Primordial magnetic fields in string cosmology

Abstract

We investigate the production of cosmological magnetic fields in models of the early Universe based on the low-energy effective action of superstring theory to lowest order in \ensuremath{\alpha}\ensuremath{'}. Even in a conformally flat background, massless gauge bosons are created during inflation thanks to the contribution of the dilaton scalar field to the gauge kinetic function. We discuss whether the resulting magnetic fluctuations may (i) serve as a seed field for subsequent amplification by a galactic dynamo, and (ii) saturate the nucleosynthesis bounds on the strength of a primordial field. It turns out to be difficult, in the end, to obtain significant field strengths on cosmological scales in the case of dilaton-driven inflation.