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Where is 'dark matter'?

  1. Mar 14, 2007 #1
    Ok, one more question.

    (The responses to my first two far exceeded my expectations, by the way. This forum is clearly populated by some very well informed and passionate people. Thanks.)

    Is there a detectable effect of DM on the motion of planets in our solar system? If not, why not?
  2. jcsd
  3. Mar 14, 2007 #2
    On scales as small as the solar system, it is expected that the density of dark matter will be pretty much uniform, which would lead to no net gravitational force from the dark matter. Additionally, the density should be low enough that any effects from non-uniformity would be extremely small compared to effects from the baryonic matter composing the sun and other planets.
  4. Mar 14, 2007 #3
    Sorry to be so dense (no pun intended), but I assumed DM would aggregate gravitationally just as 'normal' matter does. What am I missing?
  5. Mar 14, 2007 #4


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    Dark matter may actually be a non-baryonic entity that interacts gravitationally with normal matter, but is absolutely undetectable in every other way.

    Dark matter may also be a fiction to explain why galaxies and cluster of galaxies exhibit much stronger than anticipated gravitational effects (lensing and cluster binding) than predicted by Newtonian/Einsteinian gravitational theory. This would require that g is not a constant, but a variable, and that would require that the inverse-square law is not inviolable.

    The majority now believes in the existence of collisionless non-baryonic dark matter, despite no experimental/observational evidence. If dark matter is not observed, it is reasonable to infer that the magnitude of gravitational attraction is highly dependent on local conditions and that the observations of excess cluster lensing and excess cluster binding may lead us to an understanding of variable g in mass-rich environments.
  6. Mar 14, 2007 #5
    In principle, yes. But, dark matter has a thermal velocity distribution which will give almost any dark matter particle enough energy that it won't be gravitationally bound to the sun. For a gas of normal matter, we would still expect a significant amount to become bound, even with such a distribution; but, this requires that the particles that become bound radiate away energy. Since dark matter particles can't do this (at least, not directly), it is much, much rarer for them to become bound. But, as long as their energies are significantly high that that small a fraction end up bound, you'll find that there ends up being far less variation in their density over the space of the solar system than you might expect.
  7. Mar 14, 2007 #6
    It would take quite an unusual modification to gravity for such variation in g to have its greatest effect other than where the greatest concentration of mass is, don't you think? But, for such a MOND theory to explain something like the recent bullet cluster observations, wouldn't it need to do just that?
  8. Mar 14, 2007 #7


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    The total mass of dark matter in our solar system cannot be much more than a moon's worth without perturbing the orbits of the outer planets.
  9. Mar 15, 2007 #8
    If, for sake of argument, we assume DM does exist and that it interacts gravitationally with baryonic matter, doesn't that provide it a way to dissipate its KE? Is that so inefficient that even 1% of the DM in our solar system could not be captured by a body with the mass of the sun?

    If there is 10X more DM than the matter we detect, I assumed that even a normal star such as ours would have captured enough to be detectable. I'm clearly still missing something.
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