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CNB (Cosmic Neutrino Background)

  1. Nov 11, 2008 #1
    From a thread on “void theory vs. acceleration” (link in quote below);
    Rather than hijack the other thread for more detail on a side topic, I’ve opened this new thread.

    I’m interested in how science hopes to use “Neutrino observatories”.
    I’ve not seen much on the hopeful physics of making these observations which I see as based on two parts 1) What we hope to observe and 2) How we hope to observe it.

    Fundamentally I assume somewhere between the Big Bang Time of t=0 and the 380,000 years for the time of Last Scattering for the CMB there is a time of Big Bang Neutrino Generation. This should have created a CNB (Cosmic Neutrino Background) with a surface larger than the SLS. Granting that detecting them is a major observational challenge, do we have some idea of what we should be looking for in those observations?

    That is during the time prior to SLS do current theories offer expectations of how many neutrinos should have been generated and when so as to define the size CNB surface, and how many neutrinos from a CNBS (Cosmic Neutrino Background Surface) should be expected now?
    I would expect that various current theoretical views of how to apply the Standard Model and Inflation during the time of Big Bang Plasma involved sould generate differences in theory predictions.

    In addition to how many are generate, an issue I see as in need of predicting is over what time period that CNB would be generated. Unlike the SLS being created during just a few thousand years creating a relatively “thin” observable surface; since neutrinos are very ‘transparent’ thus the observable CNB surface could be notably “thick”.
    Also, this Thick Surface of CNB would clearly be much larger than SLS prior to 500,000 years post Big Bang. However, since we seem to have pretty good confirmation that neutrinos do not travel at “c” the SLS photons at some point may catch up with and pass the CNBS. Meaning the observable CNBS by now might actually be smaller than the SLS.

    So to the question:
    Are there any peer reviewed speculations applying current theories addressing some of the components needed to define a CNB and what might be available to current observation of a CNBS?
    That should be a necessary base from which to design a system to observe those neutrinos.
  2. jcsd
  3. Nov 11, 2008 #2


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    I don't have time to think this thru and give an intelligent answer.

    I think last scatter for neutrinos is around 1 second after putative bang (but inflation scenarios complicate the whole issue of when you say it was---is the bang the moment when inflation stops and the putative inflaton decays, or what).

    Anyway setting aside all this scenario vagueness, neutrino background was released very very early.

    And expansion slows down relativistic particles and saps their energy. So the temperature of the neutrion background is estimated to be about the same as the CMB

    I read about this some years back and am trying to remember. There is some effect which I believe makes the neutrino background expected temperature to be slightly LESS. Maybe 2.4 kelvin or 1.9 kelvin instead of 2.7 kelvin like the CMB.

    Right now I can't do better. I may have the direction of the effect wrong. Maybe someone with a fresher memory will jump in and clarify. In any case it is very interesting.
    Last edited: Nov 11, 2008
  4. Nov 11, 2008 #3


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    Well! There is a Wikipedia bit about CNB

    The estimated time then was 2 seconds.
    the estimated temperature now is 1.95 kelvin

    The reason CNB is so hard to see is that low energy Ns interact with matter even more weakly than higher energy.
    So seeing the CNB is very difficult.

    All these slow neutrinos around us and we can't see them! Eeeww. Jellyfish. Icky! :biggrin:
  5. Nov 11, 2008 #4
    This seems to me that we do not as of yet from a theoretical perspective have a very good idea of how or what to predict should be available for observation as a CNB.

    First comes a reasonable description of just what a neutrino is relative to its behaviors.
    First expectations for them was they were similar to photons being without mass should always have local movement at the speed of “c”. That would be consistent with what is being described in the Wikipedia article, assuming a speed of “c” neutrinos would have a “frequency” like light or photons in the CMB that translates into a “temperature”.

    But I though science had revised that concept of the neutrino for one that does have “mass” – meaning they could be stationary or move at any speed below “c” just as any other elements of mass could do. I would not think the number of neutrinos detected at whatever speed would be translatable to a temperature under what I thought was the current view of what neutrinos are.
    For example in the experiments on sun sourced neutrinos that helped establish they did in fact have “mass” of different amounts based on family type (electron, mu, tau); I don’t recall any of those detected amounts being based on a “temperature” measurement.

    I’m I correct the current view of a neutrino would allow it to assume any speed and even slow enough to be individually captured within the gravitational control of a mass like the earth or sun etc?

    Would not be the first time wikipedia was out of date or not entirely complete in describing the current scientific understanding of something.
    – — — – – —
    Other than what a neutrino is, we have what happen after the Big Bang to generate them. Does the proposed “CνB decoupled from matter when the universe was 2 seconds old” project a total mass for them relative to some volume (like the volume then, which contained the matter and antimatter that eventually reduced to the mass of our currently observable universe)?

    Also, in addition to those "2 sec." neutrinos might there be more added as coming from electron, positron, and quark etc antimatter annihilation possess that occurred well after the first 2 seconds?
    I don’t know where I saw it, but I though some of the modern high energy experiments indicated some annihilations during the BB era should also have produced neutrinos, not just pure photons.
    This is where I derived my speculations about the CNB source “surface” having a “thickness” with neutrinos being generated at different BB time intervals unlike the SLS.
    I’ve not seen that speculation anywhere else, so I can see where my “thickness” idea would be wrong if there were no additional neutrinos produces at various times such as during antimatter annihilations.
    – — — – – —
    Just having a hard time trying to find a complete treatment somewhere of what current theory expects about the nature of neutrinos during the Big Bang and what they should look like now relative to how we could observe them.

    It seems to me that a “low temperature” may not be issue as it does not IMO apply to the travel of something with mass like it does for photons.
    Rather I think the bigger problem in detecting them is in defining how many there should be and just how very slow they might be moving relative to us now.
  6. Nov 11, 2008 #5


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    Randall, your questions make sense. There is plenty I don't understand here. I will keep an eye out for more information about the CNB (which we assume is there but are unable so far to detect). But it will probably be a while before I encounter some more satisfactory information on this. What would really be nice is if some other PFer(s) would show up and do some explaining about the CNB. If you find anything, please keep us posted.
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