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Couple of Bounce Questions Regarding Black Holes / Universe

  1. Sep 19, 2008 #1
    I've been fascinated by the posts and discussions here about "The Bounce" that seems to be emerging from some LQG models (thanks Marcus and others). I have a couple of questions, which I don't think I've seen in posts here (my apologies if they've been raised and discussed and I've missed it).

    The bounce seems to describe a contraction phase followed by an expansion phase that never goes through an infinitely dense point (or "singularity" which I understand is a model failure, and not something real).

    I also understand that it is not really appropriate to equate what occurred in this extremely dense phase prior to the expansion that resulted in our current universe with what goes on in a black hole.

    Nevertheless, I am curious as to whether any of those who are working in this field have speculated as to why we don't seem to see any expanding (exploding?) black holes among the billions that are now thought to be at the centers of galaxies. Do the LQG bounce models imply that under black hole conditions that there is a continuous, stable, pulsing cycle of contraction / expansion / contraction etc. of this very, very dense "stuff" that keeps black holes in place in a fairly confined space once they are formed?

    And if that is the case, then I suppose the next obvious question is, when applying this process at the larger scale of the universe, what might explain the current expansion phase of our universe to have proceeded these last 13 odd billion years and allow for the much less dense stuff that is our universe today?

    Thanks in advance,

  2. jcsd
  3. Sep 19, 2008 #2


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    That's right! I'm glad you are interested. In LQC the cosmological bounce (the Big bounce) is much better understood than whatever happens with black holes. The big bounce is a robust result, you get it over a lot of different cases and assumptions. As you say, the situation with understanding black holes is different.

    That's hard to say, Frank. It hasn't been resolved within the LQG context. There are several different LQG/LQC papers that draw different conclusions about the black hole bounce, depending on which simplifying assumptions are made. It might turn out to be appropriate to draw some connection between BH and BB. Or it might not.

    In the LQG/LQC context, if it turns out that black hole collapse does lead to a bounce, and a new expanding region of spacetime, then that region would not be visible to us and would not affect us. Our region would be entirely in the past of whatever happened down the hole.

    I can't offer a good explanation of this. If you want articles to look at, I can post some links to recent LQG black hole papers. You will see what I mean about issues not being settled yet. If you want, let me know.

    One possible outcome of black hole collapse (considered in LQG) is a wad of something yet to be described, something that doesn't bounce and does NOT start a new tract of expanding universe disjoint from ours. A new state of space+matter. this black hole bounce business is unsettled.
    Last edited: Sep 19, 2008
  4. Sep 20, 2008 #3
    It is to be hoped that LQG theorists remain scientist in that things are not considered settled until LQG predicts something that can be verified by experiment and disagrees with GR or QM. Until then LQG is simply a speculative mathematical theory.

    Personally I think that in a few years Smolin's book "The Trouble with Physics" will get an extra chapter on LQG in that LQG will be recognized as the same mathematical masturbation as string theory is.
    Last edited: Sep 20, 2008
  5. Sep 20, 2008 #4
    Hi Jen,
    I love your description.

    I just read this month's Scientific American cover article "Follow the Bouncing Universe" by Martin Bojowald. It strikes me as the most vacuous (no pun intended) lead article I've read there in years. The article hypothesizes that the universe collapses until it becomes a mega black hole of a certain critical density, and then gravity becomes repulsive like a sponge full of water. I can't quite picture a soggy sponge as the source of the universe's kinetic expansion momentum. To wit:

    "Imagine space as a sponge and mass and energy as water. The porous sponge can store water but only up to a certain amount. Fully soaked, it can absorb no more and instead repels water. Similarly, an atomic quantum space is porous and has a finite amount of storage space for energy. When energy densities become too large, repulsive forces come into play."

    Isn't there a big distinction between inability to absorb vs. strong repulsion? If the spacetime "sponge" goes slightly above critical energy density, does it release a tiny amount of repulsion to return just to the critical density (and normally attractive gravity), or does it release a lot of repulsion and thereby expel all ALL of the big black hole's mass energy? The article implies that the repulsion "overshoots" far past the critical density point, but doesn't mention the mechanism for overshooting, or any ballpark figures for the repulsion force. Maybe it should be pictured like a fission bomb in which uranium is squeezed to critical density and explodes.

    The author makes no attempt to justify why the universe reliably recollapses into a mega black hole in each cycle -- as opposed, say, to continuing to expand forever. Is the amount of repulsion force released automatically less than the escape velocity of the mass-energy density? Presumably, but the article doesn't say.

    The article also speaks very broadly about spacetime being comprised of a dense mesh of Planck-scale "spacetime atoms", but makes no effort to explain how the "time" component of spacetime makes these atoms fundamentally different from purely "spatial" conventional atoms. The article suggests that individual spacetime atoms come and go as required by the dynamic geometric fabric of General Relativity, but does not mention where they come from or go to, or how their atomic energy might be physically transferred between different locations in space and time.

    Presumably the technical papers fill some of these glaring gaps, but I was surprised that the SciAm article's popular advocacy for the theory is so uncompelling. Ironically, the article is written in a very confident tone and predicts that the details will soon be understood.

    Last edited: Sep 20, 2008
  6. Sep 20, 2008 #5
    That is the irritating thing about it.
  7. Sep 22, 2008 #6
    excues me if my questions seem a little cluless, my degree isn't in physics.

    1. The sponge is a bad analogy, obviously a spong doesn't repel anything. if it is saturated it only fails to absorb more.

    2. multiple cycles with a bounce in between each? Are we the last such cycle meaning no more? hasnt the big crunch idea been discarded as the fate of our universe? or am I wrong about that?

    3. "The article also speaks very broadly about spacetime being comprised of a dense mesh of Planck-scale "spacetime atoms", but makes no effort to explain how the "time" component of spacetime makes these atoms fundamentally different from purely "spatial" conventional atoms. "
    I don't quite understand what you mean in this quote.
    Isn't the article implying discrete indivisable units or granularity of dimension, ie. plank distance? rather than conventional atoms with neutrons, protons etc. ? and right or wrong, if the article is going to suggest atoms of space then shouldn't that include atoms of time also?

    4. "no mention of where they come and go to, or how energy is transfered"
    Why would the article need to specify these things when we don't attempt to describe how energy is transfered in discrete quanta, say when electrons change from one energy level to another for example.?
    Last edited: Sep 22, 2008
  8. Sep 22, 2008 #7
    Hi Talon,
    Well, of course the standard LCDM model now predicts that the universe is either flat, meaning that it expands forever at an ever-declining rate, or that it has slight negative curvature. Even in the latter case, the current model for Lambda (Dark Energy, cosmological constant) predicts that the universe will expand indefinitely at an ever more rapid rate. I believe the standard model is now entirely inconsistent with any big crunch, although of course it's subject to change. So yes, it's surprising and annoying that the SciAm article seems to take repeated crunches (including into the future) for granted without commenting on this departure from the standard model. SciAm had a cover article just a couple months ago about how the universe is believed to be expanding at an accelerating rate.
    I interpret the article to describe discrete indivisible units or granularity of spacetime, with their spatial dimensions approximating the Planck length. So clearly these are not conventional atoms. I don't know that the theory needs to predict separate atoms for time and for space, but they need to say something about what the time component of "spacetime atoms" means.
    Not sure I follow this question. When electrons change energy level, it's normally because they've absorbed or emitted a discrete particle, a photon or virtual photon. So are there even tinier little particles absorbed by or emitted from a "spacetime atom"? If so, what is their nature, and how do they move through space and time? Do they imply the existence of subparticles which combine to create "spacetime atoms?" Do the atoms move around or stay in a fixed coordinate space? Do they change in size and energy content? Do they come and go entirely? I just don't think the confidence the author expresses about the existence of "spacetime atoms" is warranted in the absence of at least a cursory description of what they are and how they work.

  9. Sep 24, 2008 #8
    Thanks Marcus and others for weighing in on this. I was sure hoping that the reserachers had more to say about my questions given how much LQG and the bounce idea appears to be taking center stage these days.

    It seems that with the bounce idea the LQG modelers may have found a way out of the singularity dead end, but without offereing (yet anyway) much in terms of a clear physical description of the process that takes things from expansion to contraction to expansion etc., about the "stuff" that exists in this transitional stage, or as MeJennifer notes, a way to test some aspects of the model implications to lend credence to them. Or how it squares with the universe seeming not to be heading for another crunch.

    Well, to borrow a Shakespeare passage, it leaves me in perplexity and doubtful dilemma. But, I will keep checking back.

    Thanks again.

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