Unconventional cosmology on the (thick) brane

TL;DR

This paper explores the cosmology of a thick brane with matter that includes pressure along the extra dimension, leading to non-standard acceleration effects and an effective dark energy component in a 5D framework.

Contribution

It introduces a new model of thick brane cosmology with pressure along the extra dimension, resulting in non-standard acceleration and an effective scalar field description.

Findings

Non-standard cosmological evolution with acceleration from dust matter.

Effective 4D energy-momentum includes a dark energy component.

Conservation laws hold in 5D, not in naive 4D interpretation.

Abstract

We consider the cosmology of a thick codimension 1 brane. We obtain the matching conditions leading to the cosmological evolution equations and show that when one includes matter with a pressure component along the extra dimension in the brane energy-momentum tensor, the cosmology is of non-standard type. In particular one can get acceleration when a dust of non-relativistic matter particles is the only source for the (modified) Friedman equation. Our equations would seem to violate the conservation of energy-momentum from a 4D perspective, but in 5D the energy-momentum is conserved. One could write down an effective conserved 4D energy-momentum tensor attaching a ``dark energy'' component to the energy-momentum tensor of matter that has pressure along the extra dimension. This extra component could, on a cosmological scale, be interpreted as matter-coupled quintessence. We comment on the effective 4D description of this effect in terms of the time evolution of a scalar field (the 5D radion) coupled to this kind of matter.