Massive string modes and non-singular pre-big-bang cosmology

TL;DR

This paper explores how massive string modes might non-perturbatively regularize the singularities in pre-big-bang cosmology, highlighting the limitations of perturbative corrections and the potential role of string thermodynamics.

Contribution

It proposes a non-perturbative mechanism involving massive string modes that could resolve cosmological singularities in string theory.

Findings

Perturbative $\alpha '$ corrections are scheme-dependent and insufficient for regularization.

Massive string modes can produce a non-perturbative regularization effect.

Hawking temperature and Hagedorn temperature play a key role in the regularization process.

Abstract

Perturbative $\alpha '$ corrections to the low energy string effective action have been recently found to have a potentially regularizing effect on the singularity of the lowest order pre-big-bang solutions. Whether they actually regularize it, however, cannot be determined working at any finite order in a perturbative expansion in powers of the string constant $\alpha '$, because of scheme dependence ambiguities. Physically, these corrections are dominated by the integration over the first few massive string states. Very massive string modes, instead, can have a regularizing effect which is non-perturbative in $\alpha '$ and which basically comes from the fact that in a gravitational field with Hubble constant $H$ they are produced with an effective Hawking temperature $T=H/(2\pi)$, and an infinite production rate would occur if this temperature exceeded the Hagedorn temperature. We discuss technical and conceptual difficulties of this non-perturbative regularization mechanism.