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EHT tests of the strong-field regime of General Relativity

Written by Sebastian H. Völkel and Nicola Franchini


The name “black hole” describes the incredible phenomena that light cannot escape away from it. Here, the gravitational pull is beyond any measure, meaning that there is no emission at all. The boundary of this region is called the event horizon. On the other hand, the light that only approaches but never passes the event horizon is strongly deflected and eventually follows highly bent orbits around the black hole, until some of it can be observed on Earth.


Albert Einstein’s general theory of relativity predicts the existence of black holes. The mass of these objects can span from tens to millions or even billions of times the mass of the Sun. The latter is referred to by astrophysicists as supermassive black holes. These monsters have a lot of astrophysical relevance since they dwell in the centers of most galaxies, including the Milky Way.


According to scientists, black holes are surrounded by very hot gas in the shape of disks that are constantly rotating around the black hole. While some gas spirals inwards from far-out regions of the disk, other gas of the inner region falls into the black hole. These mechanisms produce light, which either fall in the black hole or escape from the system. The closest thing to a picture of a black hole (which itself does not emit light) is a detailed measurement of the shining gas being swallowed by it. Ongoing developments in radio astronomy, computer simulations, and data analysis techniques make it possible to take images of supermassive black holes, as demonstrated by the Event Horizon Telescope Collaboration in the last few years.


So far, in 2019, the Event Horizon Telescope Collaboration has managed to publish a picture of the supermassive black hole which lives in the middle of the M87 galaxy. The captured image consists of a bright disk around a dark central region, the so-called “shadow” of the black hole. The most relevant property one can infer from this picture is the size of such a shadow. Using computer simulations that describe the complex details of matter falling in the black hole, one can predict the size of this shadow and directly relate it to the properties of the black hole.


Credit: EHT Collaboration, [CC BY 4.0]



In our latest work, we follow up on the recent study of the Event Horizon Telescope Collaboration that combines the observed shadow size and independent measurements of black hole properties to put Einstein’s theory to a test. We quantify and loosen some of the limiting assumptions of this study to provide a more comprehensive and careful analysis of which aspects of general relativity can be tested, and how well. We analyzed the extension of general relativity focusing on multiple aspects in contrast to only one varied at time. A confrontation between these two cases confirms general relativity as a prevailing theory of gravity but also points out that the shadow-size alone is not a very stringent probe because other types of black holes in alternative theories of gravity are also in agreement with the given observations. We conclude that additional tests, especially those that are becoming available from gravitational waves, need to be included in the future to provide more stringent bounds.



From Journal publication:

Sebastian H Völkel, Enrico Barausse, Nicola Franchini and Avery E. Broderick, Class. Quantum Grav.38 21LT012021, [ arXiv:2011.06812 ]





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