Magnetic fields from inflation: the transition to the radiation era
  We compute the contribution to the scalar metric perturbations from large-scale magnetic fields which are generated during inflation. We show that apart from the usual passive and compensated modes, the magnetic fields also contribute to the constant mode from inflation. This is different from the causal (post inflationary) generation of magnetic fields where such a mode is absent and it might lead to significant, non-Gaussian CMB anisotropies. 
Camille Bonvin, Chiara Caprini and Ruth Durrer
astro-ph.CO gr-qc hep-ph