Linearized Stability of Extreme Black Holes

TL;DR

This paper demonstrates through numerical analysis that extreme black holes are linearly stable against gravitational and scalar perturbations, challenging previous claims of instability based on divergence of certain curvature scalars.

Contribution

It shows that all curvature scalar polynomials approach finite limits at the horizon, indicating stability, and clarifies that divergence of $_4$ is due to tetrad choice.

Findings

Curvature scalars approach finite limits at the horizon.

Divergence of $_4$ is tetrad-dependent.

Extreme Kerr black holes are linearly stable.

Abstract

Extreme black holes have been argued to be unstable, in the sense that under linearized gravitational perturbations of the extreme Kerr spacetime the Weyl scalar $\psi_4$ blows up along their event horizons at very late advanced times. We show numerically, by solving the Teukolsky equation in 2+1D, that all algebraically-independent curvature scalar polynomials approach limits that exist when advanced time along the event horizon approaches infinity. Therefore, the horizons of extreme black holes are stable against linearized gravitational perturbations. We argue that the divergence of $\psi_4$ is a consequence of the choice of a fixed tetrad, and that in a suitable dynamical tetrad all Weyl scalars, including $\psi_4$, approach their background extreme Kerr values. We make similar conclusions also for the case of scalar field perturbations of extreme Kerr.