EHT array expanded to eleven observatories in the spring of 2021." 54)
The international collaboration announced the first-ever image of a black hole at the heart of the radio galaxy Messier 87 on April 10, 2019 by creating a virtual Earth-sized telescope. Supported by considerable international investment, the EHT links existing telescopes using novel systems — creating a new instrument with the highest angular resolving power that has yet been achieved.
The individual telescopes involved in the EHT collaboration are: the Atacama Large Millimeter Telescope (ALMA), the Atacama Pathfinder EXplorer (APEX), the Greenland Telescope (since 2018), the IRAM 30-meter Telescope, the IRAM NOEMA Observatory (expected 2021), the Kitt Peak Telescope (expected 2021), the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), and the South Pole Telescope (SPT).
The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan), the University of Arizona, the University of Chicago, the East Asian Observatory (EAO, located on Hilo, Hawaii), Goethe-Universität Frankfurt, Institut de Radioastronomie Millimétrique (IRAM, located on Pico Veleta in the Spanish Sierra Nevada), Large Millimeter Telescope (LMT located on top of the Sierra Negra, Mexico), Max-Planck-Institut für Radioastronomie (MPIfR, Bonn), MIT Haystack Observatory, National Astronomical Observatory of Japan (NAOJ), Perimeter Institute for Theoretical Physics (Waterloo, Ontario, Canada), Radboud University (Nijmegen, the Netherlands) and the Smithsonian Astrophysical Observatory (SAO, Cambridge, Massachusetts).
• 18 March 2020: Last April, the Event Horizon Telescope (EHT) sparked international excitement when it unveiled the first image of a black hole. Today, a team of researchers have published new calculations that predict a striking and intricate substructure within black hole images from extreme gravitational light bending. 55)
- "The image of a black hole actually contains a nested series of rings," explains Michael Johnson of the Center for Astrophysics | Harvard and Smithsonian (CfA). "Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we've caught just a glimpse of the full complexity that should emerge in the image of any black hole."
- Because black holes trap any photons that cross their event horizon, they cast a shadow on their bright surrounding emission from hot infalling gas. A "photon ring" encircles this shadow, produced from light that is concentrated by the strong gravity near the black hole. This photon ring carries the fingerprint of the black hole—its size and shape encode the mass and rotation or "spin" of the black hole. With the EHT images, black hole researchers have a new tool to study these extraordinary objects.
- "Black hole physics has always been a beautiful subject with deep theoretical implications, but now it has also become an experimental science," says Alex Lupsasca from the Harvard Society of Fellows. "As a theorist, I am delighted to finally glean real data about these objects that we've been abstractly thinking about for so long."
- The research team included observational astronomers, theoretical physicists, and astrophysicists.
- "Bringing together experts from different fields enabled us to really connect a theoretical understanding of the photon ring to what is possible with observation," notes George Wong, a physics graduate student at the University of Illinois at Urbana-Champaign. Wong developed software to produce simulated black hole images at higher resolutions than had previously been computed and to decompose these into the predicted series of sub-images. "What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits."
- The researchers also found that the black hole's image substructure creates new possibilities to observe black holes. "What really surprised us was that while the nested subrings are almost imperceptible to the naked eye on images—even perfect images—they are strong and clear signals for arrays of telescopes called interferometers," says Johnson. "While capturing black hole images normally requires many distributed telescopes, the subrings are perfect to study using only two telescopes that are very far apart. Adding one space telescope to the EHT would be enough."
- The results were published in Science Advances. 56) This research was supported by grants from the National Science Foundation, the Gordon and Betty Moore Foundation, the John Templeton Foundation, the Jacob Goldfield Foundation, the Department of Energy, and NASA.
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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 (firstname.lastname@example.org).