Cosmology of codimension-two braneworlds

TL;DR

This paper investigates 6D braneworld models with azimuthal symmetry, revealing unusual properties of their expansion rate dependence on brane tension and discussing implications for the cosmological constant problem.

Contribution

It provides a comprehensive analysis of cosmological solutions in 6D braneworlds, highlighting their unique tension dependence and challenges for realistic cosmology.

Findings

Expansion rate depends on brane tension in a nonstandard way.

Automatic cancellation of brane tension by curvature occurs.

Models offer insights into the cosmological constant problem.

Abstract

We present a comprehensive study of the cosmological solutions of 6D braneworld models with azimuthal symmetry in the extra dimensions, moduli stabilization by flux or a bulk scalar field, and which contain at least one 3-brane that could be identified with our world. We emphasize an unusual property of these models: their expansion rate depends on the 3-brane tension either not at all, or in a nonstandard way, at odds with the naive expected dimensional reduction of these systems to 4D general relativity at low energies. Unlike other braneworld attempts to find a self-tuning solution to the cosmological constant problem, the apparent failure of decoupling in these models is not associated with the presence of unstabilized moduli; rather it is due to automatic cancellation of the brane tension by the curvature induced by the brane. This provides some corroboration for the hope that these models provide a distinctive step toward understanding the smallness of the observed cosmological constant. However, we point out some challenges for obtaining realistic cosmology within this framework.