Entropy Bounds and String Cosmology

TL;DR

This paper discusses entropy bounds in cosmology, advocates for a Hubble entropy bound, and explores its implications for string cosmology, especially during pre-big bang inflation and the exit problem.

Contribution

It introduces and supports a Hubble entropy bound in cosmology and applies it to string cosmology, highlighting its significance during inflation and the exit problem.

Findings

Hubble entropy bound is naturally saturated during pre-big bang inflation

Fulfilling the bound at later times impacts the string cosmology exit problem

Supports the relevance of entropy bounds in understanding cosmological evolution

Abstract

After discussing some old (and not-so-old) entropy bounds both for isolated systems and in cosmology, I will argue in favour of a "Hubble entropy bound" holding in the latter context. I will then apply this bound to recent developments in string cosmology, show that it is naturally saturated throughout pre-big bang inflation, and claim that its fulfilment at later times has interesting implications for the exit problem of string cosmology.