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Thread: particles

  1. #31
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    Quote Originally Posted by Squink View Post
    If dark matter only interacts with gravity, shouldn't its overwhelming presence affect orbital velocity of matter in accretion disks around black holes?
    I suppose we haven't got good enough data to look for an effect yet. Being non-collisional, you'd expect dark matter to be enriched in such disks even beyond its universal average of 80% of all matter.
    Its interaction with gravity is immensely weak. That's why it's only detectable on galactic or bigger scales. Local effects overwhelm it, the dark matter data is lost in the noise of other interactions like nearby stars.
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  2. #32
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    Quote Originally Posted by Noclevername View Post
    Its interaction with gravity is immensely weak. That's why it's only detectable on galactic or bigger scales. Local effects overwhelm it, the dark matter data is lost in the noise of other interactions like nearby stars.
    Why do you say that its interaction with gravity is immensely weak? Do you mean that its interaction with gravity is different from other particles?
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  3. #33
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    Quote Originally Posted by Jens View Post
    Why do you say that its interaction with gravity is immensely weak? Do you mean that its interaction with gravity is different from other particles?
    Dark matter particles are thought to be fairly light and in general clump much, much less than baryonic matter. So you only tend to see gravitational effects at large scales and in areas of low gravitational acceleration because you don't get the kind of dense concentrations of them that you do with matter.

  4. #34
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    Quote Originally Posted by Shaula View Post
    Dark matter particles are thought to be fairly light and in general clump much, much less than baryonic matter. So you only tend to see gravitational effects at large scales and in areas of low gravitational acceleration because you don't get the kind of dense concentrations of them that you do with matter.
    Thanks. So if I understand correctly, it is that they are very light and so don't cluster. Neutrinos would be similar in that regard (but even more so)?
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  5. #35
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    Quote Originally Posted by Jens View Post
    Thanks. So if I understand correctly, it is that they are very light and so don't cluster. Neutrinos would be similar in that regard (but even more so)?
    Yes, in fact an early candidate for dark matter was thought to be neutrinos. Trouble is they are too light and tend to act like Warm/Hot Dark Matter, rather than Cold Dark Matter. So while contributing they didn't fit all the properties needed.

  6. #36
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    Quote Originally Posted by Shaula View Post
    Dark matter particles are thought to be fairly light and in general clump much, much less than baryonic matter.
    Noncollisional status would get you unclumpyness. What about black holes though? Surely dark matter must have the same Schwarzschild radius as protons or photons? Else we'd theoretically be able to use them to probe beyond the event horizon. That is bad Juju!

  7. #37
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    Quote Originally Posted by Squink View Post
    Noncollisional status would get you unclumpyness. What about black holes though? Surely dark matter must have the same Schwarzschild radius as protons or photons? Else we'd theoretically be able to use them to probe beyond the event horizon. That is bad Juju!
    The lack of clumping implies that they won't form a black hole easily - they don't form dense lumps. Yes, the Schwarzschild radius would be the same - but you just wouldn't get DM that concentrated (you can't compress it as easily as you can baryonic matter in, for example, core collapse).

    Or are you asking if DM particles can fall into black holes? I think the accepted answer is yes - but it is fairly rare. Mass tends to end up in black holes from the accretion disk as it is a place where matter with angular momentum can shed it to fall in. DM can't shed it easily (most mechanisms are electromagnetic in nature) so doesn't spiral in as much. It just tends to orbit.

  8. #38
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    Quote Originally Posted by Shaula View Post
    Or are you asking if DM particles can fall into black holes? I think the accepted answer is yes - but it is fairly rare. Mass tends to end up in black holes from the accretion disk as it is a place where matter with angular momentum can shed it to fall in. DM can't shed it easily (most mechanisms are electromagnetic in nature) so doesn't spiral in as much. It just tends to orbit.
    And of course once it's part of the black hole, it won't behave any differently from any other mass. A black hole created partly from dark matter and partly from normal matter would be indistinguishable from a black hole of the same mass created entirely from normal matter.
    Conserve energy. Commute with the Hamiltonian.

  9. #39
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    I suppose even if slow and noncollisional, most dark matter would end up approaching a black hole on a hyperbolic orbit. Orbital capture would be hard, and there'd little force to concentrate the captured particles within the accretion disk of baryonic matter. That which was captured would end up as a spherical halo, with little tendency to 'spiral' in.

  10. #40
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    Quote Originally Posted by Squink View Post
    I suppose even if slow and noncollisional, most dark matter would end up approaching a black hole on a hyperbolic orbit. Orbital capture would be hard, and there'd little force to concentrate the captured particles within the accretion disk of baryonic matter. That which was captured would end up as a spherical halo, with little tendency to 'spiral' in.
    Exactly - without EM effects you either need a path that intersects the black hole or some kind of lucky series of gravitational interactions.

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