# Cosmic Neutrino Background

1. Aug 9, 2006

### 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?

Last edited: Aug 9, 2006
2. Aug 20, 2006

### wandering.the.cosmos

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.

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.

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.

Last edited: Aug 20, 2006