The Cosmophenomenology of Axionic Dark Radiation

TL;DR

This paper explores how relativistic axions from early universe decays could serve as dark radiation, affecting cosmological processes and potentially producing observable signals today.

Contribution

It introduces a detailed analysis of axion interactions during BBN and their implications for dark matter production and cosmic axion background.

Findings

Axion-photon scattering impacts light element abundances.

Bounds on axion parameters from helium overproduction.

Existence of a cosmic axion background with specific energy and flux.

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 $\sim 10^6 cm^{-2} s^{-1}$.