Kerr Black Holes are Not Unique to General Relativity

TL;DR

This paper demonstrates that black hole solutions in extended gravity theories are nearly identical to those in general relativity, implying that observing Kerr black holes alone cannot confirm Einstein's theory over alternatives.

Contribution

It shows that black hole solutions in extended gravity theories are indistinguishable from those in general relativity, challenging the use of Kerr metrics as definitive tests.

Findings

Black hole solutions in extended theories mimic those in GR

Observations of Kerr black holes cannot confirm Einstein's theory

Deviations from Kerr would indicate new physics

Abstract

Considerable attention has recently focused on gravity theories obtained by extending general relativity with additional scalar, vector, or tensor degrees of freedom. In this paper, we show that the black-hole solutions of these theories are essentially indistinguishable from those of general relativity. Thus, we conclude that a potential observational verification of the Kerr metric around an astrophysical black hole cannot, in and of itself, be used to distinguish between these theories. On the other hand, it remains true that detection of deviations from the Kerr metric will signify the need for a major change in our understanding of gravitational physics.