Small black holes ruled out by LQG?

In summary, the paper suggests that in the heart of a black hole there is a bounce instead of a singularity. This could be one of the important few papers of the year. It was well reasoned and presented. Many people 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.
  • #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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  • #2
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
 
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  • #3
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.
 
  • #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
 
  • #5
nightcleaner said:
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,...

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 doesn't 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 doesn't seem to be any word)
 
  • #6
marcus said:
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.
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?
 

1. What is LQG?

LQG stands for Loop Quantum Gravity, which is a theory that attempts to reconcile quantum mechanics and general relativity into a single framework.

2. How does LQG rule out small black holes?

LQG predicts that the fabric of space-time is made up of discrete, indivisible units called "loops." These loops cannot be smaller than the Planck length, which is about 10^-33 centimeters. Therefore, according to LQG, it is not possible for a black hole to have a size smaller than the Planck length. This effectively rules out the existence of small black holes.

3. Are there any observational evidence for LQG's predictions?

At this point, there is no direct observational evidence for LQG's predictions. However, there are ongoing experiments, such as the LIGO experiment, that are testing aspects of LQG and its predictions.

4. What are the implications of ruling out small black holes?

Ruling out small black holes has significant implications for our understanding of the universe. It means that LQG is a viable theory and that space-time is not continuous, as predicted by general relativity. It also has implications for the behavior of matter at the quantum level and the nature of gravity.

5. Could there be any exceptions to LQG's prediction about small black holes?

While LQG predicts that small black holes cannot exist, there are still some theoretical loopholes. For example, some theories suggest that black holes may have a "fuzziness" at their event horizon, which could potentially allow for the existence of small black holes. However, these theories are still under investigation and have not been proven.

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