The Cosmology of String Theoretic Axions

TL;DR

This paper investigates the cosmological implications of string theory axions, proposing two schemes that relax traditional bounds on axion mass and explore their role in early universe cosmology and baryogenesis.

Contribution

It introduces two novel cosmological models involving string axions, addressing the moduli problem and early-time axion potential variations.

Findings

String moduli are likely more massive, resolving the moduli problem.

Axion bounds are significantly weakened in these models.

Mechanisms for baryon asymmetry generation are proposed.

Abstract

String theory posesses numerous axion candidates. The recent realization that the compactification radius in string theory might be large means that these states can solve the strong CP problem. This still leaves the question of the cosmological bound on the axion mass. Here we explore two schemes for accommodating such light axions in cosmology. In the first, we note that in string theory the universe is likely to be dominated early on by the coherent oscillations of some moduli. The usual moduli problem assumes that these fields have masses comparable to the gravitino. We argue that string moduli are likely to be substantially more massive, eliminating this problem. In such cosmologies the axion bound is significantly weakened. Plausible mechanisms for generating the baryon number density are described. In the second, we point out that in string theory, the axion potentials might be much larger at early times than at present. In string theory, if CP violation is described by a small parameter, the axion may sit sufficiently close to its true minimum to invalidate the bounds.