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

TL;DR

This paper investigates how photons propagate through a cold axion background, with and without magnetic fields, revealing radiation effects and potential astrophysical implications.

Contribution

It models photon behavior in a simplified axion background, extending previous results to long wavelengths and analyzing magnetic field effects.

Findings

Charged particles radiate in the axion background at long wavelengths.

Magnetic fields alter photon propagation properties.

Results have potential astrophysical relevance.

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 that 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.