Second order gauge invariant gravitational perturbations of a Kerr black hole

TL;DR

This paper develops a gauge-invariant waveform for second order gravitational perturbations of Kerr black holes, enabling more accurate modeling of gravitational radiation beyond first order.

Contribution

It constructs a second order gauge-invariant waveform and derives a single wave equation for it, extending perturbation theory to higher orders in Kerr black hole analysis.

Findings

Existence of a second order gauge-invariant waveform $oldsymbol{psi_I}$.

Derivation of a single wave equation with a source term for $oldsymbol{psi_I}$.

Framework for initial data, energy, and wave extraction in second order perturbations.

Abstract

We investigate higher than the first order gravitational perturbations in the Newman-Penrose formalism. Equations for the Weyl scalar $\psi_4,$ representing outgoing gravitational radiation, can be uncoupled into a single wave equation to any perturbative order. For second order perturbations about a Kerr black hole, we prove the existence of a first and second order gauge (coordinates) and tetrad invariant waveform, $\psi_I$, by explicit construction. This waveform is formed by the second order piece of $\psi_4$ plus a term, quadratic in first order perturbations, chosen to make $\psi_I$ totally invariant and to have the appropriate behavior in an asymptotically flat gauge. $\psi_I$ fulfills a single wave equation of the form ${\cal T}\psi_I=S,$ where ${\cal T}$ is the same wave operator as for first order perturbations and $S$ is a source term build up out of (known to this level) first order perturbations. We discuss the issues of imposition of initial data to this equation, computation of the energy and momentum radiated and wave extraction for direct comparison with full numerical approaches to solve Einstein equations.