Perturbed Recombination from Dark Matter Annihilation
  We show that dark matter annihilation around the time of recombination can
lead to growing ionization fraction perturbations, that track the linear
collapse of matter over-densities. This amplifies small scale cosmological
perturbations to the free electron density by a significant amount compared to
the usual acoustic oscillations. Electron density perturbations distort the
CMB, inducing secondary non-gaussianity. We calculate the CMB bispectrum from
recombination, that is marginally observable by Planck. Even though electron
perturbations can be markedly boosted compared with the Standard Model
prediction, the dark matter effect in the CMB bispectrum turns out to be small
and will be difficult to disentangle from the standard model in the foreseeable
future.
