What are the Origins and Characteristics of Cosmic Neutrinos?

[Orion1]

Cosmic Neutrino Background...


What was the temperature and age of the Universe, when it became 'transparent' to cosmic neutrinos?

From what type of nuclear reaction did cosmic neutrinos originate?

What is the peak temperature of cosmic neutrinos?
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[wandering.the.cosmos]

What was the temperature and age of the Universe, when it became 'transparent' to cosmic neutrinos?
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About a few MeV, if you use units of energy for temperature. Below this temperature, neutrinos "de-couple" from the rest of the cosmic soup, though they still react with neutrons and protons, and so still affect nucleosynthesis.

From what type of nuclear reaction did cosmic neutrinos originate?
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Originate I'm not sure, if you mean how the very first neutrinos were produced. In fact nobody knows the answer to such questions, for any type of particle; we just know that at high T we ought to have a soup consisting of all particles that exist.

But as for reactions involving neutrinos, there are for example things like

\nu_i + \bar\nu_i \leftrightarrow \bar{l}_i + l_i
\nu_i + l_i \leftrightarrow \nu_i + l_i
\nu_i + \bar\nu_i \leftrightarrow \nu_j + \bar\nu_j

where l_i denotes the ith flavor lepton (e,muon,tau). There are also analogous reactions between neutrinos and quarks.

What is the peak temperature of cosmic neutrinos?
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The temperature of the universe is a monotonically decreasing function of time, so I'm not sure if it makes sense to ask what the maximum T of neutrinos are.