Kinetic decoupling
= Kinetic decoupling
Kinetic decoupling occurs when momentum-exchange collisions cease to maintain the <temperature> and <velocity dispersion> of a particle species in equilibrium with a surrounding bath. Afterwards, a freely moving nonrelativistic particle has momentum proportional to $a^{-1}$ and effective kinetic <temperature> proportional to $a^{-2}$, where $a$ is the <scale factor>.