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Corrected third quarter 2007 M.I.P. poll

  1. Aastrup Grimstrup

    0 vote(s)
  2. Bojowald

    0 vote(s)
  3. Bonanno Reuter

    0 vote(s)
  4. Engle Pereira Rovelli

  5. Freidel Krasnov

    0 vote(s)
  6. Percacci

    0 vote(s)
  1. Sep 29, 2007 #1


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    The previous attempt had a couple of glaring omissions. Here is a corrected version of the third quarter Most Influential Paper forecast. I missed the Aastrup Grimstrup paper when it came out in 2006, so am including it now as better late than never.

    Intersecting Connes Noncommutative Geometry with Quantum Gravity
    Johannes Aastrup, Jesper M. Grimstrup
    19 pages, 4 figures
    (Submitted on 18 Jan 2006)

    "An intersection of Noncommutative Geometry and Loop Quantum Gravity is proposed. Alain Connes' Noncommutative Geometry provides a framework in which the Standard Model of particle physics coupled to general relativity is formulated as a unified, gravitational theory. However, to this day no quantization procedure compatible with this framework is known. In this paper we consider the noncommutative algebra of holonomy loops on a functional space of certain spin-connections. The construction of a spectral triple is outlined and ideas on interpretation and classical limit are presented."

    What happened before the big bang?
    Martin Bojowald
    3 pages. Nature Physics, Volume 3, Issue 8, pp. 523-525 (2007).

    "Was the Universe before the Big Bang of classical nature, described well by a smooth space-time? Or was it in a highly fluctuating quantum state? This is one of the most basic questions that we may ask once it is accepted that there was something before the Big Bang. Loop quantum gravity applied to isotropic models has shown that the quantum evolution of a wavefunction extends through the Big Bang. Although a general demonstration is still lacking, this may suggest that calculations, and possibly future indirect observations, may allow us to see the Universe as it was before the Big Bang. Here, we analyse an explicit model with a pre-Big Bang era, indicating limitations that would imply that it is practically impossible to answer some of our questions. Assumptions (or prejudice) will remain necessary for knowing the precise state of the Universe, which cannot be fully justified within science itself."
    (abstract page with citation count: http://adsabs.harvard.edu/abs/2007NatPh...3..523B )

    Entropy signature of the running cosmological constant
    Alfio Bonanno and Martin Reuter
    57 pages, 7 figures

    "Renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values are investigated using a system of cosmological evolution equations which allows for an unrestricted energy exchange between the vacuum and the matter sector. It is demonstrated that the scale dependence of the gravitational parameters, the cosmological constant in particular, leads to an entropy production in the matter system. The picture emerges that the Universe started out from a state of vanishing entropy, and that the radiation entropy observed today is essentially due to the coarse graining (RG flow) in the quantum gravity sector which is related to the expansion of the Universe. Furthermore, the RG improved field equations are shown to possess solutions with an epoch of power law inflation immediately after the initial singularity. The inflation is driven by the cosmological constant and ends automatically once the RG running has reduced the vacuum energy to the level of the matter energy density."

    Flipped spinfoam vertex and loop gravity
    Jonathan Engle, Roberto Pereira, Carlo Rovelli
    37 pages, 4 figures
    (Submitted on 9 Aug 2007)

    "We introduce a vertex amplitude for 4d loop quantum gravity. We derive it from a conventional quantization of a Regge discretization of euclidean general relativity. This yields a spinfoam sum that corrects some difficulties of the Barrett-Crane theory. The second class simplicity constraints are imposed weakly, and not strongly as in Barrett-Crane theory. Thanks to a flip in the quantum algebra, the boundary states turn out to match those of SO(3) loop quantum gravity -- the two can be identified as eigenstates of the same physical quantities -- providing a solution to the problem of connecting the covariant SO(4) spinfoam formalism with the canonical SO(3) spin-network one. The vertex amplitude is SO(3) and SO(4)-covariant. It rectifies the triviality of the intertwiner dependence of the Barrett-Crane vertex, which is responsible for its failure to yield the correct propagator tensorial structure. The construction provides also an independent derivation of the kinematics of loop quantum gravity and of the result that geometry is quantized."

    A New Spin Foam Model for 4d Gravity
    Laurent Freidel and Kirill Krasnov
    40 pages

    "Starting from the Plebanski formulation of gravity as a constrained BF theory we propose a new spin foam model for 4d Riemmanian quantum gravity that generalises the well-known model of Barrett-Crane and resolves the ultralocality problem that this model is known to possess. It is well known that the BF formulation of 4d gravity possesses two sectors: one corresponding to gravity and the other topological. The model presented here is shown to give a quantisation of the gravitational sector. The present model is dual to the recently proposed spin foam model of Engle et al. which, we show, corresponds to the topological sector of the theory. One important outcome of our approach is that it also allow us to introduce the Immirzi parameter into the framework of spin foam quantisation. We generalize some of our considerations to the Lorentzian setting and obtain a new spin foam model in that context as well."

    Asymptotic Safety
    Roberto Percacci
    21 pages + Q&A, 2 figures. To appear in "Approaches to Quantum Gravity: Towards a New Understanding of Space, Time and Matter", ed. D. Oriti, Cambridge University Press
    (Submitted on 24 Sep 2007)

    "Asymptotic safety is a set of conditions, based on the existence of a nontrivial fixed point for the renormalization group flow, which would make a quantum field theory consistent up to arbitrarily high energies. After introducing the basic ideas of this approach, I review the present evidence in favor of an asymptotically safe quantum field theory of gravity".

    some past M.I.P. prediction polls
    Last edited: Sep 29, 2007
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