Effective gravitational interactions of dark matter axions
  We investigate the structure of gravitational self-interactions of coherently
oscillating axions in the general relativistic framework. A generic action for
a massive scalar field in the Friedmann-Robertson-Walker background is first
introduced based on the effective field theory approach to cosmological
perturbations. Using the obtained setup, we evaluate the effective
gravitational interaction of the massive scalar field, i.e. scalar quartic
interactions mediated by metric perturbations. Applying the results to the
system of dark matter axions, we estimate their self-interaction rate and
discuss its implications for the axion Bose-Einstein condensate dark matter
scenario. Leading contributions for the gravitational interactions of axions
are given by the process mediated by the dynamical graviton field, which is
essentially the Newtonian potential induced by fluctuations of the background
fluids. We find that it leads to the same order of magnitude for the
interaction rate of dark matter axions in the condensed regime, compared with
the results of previous studies using the Newtonian approximation.
