The initial value problem for linearized gravitational perturbations of the Schwarzschild naked singularity

TL;DR

This paper develops a new gauge-invariant approach to analyze linearized gravitational perturbations of the Schwarzschild naked singularity, proving its linear instability through analytical and numerical methods.

Contribution

It introduces a novel gauge-invariant function to handle perturbations in non-globally hyperbolic spacetime, enabling the proof of instability for the Schwarzschild naked singularity.

Findings

Established the linear instability of the Schwarzschild naked singularity.

Developed a new regular gauge-invariant function for perturbation evolution.

Numerically demonstrated the excitation of unstable modes by initial data.

Abstract

The coupled equations for the scalar modes of the linearized Einstein equations around Schwarzschild's spacetime were reduced by Zerilli to a 1+1 wave equation with a potential $V$, on a field $\Psi_z$. For smooth metric perturbations $\Psi_z$ is singular at $r_s=-6M/(\ell-1)(\ell+2)$, $\ell$ the mode harmonic number, and $V$ has a second order pole at $r_s$. This is irrelevant to the black hole exterior stability problem, where $r>2M>0$, and $r_s <0$, but it introduces a non trivial problem in the naked singular case where $M<0$, then $r_s>0$, and the singularity appears in the relevant range of $r$. We solve this problem by developing a new approach to the evolution of the even mode, based on a {\em new gauge invariant function}, $\hat \Psi$ -related to $\Psi_z$ by an intertwiner operator- that is a regular function of the metric perturbation {\em for any value of $M$}. This allows to address the issue of evolution of gravitational perturbations in this non globally hyperbolic background, and to complete the proof of the linear instability of the Schwarzschild naked singularity, by showing that a previously found unstable mode is excitable by generic initial data. This is further illustrated by numerically solving the linearized equations for suitably chosen initial data.