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Black holes and the expanding universe

  1. Jul 21, 2006 #1
    In another thread 'Entropy, information and Omphalos cosmology' I commented on a strange fact (which I think many are aware of): namely that

    to which there was an interesting reply:

    This subject seems to me not to have an immediate bearing on questions of entropy (or Omphalos cosmology, for that matter) and to deserve consideration on its own. Hence this new thread. When it comes to black holes, cosmology becomes quite "above my fireplace", and I can't be of much use.

    I make only one remark: perhaps it is dangerous to mix symmetries. The metric of a black hole is spherically symmetric, and static; the Robertson-Walker metric of the standard model universe depends on time and is isotropic everywhere. Could this produce strange results like those mentioned above?
  2. jcsd
  3. Jul 21, 2006 #2
    Interesting. The entropy of that material inside a black hole is proportional to the surface area of the event horizon. Similarly the entropy inside the observable universe should be proportional to the area of the cosmological event horizon. If the universe continues to accelerate in its expansion, then the cosmological event horizon will shrink and the entropy inside must decrease. Perhaps this is the cause of life developing on earth (and perhaps elsewhere).

    As the cosmological event horizon shrinks, we lose matter (galaxies) behind the shrinking cosmological event horizon. We also lose space. How can entropy decrease, information increase, if we lose space and matter?
  4. Jul 22, 2006 #3


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    It is easy to show that for every flat cosmological model the Schwarzschild radius of the mass inside the Hubble sphere is equal to the Hubble radius RH = c / H. In an exactly flat model this condition will always hold, since exactly flat models remain always flat.
    Last edited: Jul 22, 2006
  5. Jul 22, 2006 #4
    Local homogeneity

    Thank you Oldman for your threads. (I am only a few month older than you and it is good to see we have the same interest. I have been thinking already times on this subject).
    Let me give you some of my thoughts:
    - In an exact homogeneous universe there can not be black holes.
    - Locally our observable universe is not homogeneous so indeed it can and it really has black holes.
    - For the radius of a black hole also its environment counts.
    - Our observable universe must be (and I suppose it is) part (name it our universe) of a larger universe with enough mass in it to be a black hole in a multi/mega/infinity-verse, which is locally enough inhomogeneous to allow for black holes just as our observable universe does. (Inhomogeneity seems a kind of fractal to me).
    - The consequences for our universe being a black hole are interesting:
    1) It then existed long before our big bang started.
    2) It gives space/room to a locally “oscillating” kernel (our observable universe being part of it), so for the “time being” no problems with this expansion.
  6. Jul 22, 2006 #5
    Thanks for your comments, hurk4. I mentioned that "when it comes to black holes, cosmology becomes quite "above my fireplace", and I can't be of much use.... ", so I'll leave others to comment on your post, which is, untranslated, "bo my vuurmaak plek", which I'm sure you'll understand!
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