Tetsutaro Higaki, Kazunori Nakayama, Fuminobu Takahashi
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. The bound can be as strong as g_a B_0 < 10^{-16} - 10^{-15} GeV^{-1} nG for Delta N_{eff} ~ O(0.1) if the axion is strongly relativistic until present.
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http://arxiv.org/abs/1306.6518
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