The Evolution of an Inhomogeneous Universe

TL;DR

This paper refines a method to analyze inhomogeneous universe dynamics, showing that acceleration requires a positive cosmological constant, with implications for precision cosmology.

Contribution

It introduces a non-perturbative numerical approach to inhomogeneous universe evolution, clarifying conditions for cosmic acceleration and the role of the cosmological constant.

Findings

No acceleration in Einstein-de Sitter universe with small perturbations

Acceleration can occur with positive curvature but not with realistic Hubble rates

A positive cosmological constant is necessary for observed acceleration

Abstract

A refined version of a recently introduced method for analysing the dynamics of an inhomogeneous irrotational dust universe is presented. A fully non-perturbative numerical computation of the time dependence of volume in this framework leads to the following results. If the initial state of the universe is Einstein-de Sitter with small Gaussian perturbations, then there is no acceleration even though the inhomogeneities strongly affect the evolution. A universe with a positive background curvature can exhibit acceleration, but not in conjunction with reasonable values for the Hubble rate. Thus the correct values for both quantities can be achieved only by introducing a positive cosmological constant. Possible loopholes to this conclusion are discussed; in particular, acceleration as an illusion created by peculiarities of light propagation in an inhomogeneous universe is still possible. Independently of the cosmological constant question, the present formalism should provide an important tool for precision cosmology.