The Cosmophenomenology of Axionic Dark Radiation
ArXiv 1304.1804 (2013)
Authors:
Joseph P Conlon, MC David Marsh
Abstract:
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 $E_{CoM} \gg T_{\gamma}$, 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
overproduction 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 $T_{\gamma} \ll T_{freezeout}$. Most axions --- or any other dark
radiation candidate from modulus decay --- still linger today as a Cosmic Axion
Background with $E_{axion} \sim \mathcal{O}(100) eV$, and a flux of $\sim 10^6
cm^{-2} s^{-1}$.
