Singularities on the brane aren't isotropic

TL;DR

This paper demonstrates that, contrary to previous suggestions, the initial singularity in brane world cosmology is not necessarily isotropic, highlighting the importance of decaying modes and instability in the early universe.

Contribution

The study shows that the isotropic singularity in brane cosmology is not a generic feature, using a perturbative approach to reveal the instability caused by decaying modes.

Findings

Decaying modes indicate instability of the isotropic singularity.

Brane universe does not necessarily start with isotropy.

Large-scale isotropy remains unexplained in this model.

Abstract

Recent studies of homogeneous anisotropic universe models in the brane world scenario show that the cosmological singularity in this context is isotropic. It has therefore been suggested that this may be a generic feature of singularities on the brane, even in the inhomogeneous case. Using a perturbative approach, we show that this is not the case. As in the GR case, the presence of decaying modes in the perturbations signal the instability (in the past) of the isotropic singularity. The brane universe is therefore not born with isotropy built in: as in standard cosmology, the observed large-scale isotropy and homogeneity remains to be explained.