  Axions with mass greater than 0.7 eV are excluded by cosmological precision
data because they provide too much hot dark matter. While for masses above 20
eV the axion lifetime drops below the age of the universe, we show that the
cosmological exclusion range can be extended from 0.7 eV till 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 limits and laboratory bounds.
