Event Horizon Telescope
The Event Horizon Telescope (EHT) is a large telescope array consisting of a global network of radio telescopes. The EHT project combines data from several very-long-baseline interferometry (VLBI) stations around Earth with angular resolution sufficient to observe objects the size of a supermassive black hole's event horizon. The project's observational targets include the two black holes with the largest angular diameter as observed from Earth: the black hole at the center of the supergiant elliptical galaxy Messier 87 (M87), and Sagittarius A* (Sgr A*) at the center of the Milky Way.
The Event Horizon Telescope project is an international collaboration launched in 2009  after a long period of theoretical and technical developments. On the theory side, work on the photon orbit  and first simulations of what a black hole would look like  progressed to predictions of VLBI imaging for the Galactic Center black hole, Sgr A*. Technical advances in radio observing moved from the first detection of Sgr A*, through VLBI at progressively shorter wavelengths, ultimately leading to detection of horizon scale structure in both Sgr A* and M87. The collaboration now comprises over 200 members, 60 institutions, working over 20 countries and regions.
The first image of a black hole, at the center of galaxy Messier 87, was published by the EHT Collaboration on April 10, 2019, in a series of six scientific publications. The array made this observation at a wavelength of 1.3 mm and with a theoretical diffraction-limited resolution of 25 microarcseconds. Future plans involve improving the array's resolution by adding new telescopes and by taking shorter-wavelength observations.
The EHT is composed of many radio observatories or radio telescope facilities around the world, working together to produce a high-sensitivity, high-angular-resolution telescope. Through the technique of very-long-baseline interferometry (VLBI), many independent radio antennas separated by hundreds or thousands of kilometres can act as a phased array, a virtual telescope which can be pointed electronically, with an effective aperture which is the diameter of the entire planet. The effort includes development and deployment of submillimeter dual polarization receivers, highly stable frequency standards to enable very-long-baseline interferometry at 230–450 GHz, higher-bandwidth VLBI backends and recorders, as well as commissioning of new submillimeter VLBI sites.
Each year since its first data capture in 2006, the EHT array has moved to add more observatories to its global network of radio telescopes. The first image of the Milky Way's supermassive black hole, Sagittarius A*, was expected to be produced in April 2017, but because the South Pole Telescope is closed during winter (April to October), the data shipment delayed the processing to December 2017 when the shipment arrived.
Data collected on hard drives are transported by commercial freight airplanes (a so-called sneakernet) from the various telescopes to the MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy, where the data are cross-correlated and analyzed on a grid computer made from about 800 CPUs all connected through a 40 Gbit/s network.
The Event Horizon Telescope Collaboration announced its first results in six simultaneous press conferences worldwide on April 10, 2019. The announcement featured the first direct image of a black hole, which showed the supermassive black hole at the center of Messier 87, designated M87*. The scientific results were presented in a series of six papers published in The Astrophysical Journal Letters.
The image provided a test for Albert Einstein's general theory of relativity under extreme conditions. Studies have previously tested general relativity by looking at the motions of stars and gas clouds near the edge of a black hole. However, an image of a black hole brings observations even closer to the event horizon. Relativity predicts a dark shadow-like region, caused by gravitational bending and capture of light, which matches the observed image. The published paper states: "Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity." Paul T.P. Ho, EHT Board member, said: "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."
The image also provided new measurements for the mass and diameter of M87*. EHT measured the black hole's mass to be 6.5±0.7 billion solar masses and measured the diameter of its event horizon to be approximately 40 billion kilometres (270 AU; 0.0013 pc; 0.0042 ly), roughly 2.5 times smaller than the shadow that it casts, seen at the center of the image. From the asymmetry in the ring, EHT inferred that the matter on the brighter south side of the disk is moving towards Earth, the observer. This is based on the theory that approaching matter appears brighter because of relativistic beaming. Previous observations of the black hole's jet showed that the black hole's spin axis is inclined at an angle of 17° relative to the observer's line of sight. From these two observations, EHT concluded the black hole spins clockwise, as seen from Earth.
Producing an image from data from an array of radio telescopes requires much mathematical work. Four independent teams created images to assess the reliability of the results. These methods included both an established algorithm in radio astronomy for image reconstruction known as CLEAN, invented by Jan Högbom, as well as self-calibrating image processing methods for astronomy such as the CHIRP algorithm created by Katherine Bouman and others. The algorithms that were ultimately used were a regularized maximum likelihood (RML) algorithm and the CLEAN algorithm.
The EHT Collaboration consists of 13 stakeholder institutes:
- the Academia Sinica Institute of Astronomy and Astrophysics
- the University of Arizona
- the University of Chicago
- the East Asian Observatory
- Goethe University Frankfurt
- Smithsonian Astrophysical Observatory (part of the Center for Astrophysics | Harvard & Smithsonian)
- Institut de radioastronomie millimétrique (IRAM, itself a collaboration between the French CNRS, the German Max Planck Society, and the Spanish Instituto Geográfico Nacional),
- Large Millimeter Telescope Alfonso Serrano
- Max Planck Institute for Radio Astronomy
- MIT Haystack Observatory
- National Astronomical Observatory of Japan
- Perimeter Institute for Theoretical Physics
- Radboud University
Institutions affiliated with the EHT include:
- Aalto University
- Boston University
- Brandeis University
- California Institute of Technology
- Canadian Institute for Advanced Research
- Canadian Institute for Theoretical Astrophysics
- Chalmers University of Technology, Onsala Space Observatory
- Chinese Academy of Sciences
- Consejo Nacional de Ciencia y Tecnología
- Cornell University, Center for Astrophysics and Planetary Science
- European Research Council
- Google Research
- The Graduate University for Advanced Studies (SOKENDAI), Department of Statistical Science / Department of Astronomical Science
- Hiroshima University, Hiroshima Astrophysical Science Center
- Huazhong University of Science and Technology, School of Physics
- Institute of Statistical Mathematics
- Instituto de Astrofísica de Andalucía, Consejo Superior de Investigaciones Científicas
- Instituto Geográfico Nacional
- Instituto Nacional de Astrofísica, Óptica y Electrónica
- Istituto Nazionale di Astrofisica (INAF) – Istituto di Radioastronomia, Italian ALMA Regional Centre
- Istituto Nazionale di Fisica Nucleare, Sezione di Napoli
- Joint Institute for VLBI in Europe
- Kogakuin University of Technology Engineering
- Korea Astronomy and Space Science Institute
- Leiden University, Leiden Observatory
- Los Alamos National Laboratory
- Max-Planck-Institut für extraterrestrische Physik
- Nanjing University, Key Laboratory of Modern Astronomy and Astrophysics / School of Astronomy and Space Science
- National Optical Astronomy Observatory
- National Radio Astronomy Observatory
- National Sun Yat-Sen University, Physics Department
- National Taiwan University, Department of Physics
- Netherlands Organisation for Scientific Research
- Peking University, Department of Astronomy, School of Physics / Kavli Institute for Astronomy and Astrophysics
- Rhodes University, Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics
- Seoul National University, Department of Physics and Astronomy
- Tohoku University, Astronomy Institute / Frontier Research Institute for Interdisciplinary Sciences
- Universidad de Concepción, Astronomy Department
- Universidad Nacional Autónoma de México, Instituto de Astronomía / Instituto de Radioastronomía y Astrofísica
- Universitat de València, Departament d'Astronomia i Astrofísica / Observatori Astronòmic
- University College London, Mullard Space Science Laboratory
- University of Amsterdam, Anton Pannekoek Institute & GRAPPA
- University of Arizona
- University of California Berkeley
- University of California Santa Barbara
- University of Chinese Academy of Sciences, School of Astronomy and Space Sciences
- University of Illinois, Department of Astronomy / Department of Physics
- University of Massachusetts Amherst, Department of Astronomy
- University of Pretoria, Department of Physics
- University of Science and Technology
- University of Science and Technology of China, Astronomy Department
- University of St. Petersburg, Astronomy Institute
- University of Tokyo, Graduate School of Science, Department of Astronomy / Kavli Institute for Physics & Mathematics of the Universe
- University of Toronto, Dunlap Institute for Astronomy and Astrophysics
- University of Waterloo, Waterloo Center for Astrophysics / Department of Physics and Astronomy
- Yonsei University, Department of Astronomy
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“For the case of M87*, which is the designation of this black hole, a (very nice) name has been proposed, but it has not received an official IAU approval,” says Christensen.
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