\(S\)-brane to thermal non-singular string cosmology (Q2892068)

The authors present a new class of non-singular string cosmologies in \(d\) spacetime dimensions. At very early times, the Universe is described by a flat sigma-model metric, a constant maximal temperature \(T_c\) and super-weak string interactions. During the evolution, the metric remains flat up to times comparable with the critical time, while the string coupling grows and reaches a critical value. This phase is characterized by a uniform temporal distribution of spacelike branes. At later times (times larger than the critical one), the Universe enters a new phase of expansion, with radiation. The string coupling decreases due to the dilaton motion and asymptotes to a constant for times much larger than the critical time. Throughout the evolution, the string coupling remains smaller than the coupling constant. In the Einstein frame, the cosmologies describe bouncing Universes, where two distinct phases are connected at the critical time. 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 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.