Event Horizon Telescope Collaboration scientists use data which produced the first image of a black hole to constrain its fundamental properties.
In 2019, the EHT Collaboration published the first image of a black hole located at the centre of the galaxy M87. Now a collaboration team led by theoretical physicists at the Goethe University Frankfurt have analysed data from the black hole M87* to test Albert Einstein's theory of general relativity. According to the tests, the size of the shadow from M87* is in excellent agreement with a black hole predicted by general relativity, but narrows the properties of black holes in other theories down. These results are presented in today’s issue of the Physical Review D journal.
A team of scientists from the Event Horizon Telescope Collaboration led by Prashant Kocherlakota and Luciano Rezzolla from the Institute of Theoretical Physics at the Goethe University Frankfurt in Germany has investigated for the first time how the different theories fit with the observational data of the black hole M87* at the centre of the galaxy Messier 87. The image of M87*, taken in 2019 was a further evidence of the actual existence of black holes after, e.g., the measurement of gravitational waves in 2015.
The result of these investigations: the data from M87* are in excellent agreement with the Einstein-based theories and to a certain extent with the string-based theories. Prashant Kocherlakota, scientist at Frankfurt University and EHT Collaboration member, explains: "With the data recorded by the EHT Collaboration, we can now test different theories of physics with black hole images. Currently, we cannot reject these theories when describing the shadow size of M87*, but our calculations constrain the range of validity of these black hole models."
Luciano Rezzolla, Chair for Theoretical Physics at Frankfurt University and EHT Collaboration Board Member, says: “The idea of black holes for us theoretical physicists is at the same time a source of concern and of inspiration. While we still struggle with some of the consequences of black holes –such as the event horizon or the singularity –we seem always keen to find new black hole solutions also in other theories. It is therefore very important to obtain results like ours, which determine what is plausible and what is not. This was an important first step and our constraints will be improved as new observations are made.”
As first pointed out by the German astronomer Karl Schwarzschild, black holes bend space-time to an extreme degree due to their extraordinary concentration of mass, and heat up the matter in their vicinity so that it begins to glow. New Zealand physicist Roy Kerr showed rotation can change the black hole’s size and the geometry of its surroundings. The "edge" of a black hole is known as the event horizon, the boundary around the concentration of mass beyond which light and matter cannot escape and which makes the black hole “black”. Black holes, theory predicts, can be described by a handful of properties: mass, spin, and a variety of possible charges.
In the Event Horizon Telescope collaboration, telescopes from 19 observatories around the globe are interconnected to form a virtual giant telescope with a dish as big as the Earth itself. With the precision of this telescope, a newspaper in New York could be read from a street café in Berlin.
Figure: Event horizon sizes for different theories of gravity. All of these black holes cast dark shadows that are distinguishable from each other in size, but only those that fall in the gray band are compatible with the 2017 EHT measurements of M87*, and in this image, the one represented in red at the bottom is too small to be a viable model for M87*. © P. Kocherlakota (Univ. Frankfurt), EHT Collaboration & Fiks Film 2021 [JPG]
Video (Thumbnail): Constraining black hole models with EHT observations | Event Horizon Telescope (See YouTube version and MP4) - © P. Kocherlakota & L. Rezzolla (Univ. Frankfurt), EHT Collaboration & Fiks Film 2021, Directed by Nelis Claasen,
The Physical Review D publication (volume 103) reporting these results, with title Constraints on black-hole charges with the 2017 EHT observations of M87*, is available at doi:10.1103/PhysRevD.103.104047 (open access). Leading authors of the paper are Prashant Kocherlakota and Luciano Rezzolla as members of the EHT Collaboration.
The individual EHT telescopes involved are: ALMA, APEX, the IRAM 30-meter Telescope, 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 Greenland Telescope, the Kitt Peak Telescope, and NOEMA joined EHT after the 2017 observations.
The EHT consortium 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-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique (MPG/CNRS/IGN), Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics, Radboud University and the Smithsonian Astrophysical Observatory.