How dark is dark matter?
Robust limits on dark matter - radiation interactions from cosmological observations

Abstract

Extensions to the canonical LCDM model which incorporate elastic scattering between dark matter and Standard Model radiation predict the suppression of matter perturbations on small scales and modifications to the cosmic microwave background anisotropies. Studying these scenarios not only allows us to constrain the particle physics properties of dark matter, it also reveals if they can alleviate remaining tensions in cosmological data sets. In this context, the present thesis considers several aspects of dark matter-photon and dark matter-neutrino interactions.

One central aspect of our work is the accurate description of additional scattering terms in the numerical solutions. In the context of dark matter-photon interactions, we demonstrate the robustness of earlier studies with respect to inconsistencies in the tight coupling approximation and to the negligence of the dark matter sound speed. Accounting for the latter, however, potentially tightens limits from large-scale structure observations for light dark matter candidates. Our updated constraints, derived from the Planck 2015 data release, are about 20% tighter than previous CMB limits in the most conservative case.

We further extend dark matter-photon interactions to a mixed dark matter scenario in which two components, one collisional and one collisionless, contribute to the relic abundance. In particular a small fraction of interacting dark matter impacts the matter power spectrum in a fashion very similar to massive neutrinos. In this case, CMB data only imposes weak constraints on the interaction strength, and our Fisher forecast for the DESI survey predicts notably larger error margins on the neutrino mass.

Dark matter-neutrino interactions alter the initial conditions and cause inconsistencies in the ultra-relativistic fluid approximation, which impact the matter power spectrum on small scales. Still, we can reinforce the validity of previous studies that neglected those effects. In addition to the canonical collisional damping, dark matter-neutrino interactions imply the suppression of structure by mixed damping. Indeed, our exploration of the parameter space reveals the relevance of mixed damping for observational constraints. To provide insight into the underlying physical processes, we derive an analytical approximation to the evolution of dark matter perturbations in the mixed damping regime, which accomplishes to capture all qualitative features of our full numerical results.