# Our picks for second quarter 2013 MIP (most important QG paper)

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

21.4%

0 vote(s)
0.0%

21.4%

35.7%

28.6%

14.3%

14.3%

35.7%

7.1%

7.1%

14.3%

42.9%

14.3%

7.1%

0 vote(s)
0.0%

7.1%

14.3%
18. ### Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries

14.3%
1. Jun 30, 2013

### marcus

Of the eighteen candidates, please choose the one(s) 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/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh

http://arxiv.org/abs/1306.6126
The generator of spatial diffeomorphisms in the Koslowski-Sahlmann representation

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti

http://arxiv.org/abs/1305.6315
Why gravity codes the renormalization of conformal field theories
Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati, Lee Smolin

http://arxiv.org/abs/1305.3326
A Discrete and Coherent Basis of Intertwiners
Laurent Freidel, Jeff Hnybida

http://arxiv.org/abs/1305.2207
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski

http://arxiv.org/abs/1305.0822
On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

http://arxiv.org/abs/1304.7686
A quantum gravitational inflationary scenario in Bianchi-I spacetime
Brajesh Gupt, Parampreet Singh

http://arxiv.org/abs/1304.7247
Probing the quantum nature of spacetime by diffusion
Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig

http://arxiv.org/abs/1304.6688
Towards Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski

2. Jun 30, 2013

### marcus

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh
(Submitted on 26 Jun 2013)
A fundamental issue for any quantum cosmological theory is to specify how probabilities can be assigned to various quantum events or sequences of events such as the occurrence of singularities or bounces. In previous work, we have demonstrated how this issue can be successfully addressed within the consistent histories approach to quantum theory for Wheeler-DeWitt-quantized cosmological models. In this work, we generalize that analysis to the exactly solvable loop quantization of a spatially flat, homogeneous and isotropic cosmology sourced with a massless, minimally coupled scalar field known as sLQC. We provide an explicit, rigorous and complete decoherent histories formulation for this model and compute the probabilities for the occurrence of a quantum bounce vs. a singularity. Using the scalar field as an emergent internal time, we show for generic states that the probability for a singularity to occur in this model is zero, and that of a bounce is unity, complementing earlier studies of the expectation values of the volume and matter density in this theory. We also show from the consistent histories point of view that all states in this model, whether quantum or classical, achieve arbitrarily large volume in the limit of infinite past' or future' scalar time', in the sense that the wave function evaluated at any arbitrary fixed value of the volume vanishes in that limit. Finally, we briefly discuss certain misconceptions concerning the utility of the consistent histories approach in these models.
22 pages, 3 figures

http://arxiv.org/abs/1306.6126
The generator of spatial diffeomorphisms in the Koslowski-Sahlmann representation
(Submitted on 26 Jun 2013)
A generalization of the representation underlying the discrete spatial geometry of Loop Quantum Gravity, to accomodate states labelled by smooth spatial geometries, was discovered by Koslowski and further studied by Sahlmann. We show how to construct the diffeomorphism constraint operator in this Koslowski- Sahlmann (KS) representation from suitable connection and triad dependent operators. We show that the KS representation supports the action of hitherto unnoticed "background exponential" operators which, in contrast to the holonomy- flux operators, change the smooth spatial geometry label of the states. These operators are shown to be quantizations of certain connection dependent functions and play a key role in the construction of the diffeomorphism constraint operator.
8 pages

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah
(Submitted on 24 Jun 2013)
Since black holes can be formed through widely varying processes, the horizon structure is highly complicated in the dynamical phase. Nonetheless, as numerical simulations show, the final state appears to be universal, well described by the Kerr geometry. How are all these large and widely varying deviations from the Kerr horizon washed out? To investigate this issue, we introduce a well-suited notion of horizon multipole moments and equations governing their dynamics, thereby providing a coordinate and slicing independent framework to investigate the approach to equilibrium. In particular, our flux formulas for multipoles can be used as analytical checks on numerical simulations and, in turn, the simulations could be used to fathom possible universalities in the way black holes approach their final equilibrium.
27 pages, 1 figure

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli
(Submitted on 21 Jun 2013)
We show that Oeckl's boundary formalism incorporates quantum statistical mechanics naturally, and we formulate general-covariant quantum statistical mechanics in this language. We illustrate the formalism by showing how it accounts for the Unruh effect. We observe that the distinction between pure and mixed states weakens in the general covariant context, and surmise that local gravitational processes are indivisibly statistical with no possible quantal versus probabilistic distinction.
8 pages, 2 figures

http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis
(Submitted on 13 Jun 2013)
The trace-free version of the Einstein Gravitational equations, essentially equivalent to unimodular gravity, can solve the troubling issue of the huge discrepancy between quantum field theory estimates of the vacuum energy density and the astronomically observed value of the cosmological constant. However it has been suggested that this proposal cannot work because it prevents the inflaton potential energy from driving inflation. It is shown here that that concern is unjustified: inflation proceeds as usual if we adopt the trace free gravitational equations.
10 pages

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter
(Submitted on 12 Jun 2013)
Spin foams are models of quantum gravity and therefore quantum space time. A key open issue is to determine the possible continuum phases of these models. Progress on this issue has been prohibited by the complexity of the full four--dimensional models. We consider here simplified analogue models, so called spin nets, that retain the main dynamical ingredient of spin foams, the simplicity constraints. For a certain class of these spin net models we determine the phase diagram and therefore the continuum phases via a coarse graining procedure based on tensor network renormalization. This procedure will also reveal an unexpected fixed point, which turns out to define a new triangulation invariant vertex model.
33 pages, 17 figures

http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel
(Submitted on 4 Jun 2013)
The Hamiltonian constraint is the key element of the canonical formulation of LQG coding its dynamics. In Ashtekar-Barbero variables it naturally splits into the so called Euclidean and Lorentzian parts. However, due to the high complexity of this operator, only the matrix elements of the Euclidean part have been considered so far. Here we evaluate the action of the full constraint, including the Lorentzian part. The computation requires an heavy use of SU(2) recoupling theory and several tricky identities among n-j symbols are used to find the final result: these identities, together with the graphical calculus used to derive them, also simplify the Euclidean constraint and are of general interest in LQG computations.
36 pages

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti
(Submitted on 29 May 2013)
By reintroducing Lorentz invariance via a complex connection formulation in canonical loop quantum gravity, we define a geometrical notion of temperature for quantum isolated horizons. Upon imposition of the reality conditions in the form of the linear simplicity constraints for an imaginary Barbero-Immirzi parameter, the exact formula for the temperature can be derived by demanding that the horizon state satisfying the boundary conditions be a KMS-state. In this way, our analysis reveals the connection between the passage to the Ashtekar self-dual variables and the thermality of the horizon. The horizon equilibrium state can then be used to compute both the von Neumann and the Boltzmann entropies. By means of a natural cut-off introduced by the topological theory on the boundary, we show that the two provide the same finite answer which allows us to recover the Bekenstein-Hawking formula in the semi-classical limit. The connection with Connes-Rovelli thermal time proposal for a general relativistic statistical mechanics is worked out.
10 pages, 1 figure

http://arxiv.org/abs/1305.6315
Why gravity codes the renormalization of conformal field theories
Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati, Lee Smolin
(Submitted on 27 May 2013)
We give a new demonstration that General Relativity in d+1 dimensions with negative or positive cosmological constant codes the renormalization group behaviour of conformal field theories (CFT) in d dimensions. This utilizes Shape Dynamics, which is a conformally invariant theory known to be equivalent to General Relativity. A key result of Shape Dynamics is that the evolution of observables under local conformal transformations and spatial diffeomorphisms is shown to be equivalent to many fingered time, i.e., d+1-dimensional spacetime diffeomorphisms. This relationship explains why the renormalization group flow of a CFT is governed by a geometry with d+1-dimensional spacetime diffeomorphism invariance.
25 pages

http://arxiv.org/abs/1305.3326
A Discrete and Coherent Basis of Intertwiners
Laurent Freidel, Jeff Hnybida
(Submitted on 15 May 2013)
We construct a new discrete basis of 4-valent SU(2) intertwiners. This basis possesses both the advantage of being discrete, while at the same time representing accurately the classical degrees of freedom; hence it is coherent. The closed spin network amplitude obtained from these intertwiners depends on twenty spins and can be evaluated by a generalization of the Racah formula for an arbitrary graph. The asymptotic limit of these amplitudes is found. We give, for the first time, the asymptotics of 15j symbols in the real basis. Remarkably it gives a generalization of the Regge action to twisted geometries.
31 pages.

http://arxiv.org/abs/1305.2207
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman
(Submitted on 9 May 2013)
We study the imaginary part of the Lorentzian gravitational action for bounded regions, as described in arXiv:1301.7041. By comparing to a Euclidean calculation, we explain the agreement between the formula for this imaginary part and the formula for black hole entropy. We also clarify the topological structure of the imaginary part in Lovelock gravity. We then evaluate the action's imaginary part for some special regions. These include cylindrical slabs spanning the exterior of a stationary black hole spacetime, 'maximal diamonds' in various symmetric spacetimes, as well as local near-horizon regions. In the first setup, the black hole's entropy and conserved charges contribute to the action's imaginary and real parts, respectively. In the other two setups, the imaginary part coincides with the relevant entropy.
34 pages, 10 figures.

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski
(Submitted on 7 May 2013)
Shape Dynamics is a gauge theory based on spatial diffeomorphism- and Weyl-invariance which is locally indistinguishable form classical General Relativity. If taken seriously, it suggests that the spacetime--geometry picture that underlies General Relativity can be replaced by a picture based on spatial conformal geometry. This classically well understood trading of gauge symmetries opens new conceptual avenues in many approaches to quantum gravity. I focus on the general implications for quantum gravity and effective field theory and consider the application of the Shape Dynamics picture in the exact renormalization group approaches to gravity, loop- and polymer- quantization approaches to gravity and low energy effective field theories. I also discuss the interpretation of known results through in the Shape Dynamics picture, in particular holographic renormalization and the problem of time in canonical quantum gravity.
56 pages, 1 figure

http://arxiv.org/abs/1305.0822
On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley
(Submitted on 3 May 2013)
We provide evidence that general relativity is the unique spatially covariant effective field theory of the transverse, traceless graviton degrees of freedom. The Lorentz covariance of general relativity, having not been assumed in our analysis, is thus plausibly interpreted as an accidental or emergent symmetry of the gravitational sector.
40 pages.

http://arxiv.org/abs/1304.7686
A quantum gravitational inflationary scenario in Bianchi-I spacetime
Brajesh Gupt, Parampreet Singh
(Submitted on 29 Apr 2013)
We investigate the φ2 inflationary model in the Bianchi-I spacetime using effective spacetime description of loop quantum cosmology to understand the issues of the resolution of initial singularity, isotropization, effect of anisotropies on amount of inflation, and the phase space attractors in the presence of non-perturbative quantum gravitational modifications. A comparative analysis with the classical theory by including more general initial conditions than the ones previously considered in the latter is also performed. We show that, in general, the classical singularity is replaced by a bounce of the mean scale factor in loop quantum cosmology. Due to the underlying quantum geometric effects, the energy density of the inflaton and the anisotropic shear remain bounded throughout the non-singular evolution. Starting from arbitrary anisotropic initial conditions, a loop quantum universe isotropizes either before or soon after the onset of slow-roll inflation. We find a double attractor behavior in the phase space dynamics of loop quantum cosmology, similar to the one in classical theory, but with some additional subtle features. Quantum modifications to the dynamical equations are such that, unlike the classical theory, the amount of inflation does not monotonically depend on the initial anisotropy in loop quantum cosmology. Our results suggest that a viable non-singular inflationary model can be constructed from highly anisotropic initial conditions in the Planck regime.
34 pages, 19 figures

http://arxiv.org/abs/1304.7247
Probing the quantum nature of spacetime by diffusion
Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig
(Submitted on 26 Apr 2013)
Many approaches to quantum gravity have resorted to diffusion processes to characterize the spectral properties of the resulting quantum spacetimes. We critically discuss these quantum-improved diffusion equations and point out that a crucial property, namely positivity of their solutions, is not preserved automatically. We then construct a novel set of diffusion equations with positive semi-definite probability densities, applicable to Asymptotically Safe gravity, Horava-Lifgarbagez gravity and Loop Quantum Gravity. These recover all previous results on the spectral dimension and shed further light on the structure of the quantum spacetimes by assessing the underlying stochastic processes. Pointing out that manifestly different diffusion processes lead to the same spectral dimension, we propose the probability distribution of the diffusion process as a refined probe of quantum spacetime.
14 pages, 5 figures

http://arxiv.org/abs/1304.6688
Towards Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan
(Submitted on 24 Apr 2013)
We examine spinfoam cosmology by use of a simple graph adapted to homogeneous cosmological models. We calculate dynamics in the isotropic limit, and provide the framework for the aniostropic case. The dynamical behaviour is calculating transition amplitudes between holomorphic coherent states on a single node graph. The resultant dynamics is peaked on solutions which have no support on the zero volume state, indicating that big bang type singularities are avoided within such models.
19 pages, 4 figures

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich
(Submitted on 22 Apr 2013)
The diffeomorphism symmetry of general relativity leads in the canonical formulation to constraints, which encode the dynamics of the theory. These constraints satisfy a complicated algebra, known as Dirac's hypersurface deformation algebra. This algebra has been a long standing challenge for quantization. One reason is that discretizations, on which many quantum gravity approaches rely, generically break diffeomorphism symmetry. In this work we find a representation for the Dirac constraint algebra of hypersurface deformations in a formulation of discrete 3D gravity and for the flat as well as homogeneously curved sector of discrete 4D gravity. In these cases diffeomorphism symmetry can be preserved. Furthermore we present different versions of the hypersurface deformation algebra for the boundary of a simplex in arbitrary dimensions.
30 pages

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski
(Submitted on 20 Apr 2013)
A path integral representation of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model is proposed as a starting point of semiclassical analysis. The relation between the spinfoam model and classical simplicial geometry is studied via the large spin asymptotic expansion of the spinfoam amplitude with all spins uniformaly large. More precisely in the large spin regime, there is an equivalence between the spinfoam critical configuration (with certain nondegeneracy assumption) and a classical Lorentzian simplicial geometry. Such an equivalence relation allows us to classify the spinfoam critical configurations by their geometrical interpretations, via two types of solution-generating maps. The equivalence between spinfoam critical configuration and simplical geometry also allows us to define the notion of globally oriented and time-oriented spinfoam critical configuration. It is shown that only at the globally oriented and time-oriented spinfoam critical configuration, the leading order contribution of spinfoam large spin asymptotics gives precisely an exponential of Lorentzian Regge action of General Relativity. At all other (unphysical) critical configurations, spinfoam large spin asymptotics modifies the Regge action at the leading order approximation.
36 pages

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

Last edited: Jun 30, 2013
3. Jul 1, 2013

### Chronos

It seems natural to view shape dynamics as a leading edge concept of geometry under GR.

4. Jul 4, 2013

### marcus

Thanks to Atyy, Chronos, Jason_0, Martinbn, Shablong, and Skydive for making a strong start on the current poll!

With seven of us voting, some papers have emerged as possible front-runners. I'll only list those who've gotten two or more votes, so far.

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah

http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski

http://arxiv.org/abs/1305.0822
On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski

Last edited: Jul 4, 2013
5. Jul 10, 2013

### marcus

Here's how the lineup looks at present, with eight of us voting. As before I'll only list those who've gotten two or more votes, so far.

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski

http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel

http://arxiv.org/abs/1305.2207
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman

http://arxiv.org/abs/1305.0822
On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski

6. Jul 13, 2013

### marcus

Updated tally. Nine of us have voted.

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski

http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel

http://arxiv.org/abs/1305.2207
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman

http://arxiv.org/abs/1305.0822
On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski

7. Aug 10, 2013

### marcus

Looking ahead to the third quarter MIP poll, it's time for a tentative listing of the most interesting/significant papers that have appeared since the beginning of July.

BTW collectively we performed as a group really well last time. I'm only gradually coming to realize how right-on the ranking turned out. The combined judgment of the 9 of us who took part was definitely more insightful and prescient than I personally could have managed six weeks ago on my own. Anyway, I'll start making a "short list" of what has appeared in third quarter (July-Sept.) so far.

This first attempt at a "short" list is clearly way too long. Will have to prune it down to half or less.

Here's one MTd2 spotted, not sure it's close enough to main Loop-and-allied QG topic to keep on the poll, but seems very interesting from unification standpoint:
http://arxiv.org/abs/1308.1278
Second Order Standard Model
Johnny Espin, Kirill Krasnov
(Submitted on 6 Aug 2013)
We rewrite the Lagrangian of the fermionic sector of the Standard Model in a novel compact form. The new Lagrangian is second order in derivatives, and is obtained from the usual first order Lagrangian by integrating out all primed (or dotted) 2-component spinors. The Higgs field enters the new Lagrangian non-polynomially, very much like the metric enters the Einstein-Hilbert Lagrangian of General Relativity. We also discuss unification in the second order formalism, and describe a natural in this framework SU(2)xSU(4) unified theory.
21 pages.

There's a strong research current in the direction of applying the full spinfoam theory to cosmology:
http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan
(Submitted on 3 Aug 2013)
The dynamics of a homogeneous, anisotropic universe are investigated within the context of spinfoam cosmology. Transition amplitudes are calculated for a graph consisting of a single node and three links - the Daisy graph' - probing the behaviour a classical Bianchi I spacetime. It is shown further how the use of such single node graphs gives rise to a simplification of states such that all orders in the spin expansion can be calculated, indicating that it is the vertex expansion that contains information about quantum dynamics.
28 pages, 1 figure

Atyy spotted some really interesting work by Laurent Freidel and others:
http://arxiv.org/abs/1308.0300
Snyder Momentum Space in Relative Locality
Andrzej Banburski, Laurent Freidel
(Submitted on 1 Aug 2013)
The standard approaches of phenomenology of Quantum Gravity have usually explicitly violated Lorentz invariance, either in the dispersion relation or in the addition rule for momenta. We investigate whether it is possible in 3+1 dimensions to have a non local deformation that preserves fully Lorentz invariance, as it is the case in 2+1D Quantum Gravity. We answer positively to this question and show for the first time how to construct a homogeneously curved momentum space preserving the full action of the Lorentz group in dimension 4 and higher, despite relaxing locality. We study the property of this relative locality deformation and show that this space leads to a noncommutativity related to Snyder spacetime.
22 pages, 6 figures.

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick
(Submitted on 31 Jul 2013)
It is well known that the SU(2)-gauge invariant phase space of loop gravity can be represented in terms of twisted geometries. These are piecewise-linear-flat geometries obtained by gluing together polyhedra, but the resulting geometries are not continuous across the faces. Here we show that this phase space can also be represented by continuous, piecewise-flat three-geometries called spinning geometries. These are composed of metric-flat three-cells glued together consistently. The geometry of each cell and the manner in which they are glued is compatible with the choice of fluxes and holonomies.
We first remark that the fluxes provide each edge with an angular momentum. By studying the piecewise-flat geometries which minimize edge lengths, we show that these angular momenta can be literally interpreted as the spin of the edges: the geometries of all edges are necessarily helices. We also show that the compatibility of the gluing maps with the holonomy data results in the same conclusion. This shows that a spinning geometry represents a way to glue together the three-cells of a twisted geometry to form a continuous geometry which represents a point in the loop gravity phase space.
20 pages, 5 figures

Though not all that close to Loop QG, the next paper presents an interesting variation on causal sets.
http://arxiv.org/abs/1307.6167
The Universe as a Process of Unique Events
Marina Cortês, Lee Smolin
(Submitted on 23 Jul 2013)
We describe a new class of models of quantum space-time based on energetic causal sets and show that under natural conditions space-time emerges from them. These are causal sets whose causal links are labelled by energy and momentum and conservation laws are applied at events. The models are motivated by principles we propose govern microscopic physics which posit a fundamental irreversibility of time. One consequence is that each event in the history of the universe has a distinct causal relationship to the rest; this requires a novel form of dynamics which an be applied to uniquely distinctive events. We hence introduce a new kind of deterministic dynamics for a causal set in which new events are generated from pairs of progenitor events by a rule which is based on extremizing the distinctions between causal past sets of events. This dynamics is asymmetric in time, but we find evidence from numerical simulations of a 1+1 dimensional model, that an effective dynamics emerges which restores approximate time reversal symmetry. Finally we also present a natural twistorial representation of energetic causal sets.
26 pages, 5 figures

This guy Schliemann has 86 papers but they are almost all in condensed matter. If you dip into the paper, though, you see he is working with the LQG tetrahedron. Go figure
http://arxiv.org/abs/1307.5979
The Large-Volume Limit of a Quantum Tetrahedron is a Quantum Harmonic Oscillator
John Schliemann
(Submitted on 23 Jul 2013)
It is shown that the volume operator of a quantum tetrahedron is, in the sector of large eigenvalues, accurately described by a quantum harmonic oscillator. This result relies on the fact that (i) the volume operator couples only neighboring states of its standard basis, and (ii) its matrix elements show a unique maximum as a function of internal angular momentum quantum numbers. These quantum numbers, considered as a continuous variable, are the coordinate of the oscillator describing its quadratic potential, while the corresponding derivative defines a momentum operator. We also analyze the scaling properties of the oscillator parameters as a function of the size of the tetrahedron, and the role of different angular momentum coupling schemes.
10 pages, 3 figures

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel
(Submitted on 22 Jul 2013)
In a seminal paper, Kaminski, Kisielowski an Lewandowski for the first time extended the definition of spin foam models to arbitrary boundary graphs. This is a prerequisite in order to make contact to the canonical formulation of Loop Quantum Gravity (LQG) and allows to investigate the question whether any of the presently considered spin foam models yield a rigging map for any of the presently defined Hamiltonian constraint operators. The KKL extension cannot be described in terms of Group Field Theory (GFT) since arbitrary foams are involved while GFT is tied to simplicial complexes. Therefore one has to define the sum over spin foams with given boundary spin networks in an independent fashion using natural axioms, most importantly a gluing property for 2-complexes. These axioms are motivated by the requirement that spin foam amplitudes should define a rigging map (physical inner product) induced by the Hamiltonian constraint. This is achieved by constructing a spin foam operator based on abstract 2-complexes that acts on the kinematical Hilbert space of Loop Quantum Gravity. In the analysis of the resulting object we are able to identify an elementary spin foam transfer matrix that allows to generate any finite foam as a finite power of the transfer matrix. It transpires that the sum over spin foams, as written, does not define a projector on the physical Hilbert space. This statement is independent of the concrete spin foam model and Hamiltonian constraint. However, the transfer matrix potentially contains the necessary ingredient in order to construct a proper rigging map in terms of a modified transfer matrix.
62 pages, 14 figures

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll
(Submitted on 20 Jul 2013)
We present a detailed analysis of a recently introduced version of Causal Dynamical Triangulations (CDT) that does not rely on a distinguished time slicing. Focussing on the case of 2+1 spacetime dimensions, we analyze its geometric and causal properties, present details of the numerical set-up and explain how to extract "volume profiles". Extensive Monte Carlo measurements of the system show the emergence of a de Sitter universe on large scales from the underlying quantum ensemble, similar to what was observed previously in standard CDT quantum gravity. This provides evidence that the distinguished time slicing of the latter is not an essential part of its kinematical set-up.
44 pages, 29 figures

http://arxiv.org/abs/1307.5461
Quantum hyperbolic geometry in loop quantum gravity with cosmological constant
Maite Dupuis, Florian Girelli
(Submitted on 20 Jul 2013)
Loop Quantum Gravity (LQG) is an attempt to describe the quantum gravity regime. Introducing a non-zero cosmological constant Λ in this context has been a withstanding problem. Other approaches, such as Chern-Simons gravity, suggest that quantum groups can be used to introduce Λ in the game. Not much is known when defining LQG with a quantum group. Tensor operators can be used to construct observables in any type of discrete quantum gauge theory with a classical/quantum gauge group. We illustrate this by constructing explicitly geometric observables for LQG defined with a quantum group and show for the first time that they encode a quantized hyperbolic geometry. This is a novel argument pointing out the usefulness of quantum groups as encoding a non-zero cosmological constant. We conclude by discussing how tensor operators provide the right formalism to unlock the LQG formulation with a non-zero cosmological constant.
6 pages, 1 figure

http://arxiv.org/abs/1307.5281
Double Scaling in Tensor Models with a Quartic Interaction
Stephane Dartois, Razvan Gurau, Vincent Rivasseau
(Submitted on 19 Jul 2013)
In this paper we identify and analyze in detail the subleading contributions in the 1/N expansion of random tensors, in the simple case of a quartically interacting model. The leading order for this 1/N expansion is made of graphs, called melons, which are dual to particular triangulations of the D-dimensional sphere, closely related to the "stacked" triangulations. For D<6 the subleading behavior is governed by a larger family of graphs, hereafter called cherry trees, which are also dual to the D-dimensional sphere. They can be resummed explicitly through a double scaling limit. In sharp contrast with random matrix models, this double scaling limit is stable. Apart from its unexpected upper critical dimension 6, it displays a singularity at fixed distance from the origin and is clearly the first step in a richer set of yet to be discovered multi-scaling limits.
40 pages.

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau
(Submitted on 19 Jul 2013)
This article addresses the issue of the closure of the algebra of constraints for generic (cosmological) perturbations when taking into account simultaneously the two main corrections of effective loop quantum cosmology, namely the holonomy and the inverse-volume terms. Previous works on either the holonomy or the inverse volume case are reviewed and generalized. In the inverse-volume case, we point out new possibilities. An anomaly-free solution including both corrections is found for perturbations, and the corresponding equations of motion are derived.
19 pages.

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer
(Submitted on 18 Jul 2013)
We propose a derivation for computing black hole entropy for spherical non-rotating isolated horizons from loop quantum gravity in four and higher dimensions. The state counting problem effectively reduces to the well studied 3+1-dimensional one based on an SU(2)-Chern-Simons theory, differing only in the precise form of the area spectrum and the restriction to integer spins.
5 pages

http://arxiv.org/abs/1307.5026
Melonic phase transition in group field theory
Aristide Baratin, Sylvain Carrozza, Daniele Oriti, James P. Ryan, Matteo Smerlak
(Submitted on 18 Jul 2013)
Group field theories have recently been shown to admit a 1/N expansion dominated by so-called 'melonic graphs', dual to triangulated spheres. In this note, we deepen the analysis of this melonic sector. We obtain a combinatorial formula for the melonic amplitudes in terms of a graph polynomial related to a higher dimensional generalization of the Kirchhoff tree-matrix theorem. Simple bounds on these amplitudes show the existence of a phase transition driven by melonic interaction processes. We restrict our study to the Boulatov-Ooguri models, which describe topological BF theories and are the basis for the construction of four dimensional models of quantum gravity.
8 pages, 4 figures

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto
(Submitted on 11 Jul 2013)
A simple argument indicates that covariant loop gravity (spinfoam theory) predicts a maximal acceleration, and hence forbids the development of curvature singularities. This supports the results obtained for cosmology and black holes using canonical methods.
4 pages, 1 figure

http://arxiv.org/abs/1307.2719
Deformations of Polyhedra and Polygons by the Unitary Group
Etera R. Livine
(Submitted on 10 Jul 2013)
We introduce the set of framed (convex) polyhedra with N faces as the symplectic quotient C2N //SU(2). A framed polyhedron is then parametrized by N spinors living in C2 satisfying suitable closure constraints and defines a usual convex polyhedron plus extra U(1) phases attached to each face. We show that there is a natural action of the unitary group U(N) on this phase space, which changes the shape of faces and allows to map any (framed) polyhedron onto any other with the same total (boundary) area. This identifies the space of framed polyhedra to the Grassmannian space U(N )/ (SU(2)×U(N −2)). We show how to write averages of geometrical observables (polynomials in the faces’ area and the angles between them) over the ensemble of polyhedra (distributed uniformly with respect to the Haar measure on U(N)) as polynomial integrals over the unitary group and we provide a few methods to compute these integrals systematically. We also use the Itzykson-Zuber formula from matrix models as the generating function for these averages and correlations.
In the quantum case, a canonical quantization of the framed polyhedron phase space leads to the Hilbert space of SU(2) intertwiners (or, in other words, SU(2)-invariant states in tensor products of irreducible representations). The total boundary area as well as the individual face areas are quantized as half-integers (spins), and the Hilbert spaces for fixed total area form irreducible representations of U(N). We define semi-classical coherent intertwiner states peaked on classical framed polyhedra and transforming consistently under U(N) transformations. And we show how the U(N) character formula for unitary transformations is to be considered as an extension of the Itzykson-Zuber to the quantum level and generates the traces of all polynomial observables over the Hilbert space of intertwiners.
We finally apply the same formalism to two dimensions and show that classical (convex) polygons can be described in a similar fashion trading the unitary group for the orthogonal group. We conclude with a discussion of the possible (deformation) dynamics that one can define on the space of polygons or polyhedra. This work is a priori useful in the context of discrete geometry but it should hopefully also be relevant to (loop) quantum gravity in 2+1 and 3+1 dimensions when the quantum geometry is defined in terms of gluing of (quantized) polygons and polyhedra.
33 pages

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski
(Submitted on 5 Jul 2013)
We study the asymptotic geometry of the spin foam partition function for a large class of models, including the models of Barrett and Crane, Engle, Pereira, Rovelli and Livine, and, Freidel and Krasnov.
The asymptotics is taken with respect to the boundary spins only, no assumption of large spins is made in the interior. We give a sufficient criterion for the existence of the partition function. We find that geometric boundary data is suppressed unless its interior continuation satisfies certain accidental curvature constraints. This means in particular that most Regge manifolds are suppressed in the asymptotic regime. We discuss this explicitly for the case of the configurations arising in the 3-3 Pachner move. We identify the origin of these accidental curvature constraints as an incorrect twisting of the face amplitude upon introduction of the Immirzi parameter and propose a way to resolve this problem, albeit at the price of losing the connection to the SU(2) boundary Hilbert space.
The key methodological innovation that enables these results is the introduction of the notion of wave front sets, and the adaptation of tools for their study from micro local analysis to the case of spin foam partition functions.
63 pages, 5 figures

Last edited: Aug 10, 2013
8. Aug 13, 2013

### marcus

My first attempt at a "short" list for the 3rd quarter MIP poll was way too long. I've had to exclude several quite interesting and potentially significant papers to reduce it to manageable size, and almost certainly will need to do more pruning. We are now about half way through the quarter so the pool of candidates we draw from can be expected to approximately double, by the end of September.

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Chaharsough Shirazi, Jonathan Engle
(Submitted on 13 Aug 2013)
Spin-foams are a proposal for defining the dynamics of loop quantum gravity via path integral. In order for a path integral to be at least formally equivalent to the corresponding canonical quantization, at each point in the space of histories it is important that the integrand have not only the correct phase -- a topic of recent focus in spin-foams -- but also the correct modulus, usually referred to as the measure factor. The correct measure factor descends from the Liouville measure on the reduced phase space, and its calculation is a task of canonical analysis.
The covariant formulation of gravity from which spin-foams are derived is the Plebanski-Holst formulation, in which the basic variables are a Lorentz connection and a Lorentz-algebra valued two-form, called the Plebanski two-form. However, in the final spin-foam sum, one sums over only spins and intertwiners, which label eigenstates of the Plebanski two-form alone. The spin-foam sum is therefore a discretized version of a Plebanski-Holst path integral in which only the Plebanski two-form appears, and in which the connection degrees of freedom have been integrated out. We call this a purely geometric Plebanski-Holst path integral.
In prior work in which one of the authors was involved, the measure factor for the Plebanski-Holst path integral with both connection and two-form variables was calculated. Before one discretizes this measure and incorporates it into a spin-foam sum, however, one must integrate out the connection in order to obtain the purely geometric version of the path integral. To calculate this purely geometric path integral is the principal task of the present paper, and it is done in two independent ways. Gauge-fixing and the background independence of the resulting path integral are discussed in the appendices.
21 pages

http://arxiv.org/abs/1308.2934
The Fundamental Group of a Spatial Section Represented by a Topspin Network
Christopher L Duston
(Submitted on 13 Aug 2013)
We present an algorithm which determines the fundamental group of a spatial section using topspin networks. Tracking the topology of the spatial section is a unique feature of this approach, which is not possible in standard Loop Quantum Gravity. This leads to an example of spatial topology change in a smooth 4-manifold represented by a topspin foam.
7 pages. Based on work presented at the LOOPS 13 conference at the Perimeter Institute

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin
(Submitted on 9 Aug 2013)
We propose an approach to quantum theory based on the energetic causal sets, introduced in Cortês and Smolin (2013). Fundamental processes are causal sets whose events carry momentum and energy, which are transmitted along causal links and conserved at each event. Fundamentally there are amplitudes for such causal processes, but no space-time. An embedding of the causal processes in an emergent space-time arises only at the semiclassical level. Hence, fundamentally there are no commutation relations, no uncertainty principle and, indeed, no hbar. All that remains of quantum theory is the relationship between the absolute value squared of complex amplitudes and probabilities. Consequently, we find that neither locality, nor non locality, are primary concepts, only causality exists at the fundamental level.
9 pages. Article companion to http://arxiv.org/abs/1307.6167

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan
(Submitted on 3 Aug 2013)
The dynamics of a homogeneous, anisotropic universe are investigated within the context of spinfoam cosmology. Transition amplitudes are calculated for a graph consisting of a single node and three links - the Daisy graph' - probing the behaviour a classical Bianchi I spacetime. It is shown further how the use of such single node graphs gives rise to a simplification of states such that all orders in the spin expansion can be calculated, indicating that it is the vertex expansion that contains information about quantum dynamics.
28 pages, 1 figure

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick
(Submitted on 31 Jul 2013)
It is well known that the SU(2)-gauge invariant phase space of loop gravity can be represented in terms of twisted geometries. These are piecewise-linear-flat geometries obtained by gluing together polyhedra, but the resulting geometries are not continuous across the faces. Here we show that this phase space can also be represented by continuous, piecewise-flat three-geometries called spinning geometries. These are composed of metric-flat three-cells glued together consistently. The geometry of each cell and the manner in which they are glued is compatible with the choice of fluxes and holonomies.
We first remark that the fluxes provide each edge with an angular momentum. By studying the piecewise-flat geometries which minimize edge lengths, we show that these angular momenta can be literally interpreted as the spin of the edges: the geometries of all edges are necessarily helices. We also show that the compatibility of the gluing maps with the holonomy data results in the same conclusion. This shows that a spinning geometry represents a way to glue together the three-cells of a twisted geometry to form a continuous geometry which represents a point in the loop gravity phase space.
20 pages, 5 figures

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel
(Submitted on 22 Jul 2013)
In a seminal paper, Kaminski, Kisielowski an Lewandowski for the first time extended the definition of spin foam models to arbitrary boundary graphs. This is a prerequisite in order to make contact to the canonical formulation of Loop Quantum Gravity (LQG) and allows to investigate the question whether any of the presently considered spin foam models yield a rigging map for any of the presently defined Hamiltonian constraint operators. The KKL extension cannot be described in terms of Group Field Theory (GFT) since arbitrary foams are involved while GFT is tied to simplicial complexes. Therefore one has to define the sum over spin foams with given boundary spin networks in an independent fashion using natural axioms, most importantly a gluing property for 2-complexes. These axioms are motivated by the requirement that spin foam amplitudes should define a rigging map (physical inner product) induced by the Hamiltonian constraint. This is achieved by constructing a spin foam operator based on abstract 2-complexes that acts on the kinematical Hilbert space of Loop Quantum Gravity. In the analysis of the resulting object we are able to identify an elementary spin foam transfer matrix that allows to generate any finite foam as a finite power of the transfer matrix. It transpires that the sum over spin foams, as written, does not define a projector on the physical Hilbert space. This statement is independent of the concrete spin foam model and Hamiltonian constraint. However, the transfer matrix potentially contains the necessary ingredient in order to construct a proper rigging map in terms of a modified transfer matrix.
62 pages, 14 figures

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll
(Submitted on 20 Jul 2013)
We present a detailed analysis of a recently introduced version of Causal Dynamical Triangulations (CDT) that does not rely on a distinguished time slicing. Focussing on the case of 2+1 spacetime dimensions, we analyze its geometric and causal properties, present details of the numerical set-up and explain how to extract "volume profiles". Extensive Monte Carlo measurements of the system show the emergence of a de Sitter universe on large scales from the underlying quantum ensemble, similar to what was observed previously in standard CDT quantum gravity. This provides evidence that the distinguished time slicing of the latter is not an essential part of its kinematical set-up.
44 pages, 29 figures

http://arxiv.org/abs/1307.5461
Quantum hyperbolic geometry in loop quantum gravity with cosmological constant
Maite Dupuis, Florian Girelli
(Submitted on 20 Jul 2013)
Loop Quantum Gravity (LQG) is an attempt to describe the quantum gravity regime. Introducing a non-zero cosmological constant Λ in this context has been a withstanding problem. Other approaches, such as Chern-Simons gravity, suggest that quantum groups can be used to introduce Λ in the game. Not much is known when defining LQG with a quantum group. Tensor operators can be used to construct observables in any type of discrete quantum gauge theory with a classical/quantum gauge group. We illustrate this by constructing explicitly geometric observables for LQG defined with a quantum group and show for the first time that they encode a quantized hyperbolic geometry. This is a novel argument pointing out the usefulness of quantum groups as encoding a non-zero cosmological constant. We conclude by discussing how tensor operators provide the right formalism to unlock the LQG formulation with a non-zero cosmological constant.
6 pages, 1 figure

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau
(Submitted on 19 Jul 2013)
This article addresses the issue of the closure of the algebra of constraints for generic (cosmological) perturbations when taking into account simultaneously the two main corrections of effective loop quantum cosmology, namely the holonomy and the inverse-volume terms. Previous works on either the holonomy or the inverse volume case are reviewed and generalized. In the inverse-volume case, we point out new possibilities. An anomaly-free solution including both corrections is found for perturbations, and the corresponding equations of motion are derived.
19 pages.

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer
(Submitted on 18 Jul 2013)
We propose a derivation for computing black hole entropy for spherical non-rotating isolated horizons from loop quantum gravity in four and higher dimensions. The state counting problem effectively reduces to the well studied 3+1-dimensional one based on an SU(2)-Chern-Simons theory, differing only in the precise form of the area spectrum and the restriction to integer spins.
5 pages

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto
(Submitted on 11 Jul 2013)
A simple argument indicates that covariant loop gravity (spinfoam theory) predicts a maximal acceleration, and hence forbids the development of curvature singularities. This supports the results obtained for cosmology and black holes using canonical methods.
4 pages, 1 figure

http://arxiv.org/abs/1307.2719
Deformations of Polyhedra and Polygons by the Unitary Group
Etera R. Livine
(Submitted on 10 Jul 2013)
We introduce the set of framed (convex) polyhedra with N faces as the symplectic quotient C2N //SU(2). A framed polyhedron is then parametrized by N spinors living in C2 satisfying suitable closure constraints and defines a usual convex polyhedron plus extra U(1) phases attached to each face. We show that there is a natural action of the unitary group U(N) on this phase space, which changes the shape of faces and allows to map any (framed) polyhedron onto any other with the same total (boundary) area. This identifies the space of framed polyhedra to the Grassmannian space U(N )/ (SU(2)×U(N −2)). We show how to write averages of geometrical observables (polynomials in the faces’ area and the angles between them) over the ensemble of polyhedra (distributed uniformly with respect to the Haar measure on U(N)) as polynomial integrals over the unitary group and we provide a few methods to compute these integrals systematically. We also use the Itzykson-Zuber formula from matrix models as the generating function for these averages and correlations.
In the quantum case, a canonical quantization of the framed polyhedron phase space leads to the Hilbert space of SU(2) intertwiners (or, in other words, SU(2)-invariant states in tensor products of irreducible representations). The total boundary area as well as the individual face areas are quantized as half-integers (spins), and the Hilbert spaces for fixed total area form irreducible representations of U(N). We define semi-classical coherent intertwiner states peaked on classical framed polyhedra and transforming consistently under U(N) transformations. And we show how the U(N) character formula for unitary transformations is to be considered as an extension of the Itzykson-Zuber to the quantum level and generates the traces of all polynomial observables over the Hilbert space of intertwiners.
We finally apply the same formalism to two dimensions and show that classical (convex) polygons can be described in a similar fashion trading the unitary group for the orthogonal group. We conclude with a discussion of the possible (deformation) dynamics that one can define on the space of polygons or polyhedra. This work is a priori useful in the context of discrete geometry but it should hopefully also be relevant to (loop) quantum gravity in 2+1 and 3+1 dimensions when the quantum geometry is defined in terms of gluing of (quantized) polygons and polyhedra.
33 pages

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski
(Submitted on 5 Jul 2013)
We study the asymptotic geometry of the spin foam partition function for a large class of models, including the models of Barrett and Crane, Engle, Pereira, Rovelli and Livine, and, Freidel and Krasnov.
The asymptotics is taken with respect to the boundary spins only, no assumption of large spins is made in the interior. We give a sufficient criterion for the existence of the partition function. We find that geometric boundary data is suppressed unless its interior continuation satisfies certain accidental curvature constraints. This means in particular that most Regge manifolds are suppressed in the asymptotic regime. We discuss this explicitly for the case of the configurations arising in the 3-3 Pachner move. We identify the origin of these accidental curvature constraints as an incorrect twisting of the face amplitude upon introduction of the Immirzi parameter and propose a way to resolve this problem, albeit at the price of losing the connection to the SU(2) boundary Hilbert space.
The key methodological innovation that enables these results is the introduction of the notion of wave front sets, and the adaptation of tools for their study from micro local analysis to the case of spin foam partition functions.
63 pages, 5 figures

Last edited: Aug 13, 2013
9. Aug 16, 2013

### marcus

Ten of us have voted so far. Thanks to Atyy, Chronos, Devils, Jason_0, Martinbn, Nonlinearity, Shablong, Skydive and most recent arrival Sigma057, for sharing your perspectives on the significance of QG current research!
From my own standpoint, seeing how we as a group collectively judge the papers definitely helps me refine and balance my own assessment.

Here's an updated tally.

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah

http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel

http://arxiv.org/abs/1305.2207
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman

http://arxiv.org/abs/1305.0822
On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski

Last edited: Aug 16, 2013
10. Aug 20, 2013

### marcus

Since the maximum number allowed in a poll is 20, to keep the size manageable it has been necessary to remove several interesting and potentially valuable papers from the tentative 3rd quarter list. For compactness the abstracts have been suppressed except in the case of recent additions. So in most cases only link, title, and authorship are shown.

http://arxiv.org/pdf/1308.4348
The Echo of the Quantum Bounce
Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
(Submitted on 20 Aug 2013)
We identify a signature of quantum gravitational effects that survives from the early universe to the current era: Fluctuations of quantum fields as seen by comoving observers are significantly influenced by the history of the early universe. In particular we will show how the existence (or not) of a quantum bounce leaves a trace in the background quantum noise that is not damped and would be non-negligible even nowadays. Furthermore, we will estimate an upper bound to the typical energy and length scales where quantum effects are relevant. We will discuss how this signature might be observed and therefore used to build falsifiability tests of quantum gravity theories.
5 pages, 3 figures

http://arxiv.org/abs/1308.4063
Covariant Loop Quantum Gravity, Low Energy Perturbation Theory, and Einstein Gravity
Muxin Han
(Submitted on 19 Aug 2013)
A low-energy perturbation theory is developed from the nonperturbative framework of covariant Loop Quantum Gravity (LQG) by employing the background field method. The resulting perturbation theory is a 2-parameter expansion in the semiclassical and low-energy regime. The two expansion parameters are the large spin and small curvature. The leading order effective action coincides with the Einstein-Hilbert action. The subleading corrections organized by the two expansion parameters give the modifications of Einstein gravity in quantum and high-energy regime from LQG. The result of the paper may be viewed as the first step toward understanding the UV completeness of LQG.
4 pages, 1 figure

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Chaharsough Shirazi, Jonathan Engle

http://arxiv.org/abs/1308.2934
The Fundamental Group of a Spatial Section Represented by a Topspin Network
Christopher L Duston

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll

http://arxiv.org/abs/1307.5461
Quantum hyperbolic geometry in loop quantum gravity with cosmological constant
Maite Dupuis, Florian Girelli

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski

Last edited: Aug 21, 2013
11. Aug 20, 2013

### atyy

Darn it, I was going to vote for Bodendorfer's black hole entropy paper. At least the spinning twisted paper is still on the shortlist:)

12. Aug 21, 2013

### marcus

Have you looked at the Shirazi Engle paper?
Could still edit, so I restored the BH entropy paper on your implied recommendation :)

13. Sep 4, 2013

### marcus

Eleven of us have voted in the 2nd quarter poll, so far: Atyy, Chronos, Devils, Ftr, Jason_0, Martinbn, Nonlinearity, Shablong, Sigma057, and Skydive. Thanks all!
Here is an update of the tentative 3rd quarter list. Except in the case of recent additions, only link, title, and authorship are shown. We have 18 on this tentative slate, and the maximum that can be in a poll is 20.

http://arxiv.org/abs/1309.0777
Coupling and thermal equilibrium in general-covariant systems
Goffredo Chirco, Hal M. Haggard, Carlo Rovelli
(Submitted on 3 Sep 2013)
A fully general-covariant formulation of statistical mechanics is still lacking. We take a step toward this theory by studying the meaning of statistical equilibrium for coupled, parametrized systems. We discuss how to couple parametrized systems. We express the thermalization hypothesis in a general-covariant context. This takes the form of vanishing of information flux. An interesting relation emerges between thermal equilibrium and gauge.
8 pages, 3 figures

http://arxiv.org/abs/1309.0311
Phenomenology of Space-time Imperfection I: Nonlocal Defects
Sabine Hossenfelder
(Submitted on 2 Sep 2013)
If space-time is emergent from a fundamentally non-geometric theory it will generically be left with defects. Such defects need not respect the locality that emerges with the background. Here, we develop a phenomenological model that parameterizes the effects of nonlocal defects on the propagation of particles. In this model, Lorentz-invariance is preserved on the average. We derive constraints on the density of defects from various experiments.
25 pages, 7 figures

http://arxiv.org/abs/1309.0314
Phenomenology of Space-time Imperfection II: Local Defects
Sabine Hossenfelder
(Submitted on 2 Sep 2013)
We propose a phenomenological model for the scattering of particles on space-time defects in a treatment that maintains Lorentz-invariance on the average. The local defects considered here cause a stochastic violation of momentum conservation. The scattering probability is parameterized in the density of defects and the distribution of the momentum that a particle can obtain when scattering on the defect. We identify the most promising observable consequences and derive constraints from existing data.
18 pages, 5 figures

http://arxiv.org/abs/1308.6289
Indistinguishability of thermal and quantum fluctuations
(Submitted on 28 Aug 2013)
The existence of Davies-Unruh temperature in a uniformly accelerated frame shows that quantum fluctuations of the inertial vacuum state appears as thermal fluctuations in the accelerated frame. Hence thermodynamic experiments cannot distinguish between phenomena occurring in a thermal bath of temperature T in the inertial frame from those in a frame accelerating through inertial vacuum with the acceleration a=2π T. We show that this indisguishability between quantum fluctuations and thermal fluctuations goes far beyond the fluctuations in the vacuum state. We show by an exact calculation, that the reduced density matrix for a uniformly accelerated observer when the quantum field is in a thermal state of temperature T' is symmetric between acceleration temperature T = a/(2π) and the thermal bath temperature T'. Thus thermal phenomena cannot distinguish whether (i) one is accelerating with a = 2π T through a bath of temperature T' or (ii) accelerating with a=2π T' through a bath of temperature T. This shows that thermal and quantum fluctuations in an accelerated frame affect the observer in a symmetric manner. The implications are discussed.
4 pages

http://arxiv.org/abs/1308.5599
Why Gauge?
Carlo Rovelli
(Submitted on 26 Aug 2013)
The world appears to be well described by gauge theories; why? I suggest that gauge is more than mathematical redundancy. Gauge variables describe handles though which systems couple. Gauge-dependent quantities can not be predicted, but there is a sense in which they can be measured. This observation leads to a physical interpretation for the ubiquity of gauge: it is a consequence of a relational structure of the physical quantities.
7 pages

http://arxiv.org/pdf/1308.4348
The Echo of the Quantum Bounce
Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez

http://arxiv.org/abs/1308.4063
Covariant Loop Quantum Gravity, Low Energy Perturbation Theory, and Einstein Gravity
Muxin Han

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Chaharsough Shirazi, Jonathan Engle

http://arxiv.org/abs/1308.2934
The Fundamental Group of a Spatial Section Represented by a Topspin Network
Christopher L Duston

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski

Last edited: Sep 4, 2013
14. Sep 18, 2013

### marcus

Twelve of us have voted in the 2nd quarter poll so far. Welcome to the most recent respondent, Mentz, and thanks to all for sharing your perspectives on the significance of QG current research! From my own standpoint, seeing how we as a group judge the papers definitely helps me refine and balance my own assessment.

Here's an updated tally. The paper by cosmologist George Ellis has moved up in our collective estimation, since last time, as has the Shape Dynamics paper by Tim Koslowski.

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah

http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel

http://arxiv.org/abs/1305.2207
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman

http://arxiv.org/abs/1305.0822
On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski

============
We have 19 tentative candidates on the slate for the 3rd quarter MIP poll. The software allows a maximum of 20 so we may face a difficult job of culling out some valuable/interesting research.
I'll list the tentative candidates---the abstract is shown only for the new addition. Thanks to Atyy for spotting this one.

]http://arxiv.org/abs/1309.4563
Statistics, holography, and black hole entropy in loop quantum gravity
Amit Ghosh, Karim Noui, Alejandro Perez
(Submitted on 18 Sep 2013)
In loop quantum gravity the quantum states of a black hole horizon are produced by point-like discrete quantum geometry excitations (or punctures) labelled by spin $j$. The excitations possibly carry other internal degrees of freedom also, and the associated quantum states are eigenstates of the area $A$ operator. On the other hand, the appropriately scaled area operator $A/(8\pi\ell)$ is also the physical Hamiltonian associated with the quasilocal stationary observers located at a small distance $\ell$ from the horizon. Thus, the local energy is entirely accounted for by the geometric operator $A$.
We assume that: In a suitable vacuum state with regular energy momentum tensor at and close to the horizon the local temperature measured by stationary observers is the Unruh temperature and the degeneracy of matter' states is exponential with the area $\exp{(\lambda A/\ell_p^2)}$---this is supported by the well established results of QFT in curved spacetimes, which do not determine $\lambda$ but asserts an exponential behaviour. The geometric excitations of the horizon (punctures) are indistinguishable. In the semiclassical limit the area of the black hole horizon is large in Planck units.
It follows that: Up to quantum corrections, matter degrees of freedom saturate the holographic bound, viz. $\lambda=\frac{1}{4}$. Up to quantum corrections, the statistical black hole entropy coincides with Bekenstein-Hawking entropy $S={A}/({4\ell_p^2})$. The number of horizon punctures goes like $N\propto \sqrt{A/\ell_p^2}$, i.e the number of punctures $N$ remains large in the semiclassical limit. Fluctuations of the horizon area are small while fluctuations of the area of an individual puncture are large. A precise notion of local conformal invariance of the thermal state is recovered in the $A\to\infty$ limit where the near horizon geometry becomes Rindler.

http://arxiv.org/abs/1309.0777
Coupling and thermal equilibrium in general-covariant systems
Goffredo Chirco, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1309.0311
Phenomenology of Space-time Imperfection I: Nonlocal Defects
Sabine Hossenfelder

http://arxiv.org/abs/1309.0314
Phenomenology of Space-time Imperfection II: Local Defects
Sabine Hossenfelder

http://arxiv.org/abs/1308.6289
Indistinguishability of thermal and quantum fluctuations

http://arxiv.org/abs/1308.5599
Why Gauge?
Carlo Rovelli

http://arxiv.org/pdf/1308.4348
The Echo of the Quantum Bounce
Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez

http://arxiv.org/abs/1308.4063
Covariant Loop Quantum Gravity, Low Energy Perturbation Theory, and Einstein Gravity
Muxin Han

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Shirazi, Jonathan Engle

http://arxiv.org/abs/1308.2934
The Fundamental Group of a Spatial Section Represented by a Topspin Network
Christopher L Duston

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski

Last edited: Sep 18, 2013
15. Sep 24, 2013

### marcus

We now have 20 tentative candidates on the slate for the 3rd quarter MIP poll, the maximum allowed by the software. An abstract is shown only for the new addition.

http://arxiv.org/abs/1309.6304
Quantum-Reduced Loop-Gravity: Relation with the Full Theory
Emanuele Alesci, Francesco Cianfrani, Carlo Rovelli
(Submitted on 24 Sep 2013)
The quantum-reduced loop-gravity technique has been introduced for dealing with cosmological models. We show that it can be applied rather generically: anytime the spatial metric can be gauge-fixed to a diagonal form. The technique selects states based on reduced graphs with Livine-Speziale coherent intertwiners and could simplify the analysis of the dynamics in the full theory.
5 pages

http://arxiv.org/abs/1309.4563
Statistics, holography, and black hole entropy in loop quantum gravity
Amit Ghosh, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1309.0777
Coupling and thermal equilibrium in general-covariant systems
Goffredo Chirco, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1309.0311
Phenomenology of Space-time Imperfection I: Nonlocal Defects
Sabine Hossenfelder

http://arxiv.org/abs/1309.0314
Phenomenology of Space-time Imperfection II: Local Defects
Sabine Hossenfelder

http://arxiv.org/abs/1308.6289
Indistinguishability of thermal and quantum fluctuations

http://arxiv.org/abs/1308.5599
Why Gauge?
Carlo Rovelli

http://arxiv.org/pdf/1308.4348
The Echo of the Quantum Bounce
Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez

http://arxiv.org/abs/1308.4063
Covariant Loop Quantum Gravity, Low Energy Perturbation Theory, and Einstein Gravity
Muxin Han

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Shirazi, Jonathan Engle

http://arxiv.org/abs/1308.2934
The Fundamental Group of a Spatial Section Represented by a Topspin Network
Christopher L Duston

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski

16. Sep 26, 2013

### marcus

Nearly the end of the 3rd quarter! We have 20 tentative candidates on the slate for this quarter's MIP poll, the maximum allowed. An abstract is shown only for the new addition.

http://arxiv.org/abs/1309.6896
Observational issues in loop quantum cosmology
A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
(Submitted on 26 Sep 2013)
Quantum gravity is sometimes considered as a kind of metaphysical speculation. In this review, we show that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop Quantum Gravity as one of the best candidate for a non-perturbative and background-independant quantization of gravity, we detail some expected features.
75 pages, invited topical review for Classical and Quantum Gravity

http://arxiv.org/abs/1309.6304
Quantum-Reduced Loop-Gravity: Relation with the Full Theory
Emanuele Alesci, Francesco Cianfrani, Carlo Rovelli

http://arxiv.org/abs/1309.4563
Statistics, holography, and black hole entropy in loop quantum gravity
Amit Ghosh, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1309.0777
Coupling and thermal equilibrium in general-covariant systems
Goffredo Chirco, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1309.0311
Phenomenology of Space-time Imperfection I: Nonlocal Defects
Sabine Hossenfelder

http://arxiv.org/abs/1309.0314
Phenomenology of Space-time Imperfection II: Local Defects
Sabine Hossenfelder

http://arxiv.org/abs/1308.6289
Indistinguishability of thermal and quantum fluctuations

http://arxiv.org/pdf/1308.4348
The Echo of the Quantum Bounce
Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez

http://arxiv.org/abs/1308.4063
Covariant Loop Quantum Gravity, Low Energy Perturbation Theory, and Einstein Gravity
Muxin Han

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Shirazi, Jonathan Engle

http://arxiv.org/abs/1308.2934
The Fundamental Group of a Spatial Section Represented by a Topspin Network
Christopher L Duston

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski

17. Sep 29, 2013

### marcus

Welcome Vasudevshyam, the latest participant in our the 2nd quarter poll! who joins Atyy, Chronos, Devils, Ftr, Jason_0, Martinbn, Mentz, Nonlinearity, Shablong, Sigma057, Skydive, and myself, making quite a spread of diverse opinion on the interesting directions in current QG research. Thanks all for sharing your perspectives.
Here's an updated tally. The paper on Shape Dynamics paper by Tim Koslowski is the current leader.

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah

http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter

http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel

http://arxiv.org/abs/1305.2207
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman

http://arxiv.org/abs/1305.0822
On the Origin of Gravitational Lorentz Covariance
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski

18. Sep 29, 2013

### marcus

There've been a lot of interesting papers this quarter, making it hard to keep the list for the 3rd quarter MIP poll down to 20, which is the maximum allowed. Abstracts are shown only for the two new additions to the slate.

http://arxiv.org/abs/1309.7296
Astrophysical constraints on Planck scale dissipative phenomena
Stefano Liberati (SISSA and INFN, Trieste), Luca Maccione (LMU and MPP, Munich)
(Submitted on 27 Sep 2013)
The emergence of a classical spacetime from any quantum gravity model is still a subtle and only partially understood issue. If indeed space-time is arising as some sort of large scale condensate of more fundamental objects then it is natural to expect that matter, being a collective excitations of the spacetime constituents, will present modified kinematics at sufficiently high energies. We consider here the phenomenology of the dissipative effects necessarily arising in such a picture. Adopting dissipative hydrodynamics as a general framework for the description of the energy exchange between collective excitations and the spacetime fundamental degrees of freedom, we discuss how rates of decays for elementary particles can be derived from dispersion relations and used to provide strong constraints on the base of current astrophysical observations of high energy particles.
5 pages, 1 figure

http://arxiv.org/abs/1309.7273
Renormalization group flow of Hořava-Lifshitz gravity at low energies
Adriano Contillo, Stefan Rechenberger, Frank Saueressig
(Submitted on 27 Sep 2013)
The functional renormalization group equation for projectable Horava-Lifshitz gravity is used to derive the non-perturbative beta functions for the Newton's constant, cosmological constant and anisotropy parameter. The resulting coupled differential equations are studied in detail and exemplary RG trajectories are constructed numerically. The beta functions possess a non-Gaussian fixed point and a one-parameter family of Gaussian fixed points. One of the Gaussian fixed points corresponds to the Einstein-Hilbert action with vanishing cosmological constant and constitutes a saddle point with one IR-attractive direction. For RG trajectories dragged into this fixed point at low energies diffeomorphism invariance is restored. The emergence of general relativity from Horava-Lifshitz gravity can thus be understood as a crossover-phenomenon where the IR behavior of the theory is controlled by this Gaussian fixed point. In particular RG trajectories with a tiny positive cosmological constant also come with an anisotropy parameter which is compatible with experimental constraints, providing a mechanism for the approximate restoration of diffeomorphism invariance in the IR. The non-Gaussian fixed point is UV-attractive in all three coupling constants. Most likely, this fixed point is the imprint of Asymptotic Safety at the level of Horava-Lifshitz gravity.
32 pages, 6 figures

http://arxiv.org/abs/1309.6896
Observational issues in loop quantum cosmology
A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
Invited topical review for Classical and Quantum Gravity

http://arxiv.org/abs/1309.6304
Quantum-Reduced Loop-Gravity: Relation with the Full Theory
Emanuele Alesci, Francesco Cianfrani, Carlo Rovelli

http://arxiv.org/abs/1309.4563
Statistics, holography, and black hole entropy in loop quantum gravity
Amit Ghosh, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1309.0777
Coupling and thermal equilibrium in general-covariant systems
Goffredo Chirco, Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1309.0311
Phenomenology of Space-time Imperfection I: Nonlocal Defects
Sabine Hossenfelder

http://arxiv.org/abs/1309.0314
Phenomenology of Space-time Imperfection II: Local Defects
Sabine Hossenfelder

http://arxiv.org/pdf/1308.4348
The Echo of the Quantum Bounce
Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez

http://arxiv.org/abs/1308.4063
Covariant Loop Quantum Gravity, Low Energy Perturbation Theory, and Einstein Gravity
Muxin Han

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Shirazi, Jonathan Engle

http://arxiv.org/abs/1308.2934
The Fundamental Group of a Spatial Section Represented by a Topspin Network
Christopher L Duston

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto

Last edited: Sep 29, 2013