Axion induced angular momentum reversal in Kerr-like black holes

TL;DR

This paper investigates how a pseudoscalar axion-like field coupled to a Chern-Simons term can cause a Kerr-like black hole to reverse its angular momentum direction due to energy exchange, revealing potential observable signatures.

Contribution

It introduces a model where axion-Chern-Simons coupling leads to black hole angular momentum reversal, a novel effect not previously documented.

Findings

Black hole angular momentum decreases with increasing axion coupling.

Beyond a critical coupling, the black hole reverses its rotation direction.

Energy exchange between axionic matter and gravitational anomaly drives the reversal.

Abstract

We consider a pseudoscalar axion-like field coupled to a Chern-Simons gravitational anomaly term. The axion field backreacts on a rotating Kerr black hole background, resulting in modifications in the spacetime. In an attempt to determine potentially observable signatures, we study the angular momentum of the system of the modified Kerr-like black hole and the axionic matter outside the horizon of the black hole. As the strength of the coupling of the axion field to the Chern-Simons term is increasing, the requirement that the total angular momentum of the system remain constant forces the black hole angular momentum to decrease. There exists a critical value of this coupling beyond which the black hole starts to rotate in the opposite direction, with an increasing magnitude of its angular momentum. We interpret this effect as a consequence of the exchange of energy between the axionic matter and the gravitational anomaly, which is sourced by the rotating black hole.