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Black Holes in the center of galaxies

  1. Jul 21, 2014 #1
    Is it still true that most if not all galaxies have at their center a Black Hole? And that the estimated gravity is about 10% of the total mass galaxies. And is it still true that all stars or 99 % of them are moving out and a way from the black hole. I was just wondering if these black holes might be small bangs. And as galaxies grow so does the Black Hole. If that 10% is found to be right more then 80 % of the time ( galaxies hitting each other lets say 20 % of the time) I think it would and could be possible and worth some talk.
     
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  3. Jul 21, 2014 #2

    phinds

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    As I recall, it's more like 1.5% or 2%, never as high as 10% but yes it is now believed that all, or virtually all, galaxies have a BH at the center.

    These black holes are not "small bangs" in any meaningful sense of those words. That sounds like some pop-sci nonsense seen on TV.

    If stars were all moving away from the BH at the center of their galaxies, the galaxies would be getting bigger and would likely be unstable. Why do you think this is happening? Where did you hear this?
     
  4. Jul 21, 2014 #3
    All came from science shows and no not sifi. The data might be old even outdated I think for the most part I am correct.
     
  5. Jul 21, 2014 #4

    phinds

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    "Science shows" ARE pop-sci TV. They are absolutely not to be trusted. They get a lot of stuff right and a fair amount abysmally wrong and you'll never know which if that's where you get your science.

    I was incorrect in saying that there are NO black holes that are as much as 10% of their galactic mass but I was correct in saying that it is FAR more normal to be much smaller. The high-percent ones are apparently a fairly recent discovery.
     
  6. Jul 21, 2014 #5

    Chronos

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    What we know about black holes and gravity tells us they behave exactly as would ordinary mass at any reasonable distance. The extreme density of black holes causes odd things as you approach the event horizon, but, stars circling a black hole are generally too distant from the event horizon to suffer relativistic insults. A star that approaches too closely, not so lucky. Tidal forces will tear it apart and serve it up as an appetizer. In principle, it is generally held that a black hole is the opposite of the big bang.
     
  7. Jul 21, 2014 #6

    Chronos, In what way do you mean that? (It's just something that I haven't heard before. )

    Regards,

    Noel.
     
  8. Jul 21, 2014 #7

    PeterDonis

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    I don't think this is correct except in the obvious (and very coarse) sense that black holes are formed by collapse whereas the Big Bang involves expansion. (There is also a technical sense in which the singularities in both cases are the "opposite" of each other, but it's very weak--see below.)

    The "opposite" of a black hole (in the sense of "time reverse") is a white hole, and the Big Bang is not a white hole, because a white hole has a horizon that separates the hole's singularity from past null infinity. There is no past null infinity in the Big Bang spacetime. Similarly, the opposite (again in the sense of time reverse) of the Big Bang is a Big Crunch, and the Big Crunch is not a black hole, because a black hole has a horizon that separates the hole's singularity from future null infinity, and there is no future null infinity in a Big Crunch spacetime.

    It is true, as I mentioned above, that the Big Bang singularity and a black hole singularity are both spacelike singularities; but the similarity ends there. The Big Bang singularity had zero tidal gravity (except for the small "wrinkles" that got magnified by inflation into the small anisotropies in the CMBR that we detect today). A black hole singularity, by contrast, at least for any real black hole (as opposed to an idealized one that is formed by a perfectly spherically symmetric collapse and remains perfectly spherically symmetric for all time), has *lots* of tidal gravity, as you note. So even if we ignore the difference of horizon vs. no horizon that I described above, a black hole and the Big Bang are not "time reverses" of each other, because of the huge difference in tidal gravity. (A "real" Big Crunch, at least as far as I can tell from what I've read, is modeled as having large tidal gravity, so it isn't quite the time reverse of the Big Bang either, and its singularity would be more like a black hole singularity in that respect--though it still differs with regard to the lack of a horizon or a future null infinity.)
     
  9. Jul 21, 2014 #8

    Chronos

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    The big bang singularity is a special case, which has no analog. It resembles a white whole in some respects, but, that semblance, as you note, is strictly superficial. I brought it up to make the point the differences between a black hole and the big bang singularity are profound.
     
  10. Jul 23, 2014 #9
    I still think that if there was a big bang there would be a clear pattern to it that would point to the center. Where would I find out shape of the universe. The last I saw about it was that it is like a sheet of paper that was about 10 years ago?
     
  11. Jul 23, 2014 #10

    phinds

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    This is pop-sci nonsense. It assumes the big bang singularity event was an explosion at a single point in space, which it was not.

    The singularity happened everywhere at once and there IS no center to the universe, nor is there an edge, which there would be under your scenario.
     
  12. Jul 26, 2014 #11
    What you just said sounds more like god created the universe then like a theory. To question is to search for the truth and to not follow blindly what we do not truly know. I don't know if anyone said that before me or not but it is true. Point is at some level we are both wrong because we just don't know.
     
  13. Jul 26, 2014 #12

    phinds

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    Please refer to a specific statement and explain why you think it is more like God than a scientific theory.

    If you believe the universe has either an edge or a center then you run the risk of being banned from this forum as a crackpot. OR ... you could come up with a VALID new theory that overturns existing cosmology. Good luck with that.

    By the way, I see you're new to the forum. If you're like most of us when we first start up, you didn't read the rules yet and are probably not aware of the ban on personal speculation and the fact that the purpose of this forum is to explore and explain known science as currently understood.

    So, for example, saying something like "I don't understand why cosmologists say there is no edge to the universe" is perfectly fine. That's what this forum is for ... to help people understand such things.

    But to say "I believe that there IS an edge to the universe" is not good form and to insist on it would be to invite getting banned.
     
    Last edited: Jul 26, 2014
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