Max Planck Institute for Radio Astronomy | 2018 Apr 16
Astrophysicists at Frankfurt, the Max Planck Institute for Radio Astronomy in Bonn, and Nijmegen, collaborating in the project BlackHoleCam, answer this question by computing the first images of feeding non-Einsteinian black holes: it is presently hard to tell them apart from standard black holes.
One of the most fundamental predictions of Einstein's theory of relativity is the existence of black holes. In spite of the recent detection of gravitational waves from binary black holes by LIGO, direct evidence using electromagnetic waves remains elusive and astronomers are looking for it with radio telescopes. For the first time, collaborators in the ERC funded project BlackHoleCam, including astrophysicists at Goethe University Frankfurt, Max Planck Institute for Radio Astronomy (MPIfR) Bonn, and Radboud University Nijmegen, have compared self-consistent and realistic images of the shadow of an accreting supermassive black hole – such as the black-hole candidate Sagittarius A* (Sgr A*) in the heart of our Galaxy – both in general relativity and in a different theory of gravity. The goal was to test if Einsteinian black holes can be distinguished from those in alternative theories of gravity.
Not all of the light rays (or photons) produced by matter falling onto a black hole are trapped by the event horizon, a region of spacetime from which nothing can escape. Some of these photons will reach distant observers, so that when a black hole is observed directly, a “shadow” is expected against the background sky. The size and shape of this shadow will depend on the black hole’s properties and on the theory of gravity.
Because the largest deviations from Einstein’s theory of relativity are expected very close to the event horizon, and since alternative theories of gravity make different predictions on the properties of shadow, direct observations of Sgr A* represent a very promising approach for testing gravity in the strongest regime. Making such images of the black-hole shadow is the prime goal of the international Event Horizon Telescope Collaboration (EHTC), which combines radio data from telescopes around the world. ...
The Current Ability to Test Theories of Gravity with Black Hole Shadows - Yosuke Mizuno et al
- Nature Astronomy (online 16 Apr 2018) DOI: 10.1038/s41550-018-0449-5
arXiv.org > astro-ph > arXiv:1804.05812 > 16 Apr 2018