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Question(s) about DM and BH's

  1. Jun 16, 2012 #1
    Firstly let me reiterate for those not in the know, I am an Idiot, so if my ramblings fall short of sense making, keep that in mind :)

    I have no idea why, or where this comes from in my mind, but somehow I imagine some Dark Matter(DM) near the event horizon of a Black Hole(BH). Would this hypothetical DM once it enters also never return? I guess I'm asking if DM would 'gather' (for lack of a better term) at BH's?

    Instead of a BH, perhaps it is a sun instead, no event horizon, no crazy gravity off the scale, here the DM surely would not gather right?

    I have no idea where these thoughts came from, or to what (if anything) they pertain, but I keep thinking with DM not reacting with normal matter except through gravitational effects, it would somehow mean that the stickyness of normal matter wouldn't apply in at least one of these instances.

    Thanks in advance for consideration of my meaningless ramblings,
    Some Slacker
  2. jcsd
  3. Jun 16, 2012 #2
    As far as we know, in terms of gravitational interactions, DM acts like massive matter. That should be all you need to know.
  4. Jun 17, 2012 #3


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    Well, dark matter, if it has the right trajectory, will fall into a black hole and never return. Most of the time, however, it will simply come close and miss (black holes are small).

    Right. Dark matter cannot be captured by the Sun: it will simply shoot straight through. Now, there will be slightly more dark matter in the vicinity of a massive object like the Sun than far away, due to its gravitational attraction, but the specific dark matter particles just keep moving on by.
  5. Jun 17, 2012 #4
    This is more along the lines of my thinking (if you can call what I do Thinking) and then wouldn't that eventually mean that DM would stop orbiting in the normal sense, as DM can shoot right through the very heart of the sun where gravity is at its greatest wouldn't that end up being more like an oscillation rather than an orbit? And what is meant by 'vicinity'? Inside the orbit of mercury? further out? or simply more DM the closer you get to the sun?

    I'm sorry if I'm just not gettin it, but I would think that this holds true for Local DM but would this work also on extra-solar DM? Unfortunately I don't have the math to understand it, and even if I did not sure my pea-brain could absorb it.

    Thanks again for consideration of my senseless ramblings...
  6. Jun 17, 2012 #5


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    Orbits are oscillations. But in reality, very very little dark matter is likely to be in orbit around the Sun. The issue is that dark matter, because it interacts only very weakly with matter, has no way to slow down enough to be captured by the Sun's gravity. Instead, it will fall towards the Sun, be sped up by the attractive force of gravity, then pass the Sun, and slow down just as much as it sped up, so that it still retains the original energy it had going in (for the most part).

    The last one. Though I don't think the Sun has enough mass to really make a big difference here.

    Yes. Everything interacts with gravity.
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