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Loop Gravity cures brane collision singularity

  1. Jul 14, 2004 #1


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    Loop Quantum Gravity and the Cyclic Universe
    Martin Bojowald, Roy Maartens, Parampreet Singh
    6 pages, 4 figures

    Notice that Roy Maartens has published over 20 stringy-type papers, mostly on "brane-world" cosmology and brane-related matters.

    He now seems to have developed an interest in seeing what Loop Gravity predicts: not only about inflation (an earlier Maartens paper) but also about alternative scenarios like a hypothetical brane-collision. Apparently there used to be a concern that colliding branes might result in a singularity. Loop Gravity (which seems useful for curing singularities) has now been applied to the hypothetical brane scenario and has dispelled that possibility.

    "Loop quantum gravity introduces strong non-perturbative modifications to the dynamical equations in the semi-classical regime, which are responsible for various novel effects, including resolution of the classical singularity in a Friedman universe. Here we investigate the modifications for the case of a cyclic universe potential, assuming that we can apply the four-dimensional loop quantum formalism within the effective four-dimensional theory of the cyclic scenario. We find that loop quantum effects can dramatically alter the near-collision dynamics of the cyclic scenario. In the kinetic-dominated collapse era, the scalar field is effectively frozen by loop quantum friction, so that the branes approach collision and bounce back without actual collision."

    ----exerpts from the introduction---

    "...A bold attempt to tackle these problems is the ekpyrotic/cyclic scenario[1], which invokes ideas from M-theory to construct an alternative to the standard inflationary paradigm. A crucial issue for the cyclic scenario is how to process the cosmological dynamics and perturbations through time[2]..."

    "Loop quantum gravity is a four-dimensional nonperturbative candidate theory of quantization of space-time whose successes include prediction of a discrete spectrum for geometrical operators [3], matter Hamiltonians that are free from ultraviolet divergences [4] and derivation of the Bekenstein-Hawking entropyformula[5].

    Recently, loop quantum gravity has been applied tocosmology (for reviews see[6, 7]), leading to a resolution of cosmological singularities[8] and a new view on initial conditions [9].

    In general, singularity avoidance entails a breakdown of smooth classical spacetime structure and the quantum geometric discretization of spacetime. Loop quantum cosmology derives a difference equation for the wave function whose evolution does not stop where the classical singularity would be.

    The system then continues to a new branch after which a semiclassical description may be used[10]."

    I point out this paper partly because it breaks the "two camps" paradigm and represents crossover on the part of the authors.
    Personally I am not so captivated by the "colliding braneworlds" scenarios and think they are a bit on the pure speculation side. But here we have Roy Maartens, a recognized braneworld expert, collaborating with Martin Bojowald to apply Loop Gravity anaylysis to the braneworld scenario (purely hypothetical though it may be). It could be fruitful, this kind of crossing over and applying one thing to another---and anyway it breaks the fences. so bravo to them and also to Parampreet Singh
    who I recall has published with Maarten before and is part of the growing
    quantum gravity scene at Imperial College London
  2. jcsd
  3. Jul 15, 2004 #2
    As we know the graviton is not part of Loop Quantum Gravity.

    If we had held brane scenarios and delve further into their application, the issues of gravitons in the bulk make for strange interactions( one--> many) in those brane collisions?

    Personally I like seeing the crossing referencing going on. Makes it much easier to understand the differences. So it means LQG and String people must be very flexible and from what I have seen of Smolin and LUBOs and others, this helps greatly for a laymen.
    Last edited: Jul 15, 2004
  4. Jul 15, 2004 #3
    Note that in the last page he mentions the possibility of constructing 5D Loop Quantum gravity as a way to try to sharpen the possibilities of this new "Loop Quantum gravitized Cyclic Model"

    It really surprises me that Bojowald is flirting with string theory

    BTW, the inclusion of Bojowald in the list of quantum gravity theorists in the wikipedia page
    is due to me :approve:
    Last edited: Jul 15, 2004
  5. Jul 15, 2004 #4


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    bravo meteor! I didnt know any of us had been helping out at wikipedia
    (aside from the obvious work by Lubos Motl)

    I wouldnt even call what Bojo is doing "flirting" since he can develop a 5D version of LQG and put it to good use, why not?

    LQG in several different dimensions (1+1, 2+1...) have been studied besides the usual 3+1. I think also some work has already been done on some type of 5D.
    What was learned from 3D has been useful in arriving at conjectures in 4D by analogy.
    So it is very proper for him to work on 5D, and (whatever he thinks of stringy theories and their prospects) he can apply LQG in a highly visible way here.

    I dont know anything or assume anything about Bojo's inner state of mind or what he may or may not think are promising avenues in other fields----all I say is that if I were in his place I would love to apply Loop techniques to get a better understanding of branes (WHATEVER i might think of branes as models of the real world of nature!)

    Also Bojowald is at Albert Einstein Inst (MPI-Potsdamm) where string-theorist Hermann Nicolai has great influence and makes many of the appointments.
    and Nicolai has an expressly stated program of getting exactly this to happen----to get Loop methods tried out on stringy stuff. It provides potential tests of which Loop methods are correct, and it also promotes
    cooperation. It is also incidentally in the interest of the LQG community to get some research territory in string-land IMHO, but that is presumably not Nicolai's idea (just my humble view)

    Urs Schreiber could say the same more authoritatively, I suspect, with somewhat different bias and emphasis. being acquainted with Nicolai and with the AEI.
  6. Jul 15, 2004 #5


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    Phew! That's a relief. :biggrin:
  7. Jul 15, 2004 #6


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    platonic ekpyrotic cosmology

    I'm told some branes find it a bit frustrating :uhh:
  8. Jul 16, 2004 #7
    We just have to recognize the Branes Behind it. :smile:
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