Challenging the Stability of Light Millicharged Dark Matter
  We investigate the cosmological stability of light bosonic dark matter
carrying a tiny electric charge. In the wave-like regime of high occupation
numbers, annihilation into gauge bosons can be drastically enhanced by
parametric resonance. The millicharged particle can either be minimally coupled
to photons or its electromagnetic interaction can be mediated via kinetic
mixing with a massless hidden photon. In the case of a direct coupling current
observational constraints on the millicharge are stronger than those arising
from parametric resonance. For the (theoretically preferred) case of kinetic
mixing large regions of parameter space are affected by the parametric
resonance leading at least to a fragmentation of the dark matter field if not
its outright destruction.
