Dark matter amnesia in out-of-equilibrium scenarios
The standard paradigm for producing dark matter in the early universe is thermal production, which freezes out at a constant abundance when the interaction rate with the Standard Model becomes slower than the Hubble expansion. However, as the standard WIMP model of dark matter has been put under increasing pressure in recent years, we have needed to consider new dark matter candidates which are too weakly coupled to the Standard Model to be produced via freeze-out. These particles can be produced in the early universe by the freeze-in mechanism, which can be thought of as the opposite process of freeze-out since the dark matter is never in thermal equillibrium with the Standard Model. Freeze-in production can become very complicated when there is more than one particle in the dark sector, because it is then possible to have thermal and chemical equilibrium within the dark sector itself. In principle, this introduces complicated number-changing processes that make calculating the relic abundance of dark matter difficult. However, in this colloquium I will show that in large regimes of parameter space, these equilibration processes do not strongly affect the relic abundance of dark matter. This simplifies a large class of models of a non-minimal dark sector, which are becoming more relevant as experimental dark matter exclusion bounds tighten.