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Dark Matter Clumping

  1. Oct 16, 2011 #1
    This has been discussed in other topics but not without leaving some issues unanswered. So DM has only gravity and does clump. Clearly even if it reached stellar density it would not intiate the usual nuclear 'burn' point because there are no electromagnetic/nuclear charges involved. But if it cannot be held apart by radiation pressure then why would it not carry on becoming denser - even to black hole proportions? I've read various vague notions of 'the particles pass through each other' etc but the consequences of gravitational attraction are increasing density or orbiting behavoir. This outcome has not been observed to be the case with DM so whats happening?
    Thanks.
     
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  3. Oct 16, 2011 #2

    BillSaltLake

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    A related, but opposite, question is "why does DM clump at all"? Compare the behavior of a hydrogen atom that is in space but is attracted to a planet a few thousand miles away. The atom may be attracted and will then pick up significant speed before hitting the atmosphere or surface. Then it will stop relative to the surroundings. Thus baryonic matter will clump.
    DM particles don't stop like that. If a DM particle is at rest with respect to a large nearby mass, the DM will accelerate and pass through, reaching max speed in the center and then going all the way through. It will stop at an opposite point the same distance away as origionally. The only energy loss (probably) is gravitational radiation, which in most cases is negligible (I think it would work out to only a small loss within billions of years for a planet the size of Earth). Then this "orbit" (straight line in this example) will repeat periodically. The DM particle's average distance from the planet center is only somewhat decreased by this attracton, but if the planetary mass continues to increase, the average DM particle distance will decrease further.
     
  4. Oct 16, 2011 #3
    I see. This would then be the sort of invisible cloud of DM in and around galaxies detected by gravitational lensing observations. The DM being in various forms of near perpetual 'orbit' like what you describe. So I guess the decay is so slight that dense clumping has not yet happened? Wonder if anyone has modelled this for various parameters of 'mass' and decay energy to see how it should have played out over time - observations at increasing distances (ages) would show potential anomolies in these 'clouds' of DM, wrt the baryonic matter behaviour that is.
     
  5. Oct 16, 2011 #4

    phyzguy

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    There are a large number of people doing such simulations, and they reproduce the statistical behavior of our current universe pretty well. This is one of the (many!) reasons that most astrophysicists believe that the Lambda-CDM model is a good model and that dark matter really exists. I believe that these simulations assume that the dark matter does not decay, that it interacts with itself and with baryonic matter only through gravity, and that the dark matter particles are 'cold', meaning non-relativistic. Unfortunately, these simulations do not constrain the mass of the dark matter particles.

    Here are some links to two different modeling runs:

    http://www.mpa-garching.mpg.de/galform/millennium/
    http://arxiv.org/abs/astro-ph/0504097
    http://astro.kias.re.kr/Horizon-Run/
     
  6. Oct 16, 2011 #5

    BillSaltLake

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  7. Oct 16, 2011 #6
    Thanks for that. I should have made clear that I meant decay of the DM 'orbits' over time not actual DM particle decay.
     
  8. Oct 16, 2011 #7

    Chronos

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    Dark matter is believed to be weakly collisional, which implies dark matter halos surrounding galaxies should have a density profile - i.e, overdense regions tend to form. One of the seminal papers on this phenomenon is by Navarro, et al -

    The Structure of Cold Dark Matter Halos
    http://arxiv.org/abs/astro-ph/9508025

    This is the basis for the NFW profile that is still widely used in dark matter modeling. Another interesting discussion can be found here -

    http://scienceblogs.com/startswithabang/2010/06/convincing_a_young_scientist_t.php [Broken]
     
    Last edited by a moderator: May 5, 2017
  9. Oct 17, 2011 #8
    ..'related, but opposite, question is "why does DM clump at all"?' .. thats right Billsaltlake. However weak the interaction, it has reached the point of clumping, presumably from a more diffuse state. And therefore (assuming the attraction continues) on its way to some even more dense 'clumpy' state. So the observations over various distances of DM infused objects should show this clumpiness process evolving over time? Again, presumably then, a clump being the sum of the attraction forces of the individual DM particles (however weak) should display greater net attraction force - and grow/attract other clumps.
    Sorry, I'm now 'rambling' too far ahead of the evidence - but interesting speculation nevertherless.
     
  10. Oct 17, 2011 #9

    phyzguy

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    I don't understand what you are saying here. DM definitely interacts through gravity, that's how we know it exists. Gravity, being an attractive-only force, will cause regions of higher density to grow denser and regions of lower density to grom less dense. The only question is whether there are other, dissipative interactions between the DM particles, and my understanding is that today's observations are consistent with the only interaction being gravity.
     
  11. Oct 17, 2011 #10
    As I said I'm rambling ahead of the evidence with a few unfinished thought trains and need to go back to some of the research cited above to get a better grasp of the issues.
     
  12. Oct 17, 2011 #11
    --------------------------------------------------------------------------------

    Thanks 'Billsaltlake' and 'phyzguy' for the reference below, it takes care of most of my question (for now!)
    'http://www.mpa-garching.mpg.de/galform/millennium/0504097.pdf' [Broken]
     
    Last edited by a moderator: May 5, 2017
  13. Oct 17, 2011 #12

    Wez

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    Just a thought, but we conceive what we see in our universe is sitting on a fabric of space time, but a fabric has two sides, so what sits on the other side of the fabric ?, I would suggest an oppersite, but that in turn must have a fabric, but not ours so it too sits on a fabric of it's own and so on, at diminishing "influential " returns so is it possible dark matter is a single or multiple reflective universe(s) , what I'm suggesting is there are more than one universe that is some how linked, the mass we desribe as dark matter is a matress of multible time space fabrics that were born at the same time or perhaps and more likely at a different time but some how there is a "physics" why they still influence each other hence the " dark matter" or for that dark energy is perceived to exist.
     
  14. Oct 20, 2011 #13
    Maybe I'm missing something here (which is quite possible I might add) but we have inferred the existence of dark matter due to the velocities of stars at the edges of galaxies being too fast. We see this in contrast to the velocities we see in the solar system, for example, where the velocity of planets fall in line with Keppler's Law and the farther a planet is from a star the slower it traverses its orbit around the star and hence the planets have different orbital velocities.

    But why would this contrast actually exist? I don't understand how there can be so much dark matter around a galaxy such that the stars do not orbit the galactic centre in line with Keppler, yet there is not enough dark matter to influence the planets around stars so that Keppler's law applies. If dark matter is affecting the velocites of the stars themselves around the galactic centre then why does this effect not transmit to planets around stars as well?
     
  15. Oct 20, 2011 #14

    phyzguy

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    This is a good question. It has to do with the low density of dark matter and the huge difference in volumes between the solar system and the galaxy. The volume of the solar system inside Pluto's orbit is ~10^39 m^3, while the volume of the galaxy inside a radius of 10 kpc is ~10^62 m^3. Assume for the sake of argument that the density of dark matter in the galaxy is ~.01 Msun/pc^3. It isn't a constant density, but this will illustrate the point. Then the total mass of dark matter inside Pluto's orbit is ~10^-13 Msun, which is completely negligible. On the other hand, the total mass of dark matter inside a radius of 10 kpc is >10^10 Msun, which is comparable to the mass of stars. The dark matter density falls of slowly as you go to larger and larger radii, so it becomes more and more dominant the further out you go.
     
  16. Oct 20, 2011 #15

    phinds

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    wez, welcome to the forum. You might want to actually READ the rules you agreed to when you signed on. I think most people don't at first (I know I didn't) but there is a definite rule against overly speculative personal opinions and that's what you have in the paragraph above. The moderators have a low tolerance for this sort of thing.
     
  17. Oct 20, 2011 #16

    marcus

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    Bigmig, Phyz, Bill, Phinds...

    I don't think anyone has mentioned that expansion of distance by itself can drain momentum (and KE) from particles. We already know it drains energy from light---the CMB has been redshifted by factor of 1100 so a CMB photon has lost 99.9% of its energy.

    So a dark matter particle can lose kinetic energy gradually over a long period of time just because of the geometry expanding. It slows down relative to universe rest.

    The significance for clumping I guess you could say is this. Clouds, when they condense, need a way to dump energy. Ordinary matter has collisions and gets warm and radiates heat. But DM cannot.

    DM can interact gravitationally with itself by "gravity bootstrap" encounters that give some DM particles excess velocity so they escape, while other particles lose velocity and are then more securely bound.

    The DM particles that carry away the excess energy then may travel for a long time until they are slowed down by expansion. then they may participate in formation of another cloud.

    The point is that condensing DM has a way to dump excess energy analogous to ordinary matter radiating heat. It can do it entirely by gravitational interaction---with a portion of the DM carrying away excess energy.

    So you don't HAVE to assume some other mechanism---although I don't doubt that some of the simulations do assume some other kind(s) of interaction besides gravitational.
     
  18. Oct 29, 2011 #17
    Marcus I was intrigued by the SA article dealing with models of space-time, and ending with speculation on a possible fractal/size dependent dimensionality to space-time you referenced in your reply (hopefully still above). To save me wading through lots of stuff have you any references for more recent development of these ides?

    Thanks.
     
  19. Oct 30, 2011 #18

    phinds

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    Marcus, I'd appreciate some small expansion (no pun intended) on that statement regarding the bolded part. I don't get how the movement of a DM particle through the expansion slows it down. Or is is just that is that it is traveling away from its point of origin less rapidly than other further away objects which means it slows down in a relative sense? Seems to me only gravity would actually slow it down in any absolute sense, relative to its point of origin.
     
    Last edited: Oct 30, 2011
  20. Oct 30, 2011 #19

    BillSaltLake

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    On average, the momentum of particles (both with and w/o mass) decreases over time, and is ~proportional to the inverse of the expansion parameter. This means that particles with mass will slow down relative to the CMB frame.
     
  21. Oct 30, 2011 #20

    phinds

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    OK, I can accept that what you say is true (thanks for the response, by the way) but I still don't get the ramifications. Let me rephrase the question.

    A DM particle is in a galaxy and is expelled outward at a given speed relative to the edge of the galaxy. Are you saying that DUE TO EXPANSION its speed relative to that edge decreases? Are you even SAYING that its speed relative to that edge decreases? I can't get how to relate that to "slowing down relative to the CMB"
     
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