J
jeremy
33 minutes
WIMP Dark Matter and the Thermal Relic Abundance
How the observed dark-matter density is set for a weakly interacting massive particle (WIMP). Covers thermal freeze-out: a dark-matter species in equilibrium with the Standard Model plasma decouples as the universe cools and its annihilation rate drops below the Hubble rate, freezing in a relic abundance fixed by the thermally averaged annihilation cross section. Includes the Boltzmann equation for the number density, why the relic density scales inversely with the annihilation cross section, the tension between the required cross section and null direct-detection results, and how multi-component dark matter (with DM-DM interactions) can replenish an under-abundant candidate without changing DM-nucleon scattering rates.