PRIMORDIAL MAGNETIC-FIELDS IN STRING COSMOLOGY

We investigate the production of cosmological magnetic fields in model s of the early Universe based on the low-energy effective action of su perstring theory to lowest order in alpha'. Even in a conformally hat background, massless gauge bosons are created during inflation thanks to the contribution of the dilaton scalar field to the gauge kinetic f unction. We discuss whether the resulting magnetic fluctuations may (i ) serve as a seed field for subsequent amplification by a galactic dyn amo, 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 held strengths on cosmological scales in the case of dila ton-driven inflation.