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.