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Dark matter and black holes!

  1. Jul 31, 2011 #1
    When it comes to dark matter; Has the mass of the super-massive black holes residing at the core of every galaxy been taken into consideration? Could the total mass of all black holes account for the missing mass in the universe?
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  3. Jul 31, 2011 #2
    Dark matter is responsible for the expanding universe, creating a cosmological constant that is greater than 1, meaning a cosmos that is expanding at an accelerated rate. If we just used the black holes mass as the extra mass of the universe, gravity would overcome the positive cosmological constant and start the big collapse
  4. Jul 31, 2011 #3


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    Yes it has, people aren't that stupid.

    Dark matter, not dark energy. They're different things.
  5. Jul 31, 2011 #4
    But if we consider one to be a condensed form of the other they may be responsible at different times in their existance to act out the same affect
  6. Jul 31, 2011 #5


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    They're completely different phenomena.
  7. Jul 31, 2011 #6


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    About the supermassive black holes: There is other 'not very visible' matter in the universe like dwarf stars, neutron stars, ordinary gasses, black holes, etc which are called MACHOs. Evidence so far seems to say that these MACHOs couldn't possibly account for dark matter.

    Also, dark matter and dark energy are two different things.

    I think there is a misconception about how great the mass of a supermassive black hole is. Their mass is small, as compared to the total mass of the galaxy.
    Last edited: Jul 31, 2011
  8. Jul 31, 2011 #7
    How can this be if it is the supermassive black hole keeping the galaxy together and have a mass smaller than the galaxy?
  9. Jul 31, 2011 #8


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    The galaxy is HUGE. Also, in any orbiting system, it actually doesn't matter much how big the central object is when it comes to how much can orbit around that central object. If you have a million solar mass black hole at the center of a galaxy, you could have 1 billion sollar masses worth of stuff orbiting it. As long as it's spread out, everything would still orbit around the one giant concentrated mass that is the super-massive black hole.

    The sum of the masses of the planets in our solar system don't really compare to the mass of the Sun, but you can do the same thing with our solar system. You could scatter jupiter sized planets all over the place and as long as they don't get clumped up or are very close to the Sun, you still could have everything orbiting the Sun.
  10. Jul 31, 2011 #9
    OK; Thanks.

    Last Question: Is there a limit to how much matter can be compressed (like in black holes)?
  11. Jul 31, 2011 #10
    I've never heard of such an upper limit to a mass of a black hole.
  12. Jul 31, 2011 #11
    I did not mean how much matter can be ingested but how much compression can matter accept!
  13. Jul 31, 2011 #12


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    Infinite compression. Of course there could be a finite amount before matter turns into something else, but who knows.
  14. Jul 31, 2011 #13
    Ah but that is a mystery worth looking into! If a black hole has no limit to the amount of matter it ingests and yet matter is compressed into a singularity at the core of a black hole then we have a problem:

    Either matter is infinitely compressible or it ends up elsewhere; like into another universe! I mean theoretically a black hole can swallow whole galaxies.

    Does anyone know the limits of compressibility of matter?
  15. Jul 31, 2011 #14


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    It really doesn't matter. We cannot detect anything past the event horizon, thus as far as we are concerned the "size" of the black hole is defined by its event horizon. The event horizon of a black hole is directly proportional to its mass. If a black hole swallows a mass equal to its own, its event horizon doubles in radius. This means that the volume enclosed by the BH more than doubles. If we take the mass of a black hole an divide it into this volume, we find that the effective density of the black hole goes down as it gets more massive.

    To us, outside of the black hole, it does not matter if the mass stops compressing right after crossing the event horizon or continues on towards a singularity. All we can know is that it got larger and more massive.
  16. Jul 31, 2011 #15
    OK now I understand. Thank you for clarifying. Now to ask yet one more relevant question: Since information cannot be destroyed then would I be correct in saying that all information stays at the event horizon? If this so then what is beyond the event horizon is basically nothing?
  17. Jul 31, 2011 #16
    I think that's the whole point: the supermassive black hole as well as everything else we see in the galaxy is NOT enough to keep the galaxy together. Something else must be providing the gravity to do so...
  18. Jul 31, 2011 #17
    There is at least 1 effect that prevents the formation of singularities. As the matter compresses it causes greater and greater gravity. Gravity can be modeled as warped spacetime. The more intense the gravity, the more time slows down as compared to an outside observer. From our perspective, outside the black hole, the time needed for a star to collaps to a singularity would be infinite.

    What you would observe as you fell into a black hole I don't know.
  19. Aug 1, 2011 #18


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    I thought that issue had been taken care of by someone already?
  20. Aug 1, 2011 #19
    But does not mass require time in order to exist? If time at the core of a black hole is zero then mass cannot exist and thus not even a singularity can exist. Besides if the information remains at the event horizon then so too mass cannot exist even in an extremely compressed state.

    How can non mass create such a gravitational effect that nothing can escape it?
  21. Aug 1, 2011 #20


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    Time only slows down for an observer in a frame of reference DIFFERENT from another frame. If you were to fall into a black hole (and survive) YOU would not experience any time dilation for yourself, ever.
  22. Aug 2, 2011 #21
    Various surveys of the Galactic Halo have set upper bounds of the percentage of galactic dark matter that can be found in black holes (and other compact objects):

    OGLE: http://adsabs.harvard.edu/abs/2011MNRAS.413..493W
    EROS 2: http://adsabs.harvard.edu/abs/2007A&A...469..387T
    MACHO: http://adsabs.harvard.edu/abs/2000ApJ...542..281A

    Generally speaking, all surveys say less than 10% of dark matter can be found in black holes/compact objects (ranging in mass from 0.0001 to 100 solar masses generally.)

    Assuming, of course, that the Cosmic Censorship Conjecture is correct (which is increasing shaky).
    Last edited: Aug 2, 2011
  23. Aug 16, 2011 #22
    So, if we were to observe a black hole from the outside, would it be possible that we could see an echo of some sort representing the things that have "fallen" in?

    If i were to fall into a black hole, would my image be preserved for all eternity, seeing as it would take an infinite amount of time for me to collaps?

    (spelling may be of, since English is not my first language.)
  24. Nov 10, 2011 #23
    Please tell me if I am wrong about it..................since now reading about a black hole i could have a perception that a black hole nothing other than a star who has got its plasma cooled down and thus created a surface and due to high density it has a very large magnitude of gravitational pull which creates a escape velocity which is even greater than the speed of light.........so when anything which is pulled inside a black hole must hit a ground but due to the immense gravitational pull all the subatomic partices gets distorted and hence lead to the body of that object converts into singularity.................
  25. Nov 10, 2011 #24


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    A 'singularity' is another way of saying we don't know. We already know that GR and QT do not play well together at planck scales.
  26. Nov 10, 2011 #25
    Information cannot be destroyed and it is suspected that all information remains at the event horizon. I suspect that the answer will come from CERN within the next decade.
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