Magnetic fields from cosmological bulk flows

Abstract

ABSTRACT We explore the possibility that matter bulk flows could generate the required vorticity in the electron–proton–photon plasma to source cosmic magnetic fields through the Harrison mechanism. We analyse the coupled set of perturbed Maxwell and Boltzmann equations for a plasma in which the matter and radiation components exhibit relative bulk motions at the background level. These background bulk motions induce a relative velocity between the matter and cosmic microwave background rest frames at the present time, i.e. a bulk flow, with an amplitude β. We find that, to first order in cosmological perturbations, bulk flows with velocities compatible with current Planck limits (β < 8.5 × 10−4 at $95{{\ \rm per\ cent}}$ CL) could generate magnetic fields with an amplitude 10−21 G on 10 kpc comoving scales at the time of completed galaxy formation that could be sufficient to seed a galactic dynamo mechanism.