Here, it mentions that the total number of electrons in the observable universe is around 10^80, while this states that the total number of baryons are again, around 10^80. This article says that the total number of neutrinos seems to be around 1.2*10^89. It follows that the total number of neutrinos are a billion times more than the total number of baryons and electrons combined. Is this ballpark estimate correct ?
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The neutrinos in question are primordial neutrinos, often called the cosmic neutrino background by analogy with the cosmic microwave background.
Like the CMB, these neutrinos originate from the Big Bang and specifically from the moment when the temperature fell too low for the weak force interactions to keep the neutrinos in equilibrium with everything else. Calculating the number density of these neutrinos is complicated but reasonably well understood as it involves only energy regimes that we have probed experimentally. The result is that the ratio of the primordial neutrinos to baryons is indeed about $10^9$.
answered Dec 24, 2016 at 11:01
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$\begingroup$ Is the reason for the imbalance primarily due to the miniscule cross section for low energy neutrino/antineutrino annihilation? The electron and baron numbers are after all just the survivors of massive matter/antimatter annihilations. $\endgroup$ Commented Dec 24, 2016 at 15:41
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$\begingroup$ @LewisMiller No - my understanding is this: at some temperature in the early universe (where there is already more matter than antimatter) you get weak interactions such as p + e <-> n + $\nu$ in thermal equilibrium. At some lower temperature these interactions stop and freeze out the neutrino number at whatever number it happens to be at that temperature (which as stated is complicated). $\endgroup$– isometryCommented Dec 24, 2016 at 18:11