The cosmophenomenology of axionic dark radiation

A bstract Relativistic axions are good candidates for the dark radiation for which there are mounting observational hints. The primordial decays of heavy fields produce axions which are ultra-energetic compared to thermalised matter and inelastic axion-matter scattering can occur with E CoM ≫ T γ , thus accessing many interesting processes which are otherwise kinematically forbidden in standard cosmology. Axion-photon scattering into quarks and leptons during BBN affects the light element abundances, and bounds on over-production of 4 He constrain a combination of the axion decay constant and the reheating temperature. For supersymmetric models, axion scattering into visible sector superpartners can give direct non-thermal production of dark matter at T γ ≪ T freezeout . Most axions — or any other dark radiation candidate from modulus decay — still linger today as a Cosmic Axion Background with E axion ~ (100) eV, and a flux of ~ 10 6 cm −2 s −1 .