Are braneworlds born isotropic?

TL;DR

This paper demonstrates that brane cosmologies naturally evolve towards isotropy in the early universe, providing a potential explanation for the observed large-scale homogeneity and isotropy.

Contribution

It shows that the isotropic brane universe model is asymptotically stable in the past for certain matter conditions, supporting the idea that brane worlds are born isotropic.

Findings

Anisotropic and inhomogeneous perturbations decay as t→0 for γ > 1.

The isotropic model ${\

The model ${\

Abstract

It has recently been suggested that an isotropic singularity may be a generic feature of brane cosmologies, even in the inhomogeneous case. Using the covariant and gauge-invariant approach we present a detailed analysis of linear perturbations of the isotropic model ${\cal F}_b$ which is a past attractor in the phase space of homogeneous Bianchi models on the brane. We find that for matter with an equation of state parameter $\gamma > 1$, the dimensionless variables representing generic anisotropic and inhomogeneous perturbations decay as $t\to 0$, showing that the model ${\cal F}_b$ is asymptotically stable in the past. We conclude that brane universes are born with isotropy naturally built-in, contrary to standard cosmology. The observed large-scale homogeneity and isotropy of the universe can therefore be explained as a consequence of the initial conditions if the brane-world paradigm represents a description of the very early universe.