Density perturbations in a brane-world universe with dark radiation

TL;DR

This paper studies how dark radiation in a brane-world universe influences cosmological density perturbations, revealing subtle effects during the radiation era and significant suppression in a stiff era.

Contribution

It provides a detailed analysis of dark radiation's impact on density perturbations within a brane-world cosmology, highlighting new qualitative behaviors.

Findings

Dark radiation slightly suppresses radiation perturbations on large scales.

Dark radiation boosts dark radiation perturbations.

In a stiff era, density perturbations are strongly suppressed to zero.

Abstract

We investigate the effects on cosmological density perturbations of dark radiation in a Randall-Sundrum 2 type brane-world. Dark radiation in the background is limited by observational constraints to be a small fraction of the radiation energy density, but it has an interesting qualitative effect in the radiation era. On large scales, it serves to slightly suppress the radiation density perturbations at late times, while boosting the perturbations in dark radiation. In a kinetic (stiff) era, the suppression is much stronger, and drives the density perturbations to zero.