We find that for Peccei-Quinn symmetry-breaking scales {}4×{}10⁸ GeV (corresponding to axion masses {}0.03 eV) thermal production of axions in the early Universe (via the Primakoff and photoproduction processes) dominates coherent production by a factor of about 1200[mₐ(1 eV)]2.175. The photon luminosity from the decays of these relic axions leads to an upper limit to the axion mass of order 2-5 eV. If the axion mass saturates this bound, relic axion decays may well be detectable.
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Michael S. Turner (1987) studied this question.
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