Photon Production of Axionic Cold Dark Matter

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 pseudoscalar 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. PACS number(s): 14.80.Mz, 95.35.+d, 98.80. Cq Typeset using REVTEX E-mail address: [email protected] E-mail address: [email protected] 1