In Big Bang cosmology, neutrino decoupling was the epoch at which neutrinos ceased interacting with other types of matter, and thereby ceased influencing the dynamics of the universe at early times. Prior to decoupling, neutrinos were in thermal equilibrium with protons, neutrons and electrons, which was maintained through the weak interaction. Decoupling occurred approximately at the time when the rate of those weak interactions was slower than the rate of expansion of the universe. Alternatively, it was the time when the time scale for weak interactions became greater than the age of the universe at that time. Neutrino decoupling took place approximately one second after the Big Bang, when the temperature of the universe was approximately 10 billion kelvin, or 1 MeV. As neutrinos rarely interact with matter, these neutrinos still exist today, analogous to the much later cosmic microwave background emitted during recombination, around 377,000 years after the Big Bang. They form the cosmic neutrino background (abbreviated CνB or CNB). The neutrinos from this event have a very low energy, around 10−10 times smaller than is possible with present-day direct detection. Even high energy neutrinos are notoriously difficult to detect, so the CNB may not be directly observed in detail for many years, if at all. However, Big Bang cosmology makes many predictions about the CNB, and there is very strong indirect evidence that the CNB exists.
