Testing dark matter interactions with CMB spectral distortions

Possible interactions of dark matter (DM) with Standard Model (SM) particles can be tested with spectral distortions (SDs) of the cosmic microwave background (CMB). In particular, a nonrelativistic DM particle that scatters elastically with photons, electrons, or nuclei imprints a negative chemical potential mu to the CMB spectrum. This article revisits the first study of this effect, with an accurate treatment of heat exchange between DM and SM particles. We show that the instantaneous-decoupling approximation made in the original study systematically and significantly underestimates the amplitude of SDs. As a consequence, we derive tighter upper bounds to the DM-SM elastic-scattering cross section for DM masses m(chi) less than or similar to 0.1 MeV, from the nondetection of mu-distortions by FIRAS. We also show that a future instrument like PIXIE, sensitive to vertical bar mu vertical bar similar to 10(-8), would be able to probe DM-SM cross sections much smaller than first forecasted, and orders of magnitude below current upper limits from CMB-anisotropy data, up to DM masses of similar to 1 GeV. Lastly, we study the sensitivity of SDs to the electric and magnetic dipole moments of the DM. Although SDs can place nontrivial constraints on these models, we find that even future SD experiments are unlikely to improve upon the best current bounds. This article is accompanied by the public code DMDIST, which allows one to compute CMB SDs for generic particle-DM models, specified by their cross sections for elastic scattering with and annihilation into SM particles.

Automated summary

This article revisits the possible interactions of dark matter with Standard Model particles through spectral distortions of the cosmic microwave background. It finds that previous studies underestimated the amplitude of distortions and derives tighter upper bounds to the DM-SM elastic-scattering cross section based on non-detection of distortions. Future instruments like PIXIE may probe much smaller cross sections than initially forecasted, and current bounds on electric and magnetic dipole moments of DM are unlikely to be improved upon even with future experiments.