 Neutrino decoupling triggers the start of nucleosynthesis. Both processes happen in the unobservable realm, so we have to rely entirely on theory. But the theory is well established in nuclear physics laboratories. When the universe was just one-tenth of a second old, its temperature was around 30 billion degrees. That's about twice the temperature of the interior of our sun. At this temperature, photons can create electrons and positrons, and neutrinos are coupled to protons and neutrons. That coupling enables a fluid flow of protons to neutrons and neutrons to protons. This puts the entire mix of photons, neutrinos, electrons, positrons, protons and neutrons into thermal equilibrium. Given the mass difference between neutrons and protons, we can calculate the expected ratio of neutrons to protons for any given temperature. Note that the number of neutrons decreases exponentially as the temperature cools. Left unchecked, the universe would have had only one neutron left for every million protons by the time it was only six minutes old. But as the universe continued to expand and cool, the neutrinos decoupled from the protons and neutrons. Using the best available laboratory information, this would have occurred when the temperature cooled to nine billion degrees when the universe was around one second old. At that point, two things happened. One, without neutrino interactions, the ratio of neutrons to protons froze. Computations show that at the time of neutrino decoupling, the ratio would have been one neutron for every five protons. This neutrino decoupling process would have lasted around one second. And two, while the neutrinos were coupled to the protons and neutrons, they could not travel far. But once decoupled, they were free to travel across the universe, like photons did at their decoupling.