Cosmological bounds on sub-MeV mass axions

Axions with mass m(a) greater than or similar to 0.7 eV are excluded by cosmological precision data because they provide too much hot dark matter. While for m(a) greater than or similar to 20 eV the a -> 2 gamma lifetime drops below the age of the universe, we show that the cosmological exclusion range can be extended to 0.7 eV less than or similar to m(a) less than or similar to 300 keV, primarily by the cosmic deuterium abundance: axion decays would strongly modify the baryon-to-photon ratio at BBN relative to the one at CMB decoupling. Additional arguments include neutrino dilution relative to photons by axion decays and spectral CMB distortions. Our new cosmological constraints complement stellar-evolution and laboratory bounds.