Momentum distributions of cosmic relics: Improved analysis

We solve coupled momentum-dependent Boltzmann equations for the phase space distribution of cosmic relic particles, without resorting to approximations of assuming kinetic equilibrium or neglecting backscattering or elastic interactions. Our method is amendable to precision numerical computations. To test it, we consider two benchmark models where the momentum dependence of dark matter distribution function is potentially important: a real singlet scalar extension near the Higgs resonance and a sterile neutrino dark matter model with a singlet scalar mediator. The singlet scalar example shows that the kinetic equilibrium may hold surprisingly well even near sharp resonances. However, the integrated method may underestimate the relic density by up to 40% in extreme cases. In the sterile neutrino dark matter model, we studied how the inclusion of previously ignored elastic interactions and processes with initial state sterile neutrinos could affect the nonthermal nature of their resulting distributions. Here the effects turned out to be negligible, proving the robustness of the earlier predictions.