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Conceptualizing the Big Bounce in LQC

  1. Mar 28, 2014 #1
    Could someone please settle something conceptually for me if possible? marcus, if you catch this, please help me out.

    Consistent probabilities in loop quantum cosmology - Craig, Sing


    "Consistent with the prediction that "all states bounce", as noted all states in sLQC look like a particular symmetric superposition of expanding and collapsing Wheeler-DeWitt universes at large volume. While this is certainly a necessary condition for a theory in which a bounce is a generic feature, one may be led to inquire whether the presence of this superposition is in some sense the reason for the bounce. The answer is definitely NO. In fact, it was shown in Ref. 10. that a superposition of expanding and contracting universes in the Wheeler-DeWitt quantization of this same physical model does not, and indeed cannot bounce: all states are sucked in to the singularity at large. If a physical reason for the bounce is to be sought, it is in the "quantum repulsion" generated at small volume in loop quantum gravity, and manifested in this model in the ultraviolet cutoff in the dynamical eigenfunctions, not in the superposition of large expanding and contracting classical universes."

    From a related paper "Consistent probabilities in Wheeler-DeWitt quantum cosmology" also by Craig, Singh:

    "Of some special interest is the case where the quantum state is a superposition of contracting and expanding universes. Such a state might be regarded as an analog of a "Schrodinger's cat" state in ordinary quantum mechanics - a quantum superposition of macroscopically distinct states."

    First, can you summarize what the above means conceptually? I find it incredibly interesting, but I feel like I am being given two different ways of conceptualizing the model. Admittedly, I am not mathematically-inclined, but what I am looking for is a visual way of understanding this.

    According to these papers, I am visualizing a Universe that expands (bounces) from a quantum state, reaches a maximum area and then contracts back to a quantum state in the same "space." The expanding and contracting "branches" extending from the quantum state share the same spatio-"temporal" area, i.e., they are macroscopically superposed. I am seeing this as a sphere that inflates rapidly from a Planckian space to a maximum area, and then contracts inward back toward the same Planckian space and bounces, over and over in the same "space" localized from the same quantum "point."

    So I admittedly could be totally confused, because the concept has been referred to MUCH differently...

    So is the Universe bouncing "up" and then "down" in the same "space" as I stated above? (creating new Universes with each bounce)... Or is the Universe bouncing from "left" to "right" (as much as these terms can make sense) creating "new" space with endless branches of expansion and contraction conjoined at endless quantum bridges?

    This is how the model has been shown graphically:


    ^is this what is the bounce physically represents, the "quantum bridge" literally between a contracting previous Universe (on the "left", in a different area) and an expanding "new" Universe on the "right" in a spatially different area?

    "This includes, in particular, Schrodinger's Cat" states, generic (but possibly macroscopic) superpositions of left-moving (contracting) and right-moving (expanding) states."

    ^ from one of the above papers... Is this just mathematically speaking?

    I created this image to help "ask" my question:


    Is this how you visualize the idea? Universes being created endlessly in different "areas"? With each Universe connected to another former Universe ad infinitum via quantum bridges?

    Or is it all "self contained" as the superposition paper led me to believe, as this:


    ....or, as below, does it bounce from "one side to the other" and create new Universes? (Would each side have a superposition of expansion and contraction as discussed above? ...one side has a physical Universe, while the other does not, and switches back and forth, forever?):


    ^This is the model that Alejandro Corichi uses to show the idea

    How do you conceptualize the bounce? Why do they sometimes refer to this "Universe on the other side" as here:

    “The significance of this concept is that it answers what happened to the universe before the Big Bang,” Singh told PhysOrg.com. “It has remained a mystery, for models that could resolve the Big Bang singularity, whether it is a quantum foam or a classical space-time on the other side. For instance, if it were a quantum foam, we could not speak about a space-time, a notion of time, etc. Our study shows that the universe on the other side is very classical as ours.”

    “This means that the twin universe will have the same laws of physics and, in particular, the same notion of time as in ours,” Singh said. “The laws of physics will not change because the evolution is always unitary, which is the nicest way a quantum system can evolve. In our analogy, it will look identical to its twin when seen from afar; one could not distinguish them.”

    “In the universe before the bounce, all the general features will be the same,” said Singh. “It will follow the same dynamical equations, the Einstein’s equations when the universe is large. Our model predicts that this happens when the universe becomes of the order 100 times larger than the Planck size. Further, the matter content will be the same, and it will have the same evolution. Since the pre-bounce universe is contracting, it will look as if we were looking at ours backward in time.”

    “Such a universe will have many bounces from one branch to another,” Singh said. “It is also possible that universes in different branches will be identical.”

    I am sorry for writing so much, but I am insanely curious.

    Any and all help appreciated and again, I am sorry for being redundant and ignorant.
    Last edited: Mar 28, 2014
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  3. Mar 29, 2014 #2


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    You're doing all right. It isn't easy. You're asking questions. You've assembled some beautiful (schematic, simplified) graphics. It looks like you've done some (photoshop?) manipulation.

    I'm not sure I personally can help very much. I might get you off track. Instead of listening to words of mine or anyone else, maybe you should just keep on trying to imagine
    what it would be like to be in a universe where everything was getting closer to everything else until a calamitous crash-bounce happens, a blinding instant of unimaginably extreme density,
    and then what it would be like to be in a universe where everything was getting farther from everything else---the light thinning and cooling, clouds gathering, blobs of matter condensing.

    To DEPICT that in a schematic diagram (instead of imagining being in the midst and seeing it "from inside") you normally have to simplify by reducing the dimension.

    Instead of 3D creatures in contracting expanding 3D space, you might picture 2D creatures and 2D stars on the 2D skin of a balloon that contracts down to almost nothing and then re-expands.
    All existence is concentrated on that infinitesimally thin sphere surface. Stars with zero thickness, everything zero thickness. The whole of existence shrinks down and then reexpands. As it reexpands all the stuff, everything on the balloon surface gets farther from everything else, without MOVING in the usual sense, no change in relative positions, nobody approaches a destination (in that world). they all just get farther apart. they have no 3rd spatial dimension.

    In your "double horn" diagrams it is even more simplified. Space is a ONE dimensional RING, which contracts and expands in time, sweeping out a horn shape as it expands. If you want to show time spatially in the diagram then you have to sacrifice a space dimension. Instead of the 2D surface of the balloon, which you WATCH contract and then expand, you go down to the 1D ring. All existence is concentrated on that zero thickness circle. Stars planets animals are tiny points and intervals along that ring. And the cone or horn is a way of showing the whole contraction and expansion history of that circle. As a still, rather than as a movie.

    You don't have to use the double horn diagram "still" of the process. You can just imagine WATCHING a balloon shrink down and then re-expand, so time is not frozen into the diagram. I like that better because the 2D universe is closer to 3D reality. But it's largely a matter of taste.

    Even better is to imagine being inside a contracting-then-expanding 3D geometry, visualize it. then you do not have to sacrifice any spatial dimensions. nothing is moving *in the conventional sense of getting somewhere* everything is just becoming closer together and then after the bounce farther apart. Analogous to the 2D case of the balloon where all the spots stay where they are on the balloon surface, but get closer together and then farther apart. Maybe the full 3D imagining is the hardest, but ultimately the most satisfactory.

    I think it is up to you, but you seem to have strong visual imagination. Maybe other people here will help.

    BTW :biggrin: you are not acting "redundant and ignorant". It's hard and takes time and concentrated thought. You seem just like me, or other people.

    BTW the basic pattern of expansion is that distances are expanding at a certain PERCENTAGE RATE (so larger distances grow proportionally faster) and this percentage rate is allowed to change very gradually according to an equation called Friedman equation. You will probably learn about that later.
    Last edited: Mar 29, 2014
  4. Jul 3, 2014 #3

    Thank you much for responding and I apologize for not responding back. I have had a crazy couple of months. The best part though, is that I've been talking to Ashtekar on the phone!

    I'll send you an mp3 of some of his amazing answers to my questions. He is by far the most intelligent, most humble and kind individual I've ever gotten to speak to.

    I asked him especially about Bojowald being a dick and where LQG is headed.

    Thank you again!

    You make me love this forum!
  5. Jul 3, 2014 #4


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    My feelings resemble yours. Not only is he deeply intelligent he is also modest, gentle, kind *and* has a sense of humor. A really sweet guy. You can't describe Ashtekar, it will always come out sounding like a cliché or too good to be true :smile:

    A Physicsforums member named Phil as made an excellent interview video of interviews with Ashtekar and one of his younger collaborators/co-authors Ivan Agullo. The interviewer is, I think, really good at her job; the editing is good because it cuts back and forth skillfully between Ashtekar's answers and Ivan's slightly different answers.

    Since you have actually been talking on the telephone with Ashtekar, this 45 minute interview video may not mean so much to you personally, but it fills a gap for anyone who has not spoken directly with him. You may want to recommend it to your friends and pass along the link.

    Actually I get quite a lot out of the interviews even watching second or third time. the interviewer gets both Ashtekar and Agullo to talk about the thermodynamics of the Loop cosmology bounce---the entropy problem, how does low entropy come out of the bounce? The fact that according to Lqg gravity repels at extreme small scale and extreme high density---and this can "iron out" relic structure from the contracting pre-bounce phase. Also an interesting explanation in terms of the observer's horizon (entropy as missing/hidden information, inaccessible to the observer, about micro states). The horizon exists in the contracting phase, somewhat analogously to a black hole horizon. and then abruptly ceases to exist when the bounce is in the past. I very much liked seeing both A and A deal alternately with the question and try to put answers in terms understandable to an intelligent layperson.

    Have you by any chance already seen the video? I forget things like that.
    Last edited by a moderator: Sep 25, 2014
  6. Jul 3, 2014 #5
    Marcus, again, I love you man.

    This forum, your words and everything are just awesome.

    I nearly died in May and I honestly believe that, though I should be dead or in a coma, my studying of quantum gravity and physics helped me in the 4th dimension; Einstein, Wheeler, Planck and Oppenheimer made me go back!

    Thank you much. I think I have seen the video before; do you have a link?
  7. Jul 4, 2014 #6


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    Here's the link

    Take care of yourself. And maybe studying physics IS good for you. what you say could be right (you are the only one who can tell, but I suspect it could well be)
    Last edited by a moderator: Sep 25, 2014
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