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B Does dark matter make black holes bigger?

  1. Jan 1, 2017 #1
    I know that a black hole it's a singularity, but its event horizon gets bigger the more mass/energy you throw in. I thought that like dark matter interacts gravitationally with regular matter , it would interact with black holes, eventually falling , increasing the mass of the black hole and therefore , its event horizon. It's "easy" to see if the event horizon gets bigger too fast, only counting regular matter, so i guess it does not happen, but why?
     
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  3. Jan 1, 2017 #2

    mathman

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    Dark matter would fall into a black hole just like ordinary matter. I don't understand your last comment.
     
  4. Jan 1, 2017 #3

    berkeman

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  5. Jan 2, 2017 #4
  6. Jan 2, 2017 #5

    Bandersnatch

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    Is it, though?
    In the case of baryonic matter, black holes grow by accrettion of material that is available in their immediate vicinity - the atmosphere of a companion star, for example.
    If one were to look at the equation for the Schwarzschild radius of an idealised non-rotating black hole, ##R_s=2GM/c^2##, it is clear that in order to double the radius, you'd have to double the mass. A solar mass BH is approx 6km accross. A small BH can feed on its companion for millions of years.
    So, one needs an instrument capable of observing a few kilometers-across object, with growth rates on the order of milimeters per year. All from many light-years away.

    Apart from the difficulty of measuring the size of the event horizon, let alone its growth, one has to consider the amount of dark matter that is available for infalling into the black hole.

    While densities of both types of matter are comparable when averaged over the volume of a galaxy, a star is a highly overdense region of space, providing a lot of 'fuel' for the black hole, whereas at any given time, there is only a tiny amount of dark matter passing through the same region.
    E.g. the current models suggest that at the distance from the galactic centre at which the Sun orbits, there is roughly a small asteroid-worth of DM contained within a 1 cubic AU volume of space, and while the density goes up closer to the galactic centre, it never approaches the densities of clumps of baryonic matter.
    This is further exacerbated by the fact that DM is at best only weakly interacting - it does not have a way to form accretion discs, so no way to shed orbital velocity. Only those particles whose trajectories cross the event horizon can be added to the BH. Near misses will be just flung out back into space.
     
  7. Jan 2, 2017 #6

    Vanadium 50

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    To expand on Bandersnatch's reply. the amount of DM in a sun-sized volume weighs only about a ton. Finding a ton-sized perturbation in a star-sized effect from half the galaxy away isn't going to happen any time soon, I'm afraid.
     
  8. Jan 3, 2017 #7
    Wasn't dark matter very massive? Or at least, didn't it interact strongly with the gravitational field, thus making it weigh more?
     
  9. Jan 3, 2017 #8

    Bandersnatch

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    No, it isn't. WIMPs are currently favoured over MACHOs. A possible candidate is something like the neutrino.
    There's just more of it, mass-wise, than baryonic matter. But again, with very diffuse distribution.
     
  10. Jan 3, 2017 #9

    Vanadium 50

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    Sure, because there's a lot of space. Mass = density * volume. Density is small, volume of a galactic cluster is large.
     
  11. Jan 3, 2017 #10

    mfb

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    Only at the scales of galaxies. It has a very uniform distribution and does not clump together on small scales like regular matter does. Within those small clumps (stars with planets orbiting them), the amount of dark matter is negligible.
     
  12. Jan 3, 2017 #11
    Dark matter also doesn't seem to have any type of self-interaction, so there is little or no friction. Dust particles in an accretion disc are sort of like being in orbit. The reason they fall into the black hole is because the disc rubs on itself and friction slows everything down, allowing it to fall inwards. Dark matter doesn't slow down when it crashes into itself as far as we can tell.
     
  13. Jan 3, 2017 #12

    Vanadium 50

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    Pretty much ruled out. They move too quickly and this would disrupt large scale structure formation.
     
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