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Penrose: Before the Big Bang-Outrageous Solution to a Profound Cosmological Puzzle

  1. Sep 21, 2006 #1


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    Penrose: Before the Big Bang--Outrageous Solution to a Profound Cosmological Puzzle

    a video of a talk given at Perimeter Institute recently by Sir Roger Penrose

    Before the Big Bang: an Outrageous Solution to a Profound Cosmological Puzzle

    go here:

    select "seminar series" from the sidebar menu
    look down the list to one dated 12 September

    I heard him give what I expect was the same talk at MSRI-Berkeley and I was, just as he indicates here, outraged. I have reasonably good tolerance of new creative ideas, but there are limits.
  2. jcsd
  3. Sep 22, 2006 #2
    I actually think he meant the title to be a bit sarcastic, since his proposal is mathematically, as well as physically way less outrageous than any quantum gravity proposal I know of. :rofl: :rofl: As he humbly admits, it is a reasonable model (based upon thoughtful considerations concerning conformal invariance) which can make predictions of the right qualitative nature ; that is good science ! Actually, many people have been playing around with the idea of quantum gravity being a classical conformally invariant theory with GR as the omega = 1 phase (I think de Broglie was amongst the first to propose this idea regarding a geometric interpretation of his guidance formula).

  4. Sep 23, 2006 #3
    Maybe he should have called it the Continual Creation model since there seems to no end to the different phases of big bangs.

    It would seem in his model that there is an infinite resource to draw from to create an infinite number of big bangs.
  5. Sep 23, 2006 #4
    Concerning your first comment : you are right but the same goes for these models with a bounce at the big crunch right (I remember similar cyclic behavior coming out of the Sorkin Rideout dynamics for causal sets). Your second remark seems to be a bit premature, I do not see why we would need some extra (energy) resource in order to create a new big bang (actually there is no notion of conserved energy in de Sitter space anyway). Sure he admits to have no dynamics for the scale factor yet which accomplishes the crunch (although he is hinting at (broken) conformally invariant theories) and the subsequent big bang. I don't know but was kind of happy to see some proposals using ``sober'' geometry.

  6. Sep 23, 2006 #5
    Actually, in bounce models, don't they claim that the mass-energy-spacetime from the previous phase go into the new creation?

    It would seem to deny cause and effect if there is nothing that serves as a premise for it. And if it were "something" physical which serves as the stuff from which further big bangs can come, then something physical would be assumed to be finite and so only a limited number of big bangs could be created.
  7. Sep 23, 2006 #6
    Could you be more precise about what kind of conservation laws you are alluding to (I just told you it is very difficult to find such conservation law) ?? Moreover, in what respect would this differ from the construction of Penrose ?

    :bugeye: Cause and effect have nothing to do with change of energy. For example, in scattering processes total energy is conserved.

    Last edited: Sep 23, 2006
  8. Sep 24, 2006 #7
    I suppose the conservation laws that apply to both spacetime and matter-energy would be expressed in terms of a quantum gravity theory which doesn't exist yet. It would seem though that such laws would explain why space expands more with higher vacuum energy and space expands less with permanent matter. And it would explain how permenant matter is pulled out of the vacuum due to the acceleration affects of expansion. The only thing I can see at the time is that expansion forms horizons which hide information, and particles are also a store of information. So perhaps there is a conservation of information law at work from one big bang to the next - as information is hiden behind horizons, information is stored in the particles of the thermal bath associated with those horizons.
  9. Sep 24, 2006 #8
    I don't buy that, one could first look for a classical gravitational self enery and see how conservation laws could be improved upon.

    What you say here is long understood in classical gravity and semiclassical quantum field theory on curved spacetime.

    True, in de Sitter for example this Horizon remains constant. Ed Witten has written a nice paper about QFT on de Sitter in the context of the cosmological constant (if I remember correctly). But I do not see how this has to do with our original discussion ...

  10. Sep 24, 2006 #9
    It would seem to me that the relation between the expansion of GR and the vacuum energy of QM is by definition a Quantum Gravity effect.

    I don't remember any explanation by Wald (or others) on how spacetime expands in those models. And so we await a deeper explanation - such as perhaps Spin Foam models.

    I'd be interested in a link on the arXiv of that paper if you have it.

    I have to wonder what happens to the concept of energy and entropy, etc, in a quantum gravity theory. For energy is described in terms of how particles travel through space. But if both spacetime and particles are described by the same QFT, then it seems the spacetime symmetry that leads to the conservation of energy is lost in a theory whose symmetry encompasses both spacetime and particles. And if any spacetime-mass/energy theory would conserve information in a region of space - the propagation of particles would be governed by field that describes how space changes properties to accommodate particle position changes. And so it seem that information of the particles trajectory would be stored in the bits of space that it has travelled through. So no information is lost in particle dispersion since it is recorded in the bits of space whose properties change to accomodate propagation.
  11. Sep 25, 2006 #10
    Right, but that was not the issue, we were talking about *conservation* laws in general and that you better figure out at a classical level first.

    Calculations of particle production in quantum field theory out of the vacuum on de Sitter space have been made for some decades now (I think Mottola did this, the paper is ``particle creation in de Sitter space'' Phys. Rev. D 31, 754–766 (1985)). There are even those who claim (wrongly I think) that gravitons created in such way will screen the cosmological constant. So, unfortunately for you, there is no quantum gravity or spin foam involved here (and I do not see how the latter will solve the cosmological constant problem anyway).

    Yes, it is hep-th/0106109, quantum gravity in de Sitter space.

    Two remarks : (a) ``energy'' could still remain constant on the avarage
    (b) again you better look for an improved *classical* concept of energy first, one which is not based upon symmetries but takes into account gravitational radiation/expansion effects and so on.

    Huh, why do you say that when you know that information is not even saved in your freezer. Moreover, even if ``energy'' is conserved, entropy will rise in general in some region of space at least it will do so classically. Quantum mechanically I don't know because of unitarity but on the other hand one could take a partial trace and obtain a non unitary evolution such as happens in the decoherence interpretation.

    Last edited: Sep 25, 2006
  12. Sep 25, 2006 #11


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    Penrose has never been one to duck controversy. I think he is not above using publicity to stir other scientists from their slumbers.
  13. Sep 26, 2006 #12
    Nah, Penrose being in his seventies still has the spirit of a young guy in his twenties : naturally that stirs things up. :wink:
  14. Sep 26, 2006 #13
    So how is his suggestion any different from the Cyclic scenarios introduced long ago? I saw the entire lecture and AFAICS, the only difference was that he smoothed out the transition between the big-bangs. Does that count for "profound" or am I missing something?
  15. Sep 26, 2006 #14
    I am beginning to think that there is some conserved quantity in the QFT of spacetime-matter/energy that is responsible for the various big bangs (if more than one). Inflation tells us that the higher vacuum energy of the false vacuum is responsible for the first accelerating expansion. And when the false vacuum energy fell to its more stable level, particles obtained their mass. Now we see that the universe is beginning to accelerate in its expansion again. And again we believe it is the vacuum energy that is causing this expansion. So I have to wonder if there will come another fall of the vacuum energy to a new, lower and more stable level accompanied with another round of particle creation or possibly a new force between particles or perhaps a new property that particles acquire. The point is that it seems that the vacuum energy is pushing the universe into expansion(s) and creating particles in the process. And since the vacuum energy seems to be falling in the process, one might expect that there can only be a limited number of particle creation phases (or big bangs).

    Another hint of conservation may come from canonical quantum gravity (LQG) where the fundamental observable is the spectrum associated with the volume operator. Correct me if I'm wrong, but I believe that this means that the conjugate momentum would be something involving the change of this volume operator. And just as a classical momentum can push a particle into an unstable higher potential energy, this conjugate momentum of the volume operator can drive the expansion of the universe into an unstable state which can fall by reducing the ability to producing virtual particles (by reducing the vacuum energy).

    Is this article free online anywhere?

    Correct me if I'm wrong here, but as I understand it, the scale factor of the expanding universe is put in by hand and it coincedentally is consistent with GR. I don't believe anyone anywhere has derived the expansion of the universe and/or the scale factor used in the FRW or de Sitter metric. And so this pair production from de Sitter space is just using the expansion of the scale factor and not deriving it in any way. They have assumed an expansion and got particle creation. They have not assumed particle creation and produced expansion. Why do they think that the vacuum energy is the cosmological constant, anyway?
    Last edited: Sep 26, 2006
  16. Sep 28, 2006 #15
    I just wonder. If it is true that the conjugate momentum to the volume operators must always be positive (at least on the average) like the momentum operator conjugate to the position operator in regular QM, then this would mean that the universe can only expand and never contract. This ideal sounds familiar. But it would contradict the bouncing universe scenario being derived from LQG put forth by some.
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