Cosmological constraints on axionic dark radiation from axion-photon
  conversion in the early Universe
  Axions seem ubiquitous in string theories and some of them may be produced
non-thermally by heavy scalar decays, contributing to dark radiation. We study
various cosmological effects of photons produced from the axionic dark
radiation through axion-photon conversion in the presence of primordial
magnetic fields, and derive tight constraints on the combination of the
axion-photon coupling and the primordial magnetic field.
