Orion1 said:
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.
Orion1 said:
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.
Orion1 said:
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.
