The Universe as a Cosmic String

TL;DR

This paper explores six-dimensional brane cosmology, revealing two distinct classes of solutions—degravitating and super-accelerating—with implications for the universe's expansion and stability.

Contribution

It introduces a detailed analysis of brane-induced gravity in six dimensions, highlighting the role of Einstein-Rosen waves and classifying cosmological solutions.

Findings

Degravitating solutions have zero Hubble parameter despite energy density.

Super-accelerating solutions exhibit unbounded Hubble growth.

Degravitating regime is not phenomenologically viable, while super-accelerating solutions are unstable.

Abstract

The cosmology of brane induced gravity in six infinite dimensions is investigated. It is shown that a brane with Friedmann-Robertson-Walker symmetries necessarily acts as a source of cylindrically symmetric gravitational waves, so called Einstein-Rosen waves. Their existence essentially distinguishes this model from its codimension-one counterpart and necessitates solving the nonlinear system of bulk and brane-matching equations. A numerical analysis is performed and two qualitatively different and dynamically separated classes of cosmologies are derived: degravitating solutions for which the Hubble parameter settles to zero despite the presence of a non-vanishing energy density on the brane and super-accelerating solutions for which Hubble grows unbounded. The parameter space of both the stable and unstable regime is derived and observational consequences are discussed: It is argued that the degravitating regime does not allow for a phenomenologically viable cosmology. On the other hand, the super-accelerating solutions are potentially viable, however, their unstable behavior questions their physical relevance.