Constraining resonant photon-axion conversions in the Early Universe
  The presence of a primordial magnetic field would have induced resonant
conversions between photons and axion-like particles (ALPs) during the thermal
history of the Universe. These conversions would have distorted the blackbody
spectrum of the cosmic microwave background (CMB). In this context, we derive
bounds on the photon-ALP resonant conversions using the high precision CMB
spectral data collected by the FIRAS instrument on board of the Cosmic
Background Explorer. We obtain upper limits on the product of the photon-ALP
coupling constant g times the magnetic field strength B down to g B > 10^{-13}
GeV^{-1} nG for ALP masses below the eV scale.
