Conservation laws and flux bounds for gravitational perturbations of the Schwarzschild metric

TL;DR

This paper establishes an energy conservation law for gravitational perturbations of the Schwarzschild spacetime, providing bounds on energy fluxes and contributing to the understanding of linear stability of black holes.

Contribution

It derives a new energy conservation law expressed with first derivatives, demonstrating flux positivity and bounding energy radiated, advancing the analysis of Schwarzschild stability.

Findings

Positivity of flux on null hypersurfaces

Bounded total energy flux from initial data

Contribution to linear stability proof

Abstract

We derive an energy conservation law for the system of gravitational perturbations on the Schwarzschild spacetime expressed in a double null gauge. The resulting identity involves only first derivatives of the metric perturbation. Exploiting the gauge invariance up to boundary terms of the fluxes that appear, we are able to establish positivity of the flux on any outgoing null hypersurface to the future of the initial data. This allows us to bound the total energy flux through any such hypersurface, including the event horizon, in terms of initial data. We similarly bound the total energy radiated to null infinity. Our estimates provide a direct approach to a weak form of stability, thereby complementing the proof of the full linear stability of the Schwarzschild solution recently obtained in [M. Dafermos, G. Holzegel and I. Rodnianski \emph{The linear stability of the Schwarzschild solution to gravitational perturbations}, arXiv:1601.06467].