Aspects of String Cosmology

TL;DR

This paper reviews advances in string cosmology, highlighting models that achieve non-singular, perturbative cosmological evolutions through string dualities, thermal configurations, and exact solutions involving T-folds and stringy S-branes.

Contribution

It introduces two novel classes of string cosmology models: one using thermal string gas configurations with fluxes to avoid singularities, and another employing exact classical solutions with T-fold structures.

Findings

Models remain perturbative throughout evolution

Thermal configurations lift Hagedorn instabilities

T-folds enable non-singular Euclidean descriptions

Abstract

We review recent progress in string cosmology, where string dualities are applied so as to obtain complete cosmological evolutions, free of any essential singularities. Two classes of models are analyzed. The first class consists of string gas cosmologies associated to certain thermal configurations of type II N=(4,0) models. Finite temperature is introduced along with non-trivial "gravito-magnetic" fluxes that lift the Hagedorn instabilities of the canonical ensemble and restore thermal T-duality symmetry. At a critical maximal temperature additional thermal states become massless sourcing stringy S-branes, which facilitate a bounce between the two dual, asymptotically cold phases. Unlike previous incarnations of pre-Big Bang cosmologies, the models remain perturbative throughout the cosmological evolution. The second class consists of exact solutions to classical string theory that admit a Euclidean description in terms of compact parafermionic worldsheet systems. The Euclidean target space corresponds to a non-singular, compact T-fold, which can be used to construct a normalizable Hartle-Hawking wavefunction for the cosmology.