# Our picks for first quarter 2009 MIP (most important QG paper)

## Which will prove most valuable for future research? (Select several if you wish.)

16.7%

16.7%

25.0%

8.3%

0 vote(s)
0.0%

8.3%

41.7%

8.3%

16.7%
10. ### Disordered Locality as an Explanation for the Dark Energy (Smolin et al)

16.7%
1. Mar 31, 2009

### marcus

The poll is set up to accept multiple choices. Check off the paper or papers you predict will have the most significant impact on future QG research. If you have one that is not on the list, post the arxiv link, title and author(s) on this thread, to be counted as a "write-in".

1. Loop quantum cosmology of Bianchi I models (Ashtekar et al)
2. Asymptotic analysis of the EPRL four-simplex amplitude (Barrett et al)
3. Taming perturbative divergences in asymptotically safe gravity (Benedetti et al)
4. Particle Topology, Braids, and Braided Belts (Bilson-Thompson et al)
5. From lattice BF gauge theory to area-angle Regge calculus (Bonzom)
6. Quantum geometry from phase space reduction (Freidel et al)
7. Quantum Gravity at a Lifgarbagez Point (Horava)
8. 4d Deformed Special Relativity from Group Field Theories (Livine et al)
9. Group field theory and simplicial quantum gravity (Oriti)
10.Disordered Locality as an Explanation for the Dark Energy (Smolin et al)

Our most recent previous MIP poll is here:

Ashtekar et al
http://arxiv.org/abs/0903.3397
Loop quantum cosmology of Bianchi I models
Abhay Ashtekar, Edward Wilson-Ewing
(Submitted on 19 Mar 2009)
"The 'improved dynamics' of loop quantum cosmology is extended to include anisotropies of the Bianchi I model. As in the isotropic case, a massless scalar field serves as a relational time parameter. However, the extension is non-trivial because one has to face several conceptual subtleties as well as technical difficulties. These include: a better understanding of the relation between loop quantum gravity (LQG) and loop quantum cosmology (LQC); handling novel features associated with the non-local field strength operator in presence of anisotropies; and finding dynamical variables that make the action of the Hamiltonian constraint manageable. Our analysis provides a conceptually complete description that overcomes limitations of earlier works. We again find that the big bang singularity is resolved by quantum geometry effects but, because of the presence of Weyl curvature, Planck scale physics is now much richer than in the isotropic case. Since the Bianchi I models play a key role in the Belinskii, Khalatnikov, Lifgarbagez (BKL) conjecture on the nature of generic space-like singularities in general relativity, the quantum dynamics of Bianchi I cosmologies is likely to provide considerable intuition about the fate of generic space-like singularities in quantum gravity. Finally, we show that the quantum dynamics of Bianchi I cosmologies projects down exactly to that of the Friedmann model. This opens a new avenue to relate more complicated models to simpler ones, thereby providing a new tool to relate the quantum dynamics of LQG to that of LQC."

Barrett et al
http://arxiv.org/abs/0902.1170
Asymptotic analysis of the EPRL four-simplex amplitude
John W. Barrett, Richard J. Dowdall, Winston J. Fairbairn, Henrique Gomes, Frank Hellmann
"An asymptotic formula for a certain 4d Euclidean spin foam 4-simplex amplitude is given for the limit of large spins. The analysis covers the model with Immirzi parameter less than one defined separately by Engle, Livine, Pereira and Rovelli (EPRL) and Freidel and Krasnov (FK). We are also able to analyse the EPRL model with Immirzi parameter greater than one. The asymptotic formula has one term which is proportional to the cosine of the Regge action for gravity, and it is shown that this term is present whenever the boundary data determines a non-degenerate Euclidean geometry for the 4-simplex. A scheme for resolving the phase ambiguity of the boundary data in these cases is also presented."

Benedetti et al
http://arxiv.org/abs/0902.4630
Taming perturbative divergences in asymptotically safe gravity
Dario Benedetti, Pedro F. Machado, Frank Saueressig
16 pages
(Submitted on 26 Feb 2009)
"We use functional renormalization group methods to study gravity minimally coupled to a free scalar field. This setup provides the prototype of a gravitational theory which is perturbatively non-renormalizable at one-loop level, but may possess a non-trivial renormalization group fixed point controlling its UV behavior. We show that such a fixed point indeed exists within the truncations considered, lending strong support to the conjectured asymptotic safety of the theory. In particular, we demonstrate that the counterterms responsible for its perturbative non-renormalizability have no qualitative effect on this feature."

Bilson-Thompson et al
http://arxiv.org/abs/0903.1376
Particle Topology, Braids, and Braided Belts
Sundance Bilson-Thompson, Jonathan Hackett, Louis H. Kauffman
21 pages, 16 figures
(Submitted on 7 Mar 2009)
"Recent work suggests that topological features of certain quantum gravity theories can be interpreted as particles, matching the known fermions and bosons of the first generation in the Standard Model. This is achieved by identifying topological structures with elements of the framed Artin braid group on three strands, and demonstrating a correspondence between the invariants used to characterise these braids (a braid is a set of non-intersecting curves, that connect one set of N points with another set of N points), and quantities like electric charge, colour charge, and so on. In this paper we show how to manipulate a modified form of framed braids to yield an invariant standard form for sets of isomorphic braids, characterised by a vector of real numbers. This will serve as a basis for more complete discussions of quantum numbers in future work."

Bonzom
http://arxiv.org/abs/0903.0267
From lattice BF gauge theory to area-angle Regge calculus
Valentin Bonzom
18 pages, 2 figures
"We consider Riemannian 4d BF lattice gauge theory, on a triangulation of spacetime. Introducing the simplicity constraints which turn BF theory into simplicial gravity, some geometric quantities of Regge calculus, areas, and 3d and 4d dihedral angles, are identified. The parallel transport conditions are taken care of to ensure a consistent gluing of simplices. We show that these gluing relations, together with the simplicity constraints, contain the constraints of area-angle Regge calculus in a simple way, via the group structure of the underlying BF gauge theory. This provides a precise road from constrained BF theory to area-angle Regge calculus. Doing so, a framework combining variables of lattice BF theory and Regge calculus is built. The action takes a form à la Regge and includes the contribution of the Immirzi parameter. In the absence of simplicity constraints, the standard spin foam model for BF theory is recovered. Insertions of local observables are investigated, leading to Casimir insertions for areas and 6j-symbols for 3d angles. The present formulation is argued to be suitable for deriving spin foam models from discrete path integrals."

Freidel et al
http://arxiv.org/abs/0902.0351
Quantum geometry from phase space reduction
31 pages, 1 figure
"In this work we give an explicit isomorphism between the usual spin network basis and the direct quantization of the reduced phase space of tetrahedra. The main outcome is a formula that describes the space of SU(2) invariant states by an integral over coherent states satisfying the closure constraint exactly, or equivalently, as an integral over the space of classical tetrahedra. This provides an explicit realization of theorems by Guillemin--Sternberg and Hall that describe the commutation of quantization and reduction. In the final part of the paper, we use our result to express the FK spin foam model as an integral over classical tetrahedra and the asymptotics of the vertex amplitude is determined."

Horava
http://arxiv.org/abs/0901.3775
Quantum Gravity at a Lifgarbagez Point
Petr Horava
29 pages
"We present a candidate quantum field theory of gravity with dynamical critical exponent equal to z=3 in the UV. (As in condensed matter systems, z measures the degree of anisotropy between space and time.) This theory, which at short distances describes interacting nonrelativistic gravitons, is power-counting renormalizable in 3+1 dimensions. When restricted to satisfy the condition of detailed balance, this theory is intimately related to topologically massive gravity in three dimensions, and the geometry of the Cotton tensor. At long distances, this theory flows naturally to the relativistic value z=1, and could therefore serve as a possible candidate for a UV completion of Einstein's general relativity or an infrared modification thereof. The effective speed of light, the Newton constant and the cosmological constant all emerge from relevant deformations of the deeply nonrelativistic z=3 theory at short distances."

Livine et al
http://arxiv.org/abs/0903.3475
4d Deformed Special Relativity from Group Field Theories
Florian Girelli, Etera R. Livine, Daniele Oriti
23 pages
(Submitted on 20 Mar 2009)
"We derive a scalar field theory of the deformed special relativity type, living on non-commutative kappa-Minkowski spacetime and with a kappa-deformed Poincare symmetry, from the SO(4,1) group field theory defining the transition amplitudes for topological BF-theory in 4 space-time dimensions. This is done at a non-perturbative level of the spin foam formalism working directly with the group field theory (GFT). We show that matter fields emerge from the fundamental model as perturbations around a specific phase of the GFT, corresponding to a solution of the fundamental equations of motion, and that the non-commutative field theory governs their effective dynamics."

Oriti
http://arxiv.org/abs/0902.3903
Group field theory and simplicial quantum gravity
Daniele Oriti
(Submitted on 23 Feb 2009)
"We present a new Group Field Theory for 4d quantum gravity. It incorporates the constraints that give gravity from BF theory, and has quantum amplitudes with the explicit form of simplicial path integrals for 1st order gravity. The geometric interpretation of the variables and of the contributions to the quantum amplitudes is manifest. This allows a direct link with other simplicial gravity approaches, like quantum Regge calculus, in the form of the amplitudes of the model, and dynamical triangulations, which we show to correspond to a simple restriction of the same."

Smolin et al
http://arxiv.org/abs/0903.5303
Disordered Locality as an Explanation for the Dark Energy
Chanda Prescod-Weinstein, Lee Smolin
12 pages
(Submitted on 30 Mar 2009)
"We discuss a novel explanation of the dark energy as a manifestation of macroscopic non-locality coming from quantum gravity, as proposed by Markopoulou. It has been previously suggested that in a transition from an early quantum geometric phase of the universe to a low temperature phase characterized by an emergent spacetime metric, locality might have been 'disordered'. This means that there is a mismatch of micro-locality, as determined by the microscopic quantum dynamics and macro-locality as determined by the classical metric that governs the emergent low energy physics. In this paper we discuss the consequences for cosmology by studying a simple extension of the standard cosmological models with disordered locality. We show that the consequences can include a naturally small vacuum energy."

Last edited: Mar 31, 2009
2. Mar 31, 2009

### MTd2

3. Mar 31, 2009

### MTd2

I think this article has stronger gives further physical foundations for Horava. I enjoyed it a lot.

http://arxiv.org/abs/0902.0590

Also, I didn't like as much most of the other articles because they have a little contact with reality.

Last edited: Apr 1, 2009
4. Apr 3, 2009

### marcus

Thanks to everyone who has responded so far! Last time I looked we have five different people's ideas showing at this point.

Eventually we will want to check citation counts so I've put "cits" links. At this point it's a bit early to be looking at cites but five of the paper have already been cited---(a couple of them repeatedly: 3-5 times.)

Ashtekar et al
http://arxiv.org/abs/0903.3397
http://arxiv.org/cits/0903.3397
Loop quantum cosmology of Bianchi I models

Barrett et al
http://arxiv.org/abs/0902.1170
http://arxiv.org/cits/0902.1170
Asymptotic analysis of the EPRL four-simplex amplitude

Benedetti et al
http://arxiv.org/abs/0902.4630
http://arxiv.org/cits/0902.4630
Taming perturbative divergences in asymptotically safe gravity

Bilson-Thompson et al
http://arxiv.org/abs/0903.1376
http://arxiv.org/cits/0903.1376
Particle Topology, Braids, and Braided Belts

Bonzom
http://arxiv.org/abs/0903.0267
http://arxiv.org/cits/0903.0267
From lattice BF gauge theory to area-angle Regge calculus

Freidel et al
http://arxiv.org/abs/0902.0351
http://arxiv.org/cits/0902.0351
Quantum geometry from phase space reduction

Horava
http://arxiv.org/abs/0901.3775
http://arxiv.org/cits/0901.3775
Quantum Gravity at a Lifgarbagez Point

Livine et al
http://arxiv.org/abs/0903.3475
http://arxiv.org/cits/0903.3475
4d Deformed Special Relativity from Group Field Theories

Oriti
http://arxiv.org/abs/0902.3903
http://arxiv.org/cits/0902.3903
Group field theory and simplicial quantum gravity

Smolin et al
http://arxiv.org/abs/0903.5303
http://arxiv.org/cits/0903.5303
Disordered Locality as an Explanation for the Dark Energy

Last edited: Apr 3, 2009
5. Apr 18, 2009

### tom.stoer

I checked "Disordered Locality as an Explanation for the Dark Energy".

Smolin explains that the basic idea applies to most quantum gravity theories with underlying discrete structure. I hope that he found some alternative for the dark energy, something that is already "there" without inventing phantom energy and other star trek stuff.

6. Apr 20, 2009

### marcus

I share your high regard for that paper. I wish that it would capture researcher's attention and spark some follow-up investigation. It would be absolutely outstanding if dark energy turned out to be something that was already there---for instance something already in the quantum geometry. Like disordered locality.

For me, this kind of prediction poll is a chance not necessarily to say what I want or hope will happen but to test my perspective on reality by forecasting what actually will happen in future research. Which papers that have appeared recently are likely to have the greatest impact?

There is no single correct way to think about a poll like this. Someone could just as well see it as a chance to identify what seems to be the boldest and most original work. Perhaps that is even the best way to look at it.

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

In any case, thanks to all who have registered their choices so far!
There are eight:
Christine Dantas
Francesca
Atyy
Dipstik
Marcus
MTd2
Tom Stoer
Vicnice

7. May 12, 2009

### Demystifier

Horava - 45 citations
All other papers on the list together - 18 citations

I think no further comments are needed.

8. May 12, 2009

### MTd2

The guy is an important string theorist so if he writes it is ok to people to persue an apparently non stringy theory, people will go after that path. After all, there is a lot negative feelings against string theory, despite of being right or wrong.

9. May 12, 2009

### Demystifier

Sure, but I am convinced that this is not the main reason why this approach is receiving such a considerable attention.
After all, Witten also recently had some non-stringy publications, but they did not received such an attention.

10. May 12, 2009

### MTd2

Did any of them actualy promoted alternatives to string theory?

11. May 12, 2009

### MTd2

I am not taking away the merits of his theory, no, not at all, just telling that part of the success it is that a possible alternative theory to string theory is being promoted by one of the most important string theorists of the Universe. Note that most of the citations comes from asian people and many of them will attend Loops 2009 (Marcus told us that somewhere) which will happen in Beijing.

12. May 12, 2009

### Demystifier

I don't think that Horava gravity and LQG are alternatives to string theory. This is because string theory is MUCH MORE than a quantum theory of gravity. No other known theory has such a power of unification as string theory does. If the only goal was to have a consistent theory of quantum gravity, then string theory would not be so interesting.

The power of Horava gravity and LQG lies in their much more modest ambition.

Last edited: May 12, 2009
13. May 12, 2009

### MTd2

I will go with the usual mantra: "That means nothing without the experiments!" ;) Besides, mathematics got "for free" the hard work of thousands of physicists because of some casual coicindence, in which many aspects of strings are equivalent to the same of tools used by mathematicians. This is why string theory is so interesting. For me, think strings (but not string theory) must not be taken for granted by any respectable theory. It has already yielded many powerful mathematical tools.

Anyway, all the papers in the poll refers to quantum gravity, so I was just refering to this alternative aspect. It's less ambitious, but I don't really know if any ambition is meaninful given that we still have a long way until the planck scale.

Last edited: May 12, 2009
14. May 25, 2009

### marcus

Thanks to all who have responded so far. We are twelve, I think.

Francesca
Christine
Atyy
Demystifier
Dipstik
Finbar
MTd2
Philm2004
Sokrates
Tom.Stoer
Vicnice
and myself.

There is now just a month to go before we should have a list of QG papers from the 2nd quarter (April-June) to evaluate and predict how they will do.

Here are some possibilities. As usual we will probably start with too many and gradually narrow it down.

Freidel, Gurau, Oriti http://arxiv.org/abs/0905.3772
Freidel, Krasnov, Livine http://arxiv.org/abs/0905.3627
Barbero, Lewandowski, Villaseñor http://arxiv.org/abs/0905.3465
Engle, Noui, Perez http://arxiv.org/abs/0905.3168
Moderato, Prémont-Schwarz (blackhole paper) http://arxiv.org/abs/0905.3170
Bahr, Dittrich http://arxiv.org/abs/0905.1670
Moderato (fractal dimensionality) http://arxiv.org/abs/0905.1665
Bonzom http://arxiv.org/abs/0905.1501
Smolin http://arxiv.org/abs/0904.4841

Group field theory renormalization - the 3d case: power counting of divergences
Laurent Freidel, Razvan Gurau, Daniele Oriti
(Submitted on 22 May 2009)
Abstract: We take the first steps in a systematic study of Group Field Theory renormalization, focusing on the Boulatov model for 3D quantum gravity. We define an algorithm for constructing the 2D triangulations that characterize the boundary of the 3D bubbles, where divergences are located, of an arbitrary 3D GFT Feynman diagram. We then identify a special class of graphs for which a complete contraction procedure is possible, and prove, for these, a complete power counting. These results represent important progress towards understanding the origin of the continuum and manifold-like appearance of quantum spacetime at low energies, and of its topology, in a GFT framework.

Holomorphic Factorization for a Quantum Tetrahedron
Laurent Freidel, Kirill Krasnov, Etera R. Livine
(Submitted on 22 May 2009)
Abstract: We provide a holomorphic description of the Hilbert space H(j_1,..,j_n) of SU(2)-invariant tensors (intertwiners) and establish a holomorphically factorized formula for the decomposition of identity in H(j_1,..,j_n). Interestingly, the integration kernel that appears in the decomposition formula turns out to be the n-point function of bulk/boundary dualities of string theory. Our results provide a new interpretation for this quantity as being, in the limit of large conformal dimensions, the exponential of the Kahler potential of the symplectic manifold whose quantization gives H(j_1,..,j_n). For the case n=4, the symplectic manifold in question has the interpretation of the space of "shapes" of a geometric tetrahedron with fixed face areas, and our results provide a description for the quantum tetrahedron in terms of holomorphic coherent states. We describe how the holomorphic intertwiners are related to the usual real ones by computing their overlap. The semi-classical analysis of these overlap coefficients in the case of large spins allows us to obtain an explicit relation between the real and holomorphic description of the space of shapes of the tetrahedron. Our results are of direct relevance for the subjects of loop quantum gravity and spin foams, but also add an interesting new twist to the story of the bulk/boundary correspondence.

Flux-area operator and black hole entropy
J. Fernando Barbero G., Jerzy Lewandowski, Eduardo J. S. Villaseñor
(Submitted on 21 May 2009)
We show that, for space-times with inner boundaries, there exists a natural area operator different from the standard one used in loop quantum gravity. This new flux-area operator has equidistant eigenvalues. We discuss the consequences of substituting the standard area operator in the Ashtekar-Baez-Corichi-Krasnov definition of black hole entropy by the new one. Our choice simplifies the definition of the entropy and allows us to consider only those areas that coincide with the one defined by the value of the level of the Chern-Simons theory describing the horizon degrees of freedom. We give a prescription to count the number of relevant horizon states by using spin components and obtain exact expressions for the black hole entropy. Finally we derive its asymptotic behavior, discuss several issues related to the compatibility of our results with the Bekenstein-Hawking area law and the relation with Schwarzschild quasi-normal modes.

Black hole entropy and SU(2) Chern-Simons theory
Jonathan Engle, Karim Noui, Alejandro Perez
4 pages, 1 figure
(Submitted on 19 May 2009)
"We show that the isolated horizon boundary condition can be treated in a manifestly SU(2) invariant manner. The symplectic structure of gravity with the isolated horizon boundary condition has an SU(2) Chern-Simons symplectic structure contribution at the horizon with level $$k=a_H/ (4\pi \beta \ell^2_p)$$. Upon quantization, state counting is expressed in terms of the dimension of Chern-Simons Hilbert spaces on a sphere with marked points (defects). In the large black hole limit quantum horizon degrees of freedom can be modelled by a single intertwiner. The coupling constant of the defects with the Chern Simons theory on the horizon is precisely given by the ratio of the area contribution of the defect to the macroscopic area $$a_H$$, namely $$\lambda= 16\pi^2 \beta \ell^2_p (j(j+1))^{1/2}/a_H$$."

Self-dual Black Holes in LQG: Theory and Phenomenology
Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 20 May 2009)
In this paper we have recalled the semiclassical metric obtained from a classical analysis of the loop quantum black hole (LQBH). We show that the regular Reissner-Nordstrom-like metric is self-dual in the sense of T-duality: the form of the metric obtained in Loop quantum Gravity (LQG) is invariant under the exchange "r <-> a0/r" where "a0" is proportional to the minimum area in LQG and "r" is the standard Schwarzschild radial coordinate at asymptotic infinity. Of particular interest, the symmetry imposes that if an observer at "r" close to infinity sees a black hole of mass "m" an observer in the other asymptotic infinity beyond the horizon (at "r" close to "0") sees a dual mass "mp/m" ("mp" is the Planck mass). We then show that small LQBH are stable and could be a component of dark matter. Ultra-light LQBHs created shortly after the Big Bang would now have a mass of approximately "10-5 mp" and emit radiation with a typical energy of about 1013 - 1014 eV but they would also emit cosmic rays of much higher energies, albeit few of them. If these small LQBHs form a majority of the dark matter of the Milky Way's Halo, the production rate of ultra-high-energy-cosmic-rays (UHECR) by these ultra light black holes would be compatible with the observed rate of the Auger detector.

(Broken) Gauge Symmetries and Constraints in Regge Calculus
Benjamin Bahr, Bianca Dittrich
32 pages, 15 figures
(Submitted on 11 May 2009)
"We will examine the issue of diffeomorphism symmetry in simplicial models of (quantum) gravity, in particular for Regge calculus. We find that for a solution with curvature there do not exist exact gauge symmetries on the discrete level. Furthermore we derive a canonical formulation that exactly matches the dynamics and hence symmetries of the covariant picture. In this canonical formulation broken symmetries lead to the replacements of constraints by so--called pseudo constraints. These considerations should be taken into account in attempts to connect spin foam models, based on the Regge action, with canonical loop quantum gravity, which aims at implementing proper constraints. We will argue that the long standing problem of finding a consistent constraint algebra for discretized gravity theories is equivalent to the problem of finding an action with exact diffeomorphism symmetries. Finally we will analyze different limits in which the pseudo constraints might turn into proper constraints. This could be helpful to infer alternative discretization schemes in which the symmetries are not broken."

Fractal Quantum Space-Time
Authors: Leonardo Modesto
(Submitted on 11 May 2009)
Abstract: In this paper we calculated the spectral dimension of loop quantum gravity (LQG) using the scaling property of the area operator spectrum on spin-network states and using the scaling property of the volume and length operators on Gaussian states. We obtained that the spectral dimension of the spatial section runs from 1.5 to 3, and under particular assumptions from 2 to 3 across a 1.5 phase when the energy of a probe scalar field decreases from high to low energy in a fictitious time T. We calculated also the spectral dimension of space-time using the scaling of the area spectrum operator calculated on spin-foam models. The main result is that the effective dimension is 2 at the Planck scale and 4 at low energy. This result is consistent with two other approaches to non perturbative quantum gravity: "causal dynamical triangulation" and "asymptotically safe quantum gravity". We studied the scaling properties of all the possible curvature invariants and we have shown that the singularity problem seems to be solved in the covariant formulation of quantum gravity in terms of spin-foam models. For a particular form of the scaling (or for a particular area operator spectrum) all the curvature invariants are regular also in the Trans-Planckian regime.

Spin foam models for quantum gravity from lattice path integrals
Valentin Bonzom
19 pages, 1 figure
(Submitted on 10 May 2009)
"Spin foam models for quantum gravity are derived from lattice path integrals. The setting involves variables from both lattice BF theory and Regge calculus. The action consists in a Regge action, which depends on areas, dihedral angles and includes the Immirzi parameter. In addition, a measure is inserted to ensure a consistent gluing of simplices, so that the amplitude is dominated by configurations which satisfy the parallel transport relations. We explicitly compute the path integral as a sum over spin foams for a generic measure. The Freidel-Krasnov and Engle-Pereira-Rovelli models correspond to a special choice of gluing. In this case, the equations of motion describe genuine geometries, where the constraints of area-angle Regge calculus are satisfied. Furthermore, the Immirzi parameter drops out of the on-shell action, and stationarity with respect to area variations requires spacetime geometry to be flat."

The quantization of unimodular gravity and the cosmological constant problem
Lee Smolin
22 pages
(Submitted on 30 Apr 2009)
"A quantization of unimodular gravity is described, which results in a quantum effective action which is also unimodular, ie a function of a metric with fixed determinant. A consequence is that contributions to the energy momentum tensor of the form of the metric times a spacetime constant, whether classical or quantum, are not sources of curvature in the equations of motion derived from the quantum effective action. This solves the first cosmological constant problem, which is suppressing the enormous contributions to the cosmological constant coming from quantum corrections. We discuss several forms of uniodular gravity and put two of them, including one proposed by Henneaux and Teitelboim, in constrained Hamiltonian form. The path integral is constructed from the latter. Furthermore, the second cosmological constant problem, which is why the measured value is so small, is also addressed by this theory. We argue that a mechanism first proposed by Ng and van Dam for suppressing the cosmological constant by quantum effects obtains at the semiclassical level."

Last edited: May 25, 2009
15. May 25, 2009

### MTd2

I completely agree with the choices and those are precisely what I would have chosen. And I guess this is the toughest quarter of all to choose the best. But if I have to choose the best author so far, that would be Leonardo Modesto!

16. May 31, 2009

### marcus

Soon we should have a list of QG papers from the 2nd quarter (April-June) to evaluate and predict how they will do. As usual we will probably start with too many and gradually narrow the list down.

http://arxiv.org/abs/0905.4916
Black holes in full quantum gravity
Kirill Krasnov, Carlo Rovelli
(Submitted on 29 May 2009)
Quantum black holes have been studied extensively in quantum gravity and string theory, using various semiclassical or background dependent approaches. We explore the possibility of studying black holes in the full non-perturbative quantum theory, without recurring to semiclassical considerations, and in the context of loop quantum gravity. We propose a definition of a quantum black hole as the collection of the quantum degrees of freedom that do not influence observables at infinity. From this definition, it follows that for an observer at infinity a black hole is described by an SU(2) intertwining operator. The dimension of the Hilbert space of such intertwiners grows exponentially with the horizon area. These considerations shed some light on the physical nature of the microstates contributing to the black hole entropy. In particular, it can be seen that the microstates being counted for the entropy have the interpretation of describing different horizon shapes. The space of black hole microstates described here is related to the one arrived at recently by Engle, Noui and Perez, and sometime ago by Smolin, but obtained here directly within the full quantum theory.

http://arxiv.org/abs/0905.4949
A geometric perspective on singularity resolution and uniqueness in loop quantum cosmology
Alejandro Corichi, Parampreet Singh
(Submitted on 29 May 2009)
We re-examine the issue of singularity resolution in homogeneous loop quantum cosmology from the perspective of geometrical entities such as expansion rate and the shear scalar. These quantities are very reliable measures of the properties of spacetime and can be defined not only at the classical and effective level, but also at an operator level in the quantum theory. From the spectrum of the corresponding operators and their behavior in the effective loop quantum spacetime, we show that one can severely restrict the ambiguities in regularization of the quantum constraint and rule out unphysical choices. We analyze this in the flat isotropic model and the Bianchi-I spacetimes. In the former case we show that the expansion rate operator has a bounded spectrum only for the so called improved quantization, a result which synergizes with uniqueness of this quantization as proved earlier. For the Bianchi-I spacetime, we show that out of the available choices, the expansion rate and shear operator are bounded for only one regularization of the quantum constraint. It turns out only this choice has a well defined quantum gravity scale.

http://arxiv.org/abs/0905.4402
The Montevideo interpretation of quantum mechanics: frequently asked questions
Rodolfo Gambini, Jorge Pullin
(Submitted on 27 May 2009)
Abstract: In a series of recent papers we have introduced a new interpretation of quantum mechanics, which for brevity we will call the Montevideo interpretation. In it, the quantum to classical transition is achieved via a phenomenon called "undecidability" which stems from environmental decoherence supplemented with a fundamental mechanism of loss of coherence due to gravity. Due to the fact that the interpretation grew from several results that are dispersed in the literature, we put together this straightforward-to-read article addressing some of the main points that may confuse readers.

http://arxiv.org/abs/0905.4222
Undecidability and the problem of outcomes in quantum measurements
Rodolfo Gambini, Luis Pedro Garcia Pintos, Jorge Pullin
(Submitted on 26 May 2009)
Abstract: We argue that it is fundamentally impossible to recover information about quantum superpositions when a system has interacted with a sufficiently large number of degrees of freedom of the environment. This is due to the fact that gravity imposes fundamental limitations on how accurate measurements can be. This leads to the notion of undecidability: there is no way to tell, due to fundamental limitations, if a quantum system evolved unitarily or suffered wavefunction collapse. This in turn provides a solution to the problem of outcomes in quantum measurement by providing a sharp criterion for defining when an event has taken place. We analyze in detail in examples two situations in which in principle one could recover information about quantum coherence: a) "revivals" of coherence in the interaction of a system with the environment and b) the measurement of global observables of the system plus apparatus plus environment. We show in the examples that the fundamental limitations due to gravity and quantum mechanics in measurement prevent both revivals from occurring and the measurement of global observables. It can therefore be argued that the emerging picture provides a complete resolution to the measurement problem in quantum mechanics.

http://arxiv.org/abs/0905.4082
LQG propagator from the new spin foams
Eugenio Bianchi, Elena Magliaro, Claudio Perini
(Submitted on 25 May 2009)
Abstract: We compute metric correlations in loop quantum gravity with the dynamics defined by the new spin foam models. The analysis is done at the lowest order in a vertex expansion and at the leading order in a large spin expansion. The result is compared to the graviton propagator of perturbative quantum gravity.

http://arxiv.org/abs/0905.3772
Group field theory renormalization - the 3d case: power counting of divergences
Laurent Freidel, Razvan Gurau, Daniele Oriti
(Submitted on 22 May 2009)
Abstract: We take the first steps in a systematic study of Group Field Theory renormalization, focusing on the Boulatov model for 3D quantum gravity. We define an algorithm for constructing the 2D triangulations that characterize the boundary of the 3D bubbles, where divergences are located, of an arbitrary 3D GFT Feynman diagram. We then identify a special class of graphs for which a complete contraction procedure is possible, and prove, for these, a complete power counting. These results represent important progress towards understanding the origin of the continuum and manifold-like appearance of quantum spacetime at low energies, and of its topology, in a GFT framework.

http://arxiv.org/abs/0905.3627
Holomorphic Factorization for a Quantum Tetrahedron
Laurent Freidel, Kirill Krasnov, Etera R. Livine
(Submitted on 22 May 2009)
Abstract: We provide a holomorphic description of the Hilbert space H(j_1,..,j_n) of SU(2)-invariant tensors (intertwiners) and establish a holomorphically factorized formula for the decomposition of identity in H(j_1,..,j_n). Interestingly, the integration kernel that appears in the decomposition formula turns out to be the n-point function of bulk/boundary dualities of string theory. Our results provide a new interpretation for this quantity as being, in the limit of large conformal dimensions, the exponential of the Kahler potential of the symplectic manifold whose quantization gives H(j_1,..,j_n). For the case n=4, the symplectic manifold in question has the interpretation of the space of "shapes" of a geometric tetrahedron with fixed face areas, and our results provide a description for the quantum tetrahedron in terms of holomorphic coherent states. We describe how the holomorphic intertwiners are related to the usual real ones by computing their overlap. The semi-classical analysis of these overlap coefficients in the case of large spins allows us to obtain an explicit relation between the real and holomorphic description of the space of shapes of the tetrahedron. Our results are of direct relevance for the subjects of loop quantum gravity and spin foams, but also add an interesting new twist to the story of the bulk/boundary correspondence.

http://arxiv.org/abs/0905.3465
Flux-area operator and black hole entropy
J. Fernando Barbero G., Jerzy Lewandowski, Eduardo J. S. Villaseñor
(Submitted on 21 May 2009)
We show that, for space-times with inner boundaries, there exists a natural area operator different from the standard one used in loop quantum gravity. This new flux-area operator has equidistant eigenvalues. We discuss the consequences of substituting the standard area operator in the Ashtekar-Baez-Corichi-Krasnov definition of black hole entropy by the new one. Our choice simplifies the definition of the entropy and allows us to consider only those areas that coincide with the one defined by the value of the level of the Chern-Simons theory describing the horizon degrees of freedom. We give a prescription to count the number of relevant horizon states by using spin components and obtain exact expressions for the black hole entropy. Finally we derive its asymptotic behavior, discuss several issues related to the compatibility of our results with the Bekenstein-Hawking area law and the relation with Schwarzschild quasi-normal modes.

http://arxiv.org/abs/0905.3168
Black hole entropy and SU(2) Chern-Simons theory
Jonathan Engle, Karim Noui, Alejandro Perez
4 pages, 1 figure
(Submitted on 19 May 2009)
"We show that the isolated horizon boundary condition can be treated in a manifestly SU(2) invariant manner. The symplectic structure of gravity with the isolated horizon boundary condition has an SU(2) Chern-Simons symplectic structure contribution at the horizon with level $$k=a_H/ (4\pi \beta \ell^2_p)$$. Upon quantization, state counting is expressed in terms of the dimension of Chern-Simons Hilbert spaces on a sphere with marked points (defects). In the large black hole limit quantum horizon degrees of freedom can be modelled by a single intertwiner. The coupling constant of the defects with the Chern Simons theory on the horizon is precisely given by the ratio of the area contribution of the defect to the macroscopic area $$a_H$$, namely $$\lambda= 16\pi^2 \beta \ell^2_p (j(j+1))^{1/2}/a_H$$."

http://arxiv.org/abs/0905.3170
Self-dual Black Holes in LQG: Theory and Phenomenology
Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 20 May 2009)
In this paper we have recalled the semiclassical metric obtained from a classical analysis of the loop quantum black hole (LQBH). We show that the regular Reissner-Nordstrom-like metric is self-dual in the sense of T-duality: the form of the metric obtained in Loop quantum Gravity (LQG) is invariant under the exchange "r <-> a0/r" where "a0" is proportional to the minimum area in LQG and "r" is the standard Schwarzschild radial coordinate at asymptotic infinity. Of particular interest, the symmetry imposes that if an observer at "r" close to infinity sees a black hole of mass "m" an observer in the other asymptotic infinity beyond the horizon (at "r" close to "0") sees a dual mass "mp/m" ("mp" is the Planck mass). We then show that small LQBH are stable and could be a component of dark matter. Ultra-light LQBHs created shortly after the Big Bang would now have a mass of approximately "10-5 mp" and emit radiation with a typical energy of about 1013 - 1014 eV but they would also emit cosmic rays of much higher energies, albeit few of them. If these small LQBHs form a majority of the dark matter of the Milky Way's Halo, the production rate of ultra-high-energy-cosmic-rays (UHECR) by these ultra light black holes would be compatible with the observed rate of the Auger detector.

http://arxiv.org/abs/0905.1665
Fractal Quantum Space-Time
Leonardo Modesto
(Submitted on 11 May 2009)
Abstract: In this paper we calculated the spectral dimension of loop quantum gravity (LQG) using the scaling property of the area operator spectrum on spin-network states and using the scaling property of the volume and length operators on Gaussian states. We obtained that the spectral dimension of the spatial section runs from 1.5 to 3, and under particular assumptions from 2 to 3 across a 1.5 phase when the energy of a probe scalar field decreases from high to low energy in a fictitious time T. We calculated also the spectral dimension of space-time using the scaling of the area spectrum operator calculated on spin-foam models. The main result is that the effective dimension is 2 at the Planck scale and 4 at low energy. This result is consistent with two other approaches to non perturbative quantum gravity: "causal dynamical triangulation" and "asymptotically safe quantum gravity". We studied the scaling properties of all the possible curvature invariants and we have shown that the singularity problem seems to be solved in the covariant formulation of quantum gravity in terms of spin-foam models. For a particular form of the scaling (or for a particular area operator spectrum) all the curvature invariants are regular also in the Trans-Planckian regime.

http://arxiv.org/abs/0904.4841
The quantization of unimodular gravity and the cosmological constant problem
Lee Smolin
22 pages
(Submitted on 30 Apr 2009)
"A quantization of unimodular gravity is described, which results in a quantum effective action which is also unimodular, ie a function of a metric with fixed determinant. A consequence is that contributions to the energy momentum tensor of the form of the metric times a spacetime constant, whether classical or quantum, are not sources of curvature in the equations of motion derived from the quantum effective action. This solves the first cosmological constant problem, which is suppressing the enormous contributions to the cosmological constant coming from quantum corrections. We discuss several forms of uniodular gravity and put two of them, including one proposed by Henneaux and Teitelboim, in constrained Hamiltonian form. The path integral is constructed from the latter. Furthermore, the second cosmological constant problem, which is why the measured value is so small, is also addressed by this theory. We argue that a mechanism first proposed by Ng and van Dam for suppressing the cosmological constant by quantum effects obtains at the semiclassical level."

Appreciation is due to MTd2 and others who first identified the majority of these papers and added them to our bibliography thread. In particular the two most recent papers, at the top of the list, were just flagged by MTd2. Also it is remarkable how many interesting QG papers are being posted this quarter. It seems like a good time for QG research.

Last edited: May 31, 2009
17. Jun 25, 2009

### marcus

Preliminary list of QG papers for the 2nd quarter (April-June) poll---to evaluate and predict how they will do.

http://arxiv.org/abs/0905.4916
http://arxiv.org/cits/0905.4916
Black holes in full quantum gravity
Kirill Krasnov, Carlo Rovelli
(Submitted on 29 May 2009)
Quantum black holes have been studied extensively in quantum gravity and string theory, using various semiclassical or background dependent approaches. We explore the possibility of studying black holes in the full non-perturbative quantum theory, without recurring to semiclassical considerations, and in the context of loop quantum gravity. We propose a definition of a quantum black hole as the collection of the quantum degrees of freedom that do not influence observables at infinity. From this definition, it follows that for an observer at infinity a black hole is described by an SU(2) intertwining operator. The dimension of the Hilbert space of such intertwiners grows exponentially with the horizon area. These considerations shed some light on the physical nature of the microstates contributing to the black hole entropy. In particular, it can be seen that the microstates being counted for the entropy have the interpretation of describing different horizon shapes. The space of black hole microstates described here is related to the one arrived at recently by Engle, Noui and Perez, and sometime ago by Smolin, but obtained here directly within the full quantum theory.

http://arxiv.org/abs/0906.3947
http://arxiv.org/cits/0906.3947
Quantum gravity as sum over spacetimes
Jan Ambjorn, Jerzy Jurkiewicz, Renate Loll
67 pages, lectures given at the summer school "New Paths Towards Quantum Gravity", May 12-16 2008. To appear as part of a Springer Lecture Notes publication
(Submitted on 22 Jun 2009)
"A major unsolved problem in theoretical physics is to reconcile the classical theory of general relativity with quantum mechanics. These lectures will deal with an attempt to describe quantum gravity as a path integral over geometries known as "Causal Dynamical Triangulations" (CDT)."

http://arxiv.org/abs/0905.3168
http://arxiv.org/cits/0905.3168
Black hole entropy and SU(2) Chern-Simons theory
Jonathan Engle, Karim Noui, Alejandro Perez
4 pages, 1 figure
(Submitted on 19 May 2009)
"We show that the isolated horizon boundary condition can be treated in a manifestly SU(2) invariant manner. The symplectic structure of gravity with the isolated horizon boundary condition has an SU(2) Chern-Simons symplectic structure contribution at the horizon with level $$k=a_H/ (4\pi \beta \ell^2_p)$$. Upon quantization, state counting is expressed in terms of the dimension of Chern-Simons Hilbert spaces on a sphere with marked points (defects). In the large black hole limit quantum horizon degrees of freedom can be modelled by a single intertwiner. The coupling constant of the defects with the Chern Simons theory on the horizon is precisely given by the ratio of the area contribution of the defect to the macroscopic area $$a_H$$, namely $$\lambda= 16\pi^2 \beta \ell^2_p (j(j+1))^{1/2}/a_H$$."

http://arxiv.org/abs/0905.3627
http://arxiv.org/cits/0905.3627
Holomorphic Factorization for a Quantum Tetrahedron
Laurent Freidel, Kirill Krasnov, Etera R. Livine
(Submitted on 22 May 2009)
Abstract: We provide a holomorphic description of the Hilbert space H(j_1,..,j_n) of SU(2)-invariant tensors (intertwiners) and establish a holomorphically factorized formula for the decomposition of identity in H(j_1,..,j_n). Interestingly, the integration kernel that appears in the decomposition formula turns out to be the n-point function of bulk/boundary dualities of string theory. Our results provide a new interpretation for this quantity as being, in the limit of large conformal dimensions, the exponential of the Kahler potential of the symplectic manifold whose quantization gives H(j_1,..,j_n). For the case n=4, the symplectic manifold in question has the interpretation of the space of "shapes" of a geometric tetrahedron with fixed face areas, and our results provide a description for the quantum tetrahedron in terms of holomorphic coherent states. We describe how the holomorphic intertwiners are related to the usual real ones by computing their overlap. The semi-classical analysis of these overlap coefficients in the case of large spins allows us to obtain an explicit relation between the real and holomorphic description of the space of shapes of the tetrahedron. Our results are of direct relevance for the subjects of loop quantum gravity and spin foams, but also add an interesting new twist to the story of the bulk/boundary correspondence.

http://arxiv.org/abs/0905.1665
http://arxiv.org/cits/0905.1665
Fractal Quantum Space-Time
Leonardo Modesto
(Submitted on 11 May 2009)
Abstract: In this paper we calculated the spectral dimension of loop quantum gravity (LQG) using the scaling property of the area operator spectrum on spin-network states and using the scaling property of the volume and length operators on Gaussian states. We obtained that the spectral dimension of the spatial section runs from 1.5 to 3, and under particular assumptions from 2 to 3 across a 1.5 phase when the energy of a probe scalar field decreases from high to low energy in a fictitious time T. We calculated also the spectral dimension of space-time using the scaling of the area spectrum operator calculated on spin-foam models. The main result is that the effective dimension is 2 at the Planck scale and 4 at low energy. This result is consistent with two other approaches to non perturbative quantum gravity: "causal dynamical triangulation" and "asymptotically safe quantum gravity". We studied the scaling properties of all the possible curvature invariants and we have shown that the singularity problem seems to be solved in the covariant formulation of quantum gravity in terms of spin-foam models. For a particular form of the scaling (or for a particular area operator spectrum) all the curvature invariants are regular also in the Trans-Planckian regime.

http://arxiv.org/abs/0905.4082
http://arxiv.org/cits/0905.4082
LQG propagator from the new spin foams
Eugenio Bianchi, Elena Magliaro, Claudio Perini
(Submitted on 25 May 2009)
Abstract: We compute metric correlations in loop quantum gravity with the dynamics defined by the new spin foam models. The analysis is done at the lowest order in a vertex expansion and at the leading order in a large spin expansion. The result is compared to the graviton propagator of perturbative quantum gravity.

http://arxiv.org/abs/0904.4841
http://arxiv.org/cits/0904.4841
The quantization of unimodular gravity and the cosmological constant problem
Lee Smolin
22 pages
(Submitted on 30 Apr 2009)
"A quantization of unimodular gravity is described, which results in a quantum effective action which is also unimodular, ie a function of a metric with fixed determinant. A consequence is that contributions to the energy momentum tensor of the form of the metric times a spacetime constant, whether classical or quantum, are not sources of curvature in the equations of motion derived from the quantum effective action. This solves the first cosmological constant problem, which is suppressing the enormous contributions to the cosmological constant coming from quantum corrections. We discuss several forms of uniodular gravity and put two of them, including one proposed by Henneaux and Teitelboim, in constrained Hamiltonian form. The path integral is constructed from the latter. Furthermore, the second cosmological constant problem, which is why the measured value is so small, is also addressed by this theory. We argue that a mechanism first proposed by Ng and van Dam for suppressing the cosmological constant by quantum effects obtains at the semiclassical level."

http://arxiv.org/abs/0905.4949
http://arxiv.org/cits/0905.4949
A geometric perspective on singularity resolution and uniqueness in loop quantum cosmology
Alejandro Corichi, Parampreet Singh
(Submitted on 29 May 2009)
We re-examine the issue of singularity resolution in homogeneous loop quantum cosmology from the perspective of geometrical entities such as expansion rate and the shear scalar. These quantities are very reliable measures of the properties of spacetime and can be defined not only at the classical and effective level, but also at an operator level in the quantum theory. From the spectrum of the corresponding operators and their behavior in the effective loop quantum spacetime, we show that one can severely restrict the ambiguities in regularization of the quantum constraint and rule out unphysical choices. We analyze this in the flat isotropic model and the Bianchi-I spacetimes. In the former case we show that the expansion rate operator has a bounded spectrum only for the so called improved quantization, a result which synergizes with uniqueness of this quantization as proved earlier. For the Bianchi-I spacetime, we show that out of the available choices, the expansion rate and shear operator are bounded for only one regularization of the quantum constraint. It turns out only this choice has a well defined quantum gravity scale.

http://arxiv.org/abs/0905.4222
http://arxiv.org/cits/0905.4222
Undecidability and the problem of outcomes in quantum measurements
Rodolfo Gambini, Luis Pedro Garcia Pintos, Jorge Pullin
(Submitted on 26 May 2009)
Abstract: We argue that it is fundamentally impossible to recover information about quantum superpositions when a system has interacted with a sufficiently large number of degrees of freedom of the environment. This is due to the fact that gravity imposes fundamental limitations on how accurate measurements can be. This leads to the notion of undecidability: there is no way to tell, due to fundamental limitations, if a quantum system evolved unitarily or suffered wavefunction collapse. This in turn provides a solution to the problem of outcomes in quantum measurement by providing a sharp criterion for defining when an event has taken place. We analyze in detail in examples two situations in which in principle one could recover information about quantum coherence: a) "revivals" of coherence in the interaction of a system with the environment and b) the measurement of global observables of the system plus apparatus plus environment. We show in the examples that the fundamental limitations due to gravity and quantum mechanics in measurement prevent both revivals from occurring and the measurement of global observables. It can therefore be argued that the emerging picture provides a complete resolution to the measurement problem in quantum mechanics.

There has not been time for these to be cited yet, to any significant extent. And we don't expect citation counts to tell the whole story of a paper's longterm importance. But nevertheless, I checked cite counts to date. The Engle-Noui-Perez paper was leading, followed by Corichi-Singh. Essentially a race between Rovelli's postdocs and Ashtekar's. Just kidding (it is admittedly more serious than an athletic contest.)

Last edited: Jun 25, 2009
18. Jun 30, 2009

### marcus

Revised list of QG papers for the 2nd quarter (April-June) poll.

http://arxiv.org/abs/0905.3168
http://arxiv.org/cits/0905.3168
Black hole entropy and SU(2) Chern-Simons theory
Jonathan Engle, Karim Noui, Alejandro Perez
4 pages, 1 figure
(Submitted on 19 May 2009)
"We show that the isolated horizon boundary condition can be treated in a manifestly SU(2) invariant manner. The symplectic structure of gravity with the isolated horizon boundary condition has an SU(2) Chern-Simons symplectic structure contribution at the horizon with level $$k=a_H/ (4\pi \beta \ell^2_p)$$. Upon quantization, state counting is expressed in terms of the dimension of Chern-Simons Hilbert spaces on a sphere with marked points (defects). In the large black hole limit quantum horizon degrees of freedom can be modelled by a single intertwiner. The coupling constant of the defects with the Chern Simons theory on the horizon is precisely given by the ratio of the area contribution of the defect to the macroscopic area $$a_H$$, namely $$\lambda= 16\pi^2 \beta \ell^2_p (j(j+1))^{1/2}/a_H$$."

http://arxiv.org/abs/0906.5477
http://arxiv.org/cits/0906.5477
Scaling behaviour of three-dimensional group field theory
Jacques Magnen (CPHT), Karim Noui (LMPT), Vincent Rivasseau (LPT), Matteo Smerlak (CPT)
(Submitted on 30 Jun 2009)
"Group field theory is a generalization of matrix models, with triangulated pseudomanifolds as Feynman diagrams and state sum invariants as Feynman amplitudes. In this paper, we consider Boulatov's three-dimensional model and its Freidel-Louapre positive regularization (hereafter the BFL model) with a 'ultraviolet' cutoff, and study rigorously their scaling behavior in the large cutoff limit. We prove an optimal bound on large order Feynman amplitudes, which shows that the BFL model is perturbatively more divergent than the former. We then upgrade this result to the constructive level, using, in a self-contained way, the modern tools of constructive field theory: we construct the Borel sum of the BFL perturbative series via a convergent 'cactus' expansion, and establish the 'ultraviolet' scaling of its Borel radius. Our method shows how the 'sum over triangulations' in quantum gravity can be tamed rigorously, and paves the way for the renormalization program in group field theory."

http://arxiv.org/abs/0905.3627
http://arxiv.org/cits/0905.3627
Holomorphic Factorization for a Quantum Tetrahedron
Laurent Freidel, Kirill Krasnov, Etera R. Livine
(Submitted on 22 May 2009)
Abstract: We provide a holomorphic description of the Hilbert space H(j_1,..,j_n) of SU(2)-invariant tensors (intertwiners) and establish a holomorphically factorized formula for the decomposition of identity in H(j_1,..,j_n). Interestingly, the integration kernel that appears in the decomposition formula turns out to be the n-point function of bulk/boundary dualities of string theory. Our results provide a new interpretation for this quantity as being, in the limit of large conformal dimensions, the exponential of the Kahler potential of the symplectic manifold whose quantization gives H(j_1,..,j_n). For the case n=4, the symplectic manifold in question has the interpretation of the space of "shapes" of a geometric tetrahedron with fixed face areas, and our results provide a description for the quantum tetrahedron in terms of holomorphic coherent states. We describe how the holomorphic intertwiners are related to the usual real ones by computing their overlap. The semi-classical analysis of these overlap coefficients in the case of large spins allows us to obtain an explicit relation between the real and holomorphic description of the space of shapes of the tetrahedron. Our results are of direct relevance for the subjects of loop quantum gravity and spin foams, but also add an interesting new twist to the story of the bulk/boundary correspondence.

http://arxiv.org/abs/0905.4916
http://arxiv.org/cits/0905.4916
Black holes in full quantum gravity
Kirill Krasnov, Carlo Rovelli
(Submitted on 29 May 2009)
Quantum black holes have been studied extensively in quantum gravity and string theory, using various semiclassical or background dependent approaches. We explore the possibility of studying black holes in the full non-perturbative quantum theory, without recurring to semiclassical considerations, and in the context of loop quantum gravity. We propose a definition of a quantum black hole as the collection of the quantum degrees of freedom that do not influence observables at infinity. From this definition, it follows that for an observer at infinity a black hole is described by an SU(2) intertwining operator. The dimension of the Hilbert space of such intertwiners grows exponentially with the horizon area. These considerations shed some light on the physical nature of the microstates contributing to the black hole entropy. In particular, it can be seen that the microstates being counted for the entropy have the interpretation of describing different horizon shapes. The space of black hole microstates described here is related to the one arrived at recently by Engle, Noui and Perez, and sometime ago by Smolin, but obtained here directly within the full quantum theory.

http://arxiv.org/abs/0906.3947
http://arxiv.org/cits/0906.3947
Quantum gravity as sum over spacetimes
Jan Ambjorn, Jerzy Jurkiewicz, Renate Loll
67 pages, lectures given at the summer school "New Paths Towards Quantum Gravity", May 12-16 2008. To appear as part of a Springer Lecture Notes publication
(Submitted on 22 Jun 2009)
"A major unsolved problem in theoretical physics is to reconcile the classical theory of general relativity with quantum mechanics. These lectures will deal with an attempt to describe quantum gravity as a path integral over geometries known as "Causal Dynamical Triangulations" (CDT)."

http://arxiv.org/abs/0905.1665
http://arxiv.org/cits/0905.1665
Fractal Quantum Space-Time
Leonardo Modesto
(Submitted on 11 May 2009)
Abstract: In this paper we calculated the spectral dimension of loop quantum gravity (LQG) using the scaling property of the area operator spectrum on spin-network states and using the scaling property of the volume and length operators on Gaussian states. We obtained that the spectral dimension of the spatial section runs from 1.5 to 3, and under particular assumptions from 2 to 3 across a 1.5 phase when the energy of a probe scalar field decreases from high to low energy in a fictitious time T. We calculated also the spectral dimension of space-time using the scaling of the area spectrum operator calculated on spin-foam models. The main result is that the effective dimension is 2 at the Planck scale and 4 at low energy. This result is consistent with two other approaches to non perturbative quantum gravity: "causal dynamical triangulation" and "asymptotically safe quantum gravity". We studied the scaling properties of all the possible curvature invariants and we have shown that the singularity problem seems to be solved in the covariant formulation of quantum gravity in terms of spin-foam models. For a particular form of the scaling (or for a particular area operator spectrum) all the curvature invariants are regular also in the Trans-Planckian regime.

http://arxiv.org/abs/0906.3731
http://arxiv.org/cits/0906.3731
Prospects for constraining quantum gravity dispersion with near term observations
Giovanni Amelino-Camelia, Lee Smolin
(Submitted on 19 Jun 2009)
"We discuss the prospects for bounding and perhaps even measuring quantum gravity effects on the dispersion of light using the highest energy photons produced in gamma ray bursts measured by the Fermi telescope. These prospects are brigher than might have been expected as in the first 10 months of operation Fermi has reported so far eight events with photons over 100 MeV seen by its Large Area Telescope (LAT). We review features of these events which may bear on Planck scale phenomenology and we discuss the possible implications for the alternative scenarios for in-vacua dispersion coming from breaking or deforming of Poincare invariance. Among these are semi-conservative bounds, which rely on some relatively weak assumptions about the sources, on subluminal and superluminal in-vacuo dispersion. We also propose that it may be possible to look for the arrival of still higher energy photons and neutrinos from GRB's with energies in the range 10^14 - 10^17 eV. In some cases the quantum gravity dispersion effect would predict these arrivals to be delayed or advanced by days to months from the GRB, giving a clean separation of astrophysical source and spacetime propagation effects."

http://arxiv.org/abs/0904.4841
http://arxiv.org/cits/0904.4841
The quantization of unimodular gravity and the cosmological constant problem
Lee Smolin
22 pages
(Submitted on 30 Apr 2009)
"A quantization of unimodular gravity is described, which results in a quantum effective action which is also unimodular, ie a function of a metric with fixed determinant. A consequence is that contributions to the energy momentum tensor of the form of the metric times a spacetime constant, whether classical or quantum, are not sources of curvature in the equations of motion derived from the quantum effective action. This solves the first cosmological constant problem, which is suppressing the enormous contributions to the cosmological constant coming from quantum corrections. We discuss several forms of uniodular gravity and put two of them, including one proposed by Henneaux and Teitelboim, in constrained Hamiltonian form. The path integral is constructed from the latter. Furthermore, the second cosmological constant problem, which is why the measured value is so small, is also addressed by this theory. We argue that a mechanism first proposed by Ng and van Dam for suppressing the cosmological constant by quantum effects obtains at the semiclassical level."

http://arxiv.org/abs/0905.4949
http://arxiv.org/cits/0905.4949
A geometric perspective on singularity resolution and uniqueness in loop quantum cosmology
Alejandro Corichi, Parampreet Singh
(Submitted on 29 May 2009)
We re-examine the issue of singularity resolution in homogeneous loop quantum cosmology from the perspective of geometrical entities such as expansion rate and the shear scalar. These quantities are very reliable measures of the properties of spacetime and can be defined not only at the classical and effective level, but also at an operator level in the quantum theory. From the spectrum of the corresponding operators and their behavior in the effective loop quantum spacetime, we show that one can severely restrict the ambiguities in regularization of the quantum constraint and rule out unphysical choices. We analyze this in the flat isotropic model and the Bianchi-I spacetimes. In the former case we show that the expansion rate operator has a bounded spectrum only for the so called improved quantization, a result which synergizes with uniqueness of this quantization as proved earlier. For the Bianchi-I spacetime, we show that out of the available choices, the expansion rate and shear operator are bounded for only one regularization of the quantum constraint. It turns out only this choice has a well defined quantum gravity scale.

When I checked cite counts to date, the Engle-Noui-Perez paper was leading, followed by Corichi-Singh. Here is a condensed list of the 9 papers:

Black hole entropy and SU(2) Chern-Simons theory (Engle Noui Perez)
Scaling behaviour of three-dimensional group field theory (Magnen Noui Rivasseau Smerlak)
Holomorphic Factorization for a Quantum Tetrahedron (Freidel Krasnov Livine)
Black holes in full quantum gravity (Krasnov Rovelli)
Quantum gravity as sum over spacetimes (Ambjorn Jurkiewicz Loll)
Fractal Quantum Space-Time (Modesto)
Prospects for constraining quantum gravity dispersion with near term observations (Amelino Smolin)
The quantization of unimodular gravity and the cosmological constant problem (Smolin)
A geometric perspective on singularity resolution and uniqueness in loop quantum cosmology (Corichi Singh)

Last edited: Jun 30, 2009