The Cosmology of Massless String Modes

TL;DR

This paper explores how massless string modes influence late-time cosmology, showing they stabilize extra dimensions and lead to a consistent universe expansion compatible with observations.

Contribution

It demonstrates that massless string modes at the self-dual radius induce stabilization of extra dimensions and a standard cosmological expansion, differing from previous models.

Findings

Massless string modes dominate late-time dynamics.

Extra dimensions stabilize at the self-dual radius.

Cosmological evolution aligns with observational bounds.

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.