Bouncing and cyclic string gas cosmologies

Understand

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.