Cosmic neutrino background

The cosmic neutrino background is the relic population of neutrinos and antineutrinos produced in the early universe. It fills space nearly uniformly and is now extremely cold and difficult to detect.

Neutrino decoupling: The early-universe transition when neutrinos stopped exchanging energy efficiently with other particles. This event left neutrinos free to stream through the universe as a relic background.
Neutrino: A neutral, weakly interacting elementary particle that comes in three known flavors. The background consists of neutrinos and their antiparticles.
Cosmological parameter estimation: The inference of universe properties by comparing observations with cosmological models. Relic-neutrino effects constrain parameters such as radiation density and neutrino mass.
Cosmic neutrino background detection: Experimental efforts to observe neutrinos left over from the early universe. Direct detection would test a central prediction that indirect cosmological evidence cannot fully replace.
Big Bang nucleosynthesis: The early-universe production of light atomic nuclei from protons and neutrons. Relic neutrinos affected the expansion rate during the era when light elements formed.
Neutrino flavor: A neutrino's weak-interaction identity: electron, muon, or tau. The primordial background began with flavor populations that oscillations later mixed.
Large-scale structure: The cosmic distribution of galaxies, galaxy clusters, and matter across vast distances. Free-streaming relic neutrinos suppress the growth of structure on small scales.
Gravitational clustering of neutrinos: The concentration of neutrinos in gravitationally bound structures as they move through the universe. The local relic density may differ from the nearly uniform cosmic average.
Cosmic microwave background: The relic radiation released when the early universe became transparent to photons. Photons decoupled much later than neutrinos, leaving a warmer and directly observed background.
Thermal relic: A population of particles left over from an earlier hot, dense phase of the universe. Decoupled neutrinos are a thermal relic whose abundance records early-universe conditions.

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