= Cosmic neutrino background last-scattering surface
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.
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