Isometries and the double copy

Abstract

In the standard derivation of the Kerr-Schild double copy, the geodicity of the Kerr-Schild vector and the stationarity of the spacetime are presented as assumptions that are necessary for the single copy to satisfy Maxwell's equations. However, it is well known that the vacuum Einstein equations imply that the Kerr-Schild vector is geodesic and shear-free, and that the spacetime possesses a distinguished vector field that is simultaneously a Killing vector of the full spacetime and the flat background, but need not be timelike with respect to the background metric. We show that the gauge field obtained by contracting this distinguished Killing vector with the Kerr-Schild graviton solves the vacuum Maxwell equations, and that this definition of the Kerr-Schild double copy implies the Weyl double copy when the spacetime is Petrov type D. When the Killing vector is taken to be timelike with respect to the background metric, we recover the familiar Kerr-Schild double copy, but the prescription is well defined for any vacuum Kerr-Schild spacetime and we present new examples where the Killing vector is null or spacelike. While most examples of physical interest are type D, vacuum Kerr-Schild spacetimes are generically of Petrov type II. We present a straightforward example of such a spacetime and study its double copy structure. Our results apply to real Lorentzian spacetimes as well as complex spacetimes and real spacetimes with Kleinian signature, and provide a simple correspondence between real and self-dual vacuum Kerr-Schild spacetimes. This correspondence allows us to study the double copy structure of a self-dual analog of the Kerr spacetime. We provide evidence that this spacetime may be diffeomorphic to the self-dual Taub-NUT solution.

Typographical errors corrected, results unchanged

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• Asymptotic Weyl double copy in Newman-Penrose formalism
• Gravito-electromagnetism, Kerr-Schild and Weyl double copies; a unified perspective
• Weyl-Lewis-Papapetrou coordinates, self-dual Yang-Mills equations and the single copy
• Weyl double copy in type D spacetime in four and five dimensions
• Background ambiguity and the Gödel double copy
• Five dimensional Weyl double copy
• New framework for classical double copies
• A note on the area of event horizon of Kleinian black hole
• Slowly rotating charges from Weyl double copy for Kerr black hole with Chern-Simons correction
• Untwisting the double copy: the zeroth copy as an optical seed
• Residual Symmetries and Their Algebras in the Kerr-Schild Double Copy
• Newman-Janis Algorithm from Taub-NUT Instantons