Perturbed Recombination from Dark Matter Annihilation

TL;DR

Dark matter annihilation during recombination can enhance electron density perturbations, affecting the CMB bispectrum, but the resulting signals are subtle and challenging to distinguish from standard cosmological models.

Contribution

This paper demonstrates how dark matter annihilation influences ionization perturbations and the CMB bispectrum, providing a novel link between dark matter physics and observable cosmological signals.

Findings

Dark matter annihilation amplifies small-scale electron density perturbations.

The CMB bispectrum from recombination is marginally detectable by Planck.

Dark matter effects on the bispectrum are small and hard to distinguish from standard models.

Abstract

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.