S-brane to thermal non-singular string cosmology

Abstract

We present a new class of non-singular string cosmologies in d space-time dimensions. At very early times, τ<< τ_c, the Universe is described by a flat σ-model metric, a constant maximal temperature T_c and super-weak string interactions, g_str << 1. During the evolution, the metric remains flat up to τ_c, while the string coupling grows and reaches a critical value g*_str at τ_c. This phase is characterized by a uniform temporal distribution of spacelike branes. At later times, τ> τ_c, the Universe enters in a new phase of expansion, with radiation. The string coupling decreases due to the dilaton motion and asymptotes to a constant for τ>> τ_c. Throughout the evolution, the string coupling remains smaller than g*_str. In the Einstein frame, the cosmologies describe bouncing Universes, where two distinct phases are connected at τ_c. In the initial contracting phase, the evolution of the scale factor is identical to that of a negatively curved Universe filled with radiation. At later times, the Universe enters in an expanding thermal phase with a running dilaton. Explicit examples are presented in a large class of thermal (4,0) type II superstring vacua, with non-trivial "gravito-magnetic" fluxes.