Light Dark Matter and Dark Radiation

Understand

Light dark-matter ($M\leq20$ MeV) particles freeze out after neutrino decoupling.

• If the dark-matter particle couples to a neutrino or an electromagnetic plasma, the late time entropy production from dark-matter annihilation can change the neutrino-to-photon temperature ratio, and equally the effective number of neutrinos $N_{eff}$.
• We study the non-equilibrium effects of dark-matter annihilation on the $N_{eff}$ and the effects by using a thermal equilibrium approximation.
• Both results are constrained with Planck observations.