freeze-out

A freeze-out (in cosmology) of a type of particle is a change away from conditions such that this type of particle is constantly destroyed and created, to conditions in which the particles generally persist, i.e., the probability of an encounter with a particle that would result in its destruction has dropped to near zero. Before the freeze-out, the particle was one of a mix of particle-types that were constantly being created and destroyed, and after, any that remain generally exist for the long term, these latter termed relics. This is basically the particle-type's decoupling from the other particles in the mix. The change in conditions that constitute a freeze-out generally consists of thresholds reached by falling density (from Hubble expansion) and temperature: the reduction of these reduces the number of encounters between particles. (The term freeze-out reflects the fact that this occurs with a temperature drop).

A freeze-out left many of the neutrinos in the universe today (the cosmic neutrino background), and the same can be said for any type of particle whose individuals generally continue to exist over cosmological time. Relic abundances from freeze-outs can be calculated using observed and calculated reaction rates of particles (particle physics) and working backward, extrapolating what would happen as the universe was growing from being very small. Subsequent observed abundances that match that of these calculations constitute a significant confirmation of the Big Bang theory.