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Dark matter pooling

  1. Mar 5, 2008 #1
    If dark matter only interacts with baryonic matter gravitationally, would it pool at the center of stars, planets, and other ordinary mass objects? That is, suppose there was a particle of dark matter, whatever that is, at rest at the surface of the earth. Since it would not interact with normal particles, it seems that it would gravitate down towards gravitational equilibrium at the center of the earth. If so, would we detect such pooling by finding gravitational forces larger than would be predicted by Newtonian physics? Or does the gravitational constant already include, without realizing it, whatever dark matter has pooled?
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  3. Mar 6, 2008 #2


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    I am not sure what you mean by "pooling". However dark matter particles, which are considered to be in motion, just pass through planets and such untouched. For example if such a particle approached the center of the earth it would speed up as in penetrated the earth and would slow down past the center. By the time it reached the surface on the other side it would have the same velocity as when it entered. Neutrinos interact very waekly with ordinary matter and are "seen" to have this kind of bahavior.
  4. Mar 7, 2008 #3
    I think I know what nesp means, because I have been wondering about the same thing. The idea would be that dark matter shouldn't just pass through large bodies like planets or stars, but because of gravitational attraction iit should spiral inward and eventually just settle near the gravitational center. Yet the models I've seen say this doesn't happen; the dark matter instead hovers in a huge homogeneous cloud in and around galaxies without pooling inside baryonic bodies. Why would this hovering occur? It seems counter intuitive to me, too.
  5. Mar 7, 2008 #4


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    The thing is, dark matter does not clump like ordinary matter. When the stuff reaches the center of a gravity well, it passes right through to the other side. It is unclear if this applies in the case of black holes, but it very well may.
  6. Mar 7, 2008 #5
    so IF I have this corect
    dark matter does not attract it's self ie ''clump''
    and normal matter does not attract dark matter ie ''pool''
    BUT dark matter attracts normal matter ONLY
    in a one way action?
  7. Mar 7, 2008 #6
    sysreset explained pretty well what I meant by "pooling." Settling at gravitational wells might be a better description. What you say about "passing through" makes sense, if the dark matter particles (again, assuming there are such things) are massless (as with the neutrino) and/or move with a velocity high enough to overcome the gravitational attraction of normal matter.

    Is there evidence that dark matter behaves in this way? If it did, it seems we would not find dark matter concentrated around galaxies, its particles would distribute throughout the universe. Chronos says dark matter doesn't cluster like normal matter, yet the recent gravitational lensing reports infer that it does cluster in the sense of there being more such matter around galaxies and, whatever it is, it creates enough of a gravitational effect to alter the galaxy's rotational speed and to bend light rays.

    So, dark matter seems to congregate around galaxies of normal matter, affects the galaxy gravitationally, but it does not interact physically with that normal matter. Yet it also affects spacetime in the sense that it bends normal photons. It still appears to me that there should be dark matter pooled within these wells, even if that matter doesn't interact with normal matter. Since the earth is one such well (very small in comparison with the galaxy), seems there might be some dark matter at its center.
    Last edited: Mar 7, 2008
  8. Mar 7, 2008 #7


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    There is no assumption that dark matter particles are masslees - neutrinos do have mass. The point I was making that things which don't interact with ordinary matter (such as dark matter and very weakly as neutrinos) would tend to go in paths that end up coming out as fast as they went in. If "pooling" means traveling in some sort of spiral path, I can't see how it occurs - what force is leading to this wierd path, when they are going in straight lines? Gravity is the only thing that effects them, so that when subject primarily to earth's gravity, the would follow some sort of two dimensional (conic section) path.
  9. Mar 7, 2008 #8


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    Think about it like this; the Earth is attracted gravitationally to the Sun yet it doesn't get any closer to the Sun as time goes on. This can be understood as being because the potential energy of the Earth-Sun system is in balance with the kinetic energy of the Earth's orbit around the Sun. If the Earth got closer to the Sun it would lose potential energy, thus gaining kinetic energy but the it would be moving too fast for its new smaller orbit.

    There are many systems that undergo gravitational collapse, such as galaxies, new stars etc. The common thread through these processes is that there must be a mechanism for getting rid of the angular momentum of the collapsing material to allow it to collapse. All of the mechanisms are processes that dark matter does not partake in, such as radiation or viscous interactions that transport angular momentum radially outwards in a collapsing system.

    Therefore, while dark matter clusters on large scales (larger than galaxies) it does not form highly dense collapsed objects such as stars or planets due to its inability to shed its kinetic energy and momentum.
  10. Mar 8, 2008 #9
    Aha! Thanks, mathman, I think I'm beginning to see where I was going wrong in my thinking. What you're saying is, if dark matter particles have no interaction with ordinary matter, there would be no external force (e.g., drag, collisions) to change whatever velocities they have, so at best they would remain in a conical orbit around some gravity well. Said another way, as Wallace points out, they would have no way of shedding kinetic energy and momentum, which would be necessary for them to spiral and pool in some gravity wells or, said in a better way, to form dense collapsed objects.

    Which brings up the question of how normal matter sheds kinetic energy and momentum, and why dark matter does not. To expand on your explanation, even with normal matter, gravity by itself does not work to collapse the conical orbits, that is done through attractions and collisions between particles, eg in stellar formation, and I suppose through disruptions at the atomic level. It seems this is all action at the electromagnetic force level and, if so, would I be correct in thinking that one essential difference between normal dark and normal matter is that the former responds to both electromagnetic and gravitational forces, wheras the latter appears to respond only to gravitational forces?

    That helps me understand how dark matter that happens to be surrounding a galaxy orbits the common center of gravity of dark + normal matter but, since it doesn't interact with the normal matter in any other way, it remains there only affecting rotational velocities. Strange stuff.
  11. Mar 8, 2008 #10


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    That's pretty much the crucial difference between 'dark' and 'normal' matter. Dark matter only interacts via gravity, not the electromagnetic, strong or weak forces.
  12. Mar 9, 2008 #11
    OK, I think I understand better now. If I may, I'd like to restate my original question, which is about the possible effects of dark matter on local and universal gravitational constants.

    When we experimentally measure g(earth)=9.81... m/s as the acceleration of a mass near the earth's surface, we're measuring the effect of the aggregate force on that mass by all other particles composing the earth. I suppose all other particles in the universe as well, but the inverse square law makes the contribution from far away particles pretty insignificant.

    If there is also dark matter in earth's neighborhood, wouldn't that 9.81 be the net acceleration due to forces from dark+normal matter? It wouldn't make any difference in a working sense, since mass at earth's surface will still accelerate at g(earth) from whatever particles attract the mass, but what does that mean for G, since we can calculate G from g and knowledge of the earth's mass and radius?

    I'm aware of MOND, but the way I understand it, MOND is an alternative theory to dark matter. My question, in comparison, is about the effect of dark matter on g and G.
  13. Mar 9, 2008 #12


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    The expected density of dark matter in the solar system is very small. I don't have the exact numbers at hand but it is not thought to be significant enough to be needed in calculations of orbits etc in the solar system.

    Maybe once upon a time someone estimated G in this way, but this is certainly not the modern way of doing it. I think these days accurate measurements are made with advanced versions of the Cavendish experiment.
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