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Small black holes ruled out by LQG?

  1. Mar 9, 2005 #1

    marcus

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    http://arxiv.org/abs/gr-qc/0503041
    A black hole mass threshold from non-singular quantum gravitational collapse
    Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
    4 pages, 3 figures

    "Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms -- quantum gravity may exclude very small astrophysical black holes."
     
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  3. Mar 9, 2005 #2

    marcus

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    the analysis also appears to confirm the suspicion that in the heart of a black hole there is a bounce (instead of a singularity)

    that is spacetime continues and re-expands

    the bounce in the pit of a black hole was a premise of Smolin's CNS theory circa 1995, but it was then merely an unproven conjecture. Smolin hypothesized at that time that LQG would eventually remove the classical black hole singularity and replace it with a bounce so that a further region of spacetime could expand to form its own universe. Only recently, some ten years later, has LQG analysis of gravitational collapse reached the point of confirming that the theory does, in fact, predict a bounce.

    I should emphasize what they say in the abstract
    "To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field."
    it is a very simplified form of matter that collapses to form the black hole, lots more work remains to be done on this
     
    Last edited: Mar 9, 2005
  4. Mar 9, 2005 #3

    Chronos

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    This could be one of the important few papers of the year. It was very well reasoned and presented. Many [me too] consider singularities to be unphysical - merely illustrating theory is incomplete. I think there are alternatives to the bounce model, but it's the best I've seen to date.
     
  5. Mar 18, 2005 #4
    Hi.

    It seems to me that bounce is a poor choice of words. Bounce implies a sudden change of direction and a resultant sudden accelleration, confined to a small time interval, so that it is expressed as t^-3, which is known as jerk.

    Instead, the expansion phase is into a space which is beyond the horizon of the infall phase. This is "another dimension," so the physical forces implied by "bounce" should not occur.

    Richard
     
  6. Mar 18, 2005 #5

    marcus

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    Hello Richard. I noticed that too. the word "bounce" is not quite right because it suggests a reversal of direction

    what I picture is a collapse to maximum density which continues on into re-expansion.

    this does not happen in the context of some static spacetime
    but rather I picture it as a new branch of spacetime that branches off
    a new future is created in which there is a new inflation creating a new universe

    so it doesnt seem like one can picture it by a physical bounce analogy of something bouncing within the set framework of 3D space
    it is more that "space itself" collapses around some location and reexpands thru there, instead of bouncing "back"

    But people call this kind of thing a bounce anyway! So maybe the word will gradually acquire a second meaning which will be in harmony with our notion of what happens (for which at present there doesnt seem to be any word)
     
  7. Mar 18, 2005 #6
    One thing troubles me about bounce cosmology. If everything is accelerating to a point, it would appear that gravity is drawing things together at that small a scale. But just after the "bounce" at that same scale we are told that inflation is causing things to accelerate away from each other when just prior to the bounce at that same scale things were accelerating towards each other. Doesn't gravity act the same way at the same scale?

    If you want the collapse to be the inverse of expansion, then near the final crunch things should be decelerating ever more slowly to a point, right?
     
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