An exact solution for a rotating black hole in modified gravity

TL;DR

This paper presents an exact analytic rotating black hole solution in a class of vector-tensor modified gravity theories, revealing significant deviations from Kerr-Newman black holes in GR, with implications for astrophysical observations.

Contribution

It introduces a new exact rotating black hole solution in vector-tensor theories of modified gravity, valid for any rotation parameter, with distinctive horizon and orbit properties.

Findings

Black holes can have larger ergospheres than in GR.

Stable orbits are located further from the horizon compared to GR.

The solution exhibits large deviations from Kerr-Newman geometry.

Abstract

Exact solutions describing rotating black holes can offer important tests for alternative theories of gravity, motivated by the dark energy and dark matter problems. We present an analytic rotating black hole solution for a class of vector-tensor theories of modified gravity, valid for arbitrary values of the rotation parameter. The new configuration is characterised by parametrically large deviations from the Kerr-Newman geometry, controlled by non-minimal couplings between vectors and gravity. It has an oblate horizon in Boyer-Lindquist coordinates, and it can rotate more rapidly and have a larger ergosphere than black holes in General Relativity (GR) with the same asymptotic properties. We analytically investigate the features of the innermost stable circular orbits for massive objects on the equatorial plane, and show that stable orbits lie further away from the black hole horizon with respect to rotating black holes in GR. We also comment on possible applications of our findings for the extraction of rotational energy from the black hole.