Thomson decoupling rate criterion
= Thomson decoupling rate criterion
{c}
{title2=$n_e\sigma_T\simeq H$}
The physical <Thomson scattering> rate equals the inverse <photon> mean scattering time. Comparing it with the <Hubble parameter> gives a local approximate decoupling criterion. A more precise last-scattering <temperature> is obtained from optical depth and the visibility maximum; these conventions need not give exactly the same <temperature>.