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Thread: Do black holes pull in dark matter?

  1. #1
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    Do black holes pull in dark matter?

    I'm just wondering if that would be likely. Or do we not know yet?

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    Yes, dark matter interacts gravitationally with other matter - the apparent presence of gravitating but invisible matter affecting galaxies and galaxy clusters is why we want dark matter in our theories in the first place.
    From a black hole's point of view, it's all mass energy, and from dark matter's point of view there's a great big gravity well to respond to.

    Grant Hutchison

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    Google this:

    black hole dark matter interaction

    Lots of (visible) info.

    Munch, munch, munch a bunch of DM, from space . . .

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    If dark matter is composed of particles which only interact gravitationally, these particles will not concentrate around black holes (or other compact objects) as strongly as baryonic matter; viscosity is a big part of what control accretion disks and streams. So whatever particles happen to have low enough angular momentum to fall across the event horizon will be accreted, but will not exchange annual momentum with truss and facilitate further accretion.

    (Exception, sort of - for a while so-called sterile neutrinos, which do not partake in the 3-way flavor cycling of known neutrinos, were seen as an attractive DM candidate. In a certain mass range, these would form degenerate configurations - kind of "neutrino stars" - with maybe thousands of solar masses. These would be fine until a stellar-mass black hole wander into one, which would accrete the whole thing from the bottom. In a degenerate confutation, so of the articles would have very low energy and be accreted very quickly. Those makes so much sense as a way to seed the rapid growth of black holes in galactic nuclei that it seems a shame that data have now pretty much ruled out all mass ranges where sterile neutrinos could hide).).

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    Can dark matter interact with dark matter?
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Quote Originally Posted by DaCaptain View Post
    Can dark matter interact with dark matter?
    Yes. Via gravity.

    Or did you mean electromagnetically? Like baryonic matter does?

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    Quote Originally Posted by DaveC426913 View Post
    Yes. Via gravity.

    Or did you mean electromagnetically? Like baryonic matter does?
    If they interact gravitationally does that mean they would be drawn together to form a larger glob of dark matter?
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Quote Originally Posted by DaCaptain View Post
    If they interact gravitationally does that mean they would be drawn together to form a larger glob of dark matter?
    Clumping requires more than just gravitational interaction. As hypothesized, there is no interaction that keeps dark matter particles from just orbiting past each other, nothing to slow them down and make them adhere.
    0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 1 0 0 1 0 1 1 0 ...
    ATM is so 20th Century.

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    Quote Originally Posted by 01101001 View Post
    Clumping requires more than just gravitational interaction. As hypothesized, there is no interaction that keeps dark matter particles from just orbiting past each other, nothing to slow them down and make them adhere.
    You would think that if the universe consists of 25% or more of dark matter that there would be plenty of instances where they'd clump together. I always thought of dark matter as more of a "pudding" rather than something that's traveling around the universe. I mean if it's so infinitely small that it can't react amongst itself, How can it push around entire galaxies?
    Last edited by DaCaptain; 2017-Apr-19 at 09:18 PM.
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Quote Originally Posted by DaCaptain View Post
    You would think that if the universe consists of 25% or more of dark matter that there would be plenty of instances where they'd clump together. I always thought of dark matter as more of a "pudding" rather than something that's traveling around the universe. I mean if it's so infinitely small that it can't react amongst itself, How can it push around entire galaxies?
    There is no reason from first principles to rule out DM particles that do not interact in ways other that gravitationally. The "stuff" of the universe is what it is and does what it does, and it does not care how hard a time we have trying to make sense of it.

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    I suppose dark matter just slides around, it would swing in close to a black hole and then swing out.
    Formerly Frog march..............

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    Quote Originally Posted by Frog march View Post
    I suppose dark matter just slides around, it would swing in close to a black hole and then swing out.
    Just like any ordinary bodies that don't pass too close to the event horizon on the way in.

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    Quote Originally Posted by DaCaptain View Post
    You would think that if the universe consists of 25% or more of dark matter that there would be plenty of instances where they'd clump together.
    Ordinary matter clumps because of collisions. This (simplistically) saps kinetic energy, spreading it out, until multiple particles have similar velocities. i.e. clumping.
    If DM doesn't interact physically, then particles can essentially pass right through each other, so no clumping.

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    Quote Originally Posted by DaveC426913 View Post
    Ordinary matter clumps because of collisions. This (simplistically) saps kinetic energy, spreading it out, until multiple particles have similar velocities. i.e. clumping.
    If DM doesn't interact physically, then particles can essentially pass right through each other, so no clumping.
    Generally 'collisions' in the sense that you are describing here are mediated by the electromagnetic force. The weak and strong forces have characteristics that make them less suitable for this kind of intermediate range interaction (essentially the weak force is too weak and the strong force is too strong at typical interaction ranges). One of the defining characteristics of DM is that it does not interact electromagnetically (if it did it would behave much more like baryonic matter and not form the structures we observe associated with it). So unless there are other forces or interactions (often termed dark sector forces) it won't clump via the currently understood mechanisms.

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    Quote Originally Posted by Shaula View Post
    Generally 'collisions' in the sense that you are describing here are mediated by the electromagnetic force. The weak and strong forces have characteristics that make them less suitable for this kind of intermediate range interaction (essentially the weak force is too weak and the strong force is too strong at typical interaction ranges). One of the defining characteristics of DM is that it does not interact electromagnetically (if it did it would behave much more like baryonic matter and not form the structures we observe associated with it). So unless there are other forces or interactions (often termed dark sector forces) it won't clump via the currently understood mechanisms.
    Shaula. So, does dark matter interact only through gravitation, but would not necessarily be emitting gravitational radiation, like normal baryonic matter does when binary pulsars coalesce, or any other gravitationally-bound baryonic matter clumps? pete

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    Quote Originally Posted by trinitree88 View Post
    Shaula. So, does dark matter interact only through gravitation, but would not necessarily be emitting gravitational radiation, like normal baryonic matter does when binary pulsars coalesce, or any other gravitationally-bound baryonic matter clumps? pete
    Gravitational radiation comes from accelerating mass. If dark matter has a gravitational field, I imagine it would have to produce gravitational radiation under the same circumstances ordinary matter would, although (because of its lack of clumping) it would be less likely to get into situations where that would occur.

    Grant Hutchison

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    It's worthwhile to note that by "not clumping" we mean that it doesn't form non-gravitationally bound objects (like dust or small solid particles), and it won't lose energy via radiation, so it won't collapse in the same way as normal matter. So it can't form highly dense objects like stars or planets, but it will still form loose gravitationally bound structures. In fact, what we find from simulation is that a group of particles interacting purely gravitationally will have a tendency to form into more or less spherically symmetric shapes, with a density profile that goes like the inverse radius squared. So it would be more concentrated toward the center, just not by as much as normal matter would be. This happens to be about the same density profile that's needed to account for galactic rotation profiles. That's not a coincidence, it's precisely why dark matter works reasonably well as a model for explaining those rotation profiles.
    Conserve energy. Commute with the Hamiltonian.

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    Quote Originally Posted by grant hutchison View Post
    Gravitational radiation comes from accelerating mass. If dark matter has a gravitational field, I imagine it would have to produce gravitational radiation under the same circumstances ordinary matter would, although (because of its lack of clumping) it would be less likely to get into situations where that would occur.

    Grant Hutchison
    Does this mean dark matter has mass? Or that it's continuously accelerating? I'm beginning to think phooey about dark matter.
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Quote Originally Posted by DaCaptain View Post
    Does this mean dark matter has mass? Or that it's continuously accelerating? I'm beginning to think phooey about dark matter.
    There's a difference between a gravitational field, which is associated with anything that has mass, including dark matter, and gravitational radiation, which is a wave in the gravitational field that carries energy away from an accelerating mass.
    There are analogies with an electromagnetic field, which surrounds any object with charge, and electromagnetic radiation, which carries energy away from accelerating charges.

    Grant Hutchison

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    I think more to the point is, can observations of black holes add any constraints to theories involving dark matter?

    I am sceptical that they can. DM falling into a black hole wouldn't give off energy like ordinary matter. All it would do is add to the BH mass over time.

    The density of DM is very low, even at the centre of the galaxy, and BH's are physically small. A DM particle falling from infinity just missing the event horizon would get deflected and carry right on past. There's no friction with other matter to slow it down.

    On the other hand, neither is it deflected from falling in by radiation pressure, as is ordinary matter.

    So all you expect to observe is a gradual increase of BH mass. What the difference would be, comparing universes with and without dark matter, I really do not know. I suspect very little because DM is so diffuse.

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    Can neutrinoes interact with each other weakly? Is a weak force mediated elastic collision between neutrinos legal or illegal?

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    Quote Originally Posted by chornedsnorkack View Post
    Can neutrinoes interact with each other weakly? Is a weak force mediated elastic collision between neutrinos legal or illegal?
    Maybe. For example, this article seems to suggest that they do not. On the other hand, this paper and this paper seem to suggest that they do.
    Conserve energy. Commute with the Hamiltonian.

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