Bouncing and cyclic string gas cosmologies

TL;DR

This paper demonstrates that string gas cosmologies with higher-derivative gravity modifications can produce bouncing and cyclic universes, addressing singularities and the horizon problem, and exploring implications for dimension decompactification.

Contribution

It introduces a class of bouncing and cyclic cosmological models based on string gas and higher-derivative gravity, providing new mechanisms for avoiding singularities and solving cosmological problems.

Findings

Higher-derivative modifications bound the expansion rate.

Models exhibit long loitering phases solving the horizon problem.

Entropy production drives transition to radiation-dominated universe.

Abstract

We show that, in the presence of a string gas, simple higher-derivative modifications to the effective action for gravity can lead to bouncing and cyclic cosmological models. The modifications bound the expansion rate and avoid singularities at finite times. In these models the scale factors can have long loitering phases that solve the horizon problem. Adding a potential for the dilaton gives a simple realization of the pre-big bang scenario. Entropy production in the cyclic phase drives an eventual transition to a radiation-dominated universe. As a test of the Brandenberger-Vafa scenario, we comment on the probability of decompactifying three spatial dimensions in this class of models.