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Do black holes swallow dark matter?

  1. Aug 2, 2010 #1
    What do we know about black holes swallowing dark matter? Dark matter exhibits gravitational effects, right [lensing and keeping star orbital speeds about galactic centers rouighly independent of their distance from a galactic center]? So it seems black holes should consume both normal and dark matter.

    Are there any theoretical ways to detect the effects of dark matter when it's consumed by black holes?? Anyone seen any discussions or papers?? Thanks?
     
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  3. Aug 2, 2010 #2

    mathman

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    Anything that is consumed by a black hole will add to the mass, charge, and angular momentum. Otherwise there is nothing to to identify it. In particular, dark matter and normal matter cannot be distinguished after being consumed by a black hole.
     
  4. Aug 2, 2010 #3

    nicksauce

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    Not to mention that there is a minuscule amount of dark matter in the galactic centre, where black holes are "swallowing matter", compared to the amount of baryonic matter.
     
  5. Aug 3, 2010 #4

    Chalnoth

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    Yes. And more important than that, dark matter doesn't experience much of any friction. This means that even if some dark matter gets in the vicinity of a black hole, it tends to just blow on by. It has to actually strike the black hole to go into it (and black holes are pretty darned small for their mass, so hitting one is difficult).

    This is contrasted to normal matter which, through friction, enters an accretion disk around the black hole, a disk that slowly collapses into the black hole through loss of energy from friction.
     
  6. Aug 3, 2010 #5
    All valid points in the above posts....yet with dark matter making up such a large proportion of all matter seems like it could have an effect....maybe somehow we could observe effects.

    Chalnoth...interesting observation

    I'm guessing Hawkings work on radiation did not reflect any dark matter ingestion...but I wonder if it could be expanded/modified to do so.
     
  7. Aug 3, 2010 #6

    Chalnoth

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    The problem is that the dark matter, within the galaxy, is at a much lower density than the normal matter. Most of the dark matter is in a "halo" that surrounds the galaxy. This isn't to say it's more dense out there, it's just a statement that the normal matter collapses through friction to form the galaxy, while the dark matter more or less stays in its orbit at after it first becomes gravitationally bound to the galaxy.

    So no, I don't think there would be much of an observable effect.

    The closest thing to this suggestion that I've seen is that because of the fact that normal matter is so much more dense, it's going to attract some amount of dark matter to it. With more dark matter comes more annihilations (dark matter is expected to be made up of equal parts matter and anti-matter, because it must interact too weakly to have annihilated in the early universe to have the properties we observe). Dark matter annihilations are expected to be one observable signal we could potentially use to understand its properties.

    One attempt at observing annihilating dark matter comes from WMAP observations:
    http://arxiv.org/abs/0802.3830

    At this point, I would place this observation of the WMAP haze at speculative at best. But at least it serves as a model of how one might go about observing dark matter directly.

    I don't think it makes any difference whatsoever.
     
  8. Aug 3, 2010 #7
    Only when they are on a diet.

    :cool:
     
  9. Aug 4, 2010 #8
    This is a question for me. Dark matter would orbit the center of mass. But since it does not interact much it would just pass to the other side without interaction. I have to think, though, that there would still be a much greater density of DM at the center than at the edges simply because the gravitational field is stronger there. Sure it would be traveling faster, but there would be more of it. Perhaps there could be a greate enough density of DM to form a BH.
     
  10. Aug 4, 2010 #9

    Chalnoth

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    Yes, the density of dark matter is expected to be larger near the galactic center. Not enough to make the density outweigh the normal matter, but still larger than it is outside the galaxy (if we define the galaxy as the normal matter part). As far as I'm aware, however, our observations of dark matter are not yet accurate enough to confirm this. But it is definitely expected from theory.
     
  11. Aug 5, 2010 #10

    Chronos

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    Dark matter is not interactive with normal matter. It would pass right through a black hole. See the bullet cluster paper.
     
  12. Aug 5, 2010 #11

    Ich

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    Huh?
    Nothing passes right through a black hole.
     
  13. Aug 5, 2010 #12

    Vanadium 50

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    It gravitates (that's how we know it's there). It would therefore be captured by a BH in exactly the same way as normal matter.
     
  14. Aug 5, 2010 #13
    If we have already established that DM can develop pockets of larger density when before it was an even distribution, then the question is why can't it become even denser and form BHs made only of DM. If it can become dense to begin with, then what's to stop it from becoming even denser?
     
  15. Aug 5, 2010 #14

    Chalnoth

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    In principle it might, but the time scale for such a thing would be exceedingly long. The reasoning is as follows:

    1. To form a black hole, you need a local overdensity of dark matter so great that it prevents the dark matter from escaping. The magnitude of this overdensity is vastly above the typical average density of dark matter within a galaxy or galaxy cluster.

    2. In order to get such a massive overdensity, you need the orbits of the dark matter particles in a potential well to decay. This can only be done through friction.

    3. Dark matter experiences almost no friction.

    If you make the assumption of perfectly non-interacting dark matter, I'm sure you could calculate just how long this would take. Given our observations, this time scale must be many times the current age of the universe (because if it weren't, dark matter would have collapsed noticeably by now: it hasn't).

    However, this may well turn out not to work for real dark matter, because real dark matter is likely to have some interaction. In particular, it is likely to be nearly equal parts matter and anti-matter, meaning that any significant overdensity of dark matter will also be very efficient at causing the dark matter particles to annihilate, likely causing the structure to evaporate long before it gets dense enough to form a black hole.
     
  16. Aug 5, 2010 #15

    George Jones

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    When normal matter collapses, it loses energy through radiation, which tends to hasten collapse. Dark matter doesn't do this. Consequently: 1) normal matter forms black holes more easily; 2) black holes present larger effective targets to normal matter than they do to dark matter.

    A quote from Weinberg's new cosmology book:
    [EDIT]Chalnoth posted much the same stuf while I was composing my post.[/edit]
     
  17. Aug 5, 2010 #16

    Chalnoth

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    By the way, as a potential point of interest, even without any interactions (other than gravity), dark matter particle orbits will still decay over very long time scales. The argument goes as follows:

    1. From time to time, dark matter particles will have close interactions with other dark matter particles that exchange energy between them. This means that the particles of dark matter in a potential well will be thermalized: their energy distribution will approach a thermal distribution with time.

    2. A thermal distribution has a high-energy tail of particles that have enough velocity to escape the potential well. Thus the particles in the high-energy tail of the thermal distribution are always escaping. Since it is the highest-energy particles that escape, the average energy per particle is reduced in this process, meaning that it causes the cloud to collapse.

    I do not know if this effect or the (weak) non-gravitational dark matter interactions turn out to be the dominant effect in controlling the long time scale behavior of dark matter.
     
  18. Aug 8, 2010 #17

    Chronos

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  19. Aug 9, 2010 #18

    Chalnoth

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    Those headlines seem to be a bit misleading. The only thing that was found, as near as I can tell from reading the articles, is that the density of dark matter in the vicinity of black holes is not large enough to produce runaway accretion of dark matter into the black hole.
     
  20. Aug 10, 2010 #19

    Chronos

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    These studies assert dark matter comprises a miniscule amount of the mass of black holes. Quoting from page 6 of arXiv:0802.2041v1:

    " . . . We found that dark matter contributes to no more than 10% of the total mass accreted by black hole seeds."

    This is the conservative upper limit and suggests black holes tend not to absorb dark matter, hence the headlines.
     
  21. Aug 10, 2010 #20

    Ich

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    This is exactly what has been said in this thread: DM is less readily absorbed by black holes because it can't form accretion disks. It contradicts the claim that DM "would pass right through a black hole".
     
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