Aspects of String-Gas Cosmology at Finite Temperature

TL;DR

This paper investigates string-gas cosmology within dilaton gravity, analyzing thermodynamics and evolution in different regimes, revealing slow evolution in the Hagedorn phase and stable expansion of large dimensions.

Contribution

It introduces a detailed thermodynamical analysis of string-gas cosmology with numerical solutions in two regimes, highlighting novel cosmological behaviors.

Findings

Slow scale factor evolution in the Hagedorn regime

Large dimensions expand while small ones remain compact

Thermodynamical equilibrium assumptions enable numerical modeling

Abstract

We study string-gas cosmology in dilaton gravity, inspired by the fact that it naturally arises in a string theory context. Our main interest is the thermodynamical treatment of the string-gas and the resulting implications for the cosmology. Within an adiabatic approximation, thermodynamical equilibrium and a small, toroidal universe as initial conditions, we numerically solve the corresponding equations of motions in two different regimes describing the string-gas thermodynamics: (i) the Hagedorn regime, with a single scale factor, and (ii) an almost-radiation dominated regime, which includes the leading corrections due to the lightest Kaluza Klein and winding modes, with two scale factors. The scale factor in the Hagedorn regime exhibits very slow time evolution with nearly constant energy and negligible pressure. By contrast, in case (ii) we find interesting cosmological solutions where the large dimensions continue to expand and the small ones are kept undetectably small.