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."
Figure 1: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more massive than the Sun. This long-sought image provides the strongest evidence to date for the existence of supermassive black holes and opens a new window onto the study of black holes, their event horizons, and gravity (image credit: Event Horizon Telescope Collaboration)
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)
Figure 2: EHT images of M87 on four different observing nights. In each panel, the white circle shows the resolution of the EHT. All four images are dominated by a bright ring with enhanced emission in the south (image credit: Shep Doeleman & EHT Collaboration)
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.
Figure 3: A map of the EHT. Stations active in 2017 and 2018 are shown with connecting lines and labeled in yellow, sites in commission are labeled in green, and legacy sites are labeled in red. From Paper II (Figure 2) image credit: EHT Collaboration
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.
Table 1: Overview of EHT publications in The Astrophysical Journal
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.
Figure 5: Six press conferences around the world revealed the black-hole images (image credit: Nature)
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.
Figure 6: Nik Spencer/Nature; Avery Broderick/University of Waterloo (IMAGES bottom)
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."
Figure 7: How to hunt for a black hole with a telescope the size of Earth (image credit: ESA advanced concepts team; S. Brunier /ESO) 5)
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".
Figure 8: From left: Paulina Bocaz, NRAO/AUI legal representative in Chile; Hugo Messias; Alejandro Sáez; Akihiko Hirota (image credit: N. Lira – ALMA (ESO/NAOJ/NRAO)
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.
Figure 9: The galaxy M87, imaged here by NASA's Spitzer Space Telescope, is home to a supermassive black hole that spews two jets of material out into space at nearly the speed of light. The inset shows a close-up view of the shockwaves created by the two jets (image credit: NASA/JPL-Caltech/IPAC)
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.
Figure 10: The galaxy M87 looks like a hazy, blue space-puff in this image from NASA's Spitzer Space Telescope. At the galaxy's center is a supermassive black hole that spews two jets of material out into space (image credit: NASA/JPL-Caltech/IPAC)
Figure 11: This wide-field image of the galaxy M87 was taken by NASA's Spitzer Space Telescope. The top inset shows a close-up of two shockwaves, created by a jet emanating from the galaxy's supermassive black hole. The Event Horizon Telescope recently took a close-up image of the silhouette of that black hole, show in the second inset (image credit: NASA/JPL-Caltech/Event Horizon Telescope Collaboration)
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)
Figure 12: Two merging black holes (image credit: ESA)
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.
Figure 13: Artist's impression of the merger of two supermassive black holes during a galaxy collision. Simulations predict that their mergers, unlike those of their stellar-mass counterparts, emit both gravitational waves and radiation – the latter originating in the hot, interstellar gas of the two colliding galaxies stirred by the black holes pair when they fall towards one another. As the two spiralling black holes modulate the motion of the surrounding gas, it is likely that the X-ray signature will have a frequency commensurate to that of the gravitational wave signal. — Combining the observing power of two future ESA missions, Athena and LISA, would allow us to study these cosmic clashes and their mysterious aftermath for the first time. After the merger, we could see the emergence of a new X-ray source, and perhaps witness the birth of an active galactic nucleus, with jets of high-energy particles being launched at close to the speed of light above and beyond the newly formed black hole (image credit: ESA)
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)
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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).