Renormalization group analysis of superradiant growth of self-interacting axion cloud

TL;DR

This paper introduces a new renormalization group method to analyze the growth and saturation of superradiant axion clouds around rotating black holes, suggesting potential explosive phenomena like Bosenova.

Contribution

It develops a novel dynamical renormalization group approach to study self-interacting axion clouds, revealing that saturation may not occur in the weakly non-linear regime.

Findings

Superradiant growth does not saturate via scattering in the weakly non-linear regime.

Relativistic axion clouds may undergo explosive Bosenova events.

The method provides insights into the evolution of axion clouds around black holes.

Abstract

There are strong interests in considering ultra-light scalar fields (especially axion) around a rapidly rotating black hole because of the possibility of observing gravitational waves from axion condensate (axion cloud) around black holes. Motivated by this consideration, we propose a new method to study the dynamics of an ultra-light scalar field with self-interaction around a rapidly rotating black hole, which uses the dynamical renormalization group method. We find that for relativistic clouds, saturation of the superradiant instability by the scattering of the axion due to the self-interaction does not occur in the weakly non-linear regime when we consider the adiabatic growth of the cloud from a single superradiant mode. This may suggest that for relativistic axion clouds, an explosive phenomenon called the Bosenova may inevitably happen, at least once in its evolutionary history.