Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 310 / 3 / d / Solution

Comparing the correction in part c with the nonrelativistic kinetic energy $m_{\nu }⟨v^2⟩/2$ gives the characteristic late-time speedThus the late-time relic-neutrino speed redshifts as $a^{-1}$.