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I know that neutrinos are particles with a very small mass and no electric charge. They infrequently interact with matter and so they can give us information about the "old" universe. But how can they do it?

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Can you clarify yoir question? Are you asking how neutrinos tell us something about the "old" universe, or how we experimentally extract this information? "How can they do it" - who/what are they? If the question is the former, I ask you this: do you know how light gives us information about the "old" universe? –  Will Jun 23 '13 at 13:05
    
@Will You are right.. my original question was about how neutrinos tell us something about the "old" universe, but I don't know how we can experimentally extract this information and I'd like to know it... And no, I don't know how light gives us informations about the "old" universe... –  sunrise Jun 23 '13 at 13:14
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The experiments are quite interesting (I recommend looking them up if you're interested). There are a range of different types of neutrino experiments, but they all usually involve a large body of material (due to the "infrequent" interaction you speak of). What kind of information are you wanting to get? Light from a distant star gives us information about that star in the past due to the finite speed of light. Similarly, neutrinos travel at $v<c$ and so give information about object in the past. –  Will Jun 23 '13 at 13:24
    
@Will I'd like to know why neutrinos are important for the knowledge of the universe and what kind of information they can give us about universe –  sunrise Jun 23 '13 at 13:29
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Are you asking about relic neutrinos? en.wikipedia.org/wiki/Cosmic_neutrino_background Have you read the wikipedia article? –  Jess Riedel Jun 23 '13 at 17:40
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2 Answers

Note: I am not an expert on neutrinos, so if I have missed anything, someone please let me know.

For one thing, the knowledge of the mere existence of neutrinos is important for a full understanding of our universe. Also you may have heard that for a long time neutrinos were thought to be massless, but the observation of neutrino flavor oscillation requires them to have mass. This is a bit of a problem as the standard model of particle physics doesn't account for non-zero neutrino masses, which is an indication of some new physics, beyond the standard model.

As you mentioned, neutrinos are very weakly interacting with other matter, making it hard to detect them, which is why neutrino detectors are built on such large scales (see http://icecube.wisc.edu and http://www-sk.icrr.u-tokyo.ac.jp/kam/kamiokande.html for example). But this can be useful in terms of things we may want to find out about our universe. This very weakly interacting behavior enables physicist to see the neutrino spectrum from a star, supernova, gamma ray bursts, etc. virtually unchanged on the way from the object to Earth. Meaning that information about objects very far from Earth (and therefore far back in time) can be found where conventional techniques using light are less successful (for example if there is dust or some other object in between Earth and the object of interest - the light would interact whereas the neutrinos pass through pretty much unchanged).

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Looks good to me. One might also add, in reference to the "old" universe, that neutrinos propagated through the early, opaque universe, so you can see primordial ones from a time before the oldest photons. That said, this cosmic neutrino background may very well never be detected. –  Chris White Jun 23 '13 at 19:15
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In the "old" universe there is interaction of matter by the the 4 fundamental forces. Neutrinos occur in the weak nuclear interactions. By studying the neutrinos one can study these weak interactions. They might not interact much, but they sure do carry energy.

So by a application of the conservation of energy one could determine the loss to neutrinos and try to link these to the interactions that they might think that occur. For example the nuclear fusion in the sun that converts hydrogen into deterium:

$p+p\rightarrow^2_1H+e^++\nu_e+0.42MeV$

gives an electron-neutrino. These neutrino's don't interact and the amount of energy of the neutrino is hence determined by the conservation of energy.

One could construct a model for the "old universe" and determine the energy of the neutrinos, as the missing energy. This could give some extra information I believe.

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If someone downvotes it might be instructive to clarify his/hers reason of the downvote. That way I can see where I went wrong and won't make the same mistake twice :). –  Nick Jun 23 '13 at 17:25
    
Hey Nick, I accidentally clicked the downvote when I was commenting on the original question. But this was a while ago so I can't change my vote unless you edit your answer. How about you change neutrino's to neutrinos in your answer and I can remove the downvote. Cheers, Will. –  Will Jun 23 '13 at 17:45
    
Wow yeah indeed, check :p –  Nick Jun 23 '13 at 17:46
    
There you go. You might also want to clarify the last sentence. I find it a little confusing. (From "try to link..." on). –  Will Jun 23 '13 at 17:52
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