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The cosmic microwave background last-scattering surface is the shell from which the photons observed in the Cosmic microwave background last scattered around photon decoupling.

The cosmic neutrino background last-scattering surface is the locus from which relic neutrinos that reach an observer underwent their final appreciable scattering. Although neutrino decoupling occurs before photon decoupling, a massive neutrino travels slower than light after becoming nonrelativistic, so its last-scattering surface need not be farther away than the Cosmic microwave background last-scattering surface.

Photon decoupling occurs when the Thomson interaction rate falls below the expansion rate,At decoupling the universe is approximately matter dominated, soUsing $n_e=X_e\eta (2\zeta (3)/{\pi }^2)T^3$ in ${\Gamma }_{\gamma }=H$ givesFor $X_e\ll 1$, the Saha equation makes $X_e(T)$ exponentially steep through $e^{-B_H/(2T)}$. Power-law changes of $H_0$ or ${\Omega }_{m,0}$ therefore cause only a small shift in $T_{\mathrm{dec}}$, explaining why $T_{\mathrm{dec}}\simeq T_{\mathrm{rec}}$ is a good approximation.