I should note that muon pair production only happens at the very beginning of the lepton era because they are really heavy and (if we invoke E=mc^2 again) it takes more energy to create some heavy muons compared to some really light electrons. Also the tauons are even heavier then muons so their production took place even earlier.
As the lepton era draws to a close the universe is expanding and the temperature is dropping. Tauon pair annihilation starts happening more frequently than pair production and soon almost all the tauons disappear. Then the same thing happens with muons later and then the same thing happens with electrons a little latter (they don’t vanish completely, but before there were crazy ridiculous amounts of them…now there are just a reasonable amount of them).
Ok now to talk about neutrinos! Neutrinos are tiny little particles that have barely any mass and travel at near the speeds of light. They don’t have any electrical charge so they tend not to interact with anything. They were predicted by theory, but it took us forever to find them. There’s a cool documentary on them. We’ll talk more about them tomorrow.
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