Novel cosmological bounds on thermally-produced axion-like particles
  We constrain the coupling of thermally-produced axion-like particles (here
axions) with photons and gluons, using data from the cosmic microwave
background (CMB) spectra and baryon acoustic oscillations. The axion possesses
an explicit soft breaking mass term and it is produced thermally in the early
Universe from either axion-photon or axion-gluon processes, accounting for the
recent progresses in the field. We derive the most stringent bounds on the
axion-gluon coupling to date on the mass range considered $10^{-4} \lesssim
m_a/{\rm eV} \lesssim 100$, superseding the current bounds from SN1987A. The
bounds on the axion-photon coupling are competitive with the results from the
CAST collaboration for the axion mass $m_a \gtrsim 3\,$eV. We comment on the
forecast reaches that will be available given the sensitivity of future CMB-S4
missions.
