The Cosmology of Massless String Modes

TL;DR

The paper addresses how to realize a viable late-time cosmology in String Gas Cosmology with a fixed dilaton by identifying massless string modes at the self-dual radius as the dominant thermodynamic drivers. Using Einstein gravity, it shows that these modes stabilize all extra dimensions isotropically at the self-dual radius $R=\sqrt{\alpha'}$ and yield a standard FRW expansion for the three large dimensions, while remaining consistent with observational bounds. The work provides a concrete radion-stabilization mechanism without a dynamical dilaton and suggests potential signatures such as metric fluctuations and nonsingular behavior. Its phenomenological analysis demonstrates that the required string-gas density can be compatible with bounds on overclosure and fifth forces, supporting the viability of the scenario for late-time cosmology.

Abstract

We consider the spacetime dynamics of a gas of closed strings in the context of General Relativity in a background of arbitrary spatial dimensions. Our motivation is primarily late time String Gas Cosmology, where such a spacetime picture has to emerge after the dilaton has stabilized. We find that after accounting for the thermodynamics of a gas of strings, only string modes which are massless at the self-dual radius are relevant, and that they lead to a dynamics which is qualitatively different from that induced by the modes usually considered in the literature. In the context of an ansatz with three large spatial dimensions and an arbitrary number of small extra dimensions, we obtain isotropic stabilization of these extra dimensions at the self-dual radius. This stabilization occurs for fixed dilaton, and is induced by the special string states we focus on. The three large dimensions undergo a regular Friedmann-Robertson-Walker expansion. We also show that this framework for late-time cosmology is consistent with observational bounds.