Superradiant instabilities in astrophysical systems

TL;DR

This paper studies how massive scalar and vector fields evolve around rotating black holes, revealing potential instabilities and unique signals that could inform dark matter research and black hole physics.

Contribution

It provides the first comprehensive analysis of massive vector fields near black holes and uncovers new beat phenomena in scalar and vector field interactions.

Findings

Massive vector fields cause strong instabilities in spinning black holes.

Bound states produce amplitude modulated signals, mistaken for exponential growth.

First exploration of vector fields in generic black hole backgrounds.

Abstract

Light bosonic degrees of freedom have become a serious candidate for dark matter. The evolution of these fields around curved spacetimes is poorly understood but is expected to display interesting effects. In particular, the interaction of light bosonic fields with supermassive black holes, key players in most galaxies, could provide colourful examples of superradiance and nonlinear bosenova-like collapse. In turn, the observation of spinning black holes is expected to impose stringent bounds on the mass of putative massive bosonic fields in our universe. Our purpose here is to present a comprehensive study of the evolution of linearized massive scalar and vector fields in the vicinities of rotating black holes. For a certain boson field mass range, the field can become trapped in a potential barrier outside the horizon and transition to a bound state. Because there are a number of such quasi-bound states, the generic outcome is an amplitude modulated sinusoidal, or beating, signal. We believe that the appearance of such beatings has gone unnoticed in the past, and in fact mistaken for exponential growth. The amplitude modulation of the signal depends strongly on the relative excitation of the overtones, which in turn is strongly tied to the bound-state geography. For the first time we explore massive vector fields in generic BH background which are hard, if not impossible, to separate in the Kerr background. Our results show that spinning BHs are generically strongly unstable against massive vector fields.