Resonant amplification of gauge fields in expanding universe

We investigate the possibility that gauge fields are amplified in an expanding universe by parametric resonance, during the oscillatory regime of a scalar field to which they are coupled. We investigate the coupling of gauge fields to a charged scalar field and to an axion. For both couplings, gauge fields fluctuations undergo exponential instabilities. We discuss how the presence of other charges or currents may counteract the resonance, but we argue that in some cases the resonance will persist and that hence this mechanism could have some relevance for the problem of large scale primordial magnetic fields.