Equilibrium, Freeze-out, and Temperature

"Freeze-out" for a particle does not mean that it disappears or no longer plays any role. It just means that the particle is no longer in thermal equilibrium with the rest of the Universe because it doesn't interact strongly enough with it. As a result, the decoupled particles can have a different temperature from the rest of the Universe. For example, as described in the right panel, the weak interactions froze out earlier in the history of the Universe than the photon interactions. For that reason, particles coupled only to the weak interactions (mostly neutrinos) have a different temperature than the photons in today's Universe.

Equilibrium and Decoupling

In a radiation-dominated era the particles and antiparticles and photons are continuously undergoing interactions with each other. One says that under these conditions the radiation and the matter are in thermal equilibrium, which means that they exist with a common temperature.

On the other hand, as the temperature drops in the expanding Universe the rates for reactions of the particles also drop. At some point the reaction rates are no longer high enough to maintain equilibrium for a given kind of particle. When this happens, we say that this kind of particle has decoupled from the general thermal equilibrium; one also sometimes refers to this decoupling as freeze-out. As we shall see, the decoupling of normal matter from thermal equilibrium with photons marked a fundamental transition in the nature of our Universe.

Animation: the decoupling of radiation and matter