Can supermassive black hole shadows test the Kerr metric?

Abstract

The unprecedented image of the M87* supermassive black hole has sparked some controversy over its usefulness as a test of the general relativistic Kerr metric. The criticism is mainly related to the black hole's quasi-circular shadow and advocates that its radius depends not only on the black hole's true spacetime properties but also on the poorly known physics of the illuminating accretion flow. In this paper we take a sober view of the problem and argue that our ability to probe gravity with a black hole shadow is only partially impaired by the matter degrees of freedom and the number of non-Kerr parameters used in the model. As we show here, a more intriguing situation arises from the mass scaling of the dimensional coupling constants that typically appear in non-GR theories of gravity. Existing limits from gravitational wave observations imply that supermassive systems like the M87* black hole would suffer a suppression of all non-GR deviation parameters in their metric, making the spacetime and the produced shadow virtually Kerr. Therefore, a supermassive black hole shadow is likely to probe only those extensions of General Relativity which are endowed with dimensionless coupling constants or other special cases with a screening mechanism for black holes or certain types of spontaneous scalarisation.

5 pages, 1 figure, accepted for publication as a Letter in Physical Review D

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• Probing the Lorentz Symmetry Violation Using the First Image of Sagittarius A*: Constraints on Standard-Model Extension Coefficients
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• Thermodynamics and Shadows of GUP-corrected Black Holes with Topological Defects in Bumblebee Gravity
• Kerr black holes with synchronised Proca hair: lensing, shadows and EHT constraints
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• Black hole photon ring beyond General Relativity: an integrable parametrization
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• Fractal signatures of non-Kerr spacetimes in the shadow of light-ring bifurcations
• Photon ring autocorrelations from gravitational fluctuations around a black hole
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• Testing beyond-Kerr spacetimes with GWTC-3
• Equatorial orbits and imaging of hairy cubic Galileon black holes
• Shadow of rotating black holes with consistent thermodynamics
• Exploring black hole shadows in axisymmetric spacetimes with coordinate-independent methods and neural networks
• Light ring images of double photon spheres in black hole and wormhole space-times