Gravitational action with null boundaries

TL;DR

This paper thoroughly analyzes the boundary terms in the gravitational action involving null segments, clarifies ambiguities related to parametrization, and applies the results to compute the action rate for black hole Wheeler-deWitt patches.

Contribution

It provides a complete treatment of null boundary contributions in gravitational actions, resolving ambiguities and establishing rules for consistent action calculations.

Findings

Null boundary ambiguities depend on parametrization choices.

Affine parametrization eliminates null boundary contributions.

Derived rules ensure additivity and consistency of the gravitational action.

Abstract

We present a complete discussion of the boundary term in the action functional of general relativity when the boundary includes null segments in addition to the more usual timelike and spacelike segments. We confirm that ambiguities appear in the contribution from a null segment, because it depends on an arbitrary choice of parametrization for the generators. We also show that similar ambiguities appear in the contribution from a codimension-two surface at which a null segment is joined to another (spacelike, timelike, or null) segment. The parametrization ambiguity can be tamed by insisting that the null generators be affinely parametrized; this forces each null contribution to the boundary action to vanish, but leaves intact the fredom to rescale the affine parameter by a constant factor on each generator. Once a choice of parametrization is made, the ambiguity in the joint contributions can be eliminated by formulating well-motivated rules that ensure the additivity of the gravitational action. Enforcing these rules, we calculate the time rate of change of the action when it is evaluated for a so-called "Wheeler-deWitt patch" of a black hole in asymptotically-anti de Sitter space. We recover a number of results cited in the literature, obtained with a less complete analysis.