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Our picks for fourth quarter 2011 MIP (most important QG paper)

  1. Curvature invariants,geodesics and the strength of singularities in Bianchi-I loop quantum cosmology

    7.1%
  2. Quantum Gravity and Renormalization: The Tensor Track

    14.3%
  3. q-Deformation of Lorentzian spin foam models

    0 vote(s)
    0.0%
  4. The geometric role of symmetry breaking in gravity

    14.3%
  5. Spin Foams and Canonical Quantization

    7.1%
  6. Positive cosmological constant in loop quantum cosmology

    7.1%
  7. Spontaneously broken Lorentz symmetry for Hamiltonian gravity

    14.3%
  8. A Renormalizable 4-Dimensional Tensor Field Theory

    21.4%
  9. Is de Sitter space a fermion?

    14.3%
  10. Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology

    14.3%
  11. A proposed proper EPRL vertex amplitude

    14.3%
  12. Higher Derivative Gravity from the Universal Renormalization Group Machine

    7.1%
  13. Path integral measure and triangulation independence in discrete gravity

    7.1%
  14. Regularized Hamiltonians and Spinfoams

    14.3%
  15. Continuous formulation of the Loop Quantum Gravity phase space

    42.9%
  16. Coupling Shape Dynamics to Matter Gives Spacetime

    28.6%
  17. A new Hamiltonian for the Topological BF phase with spinor networks

    14.3%
Multiple votes are allowed.
  1. Dec 29, 2011 #1

    marcus

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    Which paper or papers do you think will contribute most to future research in Loop-and-allied quantum gravity?
    Since the poll is multiple choice, it's possible to vote for several papers. Abstract summaries follow in the next post.

    http://arxiv.org/abs/1112.6391
    Curvature invariants, geodesics and the strength of singularities in Bianchi-I loop quantum cosmology
    Parampreet Singh

    http://arxiv.org/abs/1112.5104
    Quantum Gravity and Renormalization: The Tensor Track
    Vincent Rivasseau

    http://arxiv.org/abs/1112.2511
    q-Deformation of Lorentzian spin foam models
    Winston J. Fairbairn, Catherine Meusburger

    http://arxiv.org/abs/1112.2390
    The geometric role of symmetry breaking in gravity
    Derek K. Wise

    http://arxiv.org/abs/1112.1961
    Spin Foams and Canonical Quantization
    Sergei Alexandrov, Marc Geiller, Karim Noui

    http://arxiv.org/abs/1112.0360
    Positive cosmological constant in loop quantum cosmology
    Tomasz Pawlowski, Abhay Ashtekar

    http://arxiv.org/abs/1111.7195
    Spontaneously broken Lorentz symmetry for Hamiltonian gravity
    Steffen Gielen, Derek K. Wise

    http://arxiv.org/abs/1111.4997
    A Renormalizable 4-Dimensional Tensor Field Theory
    Joseph Ben Geloun, Vincent Rivasseau

    http://arxiv.org/abs/1111.3695
    Is de Sitter space a fermion?
    Andrew Randono

    http://arxiv.org/abs/1111.3535
    Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology
    Thomas Cailleteau, Jakub Mielczarek, Aurelien Barrau, Julien Grain

    http://arxiv.org/abs/1111.2865
    A proposed proper EPRL vertex amplitude
    Jonathan Engle

    http://arxiv.org/abs/1111.1743
    Higher Derivative Gravity from the Universal Renormalization Group Machine
    F. Saueressig, K. Groh, S. Rechenberger, O. Zanusso

    http://arxiv.org/abs/1110.6866
    Path integral measure and triangulation independence in discrete gravity
    Bianca Dittrich, Sebastian Steinhaus

    http://arxiv.org/abs/1110.6150
    Regularized Hamiltonians and Spinfoams
    Emanuele Alesci

    http://arxiv.org/abs/1110.4833
    Continuous formulation of the Loop Quantum Gravity phase space
    Laurent Freidel, Marc Geiller, Jonathan Ziprick

    http://arxiv.org/abs/1110.3837
    Coupling Shape Dynamics to Matter Gives Spacetime
    Henrique Gomes, Tim Koslowski

    http://arxiv.org/abs/1110.3272
    A new Hamiltonian for the Topological BF phase with spinor networks
    Valentin Bonzom, Etera R. Livine
     
  2. jcsd
  3. Dec 29, 2011 #2

    marcus

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    http://arxiv.org/abs/1112.6391
    Curvature invariants, geodesics and the strength of singularities in Bianchi-I loop quantum cosmology
    Parampreet Singh
    (Submitted on 29 Dec 2011)
    We investigate the effects of the underlying quantum geometry in loop quantum cosmology on spacetime curvature invariants and the extendibility of geodesics in the Bianchi-I model for matter with a vanishing anisotropic stress. Using the effective Hamiltonian approach, we find that even though quantum geometric effects bound the energy density and expansion and shear scalars, divergences of curvature invariants are potentially possible under special conditions. However, as in the isotropic models in LQC, these do not necessarily imply a physical singularity. Analysis of geodesics and strength of such singular events, point towards a general resolution of all known types of strong singularities. We illustrate these results for the case of a perfect fluid with an arbitrary finite equation of state w > -1, and show that curvature invariants turn out to be bounded, leading to the absence of strong singularities. Unlike classical theory, geodesic evolution does not break down. We also discuss possible generalizations of sudden singularities which may arise at a non-vanishing volume, causing a divergence in curvature invariants. Such finite volume singularities are shown to be weak and harmless.
    24 pages

    http://arxiv.org/abs/1112.5104
    Quantum Gravity and Renormalization: The Tensor Track
    Vincent Rivasseau
    (Submitted on 21 Dec 2011)
    We propose a new program to quantize and renormalize gravity based on recent progress on the analysis of large random tensors. We compare it briefly with other existing approaches.
    18 pages, 1 figure

    http://arxiv.org/abs/1112.2511
    q-Deformation of Lorentzian spin foam models
    Winston J. Fairbairn, Catherine Meusburger
    (Submitted on 12 Dec 2011)
    We construct and analyse a quantum deformation of the Lorentzian EPRL model. The model is based on the representation theory of the quantum Lorentz group with real deformation parameter. We give a definition of the quantum EPRL intertwiner, study its convergence and braiding properties and construct an amplitude for the four-simplexes. We find that the resulting model is finite.
    12 pages, 2 figures, Proceedings of the 3rd Quantum Gravity and Quantum Geometry School (Zakopane, 2011), to appear in PoS

    http://arxiv.org/abs/1112.2390
    The geometric role of symmetry breaking in gravity
    Derek K. Wise
    (Submitted on 11 Dec 2011)
    In gravity, breaking symmetry from a group G to a group H plays the role of describing geometry in relation to the geometry the homogeneous space G/H. The deep reason for this is Cartan's "method of equivalence," giving, in particular, an exact correspondence between metrics and Cartan connections. I argue that broken symmetry is thus implicit in any gravity theory, for purely geometric reasons. As an application, I explain how this kind of thinking gives a new approach to Hamiltonian gravity in which an observer field spontaneously breaks Lorentz symmetry and gives a Cartan connection on space.
    4 pages. Contribution written for proceedings of the conference "Loops 11" (Madrid, May 2011)

    http://arxiv.org/abs/1112.1961
    Spin Foams and Canonical Quantization
    Sergei Alexandrov, Marc Geiller, Karim Noui
    (Submitted on 8 Dec 2011)
    This review is devoted to the analysis of the mutual consistency of the spin foam and canonical loop quantizations in three and four spacetime dimensions. In the three-dimensional context, where the two approaches are in good agreement, we show how the canonical quantization à la Witten of Riemannian gravity with a positive cosmological constant is related to the Turaev-Viro spin foam model, and how the Ponzano-Regge amplitudes are related to the physical scalar product of Riemannian loop quantum gravity without cosmological constant. In the four-dimensional case, we recall a Lorentz-covariant formulation of loop quantum gravity using projected spin networks, compare it with the new spin foam models, and identify interesting relations and their pitfalls. Finally, we discuss the properties which a spin foam model is expected to possess in order to be consistent with the canonical quantization, and suggest a new model illustrating these results.
    88 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

    http://arxiv.org/abs/1112.0360
    Positive cosmological constant in loop quantum cosmology
    Tomasz Pawlowski, Abhay Ashtekar
    (Submitted on 1 Dec 2011)
    The k=0 Friedmann Lemaitre Robertson Walker model with a positive cosmological constant and a massless scalar field is analyzed in detail. If one uses the scalar field as relational time, new features arise already in the Hamiltonian framework of classical general relativity: In a finite interval of relational time, the universe expands out to infinite proper time and zero matter density. In the deparameterized quantum theory, the true Hamiltonian now fails to be essentially self-adjoint both in the Wheeler DeWitt (WDW) approach and in LQC. Irrespective of the choice of the self-adjoint extension, the big bang singularity persists in the WDW theory while it is resolved and replaced by a big bounce in loop quantum cosmology (LQC). Furthermore, the quantum evolution is surprisingly insensitive to the choice of the self-adjoint extension. This may be a special case of an yet to be discovered general property of a certain class of symmetric operators that fail to be essentially self-adjoint.
    36 pages, 6 figures

    http://arxiv.org/abs/1111.7195
    Spontaneously broken Lorentz symmetry for Hamiltonian gravity
    Steffen Gielen, Derek K. Wise
    (Submitted on 30 Nov 2011)
    In Ashtekar's Hamiltonian formulation of general relativity, and in loop quantum gravity, Lorentz covariance is a subtle issue that has been strongly debated. Maintaining manifest Lorentz covariance seems to require introducing either complex-valued fields or second class constraints, and either option presents a significant obstacle to quantization. After reviewing the sources of difficulty, we present a Lorentz covariant, real formulation free of second class constraints. Rather than a foliation of spacetime, we use a gauge field y, interpreted as a field of observers, to break the SO(3,1) symmetry down to a subgroup SO(3)y. This symmetry breaking plays a role analogous to that in MacDowell-Mansouri gravity, which is based on Cartan geometry, leading us to a picture of gravity as 'Cartan geometrodynamics.' We study both Lorentz gauge transformations and transformations of the observer field to show that the apparent breaking of SO(3,1) to SO(3) is not in conflict with Lorentz covariance.
    10 pages

    http://arxiv.org/abs/1111.4997
    A Renormalizable 4-Dimensional Tensor Field Theory
    Joseph Ben Geloun, Vincent Rivasseau
    (Submitted on 21 Nov 2011)
    We prove that an integrated version of the Gurau colored tensor model supplemented with the usual Bosonic propagator on U(1)4 is renormalizable to all orders in perturbation theory. The model is of the type expected for quantization of space-time in 4D Euclidean gravity and is the first example of a renormalizable model of this kind. Its vertex and propagator are four-stranded like in 4D group field theories, but without gauge averaging on the strands. Surprisingly perhaps, the model is of the φ6 rather than of the φ4 type, since two different φ6-type interactions are log-divergent, i.e. marginal in the renormalization group sense. The renormalization proof relies on a multiscale analysis. It identifies all divergent graphs through a power counting theorem. These divergent graphs have internal and external structure of a particular kind called melonic. Melonic graphs dominate the 1/N expansion of colored tensor models and generalize the planar ribbon graphs of matrix models. A new locality principle is established for this category of graphs which allows to renormalize their divergences through counterterms of the form of the bare Lagrangian interactions. The model also has an unexpected anomalous log-divergent (∫φ2)2 term, which can be interpreted as the generation of a scalar matter field out of pure gravity.
    41 pages, 9 figures

    http://arxiv.org/abs/1111.3695
    Is de Sitter space a fermion?
    Andrew Randono
    (Submitted on 16 Nov 2011)
    Following up on a recent model yielding fermionic geometries, I turn to more familiar territory to address the question of statistics in purely geometric theories. Working in the gauge formulation of gravity, where geometry is characterized by a symmetry broken Cartan connection, I give strong evidence to suggest that de Sitter space itself, and a class of de Sitter-like geometries, can be consistently quantized fermionically. Surprisingly, the underlying mathematics is the same as that of the Skyrme model for strongly interacting baryons. This promotes the question "Is geometry bosonic or fermionic?" [ http://arxiv.org/abs/1105.4184 ] beyond the realm of the rhetorical and places it on uncomfortably familiar ground.
    15 pages, 4 figures

    http://arxiv.org/abs/1111.3535
    Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology
    Thomas Cailleteau, Jakub Mielczarek, Aurelien Barrau, Julien Grain
    (Submitted on 15 Nov 2011)
    Holonomy corrections to scalar perturbations are investigated in the loop quantum cosmology framework. Due to the effective approach, modifications of the algebra of constraints generically lead to anomalies. In order to remove those anomalies, counter-terms are introduced. We find a way to explicitly fulfill the conditions for anomaly freedom and we give explicit expressions for the counter-terms. Surprisingly, the "new quantization scheme" naturally arises in this procedure. The gauge invariant variables are found and equations of motion for the anomaly-free scalar perturbations are derived. Finally, some cosmological consequences are discussed qualitatively.
    19 pages, 1 figure

    http://arxiv.org/abs/1111.2865
    A proposed proper EPRL vertex amplitude
    Jonathan Engle
    (Submitted on 11 Nov 2011)
    As established in a prior work of the author, the linear simplicity constraints used in the construction of the so-called 'new' spin-foam models mix three of the five sectors of Plebanski theory, only one of which is gravity in the usual sense, and this is the reason for certain 'unwanted' terms in the asymptotics of the EPRL vertex amplitude as calculated by Barrett et al.
    In the present paper, an explicit classical discrete condition is derived that isolates the desired gravitational sector, which we call (II+), following other authors. This condition is quantized and used to modify the vertex amplitude, yielding what we call the 'proper EPRL vertex amplitude'. This vertex still depends only on standard SU(2) spin-network data on the boundary, is SU(2) gauge invariant, and is linear in the boundary state, as required. In addition, the asymptotics now consist in the single desired term of the form eiSRegge, and all degenerate configurations are exponentially suppressed.
    25 pages

    http://arxiv.org/abs/1111.1743
    Higher Derivative Gravity from the Universal Renormalization Group Machine
    F. Saueressig, K. Groh, S. Rechenberger, O. Zanusso
    (Submitted on 7 Nov 2011)
    We study the renormalization group flow of higher derivative gravity, utilizing the functional renormalization group equation for the average action. Employing a recently proposed algorithm, termed the universal renormalization group machine, for solving the flow equation, all the universal features of the one-loop beta-functions are recovered. While the universal part of the beta-functions admits two fixed points, we explicitly show that the existence of one of them depends on the choice of regularization scheme, indicating that it is most probably unphysical.
    7 pages

    http://arxiv.org/abs/1110.6866
    Path integral measure and triangulation independence in discrete gravity
    Bianca Dittrich, Sebastian Steinhaus
    (Submitted on 31 Oct 2011)
    A path integral measure for gravity should also preserve the fundamental symmetry of general relativity, which is diffeomorphism symmetry. In previous work, we argued that a successful implementation of this symmetry into discrete quantum gravity models would imply discretization independence. We therefore consider the requirement of triangulation independence for the measure in (linearized) Regge calculus, which is a discrete model for quantum gravity, appearing in the semi-classical limit of spin foam models. To this end we develop a technique to evaluate the linearized Regge action associated to Pachner moves in 3D and 4D and show that it has a simple, factorized structure. We succeed in finding a local measure for 3D (linearized) Regge calculus that leads to triangulation independence. This measure factor coincides with the asymptotics of the Ponzano Regge Model, a 3D spin foam model for gravity. We furthermore discuss to which extent one can find a triangulation independent measure for 4D Regge calculus and how such a measure would be related to a quantum model for 4D flat space. To this end, we also determine the dependence of classical Regge calculus on the choice of triangulation in 3D and 4D.
    36 pages, 7 figures

    http://arxiv.org/abs/1110.6150
    Regularized Hamiltonians and Spinfoams
    Emanuele Alesci
    (Submitted on 27 Oct 2011)
    We review a recent proposal for the regularization of the scalar constraint of General Relativity in the context of LQG. The resulting constraint presents strengths and weaknesses compared to Thiemann's prescription. The main improvement is that it can generate the 1-4 Pachner moves and its matrix elements contain 15j Wigner symbols, it is therefore compatible with the spinfoam formalism: the drawback is that Thiemann anomaly free proof is spoiled because the nodes that the constraint creates have volume.
    4 pages, based on a talk given at Loops '11 in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

    http://arxiv.org/abs/1110.4833
    Continuous formulation of the Loop Quantum Gravity phase space
    Laurent Freidel, Marc Geiller, Jonathan Ziprick
    (Submitted on 21 Oct 2011)
    In this paper, we study the discrete classical phase space of loop gravity, which is expressed in terms of the holonomy-flux variables, and show how it is related to the continuous phase space of general relativity. In particular, we prove an isomorphism between the loop gravity discrete phase space and the symplectic reduction of the continuous phase space with respect to a flatness constraint. This gives for the first time a precise relationship between the continuum and holonomy-flux variables. Our construction shows that the fluxes depend on the three-geometry, but also explicitly on the connection, explaining their non commutativity. It also clearly shows that the flux variables do not label a unique geometry, but rather a class of gauge-equivalent geometries. This allows us to resolve the tension between the loop gravity geometrical interpretation in terms of singular geometry, and the spin foam interpretation in terms of piecewise flat geometry, since we establish that both geometries belong to the same equivalence class. This finally gives us a clear understanding of the relationship between the piecewise flat spin foam geometries and Regge geometries, which are only piecewise-linear flat: While Regge geometry corresponds to metrics whose curvature is concentrated around straight edges, the loop gravity geometry correspond to metrics whose curvature is concentrated around not necessarily straight edges.
    27 pages

    http://arxiv.org/abs/1110.3837
    Coupling Shape Dynamics to Matter Gives Spacetime
    Henrique Gomes, Tim Koslowski
    (Submitted on 17 Oct 2011)
    Shape Dynamics is a metric theory of pure gravity, equivalent to General Relativity, but formulated as a gauge theory of spatial diffeomporphisms and local spatial conformal transformations. In this paper we extend the construction of Shape Dynamics from pure gravity to gravity-matter systems and find that there is no obstruction for the coupling of gravity to standard matter. We use the matter gravity system to construct a clock and rod model for Shape Dynamics which allows us to recover a spacetime interpretation of Shape Dynamics trajectories.
    10 pages

    http://arxiv.org/abs/1110.3272
    A new Hamiltonian for the Topological BF phase with spinor networks
    Valentin Bonzom, Etera R. Livine
    (Submitted on 14 Oct 2011)
    We describe fundamental equations which define the topological ground states in the lattice realization of the SU(2) BF phase. We introduce a new scalar Hamiltonian, based on recent works in quantum gravity and topological models, which is different from the plaquette operator. Its gauge-theoretical content at the classical level is formulated in terms of spinors. The quantization is performed with Schwinger's bosonic operators on the links of the lattice. In the spin network basis, the quantum Hamiltonian yields a difference equation based on the spin 1/2. In the simplest case, it is identified as a recursion on Wigner 6j-symbols. We also study it in different coherent states representations, and compare with other equations which capture some aspects of this topological phase.
    40 pages
     
  4. Dec 30, 2011 #3

    marcus

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    Thanks to Tom Stoer, Atyy and John86 for getting the poll off to a good start! I'm interested by what they and others think are the most significant papers in this area of research. If anyone wants to propose or discuss a paper that I did not list feel free to post the abstract here and say what you think is especially important about it.

    I see that Tom and I agree about the prospects of the new LQG Hamiltonian proposed by Carlo Rovelli and Alesci which is a major departure from everything that has gone before. It is sensitive to the creation of new volume, that is new 4-valent vertices corresponding to new 3-simplexes in the dual triangulation.

    When one thinks about it, it is strange that LQG has had a proposed canonical dynamics for well over a dozen years but previous versions of the Hamiltonian have not explicitly involved the creation/annihilation of volume. So the Alesci-Rovelli idea has the potential to shake things up in the Hamiltonian department, with repercussions for the Loop approach as a whole.

    I definitely agree with John86 on the importance of Freidel Geiller Ziprick: I think it's a landmark paper!

    Atyy picked three papers none of which I had chosen, including one with a curious title that was completely puzzling to me. While I could not confidently say where the paper is going and therefore wouldn't have voted for it, I included it on the list because based on his past work I respect the author a lot--a young guy with definitely creative original ideas.

    Part of the value of this quarterly questionnaire, for me, is exactly when this happens---someone else has a completely different take on what the most important papers and research directions in this field are.

    Another of John's picks reminds us to keep watching the Asymptotic Safe QG program, which says that quantum relativity is no big deal since GR is renormalizable after all. It is the approach which is in some sense closest to the established QFT and GR physics that has served well all these year. Creatively un-innovative.

    It would be great if anyone who voted wants to post comment indicating their reasons for picking the ones they did. I could do that as well (but my picks this time were largely guided by what, e.g. in the third quarter poll, has been garnering citations.)
     
    Last edited: Dec 30, 2011
  5. Dec 31, 2011 #4

    marcus

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    The reason I bet heavily on cosmology this time around was what happened with the citations to the MIP poll papers last time, which confirmed my suspicion that cosmology is turning out to be of critical importance in guiding QG research.

    To be good, win or lose, a theory has to be testable and that means it has to look the Ancient Light in the face without blinking. It has to be able to say something about the map of the CMB sky.

    Then, even if the sky should declare it wrong, at least it was a theory--not just math fantasy. So I think that underlying consideration is and will be steering QG towards early universe cosmology---all types, across the board, not only Loop. AsymSafe QG is another approach that is interesting in this regard.

    What happened with the citations was if you look at the papers on the 3rd quarter MIP poll list you see

    Ashtekar Singh 16 cites
    Dittrich Eckert Martin 10 cites
    Bojowald Calcagni Tsujikawa 10 cites
    Dittrich Hoehn 7 cites
    ...

    and some of the papers on the list only came out in September so those are pretty good numbers. And obviously two out of the four topcited papers on the list are cosmo.
    https://www.physicsforums.com/showthread.php?t=535170
     
    Last edited: Dec 31, 2011
  6. Jan 1, 2012 #5

    atyy

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    Unlike marcus, I picked GFT "just maths" papers which show how current formalism may be generalized. I'm thinking of the progress that Lagrange and Hamilton did not know they made when they reformulated Newton's laws in ways that could be generalized to quantum mechanics.

    As for Andy Randono's paper, its title alone was just too good not to vote for it. :biggrin: However, it is also in the unification spirit I aesthetically prefer in which spacetime and matter have a common source.
     
    Last edited: Jan 1, 2012
  7. Jan 1, 2012 #6

    RUTA

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    I don't see anything really new, just variations on very old themes. I wouldn't equate number of citations with promise. It's probably just the opposite, these ideas have been considered by the brightest minds in physics for decades and what do we have to show for it? No new physics, that's for sure. Are there any *new* ideas out there, marcus?
     
  8. Jan 1, 2012 #7
    Which is exactly why I voted for Shape Dynamics and Shape Dynamics only.
     
  9. Jan 1, 2012 #8

    atyy

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    AdS/CFT shows that even old, old ideas like simple QFT contain new physics.

    New wine in old wineskins. We all know that's gonna burst them.
     
  10. Jan 2, 2012 #9

    tom.stoer

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    It's rather difficult - as usual.

    q-Deformation of spin foams is always interesting b/c of the cc, finiteness, dS space etc.

    Alexandrov's 'Spin Foams and Canonical Quantization' points - as usual - to the weak points of spin foam quantization (second class constraints, Dirac brackets, PI measure, anomalies, vertex apmplitudes, relation to canonical quantization, ... ) but again seems to provide no convincing way ot of this mess

    Hamiltonians and spin foams are always intersting; their relation (the missing relation?) is afaik the weak point in LQG/SF models

    Freidel's paper is one of the recent attempts to understand the issues of LQG already at the classical level i.e. to separate quantization and discretization

    Randono's paper indicates that spin-2 gravitons may not be the fundamental entities of spacetime; this is to be expected based on several other research directions, i.e. the fundamental role played by SU(2) and its fundamental spin 1/2 rep.

    It's hardly possible to say which paper is the most 'promising' one. My impression is that (just like string theory - which is not covered) all other approaches have their own difficulties and that especially in LQG/SF models there is a shift of focus from progress to obstacles ...
     
  11. Jan 3, 2012 #10

    marcus

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    Atyy, RUTA, Caramon, Tom.Stoer thanks for your comments! I tend to agree with Tom about the shift towards working out kinks and bugs in existing theory instead of seeking a completely new vision. But I think of addressing obstacles as a kind of progress.

    Tom also pointed out earlier how much interest there is in redefining the Hamiltonian and reconciling the canonical and spinfoam dynamics. Alesci's paper is about a new LQG Hamiltonian, which will take quite a lot of work to learn if it is or is not satisfactory. The Freidel Geiller Ziprick paper is also aimed at resolving the tension between Hamiltonian and spinfoam dynamics. Or so I think.

    In response to RUTA's question I think several papers are innovative (Alesci's Hamiltonian certainly, shape dynamics, Derek Wise exploration of Cartan geometry which departs from ordinary differential geometry, FGZ paper's formulation of pre-LQG classical phase space.) There seem many new ideas in this batch, from my perspective at least.

    I wonder if it would help us get a better perspective on what is currently going on in this field if we were to look down the list of most-cited 2011 papers. These are mostly Jan-June because researchers haven't yet had much time to make use of the 3rd and 4th quarter work.
    I'll get the list. Maybe something will stand out, if we look it over. We might find that the July-Dec papers even though so-far less-cited are on the whole more innovative. Confronting obstacles (as Tom calls them) can sometimes make one innovative. But I don't know what we will see in a comparison.
     
    Last edited: Jan 3, 2012
  12. Jan 3, 2012 #11

    marcus

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    Here are the (citewise) top 25 papers of 2011 which InSpire classifies as LQC, LQG, spinfoam.
    They are mostly from the first half of the year. How do these compare with the batch we just considered in the poll? Do you identify any special emphasis that stands out, in either one or both of the lists? This list does have major pedagogical and review papers namely #2 and #4. However as a first impression, even though they are the more-cited papers this list as a whole doesn't seem as innovative as the list of 4th quarter papers in our MIP poll. Could be just an "optical illusion." I'd be interested to know if someone else has a different take.

    1.
    Diffeomorphisms in group field theories.
    Aristide Baratin (Ecole Polytechnique, CPHT & Saclay, SPhT & Orsay, LPT), Florian Girelli (Sydney U.), Daniele Oriti (Potsdam, Max Planck Inst.). Jan 2011. 31 pp.
    http://arxiv.org/abs/1101.0590 [hep-th]
    Cited by 40 records

    2.
    Zakopane lectures on loop gravity.
    Carlo Rovelli (Marseille, CPT). Feb 2011. 25 pp.
    http://arxiv.org/abs/1102.3660 [gr-qc]
    Cited by 29 records

    3.
    Bubble divergences: sorting out topology from cell structure.
    Valentin Bonzom, Matteo Smerlak. Mar 2011. 19 pp.
    http://arxiv.org/abs/1103.3961 [gr-qc]
    Cited by 18 records

    4.
    Loop Quantum Cosmology: A Status Report.
    Abhay Ashtekar (Penn State U.), Parampreet Singh (Louisiana State U.). Aug 2011. 136 pp.
    http://arxiv.org/abs/1108.0893 [gr-qc]
    Cited by 17 records

    5.
    Observational constraints on loop quantum cosmology.
    Martin Bojowald (Penn State U.), Gianluca Calcagni (Potsdam, Max Planck Inst.), Shinji Tsujikawa (Tokyo U. of Sci.). Jan 2011. 4 pp.
    http://arxiv.org/abs/1101.5391 [astro-ph.CO]
    Cited by 17 records

    6.
    Perfect discretization of reparametrization invariant path integrals.
    Benjamin Bahr (Cambridge U. & Potsdam, Max Planck Inst.), Bianca Dittrich, Sebastian Steinhaus (Potsdam, Max Planck Inst.). Jan 2011. 8 pp.
    http://arxiv.org/abs/1101.4775 [gr-qc]
    Cited by 16 records

    7.
    Spin foam models with finite groups.
    Benjamin Bahr, Bianca Dittrich, James P. Ryan. Mar 2011. 47 pp.
    http://arxiv.org/abs/1103.6264 [gr-qc]
    Cited by 15 records

    8.
    Spinor Representation for Loop Quantum Gravity.
    Etera R. Livine, Johannes Tambornino. May 2011. 1 pp.
    http://arxiv.org/abs/1105.3385 [gr-qc]
    Cited by 13 records

    9.
    Regge gravity from spinfoams.
    Elena Magliaro, Claudio Perini. May 2011. 8 pp.
    http://arxiv.org/abs/1105.0216 [gr-qc]
    Cited by 13 records

    10.
    Status of Horava gravity: A personal perspective.
    Matt Visser (Victoria U., Wellington). Mar 2011. 11 pp.
    http://arxiv.org/abs/1103.5587 [hep-th]
    Cited by 13 records

    11.
    Spin foam models and the Wheeler-DeWitt equation for the quantum 4-simplex.
    Valentin Bonzom (Perimeter Inst. Theor. Phys.). Jan 2011. 16 pp.
    http://arxiv.org/abs/1101.1615 [gr-qc]
    Cited by 13 records

    12.
    The Matter Bounce Curvaton Scenario.
    Yi-Fu Cai (Arizona State U. & Beijing, Inst. High Energy Phys.), Robert Brandenberger (Beijing, Inst. High Energy Phys. & McGill U.), Xinmin Zhang (Beijing, Inst. High Energy Phys. & TPCSF, Beijing). Jan 2011. 15 pp.
    http://arxiv.org/abs/1101.0822 [hep-th]
    Cited by 13 records

    13.
    The Hamiltonian constraint in 3d Riemannian loop quantum gravity.
    Valentin Bonzom, Laurent Freidel (Perimeter Inst. Theor. Phys.). Jan 2011. 24 pp.
    http://arxiv.org/abs/1101.3524 [gr-qc]
    Cited by 12 records

    14.
    Cosmological constant in spinfoam cosmology.
    Eugenio Bianchi (Marseille, CPT), Thomas Krajewski (Marseille, CPT & Orsay, LPT), Carlo Rovelli (Marseille, CPT), Francesca Vidotto (Marseille, CPT & Pavia U. & INFN, Pavia). Jan 2011. 4 pp.
    http://arxiv.org/abs/1101.4049 [gr-qc]
    Cited by 11 records

    15.
    Coarse graining methods for spin net and spin foam models.
    Bianca Dittrich, Frank C. Eckert, Mercedes Martin-Benito (Potsdam, Max Planck Inst.). Sep 2011. 39 pp.
    http://arxiv.org/abs/1109.4927 [gr-qc]
    Cited by 10 records

    16.
    Observational test of inflation in loop quantum cosmology.
    Martin Bojowald (Penn State U.), Gianluca Calcagni (Potsdam, Max Planck Inst.), Shinji Tsujikawa (Tokyo U. of Sci.). Jul 2011. 37 pp.
    http://arxiv.org/abs/1107.1540 [gr-qc]
    Cited by 10 records

    17.
    Holomorphic Simplicity Constraints for 4d Spinfoam Models.
    Maite Dupuis (Lyon, IPN & Sydney U.), Etera R. Livine (Lyon, IPN). Apr 2011. 27 pp.
    http://arxiv.org/abs/1104.3683 [gr-qc]
    Cited by 10 records

    18.
    Chern-Simons theory, Stokes' Theorem, and the Duflo map.
    Hanno Sahlmann (APCTP, Pohang), Thomas Thiemann (Erlangen - Nuremberg U., Theorie III). Jan 2011. 26 pp.
    http://arxiv.org/abs/1101.1690 [gr-qc]
    Cited by 10 records

    19.
    Quantum simplicial geometry in the group field theory formalism: reconsidering the Barrett-Crane model.
    Aristide Baratin, Daniele Oriti. Aug 2011. 24 pp.
    http://arxiv.org/abs/1108.1178 [gr-qc]
    Cited by 9 records

    20.
    Tensor models and hierarchy of n-ary algebras.
    Naoki Sasakura (Kyoto U., Yukawa Inst., Kyoto). Apr 2011. 13 pp.
    http://arxiv.org/abs/1104.5312 [hep-th]
    Cited by 9 records

    21.
    Cosmological Constant in LQG Vertex Amplitude.
    Muxin Han (Marseille, CPT). May 2011. 4 pp.
    http://arxiv.org/abs/1105.2212 [gr-qc]
    Cited by 8 records

    22.
    Euclidean three-point function in loop and perturbative gravity.
    Carlo Rovelli, Mingyi Zhang (Marseille, CPT). May 2011. 16 pp.
    http://arxiv.org/abs/1105.0566 [gr-qc]
    Cited by 8 records

    23.
    Canonical quantization of non-commutative holonomies in 2+1 loop quantum gravity.
    K. Noui (Tours U., CNRS), A. Perez, D. Pranzetti (Marseille, CPT). May 2011.
    http://arxiv.org/abs/1105.0439 [gr-qc]
    Cited by 8 records

    24.
    Tensor models and 3-ary algebras.
    Naoki Sasakura (Kyoto U., Yukawa Inst., Kyoto). Apr 2011. 18 pp.
    http://arxiv.org/abs/1104.1463 [hep-th]
    Cited by 8 records

    25.
    Curvature in spinfoams.
    Elena Magliaro, Claudio Perini (Penn State U.). Mar 2011. 6 pp.
    http://arxiv.org/abs/1103.4602 [gr-qc]
    Cited by 8 records

    http://inspirehep.net/search?ln=en&...2y=2011&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
     
    Last edited: Jan 3, 2012
  13. Jan 3, 2012 #12

    atyy

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    I think a big theme is renormalization: 1,3,6,7 & 15. How to do it? Are the different approaches to renormalization related (eg. 3)?
     
  14. Jan 4, 2012 #13

    tom.stoer

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    Another one is 'vertex amplitudes, discretization, simplicial constraints, PI measure, relation of canonical LQG and SF models' = re-examining of the construction.

    There is definitly no well-established "LQG theory" like QCD; there are several theories still under construction
     
    Last edited: Jan 4, 2012
  15. Jan 8, 2012 #14

    marcus

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    I should have numbered the 17 items in the poll to make them easier to reference. For instance Alesci's paper on the proposed new Lqg hamiltonian (Regularized Hamiltonians and Spinfoams) number 14 in the poll, is likely to be especially relevant if it turns out that Louis Crane's plan for putting matter into EPRL geometry takes shape. So I'd like a convenient way to point to it.

    That's my hunch anyway. Because of the importance of the A4 group (symmetries of the tetrahedron) in Crane's proposal, and the fact that the Alesci-Rovelli hamiltonian is based on a "Wilson tetrahedron" one could say, instead of a loop. The Alesci-Rovelli hamiltonian allows for the creation/annihliation of tetrahedral elements--i.e. one-four Pachner moves. I could be quite wrong--don't feel at all confident about this--but suspect a connection at some level with Crane's idea for introducing matter.
    ==quote==
    http://arxiv.org/abs/1201.0525
    String Field Theory from Quantum Gravity
    Louis Crane
    (Submitted on 2 Jan 2012)
    Recent work on neutrino oscillations suggests that the three generations of fermions in the standard model are related by representations of the finite group A(4), the group of symmetries of the tetrahedron. Motivated by this, we explore models which extend the EPRL model for quantum gravity by coupling it to a bosonic quantum field of representations of A(4). This coupling is possible because the representation category of A(4) is a module category over the representation categories used to construct the EPRL model...
    ==endquote==
     
    Last edited: Jan 8, 2012
  16. Jan 11, 2012 #15

    marcus

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    Although still too early to expect much in the way of citations, I'll set up links to make checking easier later on. Taking a preliminary look, I see that, early or not, #17 and #16 each have two cites and #15, #10 and #8 each have three. A couple of others (#2 and #6) have also been cited.

    1. http://arxiv.org/cits/1112.6391
    Curvature invariants, geodesics and the strength of singularities in Bianchi-I loop quantum cosmology
    Parampreet Singh

    2. http://arxiv.org/cits/1112.5104
    Quantum Gravity and Renormalization: The Tensor Track
    Vincent Rivasseau

    3. http://arxiv.org/cits/1112.2511
    q-Deformation of Lorentzian spin foam models
    Winston J. Fairbairn, Catherine Meusburger

    4. http://arxiv.org/cits/1112.2390
    The geometric role of symmetry breaking in gravity
    Derek K. Wise

    5. http://arxiv.org/cits/1112.1961
    Spin Foams and Canonical Quantization
    Sergei Alexandrov, Marc Geiller, Karim Noui

    6. http://arxiv.org/cits/1112.0360
    Positive cosmological constant in loop quantum cosmology
    Tomasz Pawlowski, Abhay Ashtekar

    7. http://arxiv.org/cits/1111.7195
    Spontaneously broken Lorentz symmetry for Hamiltonian gravity
    Steffen Gielen, Derek K. Wise

    8. http://arxiv.org/cits/1111.4997
    A Renormalizable 4-Dimensional Tensor Field Theory
    Joseph Ben Geloun, Vincent Rivasseau

    9. http://arxiv.org/cits/1111.3695
    Is de Sitter space a fermion?
    Andrew Randono

    10. http://arxiv.org/cits/1111.3535
    Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology
    Thomas Cailleteau, Jakub Mielczarek, Aurelien Barrau, Julien Grain

    11. http://arxiv.org/cits/1111.2865
    A proposed proper EPRL vertex amplitude
    Jonathan Engle

    12. http://arxiv.org/cits/1111.1743
    Higher Derivative Gravity from the Universal Renormalization Group Machine
    F. Saueressig, K. Groh, S. Rechenberger, O. Zanusso

    13. http://arxiv.org/cits/1110.6866
    Path integral measure and triangulation independence in discrete gravity
    Bianca Dittrich, Sebastian Steinhaus

    14. http://arxiv.org/cits/1110.6150
    Regularized Hamiltonians and Spinfoams
    Emanuele Alesci

    15. http://arxiv.org/cits/1110.4833
    Continuous formulation of the Loop Quantum Gravity phase space
    Laurent Freidel, Marc Geiller, Jonathan Ziprick

    16. http://arxiv.org/cits/1110.3837
    Coupling Shape Dynamics to Matter Gives Spacetime
    Henrique Gomes, Tim Koslowski

    17. http://arxiv.org/cits/1110.3272
    A new Hamiltonian for the Topological BF phase with spinor networks
    Valentin Bonzom, Etera R. Livine

    Eight people have joined the poll so far. Have to go. back later
    Thanks to Tom Stoer, Atyy and John86
     
    Last edited: Jan 11, 2012
  17. Jan 12, 2012 #16

    marcus

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    Actually it's eleven. Thanks to all! Demystifier, Oriako, Diffeomorphism, Dav, IRobot, Caramon, and MTd2. Having more people's perceptions reflected in the poll makes it more interesting.
     
    Last edited: Jan 12, 2012
  18. Jan 21, 2012 #17
    I noticed that this paper did not make the list:

    Probing Planck-scale physics with quantum optics
    Authors: Igor Pikovski, Michael R. Vanner, Markus Aspelmeyer, Myungshik Kim, Caslav Brukner
    (Submitted on 8 Nov 2011)

    Abstract: One of the main challenges in physics today is to merge quantum theory and the theory of general relativity into a unified framework. Various approaches towards developing such a theory of quantum gravity are pursued, but the lack of experimental evidence of quantum gravitational effects thus far is a major hindrance. Yet, the quantization of space-time itself can have experimental implications: the existence of a minimal length scale is widely expected to result in a modification of the Heisenberg uncertainty relation. Here we introduce a scheme that allows an experimental test of this conjecture by probing directly the canonical commutation relation of the center of mass mode of a massive mechanical oscillator with a mass close to the Planck mass. Our protocol utilizes quantum optical control and readout of the mechanical system to probe possible deviations from the quantum commutation relation even at the Planck scale. We show that the scheme is within reach of current technology. It thus opens a feasible route for tabletop experiments to test possible quantum gravitational phenomena.

    http://arxiv.org/abs/1111.1979

    Could some experiments in the near future rule out or constrain some QG theories?

    Skippy
     
  19. Jan 21, 2012 #18

    marcus

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    Nice! thanks for pointing out this paper. Please keep us posted if you hear of some actual experiments along these lines.
    I see they thank Steffen Gielen in the acknowledgements (works with Oriti at AEI, has worked with Gary Gibbons and Neil Turok in past years.) Also they cite the relative locality paper of Freidel et al which I recall discussed the possibility of making some experimental tests.

    I may be missing something but I don't see them drawing an explicit connection with Loop QG as currently formulated. On the other hand it seems possible that some QG ideas could be ruled out or constrained by this approach, if not specifically Loop ones, and that could be helpful.
     
    Last edited: Jan 21, 2012
  20. Jan 21, 2012 #19

    marcus

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    As I see it, this paper is quite interesting. I did not notice it before Skippy pointed it out. Unless there is objection I will register a write-in vote for this paper. Please let me know if this is not OK with you Skippy.

    http://arxiv.org/abs/1111.1979
    Probing Planck-scale physics with quantum optics
    Igor Pikovski, Michael R. Vanner, Markus Aspelmeyer, Myungshik Kim, Caslav Brukner
    (Submitted on 8 Nov 2011) One of the main challenges in physics today is to merge quantum theory and the theory of general relativity into a unified framework. Various approaches towards developing such a theory of quantum gravity are pursued, but the lack of experimental evidence of quantum gravitational effects thus far is a major hindrance. Yet, the quantization of space-time itself can have experimental implications: the existence of a minimal length scale is widely expected to result in a modification of the Heisenberg uncertainty relation. Here we introduce a scheme that allows an experimental test of this conjecture by probing directly the canonical commutation relation of the center of mass mode of a massive mechanical oscillator with a mass close to the Planck mass. Our protocol utilizes quantum optical control and readout of the mechanical system to probe possible deviations from the quantum commutation relation even at the Planck scale. We show that the scheme is within reach of current technology. It thus opens a feasible route for tabletop experiments to test possible quantum gravitational phenomena.
    11 pages, 3 figures, 2 tables

    We have had write-in votes in some past MIP polls, where a significant paper was omitted from the original list. I see a lot of interesting stuff coming out of Vienna, especially quantum optics but also getting into QG as well.
     
    Last edited: Jan 21, 2012
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