Photon production of axionic cold dark matter

Using the nonequilibrium quantum field theory, photon production from the coherently oscillating axion field in a flat Robertson-Walker cosmology is reexamined. 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 pseudoscalar nature of the axion-photon coupling, we observe a circular polarization asymmetry in the photons produced. 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 pseudoscalar field coupled to a photon in a thermal background in a general curved spacetime.