Primordial magnetic fields from metric perturbations - art. no. 083006

We study the amplification of electromagnetic vacuum fluctuations induced b y the evolution of scalar metric perturbations at the end of inflation. Suc h perturbations break the conformal invariance of Maxwell equations in Frie dmann-Robertson-Walker backgrounds and allow the growth of magnetic fields on super-Hubble scales. We relate the strength of the fields generated by t his mechanism with the power spectrum of scalar perturbations and estimate the amplification on galactic scales for different values of the spectral i ndex. Finally we discuss the possible effects of finite conductivity during reheating.