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What would happen, if there was (hypothetically, for any reason) a very large number of neutrinos+antineutrinos before the inflation period. They probably dont like to annihilate too much.

I would expect they survived the inflation, their energy would probably go towards their rest mass. And they would become undetectable at low energies (is that correct?). Could such a thing be the dark matter?

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  • $\begingroup$ Look up the "cosmic neutrino background". They decoupled early and are at very low energy. Models for how they might be detected float around from time to time and I suspect that the most recent round of direct dark matter experiment may bring the technology along far enough to make some progress on that. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Apr 16, 2015 at 0:50

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I would think - but could not put my hands on any evidence to back this assertion up - that such neutrinos would be unavoidably thermally coupled to the rest of the universe prior to 1 second [before this, the neutrinos are sufficiently coupled to electrons and positrons through weak interactions that they follow their temperature]. So, even though cooled massively by the inflationary expansion, they would be reheated, become part of the thermal equluibrium mixture in the early universe and become indistinguishable from the "conventional" neutrino background.

The numbers of decoupled neutrinos deduced (from the properties of the weak interaction) in this background are too few to explain the dark matter. They are also too "hot" at the epoch of radiation/matter decoupling to explain the rapid formation of structure.

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  • $\begingroup$ Does it mean that they would be re-heated by the weak interactions after the cooling? Isnt the weak interaction too weak for that? I try to understand, is it possible to determine - from the properties of weak interaction - the existence of those supercold neutrinos? $\endgroup$
    – jaromrax
    Commented Apr 15, 2015 at 11:50
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    $\begingroup$ Prior to 1 second, weak interactions sufficiently couple the neutrinos to other leptons. Excess neutrinos/anti-neutrino pairs could interact to form electron/positron pairs and ultimately photons. $\endgroup$
    – ProfRob
    Commented Apr 15, 2015 at 12:03

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