= Residual electron freeze-out
{title2=$\alpha_{\rm eff}n_e\lesssim H$}
= Electron freeze-out
{synonym}
When the effective recombination rate per <Electron> becomes smaller than the expansion rate, the small remaining ionized fraction can no longer track <chemical equilibrium>. A nonzero <Electron> fraction remains, exceeding the exponentially small <Saha equation> prediction. The effective rate includes atomic radiative bottlenecks; chemical freeze-out is distinct from <photon decoupling> by scattering.
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