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

## Which paper(s) will contribute most to future research?

7.7%

7.7%

23.1%

0 vote(s)
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7.7%

0 vote(s)
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15.4%

30.8%

46.2%

23.1%

7.7%

7.7%

7.7%

15.4%

7.7%

0 vote(s)
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7.7%

7.7%

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20. ### Black hole entropy and isolated horizons thermodynamics

7.7%
1. Sep 29, 2011

### marcus

Of the twenty candidates, please choose those you think will prove most significant for 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/1109.6538
Lorentzian spinfoam propagator
Eugenio Bianchi, You Ding

http://arxiv.org/abs/1109.5023
The kernel and the injectivity of the EPRL map
Wojciech Kaminski, Marcin Kisielowski, Jerzy Lewandowski

http://arxiv.org/abs/1109.4927
Coarse graining methods for spin net and spin foam models
Bianca Dittrich, Frank C. Eckert, Mercedes Martin-Benito

http://arxiv.org/abs/1109.4688
Loop quantum gravity vacuum with nondegenerate geometry
Tim Koslowski, Hanno Sahlmann

http://arxiv.org/abs/1109.4239
Probing Loop Quantum Gravity with Evaporating Black Holes
Aurelien Barrau, Xiangyu Cao, Jacobo Diaz-Polo, Julien Grain, Thomas Cailleteau

http://arxiv.org/abs/1109.3572
Loop gravity in terms of spinors
Etera R. Livine, Johannes Tambornino

http://arxiv.org/abs/1109.0499
Asymptotics of Spinfoam Amplitude on Simplicial Manifold: Lorentzian Theory
Muxin Han, Mingyi Zhang

http://arxiv.org/abs/1109.0080
Emergent Braided Matter of Quantum Geometry
Sundance Bilson-Thompson, Jonathan Hackett, Louis Kauffman, Yidun Wan

http://arxiv.org/abs/1108.5224
Shape Dynamics
Tim Koslowski

http://arxiv.org/abs/1108.2258
Emergence of gravity from spinfoams
Elena Magliaro, Claudio Perini

http://arxiv.org/abs/1108.1974
Canonical simplicial gravity
Bianca Dittrich, Philipp A Hoehn

http://arxiv.org/abs/1108.0910
The black hole information paradox and relative locality
Lee Smolin

http://arxiv.org/abs/1108.0893
Loop Quantum Cosmology: A Status Report
Abhay Ashtekar, Parampreet Singh

http://arxiv.org/abs/1108.0832
On the structure of a background independent quantum theory: Hamilton function, transition amplitudes, classical limit and continuous limit
Carlo Rovelli

http://arxiv.org/abs/1108.0369
Twistor Networks and Covariant Twisted Geometries
Etera R. Livine, Simone Speziale, Johannes Tambornino

http://arxiv.org/abs/1107.5274
Holomorphic Lorentzian Simplicity Constraints
Maité Dupuis, Laurent Freidel, Etera R. Livine, Simone Speziale

http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)

http://arxiv.org/abs/1107.2633
Many-nodes/many-links spinfoam: the homogeneous and isotropic case
Francesca Vidotto

http://arxiv.org/abs/1107.1540
Observational test of inflation in loop quantum cosmology
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa

http://arxiv.org/abs/1107.1320
Black hole entropy and isolated horizons thermodynamics
Amit Ghosh, Alejandro Perez

2. Sep 29, 2011

### marcus

http://arxiv.org/abs/1109.6538
Lorentzian spinfoam propagator
Eugenio Bianchi, You Ding
(Submitted on 29 Sep 2011)
The two-point correlation function is calculated in the Lorentzian EPRL spinfoam model, and shown to match with the one in Regge calculus in a proper limit: large boundary spins, and small Barbero-Immirzi parameter, keeping the size of the quantum geometry finite and fixed. Compared to the Euclidean case, the definition of a Lorentzian boundary state involves a new feature: the notion of past- and future-pointing intertwiners. The semiclassical correlation function is obtained for a time-oriented semiclassical boundary state.

http://arxiv.org/abs/1109.5023
The kernel and the injectivity of the EPRL map
Wojciech Kaminski, Marcin Kisielowski, Jerzy Lewandowski
(Submitted on 23 Sep 2011)
In this paper we prove injectivity of the EPRL map for |gamma|<1, filling the gap of our previous paper.
17 pages, 3 figures

http://arxiv.org/abs/1109.4927
Coarse graining methods for spin net and spin foam models
Bianca Dittrich, Frank C. Eckert, Mercedes Martin-Benito
(Submitted on 22 Sep 2011)
We undertake first steps in making a class of discrete models of quantum gravity, spin foams, accessible to a large scale analysis by numerical and computational methods. In particular, we apply Migdal-Kadanoff and Tensor Network Renormalization schemes to spin net and spin foam models based on finite Abelian groups and introduce cutoff models' to probe the fate of gauge symmetries under various such approximated renormalization group flows. For the Tensor Network Renormalization analysis, a new Gauss constraint preserving algorithm is introduced to improve numerical stability and aid physical interpretation. We also describe the fixed point structure and establish an equivalence of certain models.
39 pages, 13 figures, 1 table

http://arxiv.org/abs/1109.4688
Loop quantum gravity vacuum with nondegenerate geometry
Tim Koslowski, Hanno Sahlmann
(Submitted on 22 Sep 2011)
In loop quantum gravity, states of the gravitational field turn out to be excitations over a vacuum state that is sharply peaked on a degenerate spatial geometry. While this vacuum is singled out as fundamental due to its invariance properties, it is also important to consider states that describe non-degenerate geometries. Such states have features of Bose condensate ground states. We discuss their construction for the Lie-algebra as well as the Weyl-algebra setting, and point out possible applications in effective field theory, Loop Quantum Cosmology, as well as further generalizations.
15 pages; prepared for special issue "Loop Quantum Gravity and Cosmology" of the journal SIGMA

http://arxiv.org/abs/1109.4239
Probing Loop Quantum Gravity with Evaporating Black Holes
Aurelien Barrau, Xiangyu Cao, Jacobo Diaz-Polo, Julien Grain, Thomas Cailleteau
(Submitted on 20 Sep 2011)
This letter aims at showing that the observation of evaporating black holes should allow distinguishing between the usual Hawking behavior and Loop Quantum Gravity (LQG) expectations. We present a full Monte-Carlo simulation of the evaporation in LQG and statistical tests that discriminate between competing models. We conclude that contrarily to what was commonly thought, the discreteness of the area in LQG leads to characteristic features that qualify evaporating black holes as objects that could reveal quantum gravity footprints.
5 pages, 3 figures

http://arxiv.org/abs/1109.3572
Loop gravity in terms of spinors
Etera R. Livine, Johannes Tambornino
(Submitted on 16 Sep 2011)
We show that loop gravity can equally well be formulated in in terms of spinorial variables (instead of the group variables which are commonly used), which have recently been shown to provide a direct link between spin network states and discrete geometries. This results in a new, unitarily equivalent formulation of the theory on a generalized Bargmann space. Since integrals over the group are exchanged for straightforward integrals over the complex plane we expect this formalism to be useful to efficiently organize practical calculations.
4 pages, based on a talk given at Loops '11, Madrid, to appear in Journal of Physics: Conference Series

http://arxiv.org/abs/1109.0499
Asymptotics of Spinfoam Amplitude on Simplicial Manifold: Lorentzian Theory
Muxin Han, Mingyi Zhang
(Submitted on 2 Sep 2011)
The present paper studies the large-j asymptotics of the Lorentzian EPRL spinfoam amplitude on a 4d simplicial complex with an arbitrary number of simplices. The asymptotics of the spinfoam amplitude is determined by the critical configurations. Here we show that, given a critical configuration in general, there exists a partition of the simplicial complex into three type of regions RNondeg, RDeg-A, RDeg-B, where the three regions are simplicial sub-complexes with boundaries. The critical configuration implies different types of geometries in different types of regions, i.e. (1) the critical configuration restricted into RNondeg implies a nondegenerate discrete Lorentzian geometry, (2) the critical configuration restricted into RDeg-A is degenerate of type-A in our definition of degeneracy, but implies a nondegenerate discrete Euclidean geometry on RDeg-A, (3) the critical configuration restricted into RDeg-B is degenerate of type-B, and implies a vector geometry on RDeg-B. With the critical configuration, we further make a subdivision of the regions RNondeg and RDeg-A into sub-complexes (with boundary) according to their Lorentzian/Euclidean oriented 4-simplex volume V4(v), such that sgn(V4(v)) is a constant sign on each sub-complex. Then in the each sub-complex, the spinfoam amplitude at the critical configuration gives the Regge action in Lorentzian or Euclidean signature respectively on RNondeg or RDeg-A. The Regge action reproduced here contains a sign factor sgn(V4(v)) of the oriented 4-simplex volume. Therefore the Regge action reproduced here can be viewed a discretized Palatini action with on-shell connection. Finally the asymptotic formula of the spinfoam amplitude is given by a sum of the amplitudes evaluated at all possible critical configurations, which are the products of the amplitudes associated to different type of geometries.
54 pages, 2 figures

http://arxiv.org/abs/1109.0080 (EDIT)
Emergent Braided Matter of Quantum Geometry
Sundance Bilson-Thompson, Jonathan Hackett, Louis Kauffman, Yidun Wan
(Submitted on 1 Sep 2011)
Abstract: We review and present a few new results of the program of emergent matter as braid excitations of quantum geometry that is represented by braided ribbon networks, which are a generalisation of the spin networks proposed by Penrose and those in models of background independent quantum gravity theories, such as Loop Quantum Gravity and Spin Foam models. This program has been developed in two parallel but complimentary schemes, namely the trivalent and tetravalent schemes. The former studies the trivalent braids on trivalent braided ribbon networks, while the latter investigate the tetravalent braids on tetravalent braided ribbon networks. Both schemes have been fruitful. The trivalent scheme has been quite successful at establishing a correspondence between the trivalent braids and Standard Model particles, whereas the tetravalent scheme has naturally substantiated a rich, dynamical theory of interactions and propagation of tetravalent braids, which is ruled by topological conservation laws. Some recent advances in the program indicate that the two schemes may converge to yield a fundamental theory of matter in quantum spacetime.
37 pages

http://arxiv.org/abs/1108.5224
Shape Dynamics
Tim Koslowski
(Submitted on 26 Aug 2011)
General Relativity can be reformulated as a geometrodynamical theory, called Shape Dynamics, that is not based on spacetime (in particular refoliation) symmetry but on spatial diffeomorphism and local spatial conformal symmetry. This leads to a constraint algebra that is (unlike General Relativity) a Lie algebra, where all local constraints are linear in momenta and may thus be quantized as vector fields on the geometrodynamic configuration space. The Hamiltonian of Shape Dynamics is complicated but admits simple expressions whenever spatial derivatives are negligible.
4 pages

http://arxiv.org/abs/1108.2258
Emergence of gravity from spinfoams
Elena Magliaro, Claudio Perini
(Submitted on 10 Aug 2011)
We find a nontrivial regime of spinfoam quantum gravity that reproduces classical Einstein equations. This is the double scaling limit of small Immirzi parameter (gamma), large spins (j) with physical area (gamma times j) constant. In addition to quantum corrections in the Planck constant, we find new corrections in the Immirzi parameter due to the quantum discreteness of spacetime. The result is a strong evidence that the spinfoam covariant quantization of general relativity possesses the correct classical limit.

http://arxiv.org/abs/1108.1974
Canonical simplicial gravity
Bianca Dittrich, Philipp A Hoehn
(Submitted on 9 Aug 2011)
A general canonical formalism for discrete systems is developed which can handle varying phase space dimensions and constraints. The central ingredient is Hamilton's principle function which generates canonical time evolution and ensures that the canonical formalism reproduces the dynamics of the covariant formulation following directly from the action. We apply this formalism to simplicial gravity and (Euclidean) Regge calculus, in particular. A discrete forward/backward evolution is realized by gluing/removing single simplices step by step to/from a bulk triangulation and amounts to Pachner moves in the triangulated hypersurfaces. As a result, the hypersurfaces evolve in a discrete multi-fingered' time through the full Regge solution. Pachner moves are an elementary and ergodic class of homeomorphisms and generically change the number of variables, but can be implemented as canonical transformations on naturally extended phase spaces. Some moves introduce a priori free data which, however, may become fixed a posteriori by constraints arising in subsequent moves. The end result is a general and fully consistent formulation of canonical Regge calculus, thereby removing a longstanding obstacle in connecting covariant simplicial gravity models to canonical frameworks. The present scheme is, therefore, interesting in view of many approaches to quantum gravity, but may also prove useful for numerical implementations.
52 pages, 14 figures, 3 tables

http://arxiv.org/abs/1108.0910
The black hole information paradox and relative locality
Lee Smolin
(Submitted on 3 Aug 2011)
We argue that the recently proposed principle of relative locality offers a new way to resolve the black hole information puzzle.
11 pages, one figure

http://arxiv.org/abs/1108.0893
Loop Quantum Cosmology: A Status Report
Abhay Ashtekar, Parampreet Singh
(Submitted on 3 Aug 2011)
The goal of this article is to provide an overview of the current state of the art in loop quantum cosmology for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general; and, cosmologists who wish to apply loop quantum cosmology to probe modifications in the standard paradigm of the early universe. An effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without a loss of continuity.
136 pages, 15 figures

http://arxiv.org/abs/1108.0832
On the structure of a background independent quantum theory: Hamilton function, transition amplitudes, classical limit and continuous limit
Carlo Rovelli
(Submitted on 3 Aug 2011)
The Hamilton function is a powerful tool for studying the classical limit of quantum systems, which remains meaningful in background-independent systems. In quantum gravity, it clarifies the physical interpretation of the transitions amplitudes and their truncations.
7 pages

http://arxiv.org/abs/1108.0369
Twistor Networks and Covariant Twisted Geometries
Etera R. Livine, Simone Speziale, Johannes Tambornino
(Submitted on 1 Aug 2011)
We study the symplectic reduction of the phase space of two twistors to the cotangent bundle of the Lorentz group. We provide expressions for the Lorentz generators and group elements in terms of the spinors defining the twistors. We use this to define twistor networks as a graph carrying the phase space of two twistors on each edge. We also introduce simple twistor networks, which provide a classical version of the simple projected spin networks living on the boundary Hilbert space of EPRL/FK spin foam models. Finally, we give an expression for the Haar measure in terms of spinors.
18 pages

http://arxiv.org/abs/1107.5274
Holomorphic Lorentzian Simplicity Constraints
Maité Dupuis, Laurent Freidel, Etera R. Livine, Simone Speziale
(Submitted on 26 Jul 2011)
We develop an Hamiltonian representation of the sl(2,C) algebra on a phase space consisting of N copies of twistors, or bi-spinors. We identify a complete set of global invariants, and show that they generate a closed algebra including gl(N,C) as a subalgebra. Then, we define the linear and quadratic simplicity constraints which reduce the spinor variables to (framed) 3d spacelike polyhedra embedded in Minkowski spacetime. Finally, we introduce a new version of the simplicity constraints which (i) are holomorphic and (ii) Poisson-commute with each other, and show their equivalence to the linear and quadratic constraints.
20 pages

http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)
"The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from operator spin network diagrams} we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2-cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper.
36 pages, 23 figures

http://arxiv.org/abs/1107.2633
Many-nodes/many-links spinfoam: the homogeneous and isotropic case
Francesca Vidotto
(Submitted on 13 Jul 2011)
I compute the Lorentzian EPRL/FK/KKL spinfoam vertex amplitude for regular graphs, with an arbitrary number of links and nodes, and coherent states peaked on a homogeneous and isotropic geometry. This form of the amplitude can be applied for example to a dipole with an arbitrary number of links or to the 4-simplex given by the compete graph on 5 nodes. All the resulting amplitudes have the same support, independently of the graph used, in the large j (large volume) limit. This implies that they all yield the Friedmann equation: I show this in the presence of the cosmological constant. This result indicates that in the semiclassical limit quantum corrections in spinfoam cosmology do not come from just refining the graph, but rather from relaxing the large j limit.

http://arxiv.org/abs/1107.1540
Observational test of inflation in loop quantum cosmology
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa
(Submitted on 8 Jul 2011)
We study in detail the power spectra of scalar and tensor perturbations generated during inflation in loop quantum cosmology (LQC). After clarifying in a novel quantitative way how inverse-volume corrections arise in inhomogeneous settings, we show that they can generate large running spectral indices, which generally lead to an enhancement of power at large scales. We provide explicit formulas for the scalar/tensor power spectra under the slow-roll approximation, by taking into account corrections of order higher than the runnings. We place observational bounds on the inverse-volume quantum correction δ ~ a (σ >0, a is the scale factor) and the slow-roll parameter εV for power-law potentials as well as exponential potentials by using the data of WMAP 7yr combined with other observations. We derive the constraints on δ for two pivot wavenumbers k0 for several values of δ. The quadratic potential can be compatible with the data even in the presence of the LQC corrections, but the quartic potential is in tension with observations. We also find that the upper bounds on δ (k0) for given σ and k0 are insensitive to the choice of the inflaton potentials.
37 pages, 6 figures, 1 table

http://arxiv.org/abs/1107.1320
Black hole entropy and isolated horizons thermodynamics
Amit Ghosh, Alejandro Perez
(Submitted on 7 Jul 2011)
We present a statistical mechanical calculation of the thermodynamical properties of (non rotating) isolated horizons. The introduction of Planck scale allows for the definition of an universal horizon temperature (independent of the mass of the black hole) and a well-defined notion of energy (as measured by suitable local observers) proportional to the horizon area in Planck units. The microcanonical and canonical ensembles associated with the system are introduced. Black hole entropy and other thermodynamical quantities can be consistently computed in both ensembles and results are in agreement with Hawking's semiclassical analysis for all values of the Immirzi parameter.
5 pages

Last edited: Sep 29, 2011
3. Sep 30, 2011

### marcus

Five of us have already voted! Atyy, Pallen, Kcaj, and Macho, thanks for registering your picks for third quarter MIP. Usually it takes longer, especially when there is a long list of papers to choose from. Unless you've already been thinking about the recent QG papers and are familiar with some of them. It wouldn't be surprising if other people need more time to weigh the choices.

I would welcome hearing anyone's reasons for making the picks they did.
In a year or so we'll be able to see which ones of the papers are being regularly cited by researchers, and have something to compare with our own judgment. I spread my bets rather widely, not wanting to miss any I thought had a reasonable chance.

I don't speak with any special expertise---just an observer on the sidelines---but if I had to narrow it down to one or two papers that I thought made the most significant advance this time, it would be these two:

http://arxiv.org/abs/1109.6538
Lorentzian spinfoam propagator
Eugenio Bianchi, You Ding
(Submitted on 29 Sep 2011)
The two-point correlation function is calculated in the Lorentzian EPRL spinfoam model, and shown to match with the one in Regge calculus in a proper limit: large boundary spins, and small Barbero-Immirzi parameter, keeping the size of the quantum geometry finite and fixed. Compared to the Euclidean case, the definition of a Lorentzian boundary state involves a new feature: the notion of past- and future-pointing intertwiners. The semiclassical correlation function is obtained for a time-oriented semiclassical boundary state.

http://arxiv.org/abs/1109.4927
Coarse graining methods for spin net and spin foam models
Bianca Dittrich, Frank C. Eckert, Mercedes Martin-Benito
(Submitted on 22 Sep 2011)
We undertake first steps in making a class of discrete models of quantum gravity, spin foams, accessible to a large scale analysis by numerical and computational methods. In particular, we apply Migdal-Kadanoff and Tensor Network Renormalization schemes to spin net and spin foam models based on finite Abelian groups and introduce 'cutoff models' to probe the fate of gauge symmetries under various such approximated renormalization group flows. For the Tensor Network Renormalization analysis, a new Gauss constraint preserving algorithm is introduced to improve numerical stability and aid physical interpretation. We also describe the fixed point structure and establish an equivalence of certain models.
39 pages, 13 figures, 1 table

Does anyone else want to say why these might (or might not) be important? Atyy I think we may agree about one of them but it would be interesting to hear your reasons.

Last edited: Sep 30, 2011
4. Oct 1, 2011

### atyy

I voted for Dittrich et al because it's part of AdS/LQG!

The paper from Wan and colleagues because it will be as significant in a 100 years time as Thomson and Tait are now.

Of course the most significant paper that killed Rovelli's Lorentz invariant LQG is not on the list;)

5. Oct 1, 2011

why 100

6. Oct 6, 2011

### Diffeomorphic

Glad to see Shape Dynamics on here! I thought no one noticed the block they had to present at Loops 11', really fascinating stuff with a future! I recommend reading Sean Gryb and Julian Barbour's papers.

7. Nov 9, 2011

### marcus

It's certainly remarkable how much Shape Dynamics stood out in the poll this time. As it happens (just a personal note) I did not vote for the SD paper although I find that whole development, a new quantum geometry/gravity initiative, very interesting.

About half of the 13 people who participated so far DID vote for it however.

In trying to understand the current QG situation and developments it has helped (me for one, very much) to know other people's hunches and intuitive guesses so I should thank all those who have taken part in this poll.

Atyy
Tom Stoer
Francesca
PAllen
MTd2
Chronos
Diffeomorphic
Caramon
Oriako,
John86
MACHO-WIMP
Kcajrenreb

The leading papers, at this point, are:

Shape Dynamics 6 (Tim Koslowski)
Emergent Braided Matter of Quantum Geometry 4 (Sundance Bilson-Thompson et al)
Emergence of gravity from spinfoams 3 (Elena Magliaro and Claudio Perini)
Coarse graining methods for spin net and spin foam models 3 (Bianca Dittrich et al)

In case anyone just wants to check out the four front-runners, here are their links:

(T.K.) http://arxiv.org/abs/1108.5224
(S.B-T et al) http://arxiv.org/abs/1109.0080
(E.M.&C.P.) http://arxiv.org/abs/1108.2258
(B.D. et al) http://arxiv.org/abs/1109.4927

On the whole it's still too early to expect cites to indicate anything, since by and large researchers haven't had time to write the papers yet that would cite this year's 3rd quarter work---but for our eventual convenience here are the cites listings:

(T.K.) http://arxiv.org/cits/1108.5224 2
(S.B-T et al) http://arxiv.org/cits/1109.0080
(E.M.&C.P.) http://arxiv.org/cits/1108.2258 5
(B.D. et al) http://arxiv.org/cits/1109.4927 7

It does seem a bit remarkable that Bianca Dittrich et al's paper which only came out in September already has 7 cites. Nice showing!

Last edited: Nov 9, 2011
8. Dec 26, 2011

### marcus

No "voting" or performance index is perfect, but we can compare the results. In a sense research has a life of its own and just goes where it goes. Not fully predictably. Just out of curiosity I checked to see which of the third quarter papers were most highly cited now three months after the quarter ended. I know that cites don't tell the whole story, especially early like this when there has not been time for the full impact of the paper's results to be thought through and assessed by the research community. But I am still pleased to have voted for the first two here. One that garnered 16 and another that got 10 cites. I noticed that Tom Stoer and Atyy also picked some of these papers that subsequently were more cited, and others of us may have as well.

SIXTEEN CITES
http://arxiv.org/abs/1108.0893
Loop Quantum Cosmology: A Status Report
Abhay Ashtekar, Parampreet Singh
(Submitted on 3 Aug 2011)
The goal of this article is to provide an overview of the current state of the art in loop quantum cosmology for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general; and, cosmologists who wish to apply loop quantum cosmology to probe modifications in the standard paradigm of the early universe. An effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without a loss of continuity.
136 pages, 15 figures

TEN CITES
http://arxiv.org/abs/1109.4927
Coarse graining methods for spin net and spin foam models
Bianca Dittrich, Frank C. Eckert, Mercedes Martin-Benito
(Submitted on 22 Sep 2011)
We undertake first steps in making a class of discrete models of quantum gravity, spin foams, accessible to a large scale analysis by numerical and computational methods. In particular, we apply Migdal-Kadanoff and Tensor Network Renormalization schemes to spin net and spin foam models based on finite Abelian groups and introduce cutoff models' to probe the fate of gauge symmetries under various such approximated renormalization group flows. For the Tensor Network Renormalization analysis, a new Gauss constraint preserving algorithm is introduced to improve numerical stability and aid physical interpretation. We also describe the fixed point structure and establish an equivalence of certain models.
39 pages, 13 figures, 1 table

TEN CITES
http://arxiv.org/abs/1107.1540
Observational test of inflation in loop quantum cosmology
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa
(Submitted on 8 Jul 2011)
We study in detail the power spectra of scalar and tensor perturbations generated during inflation in loop quantum cosmology (LQC). After clarifying in a novel quantitative way how inverse-volume corrections arise in inhomogeneous settings, we show that they can generate large running spectral indices, which generally lead to an enhancement of power at large scales. We provide explicit formulas for the scalar/tensor power spectra under the slow-roll approximation, by taking into account corrections of order higher than the runnings. We place observational bounds on the inverse-volume quantum correction δ ~ a (σ >0, a is the scale factor) and the slow-roll parameter εV for power-law potentials as well as exponential potentials by using the data of WMAP 7yr combined with other observations. We derive the constraints on δ for two pivot wavenumbers k0 for several values of δ. The quadratic potential can be compatible with the data even in the presence of the LQC corrections, but the quartic potential is in tension with observations. We also find that the upper bounds on δ (k0) for given σ and k0 are insensitive to the choice of the inflaton potentials.
37 pages, 6 figures, 1 table

SEVEN CITES
http://arxiv.org/abs/1108.1974
Canonical simplicial gravity
Bianca Dittrich, Philipp A Hoehn
(Submitted on 9 Aug 2011)
A general canonical formalism for discrete systems is developed which can handle varying phase space dimensions and constraints. The central ingredient is Hamilton's principle function which generates canonical time evolution and ensures that the canonical formalism reproduces the dynamics of the covariant formulation following directly from the action. We apply this formalism to simplicial gravity and (Euclidean) Regge calculus, in particular. A discrete forward/backward evolution is realized by gluing/removing single simplices step by step to/from a bulk triangulation and amounts to Pachner moves in the triangulated hypersurfaces. As a result, the hypersurfaces evolve in a discrete multi-fingered' time through the full Regge solution. Pachner moves are an elementary and ergodic class of homeomorphisms and generically change the number of variables, but can be implemented as canonical transformations on naturally extended phase spaces. Some moves introduce a priori free data which, however, may become fixed a posteriori by constraints arising in subsequent moves. The end result is a general and fully consistent formulation of canonical Regge calculus, thereby removing a longstanding obstacle in connecting covariant simplicial gravity models to canonical frameworks. The present scheme is, therefore, interesting in view of many approaches to quantum gravity, but may also prove useful for numerical implementations.
52 pages, 14 figures, 3 tables

SIX CITES
http://arxiv.org/abs/1108.2258
Emergence of gravity from spinfoams
Elena Magliaro, Claudio Perini
(Submitted on 10 Aug 2011)
We find a nontrivial regime of spinfoam quantum gravity that reproduces classical Einstein equations. This is the double scaling limit of small Immirzi parameter (gamma), large spins (j) with physical area (gamma times j) constant. In addition to quantum corrections in the Planck constant, we find new corrections in the Immirzi parameter due to the quantum discreteness of spacetime. The result is a strong evidence that the spinfoam covariant quantization of general relativity possesses the correct classical limit.

Looking back, I see that I voted for 4 of these 5. The one I missed was the Bojowald Calcagni Tsujikawa, another Loop cosmology paper.

Anyway FWIW those were the third quarter poll lineup's top five, citewise. I was kind of impressed that the Bianca Frank Mercedes paper has already racked up 10 cites even though it only came out 22 September, just three months ago. That's quick work comparatively speaking.

Also impressed that TWO of the top five (in terms of cites) were by Bianca Dittrich. It can't be just because she's beautiful, can it?

Last edited: Dec 27, 2011
9. Jul 14, 2012

### julian

Interesting set of picks to choose from.

Last edited: Jul 14, 2012
10. Jul 14, 2012

### marcus

Julian, I'm glad you checked out this 2011 poll! If any of the papers especially stand out for you (in any sense, good/bad), you might mention them. Or vote for them---I think the poll is still open.

Maybe we should check to see the citation history of some of our favorites. We could see if the researchers active in the field agree with our judgment/forecasts to some extent.

Earlier (in the previous post) I just checked cites on five or so. I'll check the top four and see if the citation numbers have changed:

Then: SIXTEEN
Now: FORTY
http://arxiv.org/abs/1108.0893
http://arxiv.org/cits/1108.0893
Loop Quantum Cosmology: A Status Report
Abhay Ashtekar, Parampreet Singh
136 pages, 15 figures

Then: TEN
Now: SIXTEEN
http://arxiv.org/abs/1107.1540
http://arxiv.org/cits/1107.1540
Observational test of inflation in loop quantum cosmology
Martin Bojowald, Gianluca Calcagni, Shinji Tsujikawa
37 pages, 6 figures, 1 table

Then: TEN
Now: THIRTEEN
http://arxiv.org/abs/1109.4927
http://arxiv.org/cits/1109.4927
Coarse graining methods for spin net and spin foam models
Bianca Dittrich, Frank C. Eckert, Mercedes Martin-Benito
39 pages, 13 figures, 1 table

Then:SEVEN
Now: ELEVEN
http://arxiv.org/abs/1108.1974
http://arxiv.org/cits/1108.1974
Canonical simplicial gravity
Bianca Dittrich, Philipp A Hoehn
52 pages, 14 figures, 3 tables
=====================

There are some others that I didn't record cites for earlier, but can now:
http://arxiv.org/abs/1107.1320
http://arxiv.org/cits/1107.1320: TWELVE
Black hole entropy and isolated horizons thermodynamics
Amit Ghosh, Alejandro Perez

http://arxiv.org/abs/1108.0369
http://arxiv.org/cits/1108.0369 :EIGHT
Twistor Networks and Covariant Twisted Geometries
Etera R. Livine, Simone Speziale, Johannes Tambornino

http://arxiv.org/abs/1107.5274
http://arxiv.org/cits/1107.5274 :SEVEN
Holomorphic Lorentzian Simplicity Constraints
Maité Dupuis, Laurent Freidel, Etera R. Livine, Simone Speziale

Last edited: Jul 15, 2012
11. Jul 15, 2012

### julian

It's going to sound like I'm sitting on the fence, but I think most of them stand out. I have to admit I'm not as up to date in my reading of the literature the last couple of years like I used to be. It's difficult to pick from the list you've provided, but isn't this a good advertisement of the merit of the whole background independent approach?

I'm still aquainting myself with what's going on now.

I'm starting to get back into the subject. I like the paper by vidotto and rovelli (stepping out of homogeneity in LQC - even though it was 4 years ago) because it involves the old densitized Hamiltonian constraint (good old days), the non-graph changing Hamiltonian (sqrt of non-graph changing Master constraint of AQG I think?), matter clock variables, possible connection with Dittrich perturbation theory, and stuff I know.

Being a reader of the good old days, I'm interested in what spinors and twistors could offer to modern canonical quantum gravity.

I'm also interested in course graining of spin foams me having a background in RG of non-Hermitian condensed matter systems and Thiemann's constraint being non-Hermitian.

Last edited: Jul 15, 2012
12. Jul 16, 2012

### tom.stoer

It would be interesting to see some papers from the string and supergravity community as well ;-)

13. Aug 2, 2012

### MTd2

For the 3rd quarter of this year:

http://arxiv.org/abs/1208.0031

On the physical mechanism underlying Asymptotic Safety

Andreas Nink, Martin Reuter
(Submitted on 31 Jul 2012)
We identify a simple physical mechanism which is at the heart of Asymptotic Safety in Quantum Einstein Gravity (QEG) according to all available effective average action-based investigations. Upon linearization the gravitational field equations give rise to an inverse propagator for metric fluctuations comprising two pieces: a covariant Laplacian and a curvature dependent potential term. By analogy with elementary magnetic systems they lead to, respectively, dia- and paramagnetic-type interactions of the metric fluctuations with the background gravitational field. We show that above 3 spacetime dimensions the gravitational antiscreening occurring in QEG is entirely due to a strong dominance of the ultralocal paramagnetic interactions over the diamagnetic ones that favor screening. (Below 3 dimensions both the dia- and paramagnetic effects support antiscreening.) The spacetimes of QEG are interpreted as a polarizable medium with a "paramagnetic" response to external perturbations, and similarities with the vacuum state of Yang-Mills theory are pointed out. As a by-product, we resolve a longstanding puzzle concerning the beta function of Newton's constant in 2+{\epsilon} dimensional gravity.

14. Aug 2, 2012

### tom.stoer

MTd2 - very good choice!

and what about string and SUGRA papers?

15. Aug 2, 2012

### MTd2

I will let its billionaire patron choose it :)