Superradiant instabilities of rotating black branes and strings

TL;DR

This paper investigates a universal superradiant instability affecting rotating black branes and strings, revealing conditions for instability and stability across different dimensions, with implications for both theoretical physics and laboratory analogs.

Contribution

It identifies a new superradiant instability mechanism in rotating black branes, analyzing its dependence on dimensionality and providing numerical instability timescales.

Findings

Black brane Kerr_4 x R^p is unstable due to superradiance.

Black branes Kerr_d x R^p are stable for d>4.

Laboratory simulation of the instability with acoustic black branes is feasible.

Abstract

Black branes and strings are generally unstable against a certain sector of gravitational perturbations. This is known as the Gregory-Laflamme instability. It has been recently argued that there exists another general instability affecting many rotating extended black objects. This instability is in a sense universal, in that it is triggered by any massless field, and not just gravitational perturbations. Here we investigate this novel mechanism in detail. For this instability to work, two ingredients are necessary: (i) an ergo-region, which gives rise to superradiant amplification of waves, and (ii) ``bound'' states in the effective potential governing the evolution of the particular mode under study. We show that the black brane Kerr_4 x R^p is unstable against this mechanism, and we present numerical results for instability timescales for this case. On the other hand, and quite surprisingly, black branes of the form Kerr_d x R^p are all stable against this mechanism for d>4. This is quite an unexpected result, and it stems from the fact that there are no stable circular orbits in higher dimensional black hole spacetimes, or in a wave picture, that there are no bound states in the effective potential. We also show that it is quite easy to simulate this instability in the laboratory with acoustic black branes.