Killing and homothetic initial data for general hypersurfaces

TL;DR

This paper develops identities relating deformation tensors on hypersurfaces and establishes conditions for the existence of homothetic Killing vectors, generalizing known results and formulating initial data conditions in various hypersurface settings.

Contribution

It introduces general identities for deformation tensors on hypersurfaces and extends homothetic KID equations to null and characteristic initial data scenarios.

Findings

Derived identities relating deformation tensors and hypersurface data.

Established necessary conditions for homothetic Killing vectors on hypersurfaces.

Generalized KID equations to null and characteristic initial data cases.

Abstract

In this paper we present a collection of general identities relating the deformation tensor $\mathcal{K}=\mathcal{L}_{\eta}g$ of an arbitrary vector field $\eta$ with the tensor $\Sigma=\mathcal{L}_{\eta}\nabla$ on an abstract hypersurface $\mathcal{H}$ of any causal character. As an application we establish necessary conditions on $\mathcal{H}$ for the existence of a homothetic Killing vector on the spacetime where $\mathcal{H}$ is embedded. The sufficiency of these conditions is then analysed in three specific settings. For spacelike hypersurfaces, we recover the well-known homothetic KID equations [10, 13] in the language of hypersurface data. For two intersecting null hypersurfaces, we generalize a previous result [7], valid for Killings, to the homothetic case and, moreover, demonstrate that the equations can be formulated solely in terms of the initial data for the characteristic Cauchy problem, i.e., without involving a priori spacetime quantities. This puts the characteristic KID problem on equal footing with the spacelike KID problem. Furthermore, we highlight the versatility of the formalism by addressing the homothetic KID problem for smooth spacelike-characteristic initial data. Other initial value problems, such as the spacelike-characteristic with corners, can be approached similarly.