Cosmology on a cosmic ring

TL;DR

This paper explores a generalized DGP cosmological model with an extra compact dimension, revealing how gravitational wave emission affects late-time cosmology and demonstrating a dynamical degravitation mechanism with observational constraints.

Contribution

It introduces a new DGP extension with a compact dimension, analyzing gravitational wave effects and showing how stabilization impacts cosmological behavior.

Findings

Stabilized compact dimension reproduces DGP cosmology.

Unstabilized model leads to no 4D regime and is observationally ruled out.

Exact solutions demonstrate dynamical degravitation with a cosmological constant.

Abstract

We derive the modified Friedmann equations for a generalization of the Dvali-Gabadadze-Porrati (DGP) model in which the brane has one additional compact dimension. The main new feature is the emission of gravitational waves into the bulk. We study two classes of solutions: First, if the compact dimension is stabilized, the waves vanish and one exactly recovers DGP cosmology. However, a stabilization by means of physical matter is not possible for a tension-dominated brane, thus implying a late time modification of 4D cosmology different from DGP. Second, for a freely expanding compact direction, we find exact attractor solutions with zero 4D Hubble parameter despite the presence of a 4D cosmological constant. The model hence constitutes an explicit example of dynamical degravitation at the full non-linear level. Without stabilization, however, there is no 4D regime and the model is ruled out observationally, as we demonstrate explicitly by comparing to supernova data.