= Cold-dark-matter kinetic Jeans scale
An instantaneous kinetic suppression estimate replaces <sound speed> by the <velocity dispersion> of <cold dark matter>. Nonrelativistic particles in thermal contact with <photons> have $\sigma\propto a^{-1/2}$; after <kinetic decoupling>, their momenta redshift and $\sigma\propto a^{-1}$. A background dynamical estimate using total density is then constant during <radiation domination> and decreases as $a^{-1/2}$ during <matter domination>. Using only the species' own matter density instead makes the self-gravitating <collisionless Jeans length> decrease as $a^{-1/2}$ throughout the decoupled nonrelativistic era. Neither instantaneous estimate is the accumulated distance traveled by <collisionless free streaming>.
Back to article page
