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Dark matter black hole

  1. Feb 18, 2015 #1
    is a dark matter black hole possible, and what would it be like ?
     
  2. jcsd
  3. Feb 18, 2015 #2
    Could dark matter be a multitude of microscopic black holes?
     
  4. Feb 18, 2015 #3

    jedishrfu

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  5. Feb 18, 2015 #4

    Drakkith

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    As far as I know it would be just like a regular black hole.
     
  6. Feb 18, 2015 #5

    PeterDonis

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    If you mean, can you make a black hole by collapsing dark matter, sure, you can make a black hole by collapsing any kind of matter.

    If you mean, can you make a black hole "made of" dark matter, black holes don't work that way. See below.

    A black hole of a given mass is the same regardless of what it was made from--once the hole is formed, there's no way to tell from the outside what kind of matter collapsed in order to make it. So a black hole that was formed from collapsing dark matter would be the same as a black hole formed from collapsing anything else.
     
  7. Feb 18, 2015 #6
    From what I've read, black holes can only be made of a very small % of dark matter.
     
  8. Feb 19, 2015 #7

    PeterDonis

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    Where did you read that?
     
  9. Feb 19, 2015 #8

    phinds

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    To add to what has already been said, although a black hole could be made of dark matter, it is extraordinarily unlikely that dark matter WOULD form a black hole because dark matter doesn't clump the way normal matter does.
     
  10. Feb 19, 2015 #9
    thanks to all
     
  11. Feb 19, 2015 #10
    Can Dark Matter Form Black Holes?
    http://www.americaspace.com/?p=33773

    In his blog the astrophysicist Ethan Siegel also wrote:
     
  12. Feb 19, 2015 #11

    PeterDonis

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    Ah, I see. It's not that a black hole can't "accept" dark matter; it's just that, since dark matter interacts so weakly with anything else, it's much harder for it to fall into a black hole the way ordinary matter does (by losing energy through interactions and radiation).

    Link please?
     
  13. Feb 19, 2015 #12
  14. Feb 19, 2015 #13

    PeterDonis

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    This is a good example of why you can't trust pop science articles, even when they're written by scientists who are experts in the field. Nothing he says in here is wrong, exactly, but it's just brimming with opportunities to be misconstrued. Examples:

    "Some days may see a black hole grow by a tremendous amount, while others may see it shed more mass-and-energy than it gains!"

    Technically true (at least if you accept that Hawking radiation is possible, which practically all physicists do even though it's never been observed); but practically false, since in order to shed mass by Hawking radiation in our current universe a black hole would have to have a temperature greater than the temperature of the CMBR--otherwise it will gain mass from the CMBR faster than it loses it from Hawking radiation. The black hole mass corresponding to this temperature is about the mass of the Moon, far smaller than any black hole we have observational evidence for (those are all stellar mass or larger, sometimes much larger); so all black holes we know of will gain mass from the CMBR far faster than they lose it from Hawking radiation, even if nothing else ever falls into them.

    (Note that the article, towards the end, talks about how long it would take a stellar mass black hole to evaporate by Hawking radiation, but still doesn't mention that it won't lose any mass at all, on net, until the CMBR temperature becomes smaller than the black hole temperature. That's governed by the expansion of the universe, not the properties of the black hole.)

    "If we had enough mass in a small enough region of space, the escape speed something would need to reach could be greater than 299,792,458 m/s (670,616,629 mph), which is the speed of light in a vacuum. Since nothing can travel faster than that speed, nothing would be able to escape from it, not even light. Hence, you’d have a black hole."

    Again, technically true, but misleading because it implies that a black hole is a static object like a planet or star, just more compact so that gravity is strong enough at its surface to keep light from escaping. A black hole is not like that at all. It has no "surface", and the event horizon, where the "escape velocity" equals the speed of light, is not a "place" in the ordinary sense (it's a null surface, so it's better thought of as an outgoing spherical shell of light). To be fair, this misleading picture is in a lot of pop science articles by scientists, so it's not just this one person's fault; but it's still misleading.

    "So initially, when they’re first formed, black holes are pretty much 100% normal (baryonic) matter, and just about 0% dark matter."

    This is true of black holes formed by the process he describes (collapse of the core of a supernova when it runs out of nuclear fuel), but we don't know if all the black holes in the universe were formed by that process, or by mergers of black holes formed by that process. In particular, we don't know that the supermassive black holes at the cores of galaxies and quasars were formed by that process; they may have been formed by a different process that involves dark matter to a greater extent. And these supermassive black holes might actually account for much more mass, in the universe as a whole, than black holes formed from stellar collapse. So this statement is not so much misleading in itself as seriously incomplete.
     
  15. Feb 19, 2015 #14

    phinds

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    Yeah, I got that far and quit reading immediately.
     
  16. Feb 21, 2015 #15
    When he says "enough mass in a small enough region of space", he does mean ordinary matter. The black hole singularity will form as a result.
     
  17. Feb 21, 2015 #16

    phinds

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    Why do you think it should be restricted to ordinary matter? You got something against dark matter ? :smile:
     
  18. Feb 21, 2015 #17
    I was defending Siegel against one of Peter's criticisms. I just don't know how to quote a post in this forum.
     
  19. Feb 21, 2015 #18
    It would definitely appear as a regular black hole, because it does not matter what material the black hole is actually composed, we can only see to the event horizon and no further.
     
  20. Feb 21, 2015 #19

    PeterDonis

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    There's a Quote button at the bottom right of each post, and if you highlight some text in a post, a "Quote" button appears below the highlighted text. Then use the "Insert Quotes" button under the text box where you enter your reply.
     
  21. Feb 21, 2015 #20
    hey, it works! Thanks!
     
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