Photon Production of Axionic Cold Dark Matter

TL;DR

This paper re-examines photon production from axionic dark matter using non-equilibrium quantum field theory, revealing suppressed effects in an expanding universe and discussing plasma effects and broader applications.

Contribution

It introduces a formalism for photon production from axions in curved spacetime, accounting for spinodal instability and polarization asymmetry, with implications for dark matter detection.

Findings

Photon production via spinodal instability and parametric resonance.

Circular polarization asymmetry in produced photons.

Photon production effects are negligible in an expanding universe.

Abstract

Using the non-equilibrium quantum field theory, photon production from the coherently oscillating axion field in a flat Robertson-Walker cosmology is re-examined. First neglecting the Debye screening of the baryon plasma to photons, we find that the axions will dissipate into photons via spinodal instability in addition to parametric resonance. As a result of the pseudo-scalar nature of the axion-photon coupling, we observe a circular polarization asymmetry in the produced photons. However, these effects are suppressed to an insignificant level in the expanding universe. We then briefly discuss a systematic way of including the plasma effect which can further suppress the photon production. We note that the formalism of the problem can be applied to any pseudo-scalar field coupled to photon in a thermal background in a general curved spacetime.