Linear Stability of Black Holes and Naked Singularities

TL;DR

This paper reviews the linear stability of black holes and naked singularities, demonstrating instability of certain solutions and establishing conditions under which black holes remain stable and settle into Kerr solutions.

Contribution

It introduces a nonmodal stability concept and proves the stability of Schwarzschild black holes with non-negative cosmological constant.

Findings

Naked singularities in Kerr--Newman are unstable.

Inner black hole regions beyond Cauchy horizons are unstable.

Schwarzschild black holes with non-negative cosmological constant are stable and settle into Kerr black holes.

Abstract

These notes follow from a course delivered at the V Jos\'e Pl\'{\i}nio Baptista School of Cosmology, held at Guarapari (Esp\'{\i}rito Santo) Brazil, from 30 September to 5 October 2021. A review of the current status of the linear stability of black holes and naked singularities is given. The standard modal approach, that takes advantage of the background symmetries and analyze separately the harmonic components of linear perturbations, is briefly introduced and used to prove that the naked singularities in the Kerr--Newman family, as well as the inner black hole regions beyond Cauchy horizons, are unstable and therefore unphysical. The proofs require a treatment of the boundary condition at the timelike boundary, which is given in detail. The nonmodal linear stability concept is then introduced, and used to prove that the domain of outer communications of a Schwarzschild black hole with a non-negative cosmological constant satisfies this stronger stability condition, which rules out transient growths of perturbations, and also to show that the perturbed black hole settles into a slowly rotating Kerr black hole. The encoding of the perturbation fields in gauge invariant curvature scalars and the effects of the perturbation on the geometry of the spacetime is discussed.