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Ashtekar Loll Pullin Randono at Salonica gravity meeting in June

  1. May 25, 2008 #1

    marcus

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    Next week will be the 13th conference in the Recent Developments in Gravity series.
    It will be held at Salonica (Thessaloniki, Greece)

    Here are titles of some of the talks

    Ashtekar--Loop Quantum Cosmology: an Overview

    Loll---The Self-Organizing deSitter Universe

    Pullin--Black Holes in Loop Quantum Cosmology

    Randono--Mesoscopic Structures in Quantum Gravity

    All four talks will be on 5 June, the first three in the main session (presumably they are invited talks) and the fourth will be in the parallel session a bit later. So there is no time conflict.
    http://www.astro.auth.gr/~neb-13/

    They are going to talking about things that currently interest a lot of people. LQC has been making remarkable progress especially since 2005 in Ashtekar's group at Penn State. Getting rid of the big bang singularity---running computer and analytical model through and back before the big bang. So it was natural to invite Ashtekar to give an overview.

    Loll's Utrecht group has also been getting results rapidly. One of the recent ones was the emergence of a deSitter universe, as a spacetime path integral: the smooth quantum "average" of many jagged geometries. The thing assembled itself out of microscopic degrees of freedom. Getting a 4D spacetime, not put in by hand, to emerge has been a major goal. So that was a clear choice as well.

    There has also been much interest in loop quantum black holes, that Jorge Pullin will be talking about.

    What is unexpected here is that Andy Randono is giving his paper on mesoscopic QG features that could be pointing to observable effects. Potentially observable effects of QG is another hot topic. Randono is postdoc at Penn State. His going to Salonica with this highlights the possibility of his research (posted only last week) eventually leading to tests. Somebody evidently thinks there is enough to it to warrent a travel grant. I'll give the abstract in case anyone wants to see it.

    http://arxiv.org/abs/0805.2955
    A Mesoscopic Quantum Gravity Effect
    Andrew Randono
    10 pages, 2 figures
    (Submitted on 19 May 2008)

    "We explore the symmetry reduced form of a non-perturbative solution to the constraints of quantum gravity corresponding to quantum de Sitter space. The system has a remarkably precise analogy with the non-relativistic formulation of a particle falling in a constant gravitational field that we exploit in our anaylsis. We find that the solution reduces to de Sitter space in the semi-classical limit, but the uniquely quantum features of the solution have peculiar property. Namely, the unambiguous quantum structures are neither of Planck scale nor of cosmological scale. Instead, we find a periodicity in the volume of the universe whose period, using the observed value of the cosmological constant, is on the order of the volume of the proton."
     
    Last edited: May 25, 2008
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  3. May 25, 2008 #2

    marcus

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    in case anyone's interested in the most recent black hole result by Pullin et al
    http://arxiv.org/abs/0805.1187
    Black holes in loop quantum gravity: the complete space-time
    Rodolfo Gambini, Jorge Pullin
    4 pages, 2 figures
    (Submitted on 8 May 2008)

    "We consider the quantization of the complete extension of the Schwarzschild space-time using spherically symmetric loop quantum gravity. We find an exact solution corresponding to the semi-classical theory. The singularity is eliminated but the space-time still contains a horizon. Although the solution is known partially numerically and therefore a proper global analysis is not possible, a global structure akin to a singularity-free Reissner--Nordstrom space-time including a Cauchy horizon is suggested."


    and here's the Loll et al paper
    http://arxiv.org/abs/0712.2485
    Planckian Birth of the Quantum de Sitter Universe
    J. Ambjorn, A. Gorlich, J. Jurkiewicz, R. Loll
    4 pages, 3 figures
    (Submitted on 17 Dec 2007)

    "We show that the quantum universe emerging from a nonperturbative, Lorentzian sum-over-geometries can be described with high accuracy by a four-dimensional de Sitter spacetime. By a scaling analysis involving Newton's constant, we establish that the linear size of the quantum universes under study is in between 17 and 28 Planck lengths. Somewhat surprisingly, the measured quantum fluctuations around the de Sitter universe in this regime are to good approximation still describable semiclassically. The numerical evidence presented comes from a regularization of quantum gravity in terms of causal dynamical triangulations."
     
    Last edited: May 25, 2008
  4. May 25, 2008 #3
    Since I intend to go to the conference, and taking into account that I haven't study these things yet, would it be possible that you provide some links to the key papers for these subjects (so that I catch a glimpse of them before going to the talks)? Thank you in advance!

    Edit: OK, you're faster than one expected...lol...Is there a link for Ashtekar's talk in lqc then?
     
  5. May 26, 2008 #4

    marcus

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    heh heh, just by accident I posted links to some articles by them, except for Ashtekar.
    I'm delighted to hear that you will attend the conference! Please share some of your impressions with us.

    For Ashtekar it is more difficult to choose one or two references as pre-conference reading for you.

    There is a very good recent paper called Robustness. But the problem with it is that it mixes some clear easy passages (stating results) in with some difficult technical passages that would only be accessible to people who already know the subject.

    With a paper like that the challenge is to be able to recognize the 25 percent that is understandable and pull those parts out. And not get discouraged by the inaccessible 75 percent.

    Maybe I can help. In any case i will give the link for this "Robustness" paper.

    for completeness here are all Ashtekar's papers that are on arxiv
    http://arxiv.org/find/grp_physics/1/au:+ashtekar/0/1/0/all/0/1
    and if you look down the list you will see that #6 on the list is titled "Introduction to LQG" so that would be a possible paper for you. But I am suggesting that we try #4 on the list instead which is this:
    http://arxiv.org/abs/0710.3565
    Robustness of key features of loop quantum cosmology
    Abhay Ashtekar, Alejandro Corichi, Parampreet Singh
    (Submitted on 18 Oct 2007, last revised 24 Jan 2008)

    "A small simplification based on well motivated approximations is shown to make loop quantum cosmology of the k=0 Friedman-Robertson-Walker (FRW) model (with a massless scalar field) exactly soluble. Analytical methods are then used i) to show that the quantum bounce is generic; ii) to establish that the matter density has an absolute upper bound which, furthermore, equals the critical density that first emerged in numerical simulations and effective equations; iii) to bring out the precise sense in which the Wheeler DeWitt theory approximates loop quantum cosmology and the sense in which this approximation fails; and iv) to show that discreteness underlying LQC is fundamental. Finally, the model is compared to analogous discussions in the literature and it is pointed out that some of their expectations do not survive a more careful examination. An effort has been made to make the underlying structure transparent also to those who are not familiar with details of loop quantum gravity."

    This paper is more up-to-date than the other and has more of the latest results because it has been revised this year for publication in the Physical Review D.

    I will look thru and grab out some sample quotes.
     
    Last edited: May 26, 2008
  6. May 26, 2008 #5

    marcus

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    In the abstract you can see reference to the FRW (friedmann robertson walker) model of cosmology. You probably know that is the standard model of cosmology and it's based on the 1920s Friedmann equation (a simplification of the Einstein GR equation). Basically it is just a simple ordinary differential equation governing the scalefactor a(t).

    And the WdW (Wheeler-deWitt) equation was an early (1970s) attempt to quantize the FRW model. It didn't work but it was a useful beginning. It gave a differential equation for the wavefunction of the size of the universe. A wavefunction for a(t). The trouble was it failed right at the beginning of expansion, just like the classical theory does.

    So LQC is basically an attempt to do better than the WdW, and to get a quantum version of the Friedmann equation that works. And indeed it appears to work in the sense that it REPRODUCES classical and semiclassical behavior away from the big bang (and yet it does not fail at the very start of expansion, as the previous models did.)

    So it is an OK cosmology model that has correct behavior away from big bang, but which doesn't break down and suffer from a singularity. Instead it continues back in time.

    Well in 2006 Ashtekar's group were doing a lot of computer runs with this model (because they didn't yet have a complete analytical solution that they could rely on instead) and they kept seeing that the behavior was essentially classical until the density got to be around 0.02 of Planck and then quantum terms began to be felt and gravity became repellent instead of attractive and the bounce would always happen when the density was about 0.82 of Planck density.

    They would vary the parameters and start with different conditions, make the universe flat sometimes and curved of various sizes other times with varying amounts of matter, and they always were getting this same behavior. So it looked like the bounce was robust----it didn't depend on some particular choice of initial conditions.

    Then in 2007 they kept on experimenting with numerical models in the computer but they also developed analytical models. They set up equations that you could solve. So you didn't have to depend on computer modeling.

    And the result was that the solvable analytical models confirmed the numerical results.

    So that is what Ashtekar et al are trying to say here. The analytical model confirms that the bounce is generic. Under very broad assumptions it always happens and it always happens when the density gets to be around 80 percent of Planck.

    And I am pretty sure that is what his Salonica talk is going to be about.

    =====================

    BTW in case anyone is interested here is a link to a PDF that gives the titles of all the talks, or a lot of the main ones.
    http://www.astro.auth.gr/~neb-13/program-NEB13.pdf
    You can see the titles of other talks besides the ones I mentioned by Ashtekar Loll Pullin Randono.
     
    Last edited: May 26, 2008
  7. Jun 4, 2008 #6

    marcus

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    PhysiSmo are you at the conference?

    PhysiSmo indicated he might be attending the Salonica conference (New Developments in Gravity). Maybe we will get a little news or impressions!a

    Tomorrow (5 June, Thursday) is the big day, from my viewpoint. The abstracts have been posted so here are some abstracts from the Thursday session

    ===============
    Loop Quantum Cosmology: An Overview
    Abhay Ashtekar
    Institute for Gravitation and the Cosmos, Penn State University
    In loop quantum cosmology (LQC) of homogeneous models with at least one massless scalar field, the scalar field serves as a natural ‘internal’ clock. When evolution is discussed with respect to this emergent time, the big bang singularity is naturally resolved. The fundamental equations of LQC provide a deterministic evolution from the pre-big-bang to the post-big-bang branch. This occurs because quantum geometry effects give rise to an unforeseen repulsive force. It is completely negligible in normal conditions but rises very quickly once the matter density rho reaches about 1% of the Planck density rhoPl and overwhelms classical gravitational attraction. As a consequence, big bang is replaced by a quantum bounce. Once rho drops below ∼ 1% of rhoPl, the new force becomes negligible. As a result, although classical general relativity is completely inadequate in the Planck regime, it is an excellent approximation right till curvatures approach ∼ 1/(lengthPl2.

    In this talk, I will present an overview of the current status of loop quantum cosmology, emphasizing recent developments. These include:

    i) results on the covariant entropy bound;

    ii) a proof that in the k = 0 models, the matter density operator has an absolute
    upper bound rhosup which is approached arbitrarily closely at the bounce point;

    iii) sharpening of similarities with and differences from the Wheeler De-Witt theory;

    iv) Inclusion of Bianchi I models which play a key role in the BKL conjecture.

    ===============

    Mesoscopic Structures in Quantum Gravity
    Andrew Randono
    Physics Department, Penn State University
    In an effort to understand the nature of quantum de Sitter space, we explore the
    symmetry reduction of the generalized Kodama state. The system has a remarkably
    precise analogue with the non-relativistic formulation of a particle falling in a
    constant gravitational field that we exploit in our anaylsis. We find that the solution
    reduces to de Sitter space in the semi-classical limit, but the uniquely quantum
    features of the solution have peculiar property. Namely, the unambiguous quantum
    structures are neither of Planck scale nor of cosmological scale, but an intermediate
    length scale some twenty orders of magnitude larger than the Planck scale. We find
    a periodicity in the volume of the universe whose period, using the observed value
    of the cosmological constant, is on the order of the volume of the proton.

    ================

    Black Holes in Loop Quantum Gravity
    Jorge Pullin
    Department of Physics and Astronomy, Louisiana State University
    We review recent work on the quantization of spherically symmetric space-times
    in loop quantum gravity including treatments of the exterior, the interior and complete
    coverings. We show how the singularity is removed and discuss other features
    of the resulting models

    ===================

    The Self-Organizing de Sitter Universe
    Renate Loll
    Institute for Theoretical Physics, University of Utrecht
    I will describe the construction of a theory of quantum gravity from a nonperturbative
    and background-independent path integral. At an intermediate stage,
    it uses a regularization in terms of Causal Dynamical Triangulations (CDT). The
    continuum limit of this formulation can be investigated with the help of computer
    simulations, which show that a de Sitter universe emerges on large scales. This
    means that the theory is able to dynamically generate “its own background”, one of
    the holy grails of quantum gravity. The emergence is of an entropic, self-organizing
    nature, with the weight of the Einstein-Hilbert action playing a minor role.

    =====================

    Several of the talks have new results which have not been presented before at any conference. The Randono result is quite surprising. It shows the scale factor of the universe growing not with monotone slope but with little RIPPLES. As if by a staircase of little jumps from one "size-level" to the next (analogous to closely spaced energylevels in an atom).

    A couple of the talks (Ashtekar, Loll) present major landmark results. The proof of the Bousso entropy bound in LQC at the former singularity (now a smooth bounce) is one such.
    And the generation of a smooth deSitter background geometry out of quantum fluctuation and jaggedness, essentially out of foam, is of course a first, and big news.

    More discussion and links to the relevant Loll and Ashtekar papers can be found here:
    https://www.physicsforums.com/showthread.php?t=238605

    I gave a link earlier in this thread to the paper which Andy Randono is presenting at Salonica:

    http://arxiv.org/abs/0805.2955
    A Mesoscopic Quantum Gravity Effect
    Andrew Randono
    10 pages, 2 figures
    (Submitted on 19 May 2008)
     
    Last edited: Jun 4, 2008
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