Photon propagation in a cold axion background with and without magnetic field

Abstract

A cold relic axion condensate resulting from vacuum misalignment in the early Universe oscillates with a frequency m , where m is the axion mass. We determine the properties of photons propagating in a simplified version of such a background where the sinusoidal variation is replaced by a square wave profile. We prove that previous results, which indicated that charged particles moving fast in such a background radiate (originally derived assuming that all momenta involved were much larger than m ), hold for long wavelengths, too. We also analyze in detail how the introduction of a magnetic field changes the properties of photon propagation in such a medium. We briefly comment on possible astrophysical implications of these results.