New insights into axion freeze-in

Freeze-in via the axion-photon coupling, g_{\phi\gamma}, can produce axions in the early Universe. At low reheating temperatures close to the minimum allowed value T_{\rm reh}\approx T_{\rm BBN}\approx 10\,{\rm MeV}, the abundance peaks for axion masses m_\phi\approx T_{\rm reh}. Such heavy axions are unstable and subsequently decay, leading to strong constraints... Show more