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Where have all the neutrinos gone?

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  1. Dec 13, 2013 #1
    Where have all the neutrinos gone?

    I’m no cosmologist, and my understanding of nuclear physics is pretty primitive. I haven’t yet
    found answers to some simple questions:

    Where have all the neutrinos created so prolifically by stars over the last 13,8 billion years gone?

    Are they still pervading the universe as a faint background flux of rarely-interacting almost imperceptible entities? Has the status of neutrinos as ‘hot’ dark matter or energy been quite ruled out?

    Does their proliferation in any way assist or drive the expansion of the Universe, possibly as mandated by the virial theorem?
     
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  3. Dec 13, 2013 #2

    Drakkith

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    They're still out this screaming through space. As far as I know, they have been ruled out has being dark matter.
     
  4. Dec 14, 2013 #3

    marcus

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    Drakkith is right to say they are still out there, Paulibus. But they would represent only a small fraction of the total.

    If you google "peebles inventory" you will get a breakdown of the contents of the universe into various sorts of particles and radiation. That should give some perspective.

    You know that ordinary matter is only a small fraction. Stars made of that small fraction have converted only a small fraction of their original hydrogen and helium into other species (so far) and only a fraction of their product is neutrinos. So the neutrino share (produced by stars) would be small. It could be counted in with dark matter as a small "hot" component but it wouldn't change the size of the problem. We still need to find out what MOST of the DM is made of. You've got me curious so I'll google "peebles energy inventory" and check out the neutrino percentage.
     
    Last edited: Dec 14, 2013
  5. Dec 14, 2013 #4

    marcus

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    When I google that I get
    http://arxiv.org/abs/astro-ph/0406095 "The Cosmic Energy Inventory"
    Category 8 is "stellar neutrinos" which is what you asked about (the ones made by stars in various ways) and they say that is less than 1/100,000 of the total inventory. Less than 1/1000 of one percent.

    They also estimate another category, "primeval thermal remnant neutrinos" left over from big bang. That is category 2.2 and it is around 1/1000 of the total.
     
  6. Dec 14, 2013 #5
    Thanks Marcus. That clears it up for me nicely. I was too impressed by the flux of solar neutrinos streaming through each of us day and night!
     
  7. Dec 14, 2013 #6

    marcus

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    The neutrino flux from stars is indeed impressive! It's just that in the overall energy makeup of the universe it's a small fraction. But in a core collapse type of supernova the energy release in form of neutrinos is huge IIRC. It would be worth looking up.

    I'm fascinated by neutrinos, including the possibility that there are right-chiral species of them which (if considerably more massive) could constitute DM.
     
    Last edited: Dec 14, 2013
  8. Dec 14, 2013 #7

    marcus

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    It's a good general area in which to keep asking questions. If you google "peebles inventory" and look at category 8 "stellar neutrinos" you see:
    nuclear burning 10−6.8
    white dwarf formation 10−7.7
    core collapse 10−5.5

    That points to something really astounding. If you add up all the normal everyday neutrino flux from all the fusion in all the stars that have ever existed, it is only about ONE TENTH the amount of neutrinos that have been produced by the comparatively few brief SUPERNOVA events that have occurred.

    I think you probably know that the majority of stars are less massive than the sun, which itself is not massive enough to go supernova. Only a tiny minority of stars ever end with a core collapse explosion. But each such event results in a huge burst of neutrinos. That burst is part of what blows off the outer layers of the star, which seems incredible because we think of neutrinos as able to pass through dense material with very low probability of collision. If they don't collide they can't exert any pressure. But in a SN explosion there are so many produced in the core that even with a very very low collision probability enough nus collide, on their way out, that they exert enough force on the outer layers to blow them off. I should fetch a source for that, it sounds so incredible.

    Here's an excerpt from the Wikip. article
    ==quote http://en.wikipedia.org/wiki/Supernova ==
    If the core mass is more than about 15 solar masses then neutron degeneracy is insufficient to stop the collapse and a black hole forms directly with no supernova explosion.
    In lower mass cores the collapse is stopped and the newly formed neutron core has an initial temperature of about 100 billion kelvin, 6000 times the temperature of the sun's core.[71] 'Thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos.[72] About 1046 joules, approximately 10% of the star's rest mass, is converted into a ten-second burst of neutrinos which is the main output of the event.[70][73] The suddenly halted core collapse rebounds and produces a shock wave that stalls within milliseconds[74] in the outer core as energy is lost through the dissociation of heavy elements. A process that is not clearly understood is necessary to allow the outer layers of the core to reabsorb around 1044 joules[75] (1 foe) from the neutrino pulse, producing the visible explosion,[76] although there are also other theories on how to power the explosion.[70]
    ==endquote==

    Anyway, I think we should stay interested in neutrino astronomy/cosmology for a while. I have a hunch it will open the door to some new physics.
     
    Last edited: Dec 14, 2013
  9. Dec 15, 2013 #8
    My interest in neutrinos is growing from my small knowledge-base, thanks Marcus. It seems as if there's more to learn about neutrinos and neutrino physics than ever met my eye!

    Neutrinos must nevertheless form a permanent but growing store of highly mobile mass/energy in the universe as stars shine and especially, as supernovae form, adding to neutrino numbers. Neutrinos don't decay? and, once created, interact effectively only rarely (as supernovae form?) with existing concentrations of mass/energy.

    But this store, as you point out, is only a tiny and, I now realise, relatively insignificant constituent of the 'Cosmic Energy Inventory'.
     
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