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If neutrinos are so easy to produce, but it rarely get absorbed or detected, doesn't that mean the universe will be full of neutrinos?

Will its density ever reach a balance where its absorption events occur as frequently as emission events? Is the universe already reaching the balance or far from it?

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    $\begingroup$ Have a look at the state of our knowledge about neutrino densities in the cosmos frontiersin.org/articles/10.3389/fphy.2017.00070/full $\endgroup$
    – anna v
    Commented Jul 16, 2023 at 11:09
  • $\begingroup$ I assume you are talking about relic ν s, as the fission-produced ones from baryonic matter are vastly fewer. The WP article estimates their number and energy contribution to the universe, but the contribution by the bulk of baryonic matter overwhelms it, and is not all convertible to neutrino energy, so, regardless of their mass, ν s cannot usurp that energy. A large number of them, then, cannot "fill up" the universe. $\endgroup$
    – Cosmas Zachos
    Commented Jul 17, 2023 at 15:39

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there are a truly gigantic number of neutrinos presently in circulation in the cosmos, but note that they only very, very rarely interact with one another so they ordinarily can't develop any sort of neutrino "pressure" that would push back on the generation of still more neutrinos.

The only place where the neutrino population is (briefly) dense enough to lead to a physical neutrino "pileup" is inside a supernova, where it gives rise to a phenomenon called "the pause that refreshes".

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