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Here are some background independent QG papers which appeared in the 2nd quarter (April-June) of this year, for us to evaluate and try predicting how they will do. The poll is set up to accept multiple choices. Please check off the paper or papers you predict will have the most significant impact on future QG research. If you have one to propose that is not on the list, you can post the arxiv link, title and author(s) on this thread, to be counted as a "write-in".
(vocabulary lookups if desired: relation exponential quantization momentum degrees of freedom black hole ) 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 [tex]k=a_H/ (4\pi \beta \ell^2_p)[/tex]. 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 modeled 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 [tex]a_H[/tex], namely [tex]\lambda= 16\pi^2 \beta \ell^2_p (j(j+1))^{1/2}/a_H[/tex]."
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)
"We provide a holomorphic description of the Hilbert space H(j1,..,jn) of SU(2)-invariant tensors (intertwiners) and establish a holomorphically factorized formula for the decomposition of identity in H(j1,..,jn). 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(j1,..,jn). 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)
"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."
Incidentally although it is too early for cite counts to mean much (if they ever do!) when I checked cites to date on these papers, the Engle-Noui-Perez paper was leading, followed by Corichi-Singh. Rivasseau will be giving a series of lectures along with Rovelli, Ashtekar and Baez, at the September QG school on the island of Corfu. The Rivasseau paper listed here may be related to what he will teach at the Corfu school.
(vocabulary lookups if desired: relation exponential quantization momentum degrees of freedom black hole ) 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 [tex]k=a_H/ (4\pi \beta \ell^2_p)[/tex]. 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 modeled 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 [tex]a_H[/tex], namely [tex]\lambda= 16\pi^2 \beta \ell^2_p (j(j+1))^{1/2}/a_H[/tex]."
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)
"We provide a holomorphic description of the Hilbert space H(j1,..,jn) of SU(2)-invariant tensors (intertwiners) and establish a holomorphically factorized formula for the decomposition of identity in H(j1,..,jn). 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(j1,..,jn). 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)
"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."
Incidentally although it is too early for cite counts to mean much (if they ever do!) when I checked cites to date on these papers, the Engle-Noui-Perez paper was leading, followed by Corichi-Singh. Rivasseau will be giving a series of lectures along with Rovelli, Ashtekar and Baez, at the September QG school on the island of Corfu. The Rivasseau paper listed here may be related to what he will teach at the Corfu school.
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