Dynamical evolution of axion condensates under stimulated decays into
  photons
  Dark matter axion condensates may experience stimulated decays into photon
pairs. This effect has been often interpreted as a parametric resonance of
photons from the axion-photon coupling, leading to an exponential growth of the
photon occupation number in a narrow instability band. Most of the previous
literature does not consider the possible evolution of the axion field due to
the photon growth. We revisit this effect presenting a mean field solution of
the axion-photon kinetic equations, in terms of number of photons and pair
correlations. We study the limit of no axion depletion, recovering the known
instability. Moreover, we extend the results including a possible depletion of
the axion field. In this case we find that the axion condensate exhibits the
behaviour of an inverted pendulum. We discuss the relevance of these effects
for two different cases: an homogeneous axion field at recombination and a
localized axion clump and discuss constraints that could result from the
induced photon background.
