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Black Holes and Dark Mattter

  1. Jan 6, 2009 #1
    If Dark Matter are particles that interact gravitationally, then what happens when/or if Dark Matter falls into a Black Hole? We would not "SEE" a Dark Matter accreation disk, but would we see any kind of radiation? Or would we see the mass grow? Thanks.
  2. jcsd
  3. Jan 6, 2009 #2
    How would you "see" it?
  4. Jan 6, 2009 #3
    Well, maybe you could see effects of gravity waves as dark matter is quickly sucked into a small region. Or perhaps WIMPS are so dense just before entering that they start reacting to normal matter, producing more radiation than can be accounted for from other visible accretion effects. Or maybe there might be changes in microlensing effect with no other accretions matter being visible.
  5. Jan 7, 2009 #4
    I think it's accepted that CMB and dark matter fall into black holes and contribute to their mass. The interesting thing is that a popular candidate for dark matter is the neutralino which is it's own anti-particle and has a habit of annihilating (turning into high energy gamma rays) when the particles come into close proximity with each other. If dark matter is the neutralino, I guess the question is, how close can they get before annihilating? Considering the concentration of matter in an accretion disk, would they make it to the event horizon or would they interact and convert into high energy gamma rays, the energy essentially escaping the black hole? It would probably depend on the size of the BH.
  6. Jan 7, 2009 #5
    Dark matter would certainly form an accretion disk around black holes. But from what I know, the dark matter can only interact gravitationally. Therefore, there would be no efficient process to transport angular momentum from the dark matter particles. Normal matter in accretion disks experience MHD turbulence, transferring angular momentum from one particle to another. This forces one particle closer to the black hole and another further from the black hole. But magnetic fields can play no role in any dark matter accretion disk, and I don't think the dark matter could even experience any dynamic fluid friction to lose energy. Perhaps the only mechanism to lose energy is through gravitational radiation, which is a slow process.
  7. Jan 7, 2009 #6


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    SpiffyKavu has it right. Dark matter does not dynamically interact to any appreciable extent with any other particles, including other DM particles, to our knowledge. That what makes the stuff so devilishly hard to detect. A DM particle could certainly fall into a black whole, but would go into the night quietly.
  8. Jan 8, 2009 #7
    If dark matter does not interact with anything else except by gravity, then what would give it any angular momentum to avoid a black hole? There would be no heat pressure against collapse. It would not be deflected by interactions to prevent it from immediately collapsing. So if there is 5 times more dark matter than regular matter and none of the usual interaction preventing colapse, then shouldn't we expect there to be more and bigger black holes than what we otherwise expect?
    Last edited: Jan 8, 2009
  9. Jan 8, 2009 #8
    How is that possible? Since the neutralino is neutral it can't couple with a photon.
    And if that was true, wouldn't neutralinos then cause a background with a peak in the gamma ray energies (ie, couldn't we detect such a scenario easily)?

    I must be missing something here.
  10. Jan 9, 2009 #9
    Hi JustinLevy

    Here's one paper-
    'Search for very high energy gamma rays from WIMP annihilations near the Sun
    with the Milagro detector'

    'The neutralino, the lightest stable supersymmetric particle, is a strong theoretical candidate for the missing astronomical ‘‘dark matter’’. A profusion of such neutralinos can accumulate near the Sun when they lose energy upon scattering and are gravitationally captured. Pair-annihilations of those neutralinos may produce very high-energy (VHE, above 100 GeV) gamma rays. Milagro is an air shower array which uses the water Cherenkov technique to detect extensive-air showers and is capable of observing VHE gamma rays from the direction of the Sun with an angular resolution of 0:75. Analysis of Milagro data with an exposure to the Sun of 1165 hours presents the first attempt to detect TeV gamma rays produced by annihilating neutralinos captured by the Solar system and shows no statistically significant signal. Resulting limits that can be set on the gamma-ray flux due to near-Solar neutralino annihilations and on the neutralino cross-section are presented.'

    A couple of other sources-

    'Neutralino annihilation in the Galactic center'

    'Neutralino gamma-ray signals from accreting halo dark matter'

  11. Jan 9, 2009 #10
    Even discounting those things, a dark matter particle isn't going to fall straight in to a black hole. It's going to have momentum before it is captured in the gravitational field that from the reference of the black hole is angular. This angular momentum should cause it to spiral in just like anything else.
  12. Jan 9, 2009 #11
    Then how would energy radiate from the dark matter particles so that it lost enough angular momentum in order to eventually fall into the black hole? Don't particles have to radiate away from dark matter in order for it to loose kinetic energy and fall in? But if nothing interacts with dark matter, then it cannot loose kinetic energy and fall in. In that case, only those DM particles who original orbit intersects the BH horizon will fall in, right? Wouldn't this predict a growth of the BH proportional to it size even without any barionic matter around?

    Or as far as that goes, wouldn't DM have to radiate some kind of particle (and thus interact with something) in order to fall at all? How does it loose potential energy? Or for that matter, how did it gain kinetic energy to begin with if it does not interact with anything?
  13. Jan 9, 2009 #12


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    DM particles can interact with other DM particles ... gravitationally.

    There is a baryonic matter analog to a ball of CDM - globular clusters. Of course it's not a perfect analog, but the characteristic time-scales for various kinds of energy exchanges by the constituent 'particles' (stars) are such that it's pretty good (star-star 'hard' collisions in globular clusters certainly happen, but the timescale over which such collisions will result in most of the mass being a central black hole is far greater than the other timescales of relevance).

    It turns out that 'close encounters of the binary kind' are quite efficient means of redistributing angular momentum and kinetic energy ... so much so that they lead to massive stars 'settling in' to the core, and low mass stars being expelled or sent into orbits which reach way into the outskirts.

    How does this relate to CDM particles? Well, as of today, no one knows ... if only because the constraints on CDM 'close binaries' are weak to non-existent.

    However, as a general mechanism for changing the angular momentum and kinetic energy of any particular CDM particle, 3-way 'collisions' work like a charm.
  14. Jan 10, 2009 #13


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    DM particles do not follow the trajectories of normal matter particles when approaching a gravitational field. They do not suffer the effects of collisions with other particles, hence, coallesce at a slower rate than normal matter particles. DM halos is, I believe, the term used to characterize this effect.
  15. Jan 10, 2009 #14
    So if DM does not interact with anything else, including itself, then it should escape a black hole because nothing slows it down, right? I suspect this would be true except that time slows down and comes to a stop at the horizon, so we never see it enter. It just seems to get deposited on the surface, right?
  16. Jan 10, 2009 #15
    But DM does interact gravitationally and so is affected by black hole gravity. It cannot escape a black hole once it enters one.
  17. Jan 11, 2009 #16
    But only because time slows down to a stop as it enters the horizon.

    But more generally, if a particle does not interact with anything, then it will climb back out of the gravity well into which it has fallen and end up at the same distance from the well as it was before it fell. That would mean that there would be the same distribution of dark matter with or without a gravity well, because those DM particles that fell in would also just as easily climb back out, leaving the same distribution as before, right?

    So that means the DM is not made of particles, or they at least interact weakly with normal matter, right?
  18. Jan 11, 2009 #17


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    No, because once you cross the event horizon, spacetime is so curved that "all roads lead to the center"

    One way to look at it is that inside the event horizon, radial movement has the same contraints as time does outside the horizon. Outside the horizon you have freedom of movement in all three spatial directions but are constrained to only moving forward in time. Inside the horizon your spatial movements are constrained to inward movement along the radial direction.
  19. Jan 13, 2009 #18
    But if time slows down for particles approaching the horizon, how can there be activity inside if nothing ever really enters it because it takes forever to get there?
  20. Jan 13, 2009 #19
    Again, be careful of proper time and coordinate time. With respect to observer far away from the black hole we see that the time of infalling particle slows down. But to the particle falling into the black hole, time runs at normal rate, and they will cross the event horizon in finite time.
  21. Jan 13, 2009 #20
    So neither does the particle observe an horizon. Horizons are only visible to those far away. So talk of horizons is only relevant to those far away who see the particle slowing down and never really entering. So for far observes there is no meaning to inside since nothing ever really enters since time slows to a stop. But for the particle itself, it never perceives a horizon, so again there is no inside.
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