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The cosmic neutrino background (CNB or CνB, aka relic neutrinos or cosmic neutrinos) refers to neutrinos from within the initial seconds after the Big Bang. Though they are theorized to be everywhere, their presence is not confirmed by detection: their much lower energy makes detection far more challenging than others (such as solar neutrinos) and there has not been a serious attempt to detect them. But there is indirect evidence for their creation e.g., the Big Bang nucleosynthesis, which would occur afterward within seconds, would produce elemental abundances now confirmed by observation. Their kinetic energy (KE) is on the order of 10-6 to 10-4 eV, corresponding to perhaps a hundredth of the speed of light, whereas other neutrinos generally travel at relativistic speed. Cosmic neutrinos' slow speed is due to Hubble expansion which has a slowing effect analogous to redshift (photons, which have no mass, lengthen their wavelength rather than slow their speed).
Cosmic neutrinos began their current activity (free streaming after decoupling) within seconds of the Big Bang, potentially offering clues to an earlier era than does the cosmic microwave background (CMB), which provides direct information only back to recombination. Regarding the origin of the decoupling neutrinos, all we can say is that given the presence of energy and mass in the universe, extrapolating back using the laws of physics we know, the universe's energy would be so confined during its first second, that the implied density and temperature would prevent atoms and even protons and neutrons from existing; what would exist would be elementary particles such as quarks, electrons, photons, and neutrinos, etc. There would be constant interactions that change particles into other types, and the interaction rates under these conditions imply certain abundances of these particle-types if equilibrium is reached or approximated, setting the ratios such as that of neutrinos to quarks, the number of the latter implied by the matter we observe. The cosmic neutrinos are cited to have a temperature of 1.95 K or 1.9 K, which is the temperature-equivalent of their kinetic energy, which has fallen due to the Hubble expansion, in the same manner as the CMB's peak photon energy. These cited temperature values are based upon the necessary ratio between this temperature and that of the CMB.
Note that the terms cosmic neutrino background and relic neutrinos are sometimes used to mean all the random background of neutrinos rather than specifically those produced by the Big Bang.