Reionization during the dark ages from a cosmic axion background

Recently it has been pointed out that a cosmic background of relativistic axion-like particles (ALPs) would be produced by the primordial decays of heavy fields in the post-inflation epoch, contributing to the extra-radiation content in the Universe today. Primordial magnetic fields would trigger conversions of these ALPs into sub-MeV photons during the dark ages. This photon flux would produce an early reionization of the Universe, leaving a significant imprint on the total optical depth to recombination $τ$. Using the current measurement of $τ$ and the limit on the extra-radiation content $ΔN_{\rm eff}$ by the Planck experiment we put a strong bound on the ALP-photon conversions. Namely we obtain upper limits on the product of the photon-ALP coupling constant $g_{aγ}$ times the magnetic field strength $B$ down to $g_{aγ} B \gtrsim 6 \times 10^{-18} \textrm{GeV}^{-1} \textrm{nG}$ for ultralight ALPs.

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