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Is Dark Matter Neutrinos

  1. Aug 26, 2009 #1
    Could dark matter be neutrinos? I'm wondering, neutrinos are weakly interacting, but they do respond to gravity. So has anyone calculated how far out of a galaxy a typical galactiaclly produced neutrino would travel before coming back into the galaxy? Are some galactically produced neutrinos gravitationally bound to their galaxy?I suppose that they would move very slowly at the outer edge of a galaxy, and very fast near the center. So most of them would spend most of their time at the edges, like dark matter, right?
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  3. Aug 26, 2009 #2

    George Jones

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  4. Aug 26, 2009 #3
    Thanks. You say neutrinos move too fast to account for structure formation. However, if neutrinos cannot escape a galaxy, then they can come to rest before falling back in. If there can be non-moving nuetrinos, then how would you detect them, and how many are around us? How many would be created by the Big Bang, and how fast would they be going today if they were created everywhere at the same time and have cooled with expansion? Since they have very little mass, wouldn't they tend to move faster under the influence of gravity and collect before other kinds of matter?
    Last edited: Aug 26, 2009
  5. Aug 27, 2009 #4


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    Well, neutrinos are thermally produced, and so we can say to a fair amount of precision how fast they tend to move. Granted, there is a lot of uncertainty due to the fact that we don't know their masses precisely, but what we do know is that their masses are far, far too low to account for the dark matter.

    So, neutrinos are basically out. But the dark matter could very well be another particle that behaves very much like a neutrino, but has more mass (such as the lightest supersymmetric partner, perhaps a gravitino or neutralino). Or it could be a particle with similar or even less mass as long as that particle is produced by some other mechanism that gets the right abundance and a lower temperature (such as an axion).
    Last edited: Aug 27, 2009
  6. Aug 27, 2009 #5
    I'm not sure how we could calculate the amount of neutrinos created by the big bang. Do we even know the original amount of antimater created in the big bang? I understand we might be able to account for nuetrinos created by fission and fusion process, but wouldn't there be some created originally before fission and fusion when all the other protons and neutrons were created? How much would that be? There would be some created when antimater was created, but when the antimatter annihilated, the neutrinos created in that process would have remained.
  7. Aug 27, 2009 #6


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    Well, like I said, it's thermal production. When you have temperatures that are much higher than the masses of the particles in question, and those particles are interacting rapidly, you can calculate quite accurately how many of the particles there are sitting around, and what their average energy is.

    What happened with the neutrinos is that when the temperature dropped low enough (meaning significantly below the masses of the W and Z bosons), the weak force "turned off", and the number of neutrinos around at that time basically just stuck around. They've been free streaming ever since.

    Therefore the current density and temperature of neutrinos is tightly constrained by how the weak nuclear force behaves, which we know from particle accelerator experiments to a rather high degree of accuracy.
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