Testing dark matter interactions with CMB spectral distortions

Understand

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 non-relativistic 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.