Photon production of axionic cold dark matter - art. no. 085003

Using the nonequilibrium quantum field theory, photon production from the c oherently oscillating axion field in a flat Robertson-Walker cosmology is r eexamined. First neglecting the Debye screening of the baryon plasma to pho tons, we find that the axions will dissipate into photons via spinodal inst ability in addition to parametric resonance. As a result of the pseudoscala r nature of the axion-photon coupling, we observe a circular polarization a symmetry 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 t he photon production. We note that the formalism of the problem can be appl ied to any pseudoscalar field coupled to a photon in a thermal background i n a general curved spacetime.