Testing Einstein’s Theory of General Relativity From the Shadows and Collisions of Black Holes

Artist’s impression of binary black holes about to collide.
Credit: Mark Myers, OzGrav-Swinburne University

General relativity, Einstein’s theory of gravity, is best tested at its most extreme — close to the event horizon of a black hole. This regime is accessible through observations of shadows of supermassive black holes and gravitational waves — ripples in the fabric of our Universe from colliding stellar-mass black holes. For the first time, scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), the Event Horizon Telescope (EHT) and the LIGOScientific Collaboration, have outlined a consistent approach to exploring deviations from Einstein’s general theory of relativity in these two different observations. This research, published in Physical Review D, confirms that Einstein’s theory accurately describes current observations of black holes, from the smallest to the largest.

One of the hallmark predictions from general relativity is the existence of black holes. The theory provides a specific description of a black hole’s effect on the fabric of space-time: a four-dimensional mesh which encodes how objects move through space and time. Known as the Kerr metric, this prediction can be related to the bending of light around a black hole, or the orbital motion of binary black holes. In this study, the deviations from the Kerr metric were linked to features in these black hole observations.

In 2019, the Event Horizon Telescope generated silhouette images of the black hole at the center of the galaxy M87, with a mass several billion times that of our Sun. The angular size of the shadow is related to the mass of the black hole, its distance from Earth, and possible deviations from general relativity’s prediction. These deviations can be calculated from the scientific data, including previous measurements of the black hole’s mass and distance.

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