Disentangling dark physics with cosmic microwave background experiments

We forecast constraints on dark matter (DM) scattering with baryons in the early Universe with upcoming and future cosmic microwave background (CMB) experiments, for DM particle masses down to 15 keV. In terms of the upper limit on the interaction cross section for a velocity-independent spin-independent elastic scattering, compared to current Planck results, we find a factor of $\sim$6 improvement with CMB-Stage 3, a factor of $\sim$26 with CMB-Stage 4, and a factor of $\sim$200 with a cosmic-variance limited experiment. Once the instrumental noise reaches the proximity of 1 $\mu$K-arcmin, the constraints are entirely driven by the lensing measurements. The constraints benefit from a wide survey, and show gradual improvement for instrumental noise levels from 10 $\mu$K-arcmin to 1 $\mu$K-arcmin and resolution from 5 arcmin to 1 arcmin. We further study degeneracies between DM interactions and various other signatures of new physics targeted by the CMB experiments. In the primary temperature and polarization only, we find moderate degeneracy between the effects of DM scattering, signals from massive neutrinos, and from the effective number of relativistic degrees of freedom. The degeneracy is almost entirely broken once the lensing convergence spectrum is included into the analyses. We discuss the implications of our findings in context of planned and upcoming CMB measurements and other cosmological probes of dark-sector and neutrino physics.