On average properties of inhomogeneous fluids in general relativity II: perfect fluid cosmologies

TL;DR

This paper derives generalized averaged equations for inhomogeneous perfect fluid cosmologies in general relativity, revealing how inhomogeneities influence cosmic evolution and the effective dynamics beyond standard models.

Contribution

It introduces a generalized averaging framework for inhomogeneous cosmologies, including backreaction effects, and explores their implications for various cosmological scenarios.

Findings

Averaged scalar curvature changes during structure formation.

Inhomogeneous radiation cosmologies deviate from standard models.

Backreaction terms can mimic scalar field sources.

Abstract

For general relativistic spacetimes filled with an irrotational perfect fluid a generalized form of Friedmann's equations governing the expansion factor of spatially averaged portions of inhomogeneous cosmologies is derived. The averaging problem for scalar quantities is condensed into the problem of finding an `effective equation of state' including kinematical as well as dynamical `backreaction' terms that measure the departure from a standard FLRW cosmology. Applications of the averaged models are outlined including radiation-dominated and scalar field cosmologies (inflationary and dilaton/string cosmologies). In particular, the averaged equations show that the averaged scalar curvature must generically change in the course of structure formation, that an averaged inhomogeneous radiation cosmos does not follow the evolution of the standard homogeneous-isotropic model, and that an averaged inhomogeneous perfect fluid features kinematical `backreaction' terms that, in some cases, act like a free scalar field source. The free scalar field (dilaton) itself, modelled by a `stiff' fluid, is singled out as a special inhomogeneous case where the averaged equations assume a simple form.