The cosmology of massless string modes

Abstract

We consider the spacetime dynamics of a gas of closed strings in the context of generalrelativity in a background of arbitrary spatial dimensions. Our motivation isprimarily late time string gas cosmology, where such a spacetime picture has toemerge after the dilaton has stabilized. We find that after accounting for thethermodynamics of a gas of strings, only string modes which are massless at theself-dual radius are relevant, and that they lead to a dynamics which is qualitativelydifferent from that induced by the modes usually considered in the literature. Inthe context of an ansatz with three large spatial dimensions and an arbitrarynumber of small extra dimensions, we obtain isotropic stabilization of these extradimensions at the self-dual radius. This stabilization occurs for fixed dilaton, andis induced by the special string states we focus on. The three large dimensionsundergo a regular Friedmann–Robertson–Walker expansion. We also show thatthis framework for late time cosmology is consistent with observational bounds.