Linear Einstein equations and Kerr-Schild maps

TL;DR

This paper proves that certain Kerr-Schild metrics constructed from solutions of the linearized Einstein equations are exact solutions of the full Einstein equations, generalizing previous vacuum and flat spacetime results.

Contribution

It establishes that Kerr-Schild metrics with autoparallel null congruences can be generated from linearized solutions, extending known theorems to more general matter fields.

Findings

Kerr-Schild metrics with autoparallel null vectors are exact solutions.

The linearized Einstein equations are sufficient to generate full solutions.

Generalization of previous vacuum and flat spacetime theorems.

Abstract

We prove that given a solution of the Einstein equations $g_{ab}$ for the matter field $T_{ab}$, an autoparallel null vector field $l^{a}$ and a solution $(l_{a}l_{c}, \mathcal{T}_{ac})$ of the linearized Einstein equation on the given background, the Kerr-Schild metric $g_{ac}+\lambda l_{a}l_{c}$ ($\lambda $ arbitrary constant) is an exact solution of the Einstein equation for the energy-momentum tensor $T_{ac}+\lambda \mathcal{T}_{ac}+\lambda ^{2}l_{(a}\mathcal{T}_{c)b}l^{b}$. The mixed form of the Einstein equation for Kerr-Schild metrics with autoparallel null congruence is also linear. Some more technical conditions hold when the null congruence is not autoparallel. These results generalize previous theorems for vacuum due to Xanthopoulos and for flat seed space-time due to G\"{u}rses and G\"{u}rsey.