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Question about dark matter

  1. Dec 19, 2008 #1
    Just a layman's question about dark matter.

    Dark matter, like regular matter, is associated with the gravitational force. Dark matter attracts other dark matter as well as regular matter. Regular matter attracts dark matter also.

    With that said, wouldn't dark matter be all around us? Wouldn't it be part of the Earth, or at least the solar system? Also, since dark matter doesn't interact with regular matter in any way except through the gravitational force, maybe the center of planets and stars contain large amounts of dark matter? The dark matter wouldn't bond chemically to regular matter and would presumably just float on by, straight to the center of large objects.

    I would think this would be the case, especially since there is supposed to be at least 5 times the amount of dark matter in the universe as regular matter.

    I'm sure I'm missing something fundamental about the nature of dark matter, so any explanations will be appreciated.

  2. jcsd
  3. Dec 19, 2008 #2


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    The problem is that just like there is nothing to prevent the dark matter from reaching the center of the Earth, neither is there any thing to cause it to stop there. Ordinary matter tends to clump together to form large bodies because it interacts electromagnetically. So when it collides due to its mutual gravitational attraction it "sticks" together and sheds its excess kinetic energy by radiating it away. Dark matter neither sticks together nor can it shed energy by radiating. As it falls toward the center of the Earth, it picks up kinetic energy, and with no interaction or radiation, it has all that kinetic energy when it reaches the center, and this kinetic energy carries it back out of the Earth. There is no way for it to collect there.
  4. Dec 19, 2008 #3
    Really? So because there is no friction of any sort, all dark matter can do is slingshot through a planet or star?

    Surely dark matter would tend to clump together with itself gravitationally, but let's say a large clump of dark matter fell into a planet. Would we be able to see any perturbation in the planet's orbit as the dark matter clump interacts with the planet and then slignshots away? It would have to be a pretty large clump of dark matter.

    Also, I guess the dark matter would scatter from this interaction since it can't clump together chemically, only gravitationally.
  5. Dec 19, 2008 #4


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    Dark matter won't tend to clump together for the same reasons that it doesn't collect at the center of planets. There is no "friction" between the particles of dark matter that allows for shedding of the kinetic energy.
  6. Dec 19, 2008 #5
    Why wouldn't dark matter clump together gravitationally?
  7. Dec 19, 2008 #6


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    It does to some extent. It's a matter:devil: of degree.

    Nobelist George Smoot has a 20 minute talk you should watch where he traces the clumping of ordinary matter back to the clumping of dark matter.
    Dark matter gathering together was essential to the formation of structure----including concentrations of ordinary matter that became galaxies and clusters thereof.

    Janus is right comparatively speaking. Dark matter doesn't clump very easily because it has only two ways to get rid of excess kinetic energy, that we know of.

    But there is a lot of dark matter, so even if it is awkward for it to clump it plays a very important role in structure formation. Computer dynamical simulations of dark matter show it forming cobwebby structures, strands and voids. It forms the kind of qualitative largescale structure that we observe.

    (This is according to standard mainstream computer models and conventional notions. It's not written in stone, new discoveries could make the pros change their mind.)

    In his talk, Smoot plays computer movies of dark matter clumping---numerical simulations by Kravtsov. Great. Links are probably in the Cosmology sticky. If not I'll dig them up.

    How does dark matter jettison excess kinetic energy?

    Gravitational (e.g. slingshot) interaction. DM can give away KE to ordinary or to other DM. Then it has less and can clump.

    The other DM that took away the extra KE will itself eventually be slowed down by the universe expansion process and may get a second chance to clump. Steven Weinberg has a proof in his Cosmo text that expansion drains kinetic energy. But Hongbao Zhang has a simpler proof posted on arxiv.

    We know that expansion drains energy from light, from radiation---that is the usual cosmo redshift. So it should not be completely surprising that particles with mass are gradually slowed down by expansion (seen from standpoint of observer at rest relative to cosmic Background.)

    We have contour maps showing concentration of vast clouds of dark matter around clusters of galaxies. DM must be able to clump otherwise it would be spread uniformly. But large differences in DM density have evolved. Smoot's talk is great on this.

    Here are some links:

    Kravtsov computer simulations
    I watched the halfsize MP4 version of the movie because it is very easy to download, only about 2.4 MB.[/QUOTE]

    The mp4 version of Smoot's talk is slow to download.
    You click on it, press pause, and go away and do something else for 5 or 10 minutes. Then come back and press run, that way the streaming has a head start.
    Last edited: Dec 19, 2008
  8. Dec 20, 2008 #7
    There is no observational evidence of DM.
    We observe normal matter in the sky, thru matter radiated energy (photons).
    We look to the patterns of distribution of matter and do gravitational simulations usually using the Newtonian approximation model (infinite speed of propagation).
    Things got wrong now, because only after injecting more mass (less than 5% observed and 95% of DM) the observed patterns (galaxy or overall observed universe) got similar to the simulation.
    It is only a parameter in the simulation code, and something yet to be observed.
    As there is no other known acceptable model we stick to LCDM that now reaches more consensuses, but several problems persist.
  9. Dec 20, 2008 #8


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    You appear to be confused, heldervelez, and to have not been paying attention to recent observations mapping DM concentrations by weak lensing.

    This is a trivial matter but your post seems to equate DM with DE (dark energy).

    DM is not estimated at 95%. That would be both darks combined.

    DM is estimated at about 24%.

    Dark matter and dark energy are very different and present different observational challenges, so it is important to distinguish between them.

    Dark matter is comparatively easy to map because it condenses into clouds of higher-than-average density surrounding galaxy clusters, and these clouds can be mapped by weak lensing of more distant galaxies in the background.

    When clusters of galaxies collide, as in the "Bullet Cluster" case, their clouds of DM behave differently from their clouds of ordinary matter.

    Try Wikipedia on "Dark Matter". I will try to help you by getting some links, but you may be able to find adequate ones on your own.

    Yeah, here's something:
    Look at Figure 1.
    By coincidence I've snorkeled in the Caribbean with Maxim, one of the authors. :biggrin: That was a couple of years before he became famous.
    They titled their article in the Astrophysical Journal A DIRECT EMPIRICAL PROOF OF THE EXISTENCE OF DARK MATTER and they weren't kidding. Their work has been supported by other studies with other systems of galaxies.

    Hey! I just tried wikipedia. The article is quite good! It gives the various kinds of evidence that have been accumulating during the past few of years.
    Last edited: Dec 20, 2008
  10. Dec 21, 2008 #9
    So, as dark matter is made of WIMPs, they dont interact - like an ideal gas.
    What is a temperature of that gas?
  11. Dec 21, 2008 #10


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    To me that seems like a beautiful question.
    Excuse me if I seem too enthusiastic. But I really like it.

    What would you say? 3/2 times the average kinetic energy?

    Maybe someone else will jump in on this one.

    I don't want to answer---if I were forced to guess I would say the same temperature as the primordial neutrino background, or almost exactly the same temperature as the CMB but not quite.

    Expansion gradually deprives massive particles of kinetic energy. So the WIMPs must have cooled quite a bit.
  12. Dec 21, 2008 #11
    Hm... But WIMPs are quite heavy.
    I am afraid in that case they would be too slow, so they would not be able to escape even from the Earth gravity. We dont know the mass of the WIMPs, so let me ask my question differently, not 'what is the temperature', but 'how fast do they move'?

    Well, in any case, no matter how fast WIMPs move, there is a very small percentage of extremely cold ones. During 4 billion years, Earth could capture some dark matter cirling inside the core now on different orbits...

    But measuring G using the balls we completely ignore the fact that Earth can contain some percentage of the dark matter.
  13. Dec 21, 2008 #12

    I know there are many simulations of the early galaxy formation. Different % of the Dark matter leads to different results. However, I believe they must somehow define a mass of a WIMP (what if there are >1 sorts of stable WIMPs???)
  14. Dec 21, 2008 #13

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  15. Dec 21, 2008 #14
    Thank you for the link
    So avg speed = 9km/s?
    Temperature = 10000K?
    Doesn't it give us an exact MASS of a WIMP?
    Looks like it is very light (based on these 2 numbers)... I expected it to be much heavier. Heavy enough, for example, to decay into "oh my God" particles.
    Last edited: Dec 21, 2008
  16. Dec 21, 2008 #15
    Well, so far this has been very educational for me. Thanks for all the links. It may take me a while to get through the video. I'll get to that later. Thanks guys.
  17. Dec 21, 2008 #16
    can a black hole capture DM?
    esp the huge galaxy center type of BH
    if so what % of a multi million solar mass BH would be DM ?
    could DM capture explain the huge mass of these BH's?

    if a BH cannot capture DM does that mean DM could be a tachyon?
  18. Dec 22, 2008 #17
    making the history of the % of DM+DE versus matter since the BB cosmology tell us something:
    The existence of DM & DE and the amount (and now Dark Flow ?) had not been derived from any principle nor predicted.

    The initial model of BB disagrees with measured data (now at 96% of darkness versus 4% of matter/light).
    Two ways to evade from this conflict.
    Get another model or supply a different data set and make a patch to the theory.
    A new model is not around the corner. So we are left with the second alternative.

    quoting http://en.wikipedia.org/wiki/Dark_Matter
    It has been noted that the names "dark matter" and "dark energy" serve mainly as expressions of human ignorance, much like the marking of early maps with "terra incognita."[2]
    Recently too there is evidence that there are 10 to 100 times fewer small galaxies than permitted by what the dark matter theory of galaxy formation predicts. There are also a small number of galaxies, like NGC 3379 whose measured orbital velocity of its gas clouds, show that it contains almost no dark matter at all.[9]

    see more puzzles at https://www.physicsforums.com/showthread.php?t=276174
    wolram and markus introduced to us this very recent article
    at http://arxiv.org/abs/0811.4684 (Submitted on 28 Nov 2008 (v1), last revised 22 Dec 2008 (this version, v2)

    I hope that sometime in future we can discuss a model that fits 100% of matter/light, no more, no less. I do preffer this kind of solution.
  19. Dec 22, 2008 #18
    Dark matter is absolutely required for the formation of the galaxies. Without dark matter background radiation would be the same from any directs and no galaxies will be formed.

    If you dont like DM how do you explain it?
  20. Dec 22, 2008 #19
    Yeah. I hadn't thought of that before. Black holes should be gobbling up huge amounts of dark matter. I don't think we'd see any evidence of this because the DM couldn't radiate anything to tell us that it was being sucked in. All that would happen is the mass of the black hole would increase. It would be very difficult if at all possible to observe that sort of thing.
  21. Dec 22, 2008 #20
    I'm not taking any position on the existence of DM, but why would DM in particular be unevenly distributed right after the BB? Wouldn't it be equally distributed just like regular matter?
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