In section 3.1 of Dodelson's "Modern Cosmology", after introducing the Boltzmann equation, in the second paragraph of page 60 the author states that: "The first, most important realization is that scattering processes typically enforce kinetic equilibrium. That is, scattering takes place so rapidly that the distributions of the various species take on the generic Bose-Einstein/Fermi-Dirac forms." QESTIONS: What is the definition of kinetic equilibrium? When achieved, why does kinetic equilibrium it imply that the distributions take on the equilibrium forms? Why does scattering processes typically enforce kinetic equilibrium? In Cercignani's "The Relativistic Boltzmann Equation: Theory and Applications" section 2.7 he derives so called equilibrium distribution functions by demanding that the entropy production rate must vanish. Is this the same as kinetic equilibrium?