A Solution to the Graceful Exit Problem in Pre-Big Bang Cosmology

TL;DR

This paper presents a method to find regular pre-big bang solutions in string cosmology that address the graceful exit problem, requiring an exotic equation of state to avoid fine-tuning.

Contribution

It introduces a generic algorithm for solutions with desired properties and discusses the necessity of an exotic equation of state for consistent dilaton evolution.

Findings

Solutions are regular at t=0, solving the graceful exit problem.

An exotic equation of state is necessary to avoid fine tuning.

The approach ensures suitable dilaton behavior at late times.

Abstract

We examine the string cosmology equations with a dilaton potential in the context of the Pre-Big Bang Scenario with the desired scale factor duality, and give a generic algorithm for obtaining solutions with appropriate evolutionary properties. This enables us to find pre-big bang type solutions with suitable dilaton behaviour that are regular at $t=0$, thereby solving the graceful exit problem. However to avoid fine tuning of initial data, an `exotic' equation of state is needed that relates the fluid properties to the dilaton field. We discuss why such an equation of state should be required for reliable dilaton behaviour at late times.