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The Self-Organized de Sitter Universe/heading the list of MVP nominees this quarter

  1. Jun 3, 2008 #1

    marcus

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    Every quarter we have a poll predicting which will be the quarter's most valuable QG paper, most important for future non-string quantum gravity research. In this case, I'm collecting nominees for the time period April thru June 2008, This simple elegant 6-page paper by members of Loll's group just came out and definitely goes on the list.

    http://arxiv.org/abs/0806.0397
    The Self-Organized de Sitter Universe
    J. Ambjorn, J. Jurkiewicz, R. Loll
    7 pp, 1 figure, entry to 2008 GRF essay competition, honourable mention
    (Submitted on 2 Jun 2008)

    "We propose a theory of quantum gravity which formulates the quantum theory as a nonperturbative path integral, where each spacetime history appears with a weight given by the exponentiated Einstein-Hilbert action of the corresponding causal geometry. The path integral is diffeomorphism-invariant (only geometries appear) and background-independent. The theory can be investigated by computer simulations, which show that a de Sitter universe emerges on large scales. This emergence is of an entropic, self-organizing nature, with the weight of the Einstein-Hilbert action playing a minor role. Also the quantum fluctuations around this de Sitter universe can be studied quantitatively and remain small until one gets close to the Planck scale. The structures found to describe Planck-scale gravity are reminiscent of certain aspects of condensed-matter systems."

    This is the first time that a whole smooth spacetime continuum (not put in by hand at the beginning) has emerged as the weighted average in a kind of Feynman path integral. (combining much variety of irregular spacetimes, whose unsmoothness cancels out in the average.) It is obviously a landmark achievement. But it comes at a time when OTHER significant results have also just appeared in other non-string QG research lines. Loop Quantum Cosmology, for instance.

    To place it in context, I will fetch the four or five others constituing the MVP (most valuable paper) nominee list so far---we still have some three weeks to go before nominations are closed. If you have some suggestions, or comments on any of the papers, please contribute them to this thread.
     
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  3. Jun 3, 2008 #2

    marcus

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    I have revised the list a little, dropping a paper out to keep the total at five.
    First I will list the abstracts in this post, without comment. Then I will condense a brief list of five titles and give some explanation why each is important.


    http://arxiv.org/abs/0805.3511
    The covariant entropy bound and loop quantum cosmology
    Abhay Ashtekar, Edward Wilson-Ewing
    15 pages, 3 figures
    (Submitted on 22 May 2008)

    "We examine Bousso's covariant entropy bound conjecture in the context of radiation filled, spatially flat, Friedmann-Robertson-Walker models. The bound is violated near the big bang. However, the hope has been that quantum gravity effects would intervene and protect it. Loop quantum cosmology provides a near ideal setting for investigating this issue. For, on the one hand, quantum geometry effects resolve the singularity and, on the other hand, the wave function is sharply peaked at a quantum corrected but smooth geometry which can supply the structure needed to test the bound. We find that the bound is respected. We suggest that the bound need not be an essential ingredient for a quantum gravity theory but may emerge from it under suitable circumstances."


    I omitted the Loll et al paper http://arxiv.org/abs/0804.0252

    These two papers by Song He and Yidun Wan can be treated as a unit
    http://arxiv.org/abs/0805.0453
    Conserved Quantities and the Algebra of Braid Excitations in Quantum Gravity
    Song He, Yidun Wan
    25 pages, 2 figures
    (Submitted on 5 May 2008)

    "We derive conservation laws from interactions of braid-like excitations of embedded framed spin networks in Quantum Gravity. We also demonstrate that the set of stable braid-like excitations form a noncommutative algebra under braid interaction, in which the set of actively-interacting braids is a subalgebra."

    http://arxiv.org/abs/0805.1265
    C, P, and T of Braid Excitations in Quantum Gravity
    Song He, Yidun Wan
    28 pages, 5 figures
    (Submitted on 9 May 2008)

    "We study the discrete transformations of four-valent braid excitations of framed spin networks embedded in a topological three-manifold. We show that four-valent braids allow seven and only seven discrete transformations. These transformations can be uniquely mapped to C, P, T, and their products. Each CPT multiplet of actively-interacting braids is found to be uniquely characterized by a non-negative integer. Finally, braid interactions turn out to be invariant under C, P, and T."


    http://arxiv.org/abs/0804.0632
    Reconstructing AdS/CFT
    Laurent Freidel
    34 pages
    (Submitted on 4 Apr 2008)

    "In this note we clarify the dictionary between pure quantum gravity on the bulk in the presence of a cosmological constant and a CFT on the boundary. We show for instance that there is a general correspondence between quantum gravity ``radial states'' and a pair of CFT's. Restricting to one CFT is argued to correspond to states possessing an asymptotic infinity. This point of view allows us to address the problem of reconstructing the bulk from the boundary. And in the second part of this paper we present an explicit formula which gives, from the partition function of any 2 dimensional conformal field theory, a wave functional solution to the 3-dimensional Wheeler-DeWitt equation. This establishes at the quantum level a precise dictionary between 2d CFT and pure gravity."

    http://arxiv.org/abs/0805.4696
    Regularization and finiteness of the Lorentzian LQG vertices
    Jonathan Engle, Roberto Pereira
    13 pages
    (Submitted on 30 May 2008)

    "We give an explicit form for the Lorentzian vertices recently introduced for possibly defining the dynamics of loop quantum gravity. As a result of so doing, a natural regularization of the vertices is found and suggested. The regularized vertices are then proven to be finite."

    Now let's add the Loll et al paper about getting the de Sitter universe, and make the condensed list. This may already be pretty close to what goes into the poll.
     
    Last edited: Jun 3, 2008
  4. Jun 3, 2008 #3

    marcus

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    Skipping details, to make it legible, here's the condensed list so far:


    1. J. Ambjorn, J. Jurkiewicz, R. Loll
    The Self-Organized de Sitter Universe
    (the title speaks for itself)


    2. Abhay Ashtekar, Edward Wilson-Ewing
    The covariant entropy bound and loop quantum cosmology
    (proving the important cov. entropy bound in LQG is a major landmark. The original conjecture by Bousso actually fails in classic cosmology near the singularity, so proving it here depends on using the quantum version, LQC.)

    The pair of braid-matter papers by
    3. Song He, Yidun Wan
    Conserved Quantities and the Algebra of Braid Excitations in Quantum Gravity

    C, P, and T of Braid Excitations in Quantum Gravity
    (If matter does enter into background independent quantum gravity this way, it's important. If it doesn't at least we will find out.)

    4. Laurent Freidel
    Reconstructing AdS/CFT
    (I quote the introduction: "The main question we want to address here is the question: What is AdS/CFT from the point of view of background independent quantum gravity?" He has found a way to start with the conformal field on the boundary and CONSTRUCT the gravity in the bulk. It may be that he can actually prove the conjecture in this new context. It is a bold question, what is AdS/CFT in the background independent QG context.)

    5. Jonathan Engle, Roberto Pereira
    Regularization and finiteness of the Lorentzian LQG vertices
    (the new spinfoam vertex formula is an important development, this paper pushes ahead with it and that could count for a lot)
     
    Last edited: Jun 3, 2008
  5. Jun 3, 2008 #4

    MTd2

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    I can't find anything new here. Isn't this just an expository paper for her work group?
     
  6. Jun 3, 2008 #5

    marcus

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    This paper is a short Reader's Digest version of the December 2007 paper.
    The December 2007 paper was entirely new. It gave technical detail and was harder reading.
    If you haven't read it. I urge you to give it a try.

    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."

    But I expect since you don't find the June 2008 paper new that you DID already read this one!:biggrin: Otherwise what you said wouldn't make sense.

    Basically I see this as a major landmark and I am looking for a paper to put in the poll that represents this important result. If by end of June all I have is this Reader's Digest easy-reading essay------written for non-specialists, for the Gravity Research Foundation annual essay competition. If that is all there is then I will use this.

    But actually I'm anticipating another paper from Loll's group about the emergence of deSitter universe. I understand they have further unreported results. We'll see.

    Thanks for the comment! If you read the December paper your comment is right on target!

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

    BTW, so glad you are following this! Did you notice that reference [6] of the December 2007 paper is
    "[6] J. Ambjørn, A. Görlich, J. Jurkiewicz and R. Loll, to appear."

    about time now, I'd say. If it comes out before end June I will swap it in for this one.
     
    Last edited: Jun 3, 2008
  7. Jun 3, 2008 #6

    marcus

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    Here's what it says on page 7
    "Awaiting publication of a full analysis of the
    data [6], suffice it to say that within measuring accuracy the fluctuations of the
    spatial volume are described by the mini-superspace action (4). This enables us
    to numerically estimate k1 ≈ 0.016, which according to (6) leads to G ≈ 0.22a2,
    or lengthPl ≈ 0.47a. In other words, the linear size of the quantum de Sitter universes
    studied here lies in the range of 17-28 Planck lengths."

    the thing is determining its own size. I find that fascinating. the full analysis should come out in their next paper.

    the only way they can make a bigger universe is to put in more atoms, it would seem. do you see another way.
    I should think there'd have to be one.
     
    Last edited: Jun 3, 2008
  8. Jun 4, 2008 #7

    MTd2

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    Heh, I wonder how and when particles will emerge from that. Certainly, group theoretical analysis should be of no concern here, since particles would just pop up from a background. There could be a kind of simulation where something resembling realistic particles appear. Maybe electron and photons at first.
     
  9. Jun 4, 2008 #8

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    I dig their association to self-organisation. I hope they keep up that line of association, it's in my taste :)

    However, their basic choice of reasoning implicit in the path integral itself is bothering me. But OTOH, maybe one step at a time is wise?

    Anyway, I would like to see also in the same spirit an emergence of feynmanns path intergral logic, as well the notions of manifolds. I don't see this as hopeless. Perhaps once a better understanding of the logic of the path integral is on the table, it will be straightforward to see how a the regularization and renormalisations come naturally.

    This is one of the basic things that both bugged and fascinated me since the first QM and stat mech courses where one could not avoid making the association between statistical reasoning and the formalism of QM. Yet, the connection is incomplete in several ways. And I'd hope that a fundamental theory would not leave such a thing unresolved. Maybe such an understanding would also make it more clear howto incorporate particle degrees of freedom in the same logic.

    This is the first point of rejection for me when reading that.

    /Fredrik
     
  10. Jun 4, 2008 #9

    marcus

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    For safety I had better add a sixth paper, with a surprising quantum cosmology result. That by Andy Randono. I will put the abstract down at the end of this list of MVP candidates for the second quarter of 2008


    1. J. Ambjorn, J. Jurkiewicz, R. Loll
    The Self-Organized de Sitter Universe
    (the title speaks for itself)


    2. Abhay Ashtekar, Edward Wilson-Ewing
    The covariant entropy bound and loop quantum cosmology
    (proving the important cov. entropy bound in LQG is a major landmark. The original conjecture by Bousso actually fails in classic cosmology near the singularity, so proving it here depends on using the quantum version, LQC.)

    The pair of braid-matter papers by
    3. Song He, Yidun Wan
    Conserved Quantities and the Algebra of Braid Excitations in Quantum Gravity

    C, P, and T of Braid Excitations in Quantum Gravity
    (If matter does enter into background independent quantum gravity this way, it's important. If it doesn't at least we will find out.)

    4. Laurent Freidel
    Reconstructing AdS/CFT
    (I quote the introduction: "The main question we want to address here is the question: What is AdS/CFT from the point of view of background independent quantum gravity?" He has found a way to start with the conformal field on the boundary and CONSTRUCT the gravity in the bulk. It may be that he can actually prove the conjecture in this new context. It is a bold question, what is AdS/CFT in the background independent QG context.)

    5. Jonathan Engle, Roberto Pereira
    Regularization and finiteness of the Lorentzian LQG vertices
    (the new spinfoam vertex formula is an important development, this paper pushes ahead with it and that could count for a lot)

    6. Andy Randono
    A Mesoscopic Quantum Gravity Effect
    (Andy delivered this paper at this week's Salonica conference, New Developments in Gravitation.)

    ================
    since I'm adding the Randono paper to the list, I'll give the abstract

    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."

    This was discussed in the thread about this week's conference
    https://www.physicsforums.com/showthread.php?t=237076

    It will shed further light to quote the abstract of the talk given at the conference on this paper. The paper contains some surprising results which could make it a valuable contribution.

    Mesoscopic Structures in Quantum Gravity (the Salonica conference talk)
    Andrew Randono

    "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."

    Putting it crudely, essentially what he found is that the universe is doing its usual bounce----what in quantum gravity can replace the big bang singularity----but the scalefactor, the index of the universe size, decreases and then increases in ripples. As if the size is coming down in little quasi-discrete quantum steps, and then there is the bounce, and it starts expanding and increases again in little quasi-discrete steps. See figure 2. The picture makes it clearer.

    And these size of the steps, although small, is much larger than planck scale. It's curious. suggests there might be something observable to look for in connection with this.
     
    Last edited: Jun 4, 2008
  11. Jun 30, 2008 #10

    marcus

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    revised list of MVP nominees this quarter

    This is the short list of nominees for the Most Valuable Paper forecast poll this quarter. I am replacing the Loll (CDT triangulations) paper, Self Organized deSitter, by a more recent Loll paper, "Shaken but not stirred..." and adding a new paper by Stephon Alexander called "Superconducting LQG...". It is nearly time to set up the poll.


    1. Stephon H.S. Alexander, Gianluca Calcagni
    http://arxiv.org/abs/0806.4382
    Superconducting loop quantum gravity and the cosmological constant
    "We argue that the cosmological constant is exponentially suppressed in a candidate ground state of loop quantum gravity as a nonperturbative effect of a holographic Fermi-liquid theory living on a two-dimensional spacetime. Ashtekar connection components, corresponding to degenerate gravitational configurations breaking large gauge invariance and CP symmetry, behave as composite fermions that condense as in Bardeen--Cooper--Schrieffer theory of superconductivity. Cooper pairs admit a description as wormholes on a de Sitter boundary."

    2. Abhay Ashtekar, Edward Wilson-Ewing
    http://arxiv.org/abs/0805.3511
    The covariant entropy bound and loop quantum cosmology
    (Proving the important covariant entropy bound in LQG is a major milestone. The original conjecture by Bousso actually fails near the singularity in classic cosmology. LQC supplies what is needed to validate it.)

    3. Laurent Freidel
    http://arxiv.org/abs/0804.0632
    Reconstructing AdS/CFT
    "What is AdS/CFT from the point of view of background independent quantum gravity?"

    4. Jonathan Engle, Roberto Pereira
    http://arxiv.org/abs/0805.4696
    Regularization and finiteness of the Lorentzian LQG vertices
    (This paper advances the development of the new spinfoam vertex formula.)

    5. Andy Randono
    http://arxiv.org/abs/0805.2955
    A Mesoscopic Quantum Gravity Effect
    (When the universe is doing its bounce----which in QG replaces the big bang singularity--- the scalefactor, indicating the universe size, decreases and then increases in ripples. As if the size is coming down in little quasi-discrete quantum steps, and then starts expanding and increases again in little quasi-discrete steps. See figure 2---the picture makes it clearer. Randono finds that the size of the steps, although small, is much larger than planck scale. A bit like finding the energy levels in the hydrogen atom.)

    6. J. Ambjorn, K.N. Anagnostopoulos, R. Loll, I. Pushkina
    http://arxiv.org/abs/0806.3506
    Shaken, but not stirred - Potts model coupled to quantum gravity
    "We investigate the critical behaviour of both matter and geometry of the three-state Potts model coupled to two-dimensional Lorentzian quantum gravity in the framework of causal dynamical triangulations. Contrary to what general arguments of the effects of disorder suggest, we find strong numerical evidence that the critical exponents of the matter are not changed under the influence of quantum fluctuations in the geometry, compared to their values on fixed, regular lattices. This lends further support to previous findings that quantum gravity models based on causal dynamical triangulations are in many ways better behaved than their Euclidean counterparts."

    7. The pair of braid-matter papers posted this quarter by Song He and Yidun Wan
    http://arxiv.org/abs/0805.0453 and http://arxiv.org/abs/0805.1265
    Conserved Quantities and the Algebra of Braid Excitations in Quantum Gravity
    and
    C, P, and T of Braid Excitations in Quantum Gravity
    (If matter enters into background independent quantum gravity as braiding, these papers could be important.)
     
    Last edited: Jun 30, 2008
  12. Jun 30, 2008 #11

    marcus

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    list of MVP nominees this quart

    We have seven already and now two more that need to be considered were just posted:

    8. Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
    http://arxiv.org/abs/0806.4640
    Path integral representation of spin foam models of 4d gravity
    29 pages, 6 figures
    (Submitted on 28 Jun 2008)

    "We give a unified description of all recent spin foam models introduced by Engle, Livine, Pereira and Rovelli (ELPR) and by Freidel and Krasnov (FK). We show that the FK models are, for all values of the Immirzi parameter, equivalent to path integrals of a discrete theory and we provide an explicit formula for the associated actions. We discuss the relation between the FK and ELPR models and also study the corresponding boundary states. For general Immirzi parameter, these are given by Alexandrov's and Livine's SO(4) projected states. For 0 <= gamma < 1, the states can be restricted to SU(2) spin networks."

    9. Eugenio Bianchi
    http://arxiv.org/abs/0806.4710
    The length operator in Loop Quantum Gravity
    33 pages
    (Submitted on 28 Jun 2008)

    "The dual picture of quantum geometry provided by a spin network state is discussed. From this perspective, we introduce a new operator in Loop Quantum Gravity - the length operator. We describe its quantum geometrical meaning and derive some of its properties. In particular we show that the operator has a discrete spectrum and is diagonalized by appropriate superpositions of spin network states. A series of eigenstates and eigenvalues is presented and an explicit check of its semiclassical properties is discussed."

    An unusually large number of potentially important papers were posted this quarter. It may make the prediction poll (forecasts of which papers will in fact turn out to be influential and valuable for future research) unwieldy and awkward to construct and answer.
    I would like some ideas about how to narrow the list down while doing justice to the papers.
     
  13. Jun 30, 2008 #12

    MTd2

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    Re: The Self-Organized de Sitter Universe/heading the list of MVP nominees this quart

    I guess I'd wait sometime before electing an article. I mean, it would be nice to peer review them a little bit.
     
  14. Jul 1, 2008 #13

    marcus

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    Re: The Self-Organized de Sitter Universe/heading the list of MVP nominees this quart

    I agree that it would excellent if we could discuss and evaluate the merits. Have some discussion of the papers. And state our differences of opinion. Ultimately everybody decides for himself and there is no one correct answer, but a discussion would certainly be useful!

    I am wondering now if it would make discussion easier if we divide the papers up into four groups and choose between the groups. there are too many articles now (ten in all I think) so it is hard to focus. How do you like it if we divide it up by LOCATION this way?:

    MARSEILLE
    Periera and Bianchi are associated with Rovelli's group at Marseille. Engle, now at Max Planck Potsdam, was there until recently.

    PENN STATE
    Abhay Ashtekar
    Stephon Alexander
    Andy Randono
    Edward Wilson-Ewing
    Gianluca Calcagni

    PERIMETER
    Laurent Freidel
    Yidun Wan (Song He was a visitor at Perimeter while the work was in progress)

    UTRECHT
    Jan Ambjorn
    Renate Loll

    So we could ask then what location has been most productive.
    I've had a long day and am too sleepy to think about it. Maybe it is best to keep to the old way and just have 9 or 10 different papers. Or maybe some other system of grouping the papers would be good. If you have any suggestions, I will think about it in the morning.
     
  15. Jul 1, 2008 #14

    marcus

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    splitting list into MVP junior and senior

    Another way to organize it would be to have two lists, and two polls. MVP-A for papers where one of the authors is faculty like Ashtekar, Loll, Freidel, Alexander. MVP-B for papers by grad students and postdocs.

    Here's how it would look, split into two polls. My comments are in parentheses, otherwise i've quoted the authors' description.

    MVP-A

    1. Florian Conrady, Laurent Freidel
    http://arxiv.org/abs/0806.4640
    Path integral representation of spin foam models of 4d gravity
    "We give a unified description of all recent spin foam models introduced by Engle, Livine, Pereira and Rovelli (ELPR) and by Freidel and Krasnov (FK). We show that the FK models are, for all values of the Immirzi parameter, equivalent to path integrals of a discrete theory and we provide an explicit formula for the associated actions. We discuss the relation between the FK and ELPR models and also study the corresponding boundary states. For general Immirzi parameter, these are given by Alexandrov's and Livine's SO(4) projected states. For 0 <= gamma < 1, the states can be restricted to SU(2) spin networks."

    2. Stephon H.S. Alexander, Gianluca Calcagni
    http://arxiv.org/abs/0806.4382
    Superconducting loop quantum gravity and the cosmological constant
    "We argue that the cosmological constant is exponentially suppressed in a candidate ground state of loop quantum gravity as a nonperturbative effect of a holographic Fermi-liquid theory living on a two-dimensional spacetime. Ashtekar connection components, corresponding to degenerate gravitational configurations breaking large gauge invariance and CP symmetry, behave as composite fermions that condense as in Bardeen--Cooper--Schrieffer theory of superconductivity. Cooper pairs admit a description as wormholes on a de Sitter boundary."

    3. Abhay Ashtekar, Edward Wilson-Ewing
    http://arxiv.org/abs/0805.3511
    The covariant entropy bound and loop quantum cosmology
    (Proving the important covariant entropy bound in LQG is a major milestone. The original conjecture by Bousso actually fails near the singularity in classic cosmology. LQC supplies what is needed to validate it.)

    4. Laurent Freidel
    http://arxiv.org/abs/0804.0632
    Reconstructing AdS/CFT
    "What is AdS/CFT from the point of view of background independent quantum gravity?"

    5. J. Ambjorn, K.N. Anagnostopoulos, R. Loll, I. Pushkina
    http://arxiv.org/abs/0806.3506
    Shaken, but not stirred - Potts model coupled to quantum gravity
    "We investigate the critical behaviour of both matter and geometry of the three-state Potts model coupled to two-dimensional Lorentzian quantum gravity in the framework of causal dynamical triangulations. Contrary to what general arguments of the effects of disorder suggest, we find strong numerical evidence that the critical exponents of the matter are not changed under the influence of quantum fluctuations in the geometry, compared to their values on fixed, regular lattices. This lends further support to previous findings that quantum gravity models based on causal dynamical triangulations are in many ways better behaved than their Euclidean counterparts."

    =================================
    MVP-B

    1. Jonathan Engle, Roberto Pereira
    http://arxiv.org/abs/0805.4696
    Regularization and finiteness of the Lorentzian LQG vertices
    (This paper advances the development of the new spinfoam vertex formula.)

    2. Andy Randono
    http://arxiv.org/abs/0805.2955
    A Mesoscopic Quantum Gravity Effect
    (When the universe is doing its bounce----which in QG replaces the big bang singularity--- the scalefactor, indicating the universe size, decreases and then increases in ripples, according to Randono's analysis. He finds that the size comes down in quasi-discrete quantum steps, and then increases again in little steps. See figure 2---the picture makes it clear. Randono calculates the size of the steps, which, although small, turns out to be many orders of magnitude larger than planck scale. A bit like finding the energy levels in the hydrogen atom.)

    3. The pair of braid-matter papers posted this quarter by Song He and Yidun Wan
    http://arxiv.org/abs/0805.0453 and http://arxiv.org/abs/0805.1265
    Conserved Quantities and the Algebra of Braid Excitations in Quantum Gravity
    and
    C, P, and T of Braid Excitations in Quantum Gravity
    (If matter enters into background independent quantum gravity as braiding, these papers could be important.)

    4. Eugenio Bianchi
    http://arxiv.org/abs/0806.4710
    The length operator in Loop Quantum Gravity
    ("The dual picture of quantum geometry provided by a spin network state is discussed. From this perspective, we introduce a new operator in Loop Quantum Gravity - the length operator. We describe its quantum geometrical meaning and derive some of its properties. In particular we show that the operator has a discrete spectrum and is diagonalized by appropriate superpositions of spin network states. A series of eigenstates and eigenvalues is presented and an explicit check of its semiclassical properties is discussed.")
     
    Last edited: Jul 1, 2008
  16. Jul 1, 2008 #15

    Demystifier

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    Re: The Self-Organized de Sitter Universe/heading the list of MVP nominees this quart

    I have one general question. Can you show us some results regarding a correlation between the prediction of importance and the actual importance (measured e.g. by the number of citations) of the papers? If there is no significant correlation, then what is the point of making such predictions?
     
  17. Jul 1, 2008 #16

    marcus

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    Re: The Self-Organized de Sitter Universe/heading the list of MVP nominees this quart

    Thanks for asking, Demy. I think citation count is a approximately fair (but not infallible) measure of importance, especially after 2 or 3 years have elapsed for the paper to be recognized and cited in subsequent research.

    I have gone back over some of the earlier polls and I have the impression that some people are better guessers than others. They tend to pick the papers that later will be recognized and cited.

    On the other hand we at PF tend to have some blind spots. Ashtekar papers regularly get a lot of citations and people don't often pick them in the poll. Maybe I haven't nominated them consistently enough.

    Anyway, I am interested in how good different people are at predicting (which people's guesses correlate with stuff like citations 2 or 3 years down the road) and I expect you are too. So I will collect some links and you can check it out.
    ====================

    Also I think the exercise of evaluating and ranking the current crop of papers is educational. At least for me it is. I learn a lot by focusing on the current research output and trying to evaluate which are the most significant and ground-breaking. It is a way of clarifying one's ideas of the current status and directions in the field.
    ====================

    Here are our 2006 MVP forecast polls. I hope we are getting better! Since it takes a while for citations to accumulate, it is probably still too early to see how we did in 2007. Check it out and see if you can identify any individuals who guess right more consistently than the rest of us.

    first quarter 2006
    https://www.physicsforums.com/showthread.php?t=116791
    second quarter 2006
    https://www.physicsforums.com/showthread.php?t=124951
    third quarter 2006
    https://www.physicsforums.com/showthread.php?t=134513
    fourth quarter 2006
    https://www.physicsforums.com/showthread.php?t=149466
     
    Last edited: Jul 1, 2008
  18. Jul 1, 2008 #17

    marcus

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    Gold Member
    2015 Award
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    best guessers of 2006

    this is slightly off topic but is in response to a question from Demy

    just taking first quarter 2006 as a sample, and using "citebase" counts, we see

    Marcus guess got 10
    Hossi guess got 11
    But Arivero, selfAdjoint, Kea, Turbo scored big, their guess got 71.

    So they are clearly the leaders at this point, so look at second quarter 2006 and see how various people do.

    Now looking at second quarter 2006, Arivero emerges as a clear winner

    His pair of Ashtekar papers has 85 cites.
    My pick got 26
    Kea's pick got 25.

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

    In the third quarter, I see that Demy played the game. He and I both had the same pick. But we did not win.

    Arivero won again, his pick got 29 cites.
    Etera Livine and Francesca's choice got 7 cites.
    Demy and I picked a paper that got 6 cites.

    I see that John Baez entered that poll and his prediction was the same as Arivero's.
    ======================

    In answer to Demy's question there is obviously a huge correlation between who is guessing and how many cites their choice of papers has accumulated after two years.
    =================

    But the point of the exercise is that we can all learn something by participating. We can each learn better how to assess the main research directions and significant results in the QG field. It isn't just about who is the best player, it is about how you individually see the QG field and where it is going and where it will be say two years from now.
    =================

    the reason I am counting cites here is that Demy specifically referred to citation counts. There are other more subjective measures of importance which we could discuss and I think they matter but that discussion would take longer and really belongs in a different thread. Cites is the quick and dirty measure.
     
    Last edited: Jul 1, 2008
  19. Jul 1, 2008 #18

    jal

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    Re: The Self-Organized de Sitter Universe/heading the list of MVP nominees this quart

    Compare
    http://arxiv.org/abs/0805.2955
    A Mesoscopic Quantum Gravity Effect
    Andy Randono
    (When the universe is doing its bounce----which in QG replaces the big bang singularity--- the scalefactor, indicating the universe size, decreases and then increases in ripples, according to Randono's analysis. He finds that the size comes down in quasi-discrete quantum steps, and then increases again in little steps. See figure 2---the picture makes it clear. Randono calculates the size of the steps, which, although small, turns out to be many orders of magnitude larger than planck scale. A bit like finding the energy levels in the hydrogen atom.)

    With
    http://arxiv.org/abs/0806.0397
    The Self-Organized de Sitter Universe
    Authors: J. Ambjorn, J. Jurkiewicz, R. Loll
    (Submitted on 2 Jun 2008)
    Look at the graph on p. 5
    Also the quantum fluctuations around this de Sitter universe can be studied quantitatively and remain small until one gets close to the Planck scale. The structures found to describe Planck-scale gravity are reminiscent of certain aspects of condensed-matter systems.
    -------
    Both are finding that there is fluctuations in the geometry.

    ----
    Both are expanding the bouncing universe.
     
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