Relaxing the limits on inflationary magnetogenesis

Abstract

Inflation has long been thought as the best way of producing primordial large-scale magnetic fields. To achieve fields strong enough to seed the galactic dynamo, most of the mechanisms operate outside conventional electromagnetic theory. The latter is typically restored after the end of the de Sitter phase. Breaking away from standard electromagnetism can lead to substantially stronger magnetic fields by the end of inflation, thus compensating for the their subsequent adiabatic depletion. We argue that the drastic magnetic enhancement during the de Sitter era may no longer be necessary because, contrary to the widespread perception, superhorizon-sized magnetic fields decay at a slower pace after inflation. The principle behind this claim is causality, which confines the post-inflationary electric currents inside the horizon. Without the currents there can be no magnetic-flux freezing on super-Hubble lengths. There, the magnetic decay-rate slows down, thus making it much easier to produce primordial fields of astrophysical interest. To quantify this qualitative statement, one can start from the current galactic-dynamo requirements and `reverse engineer' the magnetic strengths needed at the end of inflation, in order to produce astrophysically relevant residual seeds today. Our results suggest that, depending on the magnetic scale, mechanisms of inflationary magnetogenesis generating fields stronger than $10^{17}$ G by the end of the de Sitter phase, could successfully seed the galactic dynamo at present.