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CMB versus neutrino BG

  1. May 1, 2006 #1
    Hey,

    Was reading how the CMB is ~2.75 Kelvin, while the Neutrino BG is ~2 Kelvin. Clearly this means that neutrino transparency occured before photon transparency did in the early evolution of the universe. My question is why? Does is have anything to do with neutron-to-proton decay?

    I'm guessing (probably incorrectly) that after neutrons and protons cooled sufficiently to bind in nuclei, the neutron was now stable and therefore stopped decaying into protons (as free neutrons do) + electrons + electron antineutrinos. And it was these antineutrinos that ceased to be produced, which gave normal neutrinos free reign in the universe without fear of being annihilated by their antineutrino counterparts. So then neutrino transparency was achieved and the neutrinos were free to redshift to today's cool 2 degrees Kelvin? I wonder if this makes sense.

    Help appreciated, thanks.
     
  2. jcsd
  3. May 1, 2006 #2

    SpaceTiger

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    Are you sure you understand this? The argument deals with conservation of entropy and counting particles in thermal equilibrium.


    The answer to this question actually is simple. Neutrinos, lacking electric charge, can only interact through the weak force (and, presumably, gravity), so their low-energy interaction cross sections are generally much smaller than those for electrons and photons.
     
  4. May 1, 2006 #3

    Why are you picking on my first comment. Speak to this comment, 'I'm guessing (probably incorrectly) that after neutrons and protons cooled sufficiently to bind in nuclei, the neutron was now stable and therefore stopped decaying into protons (as free neutrons do) + electrons + electron antineutrinos. And it was these antineutrinos that ceased to be produced, which gave normal neutrinos free reign in the universe without fear of being annihilated by their antineutrino counterparts. So then neutrino transparency was achieved and the neutrinos were free to redshift to today's cool 2 degrees Kelvin? I wonder if this makes sense.
    '

    Does that make sense or no?
     
  5. May 1, 2006 #4
    So why did neutrino transparency occur before photon transparency then?
     
  6. May 2, 2006 #5

    SpaceTiger

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    You can find a decent explanation here.
     
  7. May 4, 2006 #6
    St

    Ya, I've read that. I really love that website, I've known about it some time now and its great at explaining physics in a semi-lay manner.
     
  8. May 21, 2006 #7

    marcus

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    hey Order, if you know the Georgia State site and you know that explanation of the factor of 1.401, then maybe what your question is you simply want to TALK ABOUT IT with someone. this is a standard non-hierarchical self-teaching approach where you just get somebody and go over it with them

    neither person is actually the "teacher" per se, in that case

    So go ahead and tell me about it:smile: I will listen gladly.

    I really like that webpage and that simple idea of a jump in temp by the 1.4 factor (if there are any people who like to quibble about correctness of language, they might quibble with my saying "jump" so I better watch out.)

    Instead of a jump it is more that the DECLINE IN TEMP that you expect with expansion is effectively DELAYED some by the e+ and e- recombination event---so that what results is that after that event all the temperatures are a factor of 1.4 HIGHER than you would have expected or that would have been the case absent that recomb event

    so after that recomb ALL THE TEMPS ARE JACKED UP BY THAT FACTOR compared with the OLD formula relating expansion temp.


    OK you can probably say it better, or correct some mistake I've made, or say something else to fill in the picture. So try.

    And neutrinos date from BEFORE that event, so their temp goes down a different curve-----as the space expands

    While photons date from AFTER that event, so their temp goes down a jacked up temperature curve

    step by step the same, except for being hotter by a 1.401 factor, and everybody getting colder and colder until photons are 2.7 and neutrinos are whatever they are (less by a 1.401 factor)

    My copy of the first 3 minutes is gathering dust upstairs, but it is a great book it seems----probably is, a lot of people think so---and that webpage appears to be a snack of Weinberg

    ======================

    now the entropy question. do you, the lil bro of Chaos, want to explain that part? it would seem appropriate considering the dichotomy of your name:smile:

    WHY does the temp have to be higher AFTER the recomb event (when there are effectively fewer particles)?

    Is this the kind of discussion you wanted to have? If so, please do some paraphrasing work yourself. (if not just ignore. 's cool)

    ================
    Georgia State
    http://www.phy-astr.gsu.edu/new_web/newmain.html

    Hyperphsics about the neutrino transparency temp
    http://hyperphysics.phy-astr.gsu.edu/Hbase/astro/neutemp.html
     
    Last edited: May 21, 2006
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