Non singular bounce in modified gravity

TL;DR

This paper demonstrates that in a modified gravity model, a non-singular bounce can occur in a flat universe without spatial curvature, allowing for flexible bounce times and connecting to standard cosmological epochs.

Contribution

It shows that non-singular bounces are possible in flat universes within a modified gravity framework, unlike in general relativity, and discusses implications for bounce times.

Findings

Bounce occurs in flat universe without spatial curvature.

Bounce can be arbitrarily short or long in duration.

Model connects contraction to standard cosmological epochs.

Abstract

We investigate bouncing solutions in the framework of the non-singular gravity model of Brandenberger, Mukhanov and Sornborger. We show that a spatially flat universe filled with ordinary matter undergoing a phase of contraction reaches a stage of minimal expansion factor before bouncing in a regular way to reach the expanding phase. The expansion can be connected to the usual radiation- and matter-dominated epochs before reaching a final expanding de Sitter phase. In general relativity (GR), a bounce can only take place provided that the spatial sections are positively curved, a fact that has been shown to translate into a constraint on the characteristic duration of the bounce. In our model, on the other hand, a bounce can occur also in the absence of spatial curvature, which means that the timescale for the bounce can be made arbitrarily short or long. The implication is that constraints on the bounce characteristic time obtained in GR rely heavily on the assumed theory of gravity. Although the model we investigate is fourth order in the derivatives of the metric (and therefore unstable vis-a-vis the perturbations), this generic bounce dynamics should extend to string-motivated non singular models which can accommodate a spatially flat bounce.