A Boundary Term for the Gravitational Action with Null Boundaries

TL;DR

This paper proposes a new boundary counter-term for null boundaries in general relativity to ensure a well-posed variational principle, extending the known Gibbons-Hawking-York term for spacelike and timelike boundaries.

Contribution

It introduces a specific boundary integral as a counter-term for null boundaries, addressing a gap in the formulation of variational principles in general relativity.

Findings

Proposes the boundary integral of 2√(-g)(Θ+κ) as a counter-term for null boundaries.

Analyzes metric variations on null boundaries and highlights the need for further investigation.

Provides a foundation for better understanding boundary conditions in gravitational theories.

Abstract

Constructing a well-posed variational principle is a non-trivial issue in general relativity. For spacelike and timelike boundaries, one knows that the addition of the Gibbons-Hawking-York (GHY) counter-term will make the variational principle well-defined. This result, however, does not directly generalize to null boundaries on which the 3-metric becomes degenerate. In this work, we address the following question: What is the counter-term that may be added on a null boundary to make the variational principle well-defined? We propose the boundary integral of $2 \sqrt{-g} \left( \Theta+\kappa \right)$ as an appropriate counter-term for a null boundary. We also conduct a preliminary analysis of the variations of the metric on the null boundary and conclude that isolating the degrees of freedom that may be fixed for a well-posed variational principle requires a deeper investigation.