Gravitational instabilities of superspinars

TL;DR

This paper demonstrates that superspinars, ultracompact objects with spin exceeding the Kerr limit, are dynamically unstable under gravitational perturbations, making them unlikely astrophysical black hole alternatives.

Contribution

It provides the first analysis of the gravitational stability of superspinars, showing their inherent instability regardless of boundary conditions.

Findings

Superspinars are unstable to gravitational perturbations.

Instability is strongest at lower spin values.

Superspinars are unlikely to be astrophysical black hole alternatives.

Abstract

Superspinars are ultracompact objects whose mass M and angular momentum J violate the Kerr bound (cJ/GM^2>1). Recent studies analyzed the observable consequences of gravitational lensing and accretion around superspinars in astrophysical scenarios. In this paper we investigate the dynamical stability of superspinars to gravitational perturbations, considering either purely reflecting or perfectly absorbing boundary conditions at the "surface" of the superspinar. We find that these objects are unstable independently of the boundary conditions, and that the instability is strongest for relatively small values of the spin. Also, we give a physical interpretation of the various instabilities that we find. Our results (together with the well-known fact that accretion tends to spin superspinars down) imply that superspinars are very unlikely astrophysical alternatives to black holes.