Adiabatic Solutions in General Relativity and Boundary Symmetries

TL;DR

This paper develops a method to find approximate solutions to Einstein's equations with boundary conditions by using slow diffeomorphisms and boundary symmetries, revealing the structure of the space of vacua.

Contribution

It introduces a novel application of the Manton approximation to general relativity with boundary, characterizing adiabatic solutions and boundary symmetries.

Findings

Boundary action functional depends on boundary diffeomorphisms.

Unique solutions are identified for given boundary data.

The space of vacua forms a (pseudo)-Riemannian homogeneous space.

Abstract

We investigate adiabatic solutions to general relativity for a spacetime with spatial slices with boundary, by Manton approximation. This approximation tells us for a theory with a Lagrangian in the natural form, a motion that is described as a slow motion on the space of vacua-static solutions that minimize the energy -- is a good approximate solution. To apply this to the case of general relativity we first bring it to the natural form by splitting space and time and choosing Gaussian normal coordinates, where a spacetime is described by the metric on its spatial slices. Then following Manton we propose slow solutions such that each slice is a slowly changing diffeomorphism of a reference slice, and thus each solution is described by a vector field on the spatial slice. These solutions will have the property that the action will become a functional of the vector fields on the boundaries of the spatial slices. Moreover using the Hodge-Morrey-Friedrichs decomposition we will show that the constraints of general relativity will identify a unique solution for a given boundary value. Then we comment on the structure of the space of vacua which we show to be a (pseudo)-Riemannian homogeneous space. We illustrate our procedure for a specific reference slice we choose: the 3d Euclidean round ball.