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Okay, clear this up for me

  1. Nov 28, 2005 #1
    I understand that dark matter if it exists is some exotic material that I've never dealt with, but I have a couple of questions
    Can I not see the dark matter because it is dark? If I had a pound of dark matter sitting on the table could I see it? could I touch it? could I see something behind a wall of dark matter?
     
  2. jcsd
  3. Nov 28, 2005 #2
    I believe that dark matter is thought to have exotic properties such as possibly only interacting via the gravitational force (hence, why it has only been 'detected' through it's gravitational influence). If that is the case, I suppose that the dark matter 'on' your table would pass through the table and accelerate towards the centre of the Earth (or, depending how much you have, both the Earth and the DM would accelerate towards each other), presumably settling in a simple harmonic oscillator type system (because of a lack of any frictional forces).
    Of course, this is a highly speculative subject since the nature of dak matter is unknown and the above could well be way off the mark. For instance, if the source of the extra mass were neutrinos (as is one hypothesis), the above situation clearly wouldn't be at all plausible.
    I hope that this is of assistance (and not strewn with errors, I am no authority on the subject).
     
  4. Nov 28, 2005 #3
    thanks, I don't know if it helped much, but that is my fault. I'm just trying to get a mental picture of an exotic material. do you know if the dark matter has to be spread out thinly or could there be planet sized chunks of it? Didn't nutrinos get crossed off the list of possibilities or at least can only account for a small percentage of dark matter
     
  5. Nov 28, 2005 #4

    mathman

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    Based on what is known about dark matter (very little), "chunking" seems highly unlikely. Ordinary matter forms solids and liquids due to electromagnetic force. Dark matter appears not to be subject to this force or anything else, except gravity.
     
  6. Nov 29, 2005 #5

    Chronos

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    Dark matter really is pretty exotic. For one, there is almost none of the stuff in our solar system. Planetary orbits are very nicely explained using plain vanilla newtonian gravity and using the measured mass of the planets [with a sprinking of asteroids, etc.]. Were there even a decent sized planets worth of 'dark matter' dispersed throughout our solar system, the outer planets would have some seriously screwed up orbits, or the theory of gravity would be as weird as quantum physics.
     
  7. Nov 29, 2005 #6

    mathman

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    Dark matter can't form planets or anything else of any size. It exists(?) as individual particles moving freely throughout each galaxy.
     
  8. Nov 30, 2005 #7

    DaveC426913

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    Yeah, mathman is right. I imagine dark matter won't be like a chunk of rock, it'll be like neutrinos or other subatomic particles.
     
  9. Nov 30, 2005 #8

    turbo

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    Review the work of Puthoff, Haisch, Rueda, et al and see if you are willing to contemplate that the particle-antiparticle virtual pairs of the quantum vacuum might fill the bill. They are practically indetectable except via the Lamb effect, the Casimir effect, and related properties. They exist for a very short time before annihilating, and the virtual particles are so bound by their shared debt that they must annihilate with one another (except perhaps when they can be separated a la Hawking Radiation, but that's another story). That satisfies the condition that the entity be invisible and weakly interactive. In the 1960's Sakharov posited that mass, gravitation, and inertia all arise from the interaction of matter with the quantum vacuum field. Hmmm. :rolleyes:
     
  10. Nov 30, 2005 #9

    Garth

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    But gravitationally they should be very strongly interactive - about 10120 times that observed cosmologically.

    Garth
     
  11. Nov 30, 2005 #10

    mathman

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    The virtual pairs in the vacuum may be related to dark energy, which is supposed to explain the observed acceleration in the expansion of the universe, sort of like negative gravity.

    Dark matter is used to explain a positive gravitational effect, such as holding galaxies together.

    Current theory has ordinary matter about 5%, dark matter about 25%, and dark energy about 70% of the total mass of the universe.
     
  12. Nov 30, 2005 #11

    JesseM

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    Would there need to be? As long as dark matter is distributed basically evenly, with very little clumping (although it apparently does clump on galactic scales, forming dark matter halos around galaxies), I would think the density of dark matter would be within a few orders of magnitude of the density of ordinary matter in interstellar space, which is quite low--around 1 hydrogen atom per cubic centimeter according to this page. Anyone know what the average density of dark matter would have to be in the neighborhood of galaxies to explain the galactic rotation curves?
     
  13. Nov 30, 2005 #12

    SpaceTiger

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    To start, let's clarify which theory of dark matter we're talking about. The most popular theories of dark matter involve weakly interacting massive particles (WIMPs). In the context of these models, the answers to your questions are:

    1) No, you couldn't see it. If it interacts very little with normal matter, there is no way for it to produce photons for you to see. Similarly, it can't absorb light from some other source.
    2) As has already been pointed out, it would pass right through the table, not being able to interact with the wood (or whatever).
    3) If it can't interact with the table, it can't interact with your hand, so you wouldn't feel it.
    4) Yes, it wouldn't absorb any of the light from the object behind it.

    One of the other popular theories of dark matter involves black holes. If this were the case, the answers would be:

    1) Depends on the mass of the black hole. A sizable one would noticably distort the light of objects behind it and accrete matter from its surroundings, producing light.
    2) A sizable one would not be good for the table...
    3) ...or your hand.
    4) Yes, but it would be all distorted by gravitational lensing.


    The black holes could be planet sized, but the WIMPs couldn't self-interact enough to produce clumps. Yes, neutrinos do make up only a small fraction of the dark matter.
     
  14. Nov 30, 2005 #13

    SpaceTiger

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    In theory, that could happen with a neutrino (or any massive particle), but your intuition is right in the sense that it would be hard to arrange the energetics such that it became bound with the earth. Neutrinos are what we would call hot (or warm) dark matter because their typical velocities are near the speed of light. The current theories favor cold (low average velocity) dark matter because hot dark matter would have difficulty collecting itself into the dark matter "halos" that we see surrounding galaxies and clusters.
     
  15. Nov 30, 2005 #14

    SpaceTiger

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    Depends on what you mean by "in the neighborhood of". The average dark matter density in the universe is of order 10-6 cm-3. It rises as you get closer to the centers of galaxies and clusters, but still spans many orders of magnitude. Near the earth, it should have roughly the same mass density as the ISM (but that won't be true throughout the galaxy).
     
  16. Nov 30, 2005 #15

    EL

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    The definition I am used to is that what determines if we call the dark matter "hot" or "cold" is wheter it is relativistic or not at freezeout! The reason why hot dark matter is favoured comes from structure formation in the early universe. Observations show that structures have been growing "from small clumps to bigger clumps", which is consistent with nonrelatvistic particles, and not from "large clumps splitting up into smaller clumps", which would be the case for relativistic particles. So in fact "hot" DM need not be relativistic right now, what matters is that it was at freezeout.
     
  17. Nov 30, 2005 #16

    SpaceTiger

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    Well, I was trying to be pedagogical, but I've not seen a consistent definition of "hot dark matter" in the literature. It would seem the most useful definition of "hot dark matter" would be if it's relativistic when galaxy scale perturbations enter the horizon.
     
    Last edited: Dec 1, 2005
  18. Dec 1, 2005 #17

    EL

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    Yepp. That would be the most logical definition, but doesn't that almost coinside with freezeout...after freezeout the DM start building there potential wells, which ordinairy matter later can fall into.
     
    Last edited: Dec 1, 2005
  19. Dec 1, 2005 #18

    JesseM

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    I don't know if it answers your question, but this paper has a nice summary of different alternatives to the "cold, collisionless dark matter" model:
    It also discusses the different predictions that would be made by each of these models, and whether they could be made to fit with the observational evidence.
     
  20. Dec 1, 2005 #19

    SpaceTiger

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    Sure, they do occur at about the same time, but I don't think they're physically related.


    The model we're discussing has since been ruled out by observations, so it's not mentioned in that paper. The closest thing is "warm dark matter", which sits in between the hot and cold dark matter models. The term "hot dark matter" is often used loosely in the literature (much like the term "dark matter" itself), but the basic idea is the same -- high-temperature dark matter particles impede the growth of structure.
     
  21. Dec 1, 2005 #20

    hellfire

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    The size of the horizon at the epoch at which the particles become non-relativistic fixes the smallest scale of the fluctuations surviving free-streaming damping. This smalles scale must be at least of galatic scale (or smaller) in order to fit with observations of matter distribution. So I would guess that the correct definition is the one that relates the transition to non-relativistic behaviour to the size of the smallest perturbations according to observations. You definition relates the transition to non-relativistic behaviour to the freeze out of that particles. If CDM particles may remain relativistic after a very early freeze out, but become non-relativistic before the horizon size is the required one to fit with observations, then both definitions are not equivalent. It it not very clear to me in which extent both definitions are actually equivalent for the currently postulated CDM.
     
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