Stationary untrapped boundary conditions in general relativity

TL;DR

This paper introduces boundary conditions in general relativity that allow defining conserved quantities for quasi-local regions by fixing the area element and angular momentum surface density on untrapped or marginal surfaces.

Contribution

It proposes a new class of boundary conditions for canonical general relativity that generalize the Hawking energy and relate to stationary untrapped boundaries.

Findings

Defines functionally differentiable Hamiltonian with new boundary conditions.

Generalizes Hawking energy using a dual expansion vector.

Reduces to known horizon boundary conditions in special cases.

Abstract

A class of boundary conditions for canonical general relativity are proposed and studied at the quasi-local level. It is shown that for untrapped or marginal surfaces, fixing the area element on the 2-surface (rather than the induced 2-metric) and the angular momentum surface density is enough to have a functionally differentiable Hamiltonian, thus providing definition of conserved quantities for the quasi-local regions. If on the boundary the evolution vector normal to the 2-surface is chosen to be proportional to the dual expansion vector, we obtain a generalization of the Hawking energy associated with a generalized Kodama vector. This vector plays the role for the stationary untrapped boundary conditions which the stationary Killing vector plays for stationary black holes. When the dual expansion vector is null, the boundary conditions reduce to the ones given by the non-expanding horizons and the null trapping horizons.