The Relic Neutrino Number-Density Budget: Per-Flavour Density, Photon Ratio, and What a Capture Target Actually Contains

Each neutrino flavour, counting neutrino plus antineutrino, fills space today with 113.1 particles per cubic centimetre, for a three-flavour total of 339.4 per cubic centimetre, against 410.7 cosmic microwave background photons per cubic centimetre. The per-flavour ratio to photons is 0.275. This paper derives those densities from the Fermi-Dirac integral at the corrected relic temperature, shows how sensitive they are to the temperature convention and to any assumed chemical potential, and translates the densities into the counts a laboratory volume actually contains. The numbers are large by laboratory standards and small by the standards capture detection requires, and both facts are quantified. This study is simulated and computed from stated assumptions, not measured.