Covariant coarse-graining of inhomogeneous dust flow in General Relativity

TL;DR

This paper introduces a covariant, quasi-local method for defining coarse-grained kinematic quantities of dust flow in General Relativity, which can be applied to finite domains and may inform cosmological backreaction studies.

Contribution

It presents a novel covariant coarse-graining approach based on isometric embedding, generalizing volume averages to curved spacetime in a coordinate-independent way.

Findings

Reproduces local quantities in the infinitesimal limit.

Derives evolution equations similar to local flow equations.

Applied to exact solutions, yields reasonable results.

Abstract

A new definition of coarse-grained quantities describing the dust flow in General Relativity is proposed. It assigns the coarse--grained expansion, shear and vorticity to finite-size comoving domains of fluid in a covariant, coordinate-independent manner. The coarse--grained quantities are all quasi-local functionals, depending only on the geometry of the boundary of the considered domain. They can be thought of as relativistic generalizations of simple volume averages of local quantities in a flat space. The procedure is based on the isometric embedding theorem for S^2 surfaces and thus requires the boundary of the domain in question to have spherical topology and positive scalar curvature. We prove that in the limit of infinitesimally small volume the proposed quantities reproduce the local expansion, shear and vorticity. In case of irrotational flow we derive the time evolution for the coarse-grained quantities and show that its structure is very similar to the evolution equation for their local counterparts. Additional terms appearing in it may serve as a measure of the backreacton of small-scale inhomogeneities of the flow on the large-scale motion of the fluid inside the domain and therefore the result may be interesting in the context of the cosmological backreaction problem. We also consider the application of the proposed coarse-graining procedure to a number of known exact solutions of Einstein equations with dust and show that it yields reasonable results.