Neutrondensity

Neutrondensity is a measure of the number of neutrinos per unit volume, commonly denoted nν. It describes the abundance of neutrinos in a given region, such as the early universe, a stellar interior, or the vicinity of a detector. In many contexts it is useful to distinguish the number density from the neutrino energy density, ρν, which is the energy per unit volume carried by neutrinos. The energy density is related to the spectrum of neutrino energies and can be converted from nν if the energy distribution is known.

In cosmology, neutrinos decoupled from the primordial plasma when the universe was about one second old, and since then they have formed a cosmic neutrino background that fills space. The present-day total neutrino number density for all flavors and antiparticles is about 336 cm^-3, with roughly 112 cm^-3 per flavor accounting for both neutrinos and antineutrinos. If neutrinos are relativistic, their energy density scales like radiation (approximately a^-4 with the cosmic expansion); if they have mass, they gradually contribute to the matter density via ρν ≈ mν nν once they become non-relativistic. The effective number of neutrino species, Neff, parameterizes their contribution to the radiation density and is constrained by cosmological data.

Direct terrestrial measurement of nν is challenging; neutrino detectors measure fluxes from specific sources and infer rates rather than local number densities. Cosmological and astrophysical observations provide the primary constraints on neutrondensity through neutrino relic abundance, neutrino oscillations, and neutrino-induced energy transport. In high-density environments such as core-collapse supernovae, neutrino densities can become extremely large, enabling collective effects that influence dynamics and transport.