Reheating and dangerous relics in pre-big bang string cosmology

TL;DR

This paper examines reheating in pre-big bang string cosmology, showing that gravitational and scattering-produced relics exceed nucleosynthesis limits, and proposes entropy production via modulus or axion decay to resolve this issue.

Contribution

It introduces a detailed analysis of relic production and proposes a mechanism involving modulus or axion decay for entropy generation in pre-big bang models.

Findings

Relic abundances surpass nucleosynthesis constraints.

Entropy production via modulus or axion decay can dilute relics.

Baryogenesis can be achieved through the Affleck-Dine mechanism.

Abstract

We discuss the mechanism of reheating in pre-big bang string cosmology and we calculate the amount of moduli and gravitinos produced gravitationally and in scattering processes of the thermal bath. We find that this abundance always exceeds the limits imposed by big-bang nucleosynthesis, and significant entropy production is required. The exact amount of entropy needed depends on the details of the high curvature phase between the dilaton-driven inflationary era and the radiation era. We show that the domination and decay of the zero-mode of a modulus field, which could well be the dilaton, or of axions, suffices to dilute moduli and gravitinos. In this context, baryogenesis can be accomodated in a simple way via the Affleck-Dine mechanism and in some cases the Affleck-Dine condensate could provide both the source of entropy and the baryon asymmetry.