Large-scale primordial magnetic fields from inflation and preheating

We consider models in which the (transverse) photon mass is non-zero during inflation and drops to zero non-adiabatically at the end of inflation. Through this process, vacuum fluctuations of the photon field are converted into physical, long-wavelength modes with high amplitude. The resulting spectrum of the field strength is approximately B-l similar to l(-1), where l is tile relevant coherence scale. With a reasonable model of field evolution we obtain, on comoving galactic scales, a magnetic field strong enough to seed a dynamo mechanism and generate the observed galactic magnetic fields.