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Big Bang nucleosynthesis forms light atomic nuclei during the first minutes of the expanding, cooling universe. Weak interactions initially maintain neutron-proton chemical equilibrium. After cosmological weak freeze-out, free-neutron decay after freeze-out reduces the available neutron supply.

The deuterium bottleneck delays efficient assembly: energetic photons break up newly formed deuterium until sufficient cooling occurs. Most surviving neutrons then enter helium-4, with smaller residual abundances of other light nuclei. If $r=n_n/n_p$ at assembly and nearly all neutrons enter helium, counting two neutrons per helium nucleus gives the approximate helium mass fractionThis links nuclear reactions and particle decay to an astronomical abundance measurement. Comparing primordial deuterium and helium with the independently inferred cosmological baryon density tests the thermal history; the cosmological lithium problem remains an important qualification to the comparison.^[ref]

The Cosmic neutrino background is the relic population of neutrinos left after thermal decoupling in cosmology. As the universe expands, their momenta redshift; the decoupled distribution retains a temperature parameter proportional to the inverse scale factor, even after massive neutrinos become nonrelativistic.

The standard ideal temperature ratio follows from cosmological entropy conservation. Assume instantaneous neutrino decoupling before electron-positron annihilation and negligible chemical potentials. The coupled photon–electron–Positron sector initially has entropy degrees of freedom $2+(7/8)4=11/2$, dropping to two after annihilation. Conserving its comoving entropy heats the photons relative to the freely redshifting neutrinos, givingFor example, a photon temperature of $2.725\mathrm{K}$ corresponds to a relic-neutrino temperature parameter near $1.95\mathrm{K}$ in this idealization. Residual interactions during non-instantaneous neutrino decoupling slightly modify the distribution, so the simple ratio is an approximation.^[ref]