Can supermassive black hole shadows test the Kerr metric?

TL;DR

This paper discusses the potential of supermassive black hole shadows, like M87*, to test the Kerr metric, highlighting limitations due to matter effects and the suppression of non-GR deviations by existing gravitational wave constraints.

Contribution

It provides a nuanced analysis showing that supermassive black hole shadows mainly test certain extensions of GR with dimensionless couplings or screening mechanisms.

Findings

Black hole shadow radius depends on spacetime and accretion physics.

Gravitational wave limits suppress non-GR deviations in supermassive black holes.

Shadows primarily probe specific modified gravity theories with screening or scalarisation.

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.