On the Wondrous Stability of ALP Dark Matter
  The very low mass and small coupling of axion-like particles (ALPs) is
usually taken as a guarantor of their cosmological longevity, making them
excellent dark matter candidates. That said, Bose enhancement could stimulate
decays and challenge this paradigm. Here, we analyze and review the
cosmological decay of ALPs into photons, taking Bose enhancement into account,
thereby going beyond the usual naive perturbative estimate. At first glance,
this calculation seems to yield an exponentially growing resonance and
therefore an extremely fast decay rate. However, the redshifting of the decay
products due to the expansion of the Universe as well as the effective plasma
mass of the photon can prevent an efficient resonance. While this result agrees
with existing analyses of the QCD axion, for more general ALPs that can feature
an enhanced photon coupling, stability is only ensured by a combination of the
expansion and the plasma effects.
