View Poll Results: Which paper(s) will contribute most significantly to future research?
A positive formalism for quantum theory in the generalized boundary formulation 0 0%
Gravitational origin of the weak interaction's chirality 2 15.38%
On the Architecture of Spacetime Geometry 5 38.46%
Black Hole Entropy from complex Ashtekar variables 4 30.77%
Asymptotic silence in loop quantum cosmology 1 7.69%
Primordial tensor power spectrum in holonomy corrected Omega-LQC 1 7.69%
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron 2 15.38%
Pentahedral volume, chaos, and quantum gravity 2 15.38%
The Matter Bounce Scenario in Loop Quantum Cosmology 2 15.38%
Holonomy-flux spinfoam amplitude 1 7.69%
The spin connection of twisted geometry 5 38.46%
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era 2 15.38%
Schwinger-Dyson Equations in Group Field Theories of Quantum Gravity 0 0%
Horizon entanglement entropy and universality of the graviton coupling 2 15.38%
Emergent Isotropy-Breaking in Quantum Cosmology 0 0%
Pure connection formalism for gravity: Feynman rules and the graviton-graviton scattering 1 7.69%
Geometric asymptotics for spin foam lattice gauge gravity on arbitrary triangulations 1 7.69%
A new perspective on cosmology in Loop Quantum Gravity 4 30.77%
Interpretation of the triad orientations in loop quantum cosmology 2 15.38%
Multiple Choice Poll. Voters: 13. You may not vote on this poll

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Our picks for fourth quarter 2012 MIP (most important QG paper)

by marcus
Tags: 2012, important, paper, picks, quarter
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marcus
#1
Dec29-12, 12:15 AM
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Of the nineteen 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/1212.5571
A positive formalism for quantum theory in the generalized boundary formulation
Robert Oeckl

http://arxiv.org/abs/1212.5246
Gravitational origin of the weak interaction's chirality
Stephon Alexander, Antonino Marciano, Lee Smolin

http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers

http://arxiv.org/abs/1212.4060
Black Hole Entropy from complex Ashtekar variables
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1212.3527
Asymptotic silence in loop quantum cosmology
Jakub Mielczarek

http://arxiv.org/abs/1212.2852
Primordial tensor power spectrum in holonomy corrected Omega-LQC
Linda Linsefors, Thomas Cailleteau, Aurelien Barrau, Julien Grain

http://lanl.arxiv.org/abs/1212.1930
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron
Christopher E. Coleman-Smith, Berndt Müller

http://arxiv.org/abs/1211.7311
Pentahedral volume, chaos, and quantum gravity
Hal M. Haggard

http://arxiv.org/abs/1211.6269
The Matter Bounce Scenario in Loop Quantum Cosmology
Edward Wilson-Ewing

http://arxiv.org/abs/1211.4807
Holonomy-flux spinfoam amplitude
Claudio Perini

http://arxiv.org/abs/1211.2166
The spin connection of twisted geometry
Hal M. Haggard, Carlo Rovelli, Francesca Vidotto, Wolfgang Wieland

http://arxiv.org/abs/1211.1354
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1211.1244
Schwinger-Dyson Equations in Group Field Theories of Quantum Gravity
Thomas Krajewski

http://arxiv.org/abs/1211.0522
Horizon entanglement entropy and universality of the graviton coupling
Eugenio Bianchi

http://arxiv.org/abs/1211.0161
Emergent Isotropy-Breaking in Quantum Cosmology
Andrea Dapor, Jerzy Lewandowski

http://arxiv.org/abs/1210.6215
Pure connection formalism for gravity: Feynman rules and the graviton-graviton scattering
Gianluca Delfino, Kirill Krasnov, Carlos Scarinci

http://arxiv.org/abs/1210.5276
Geometric asymptotics for spin foam lattice gauge gravity on arbitrary triangulations
Frank Hellmann, Wojciech Kaminski

http://arxiv.org/abs/1210.4504
A new perspective on cosmology in Loop Quantum Gravity
Emanuele Alesci, Francesco Cianfrani

http://arxiv.org/abs/1210.0418
Interpretation of the triad orientations in loop quantum cosmology
Claus Kiefer, Christian Schell

The poll follows the same general format as MIP polls in the past, for example
third quarter 2011:
http://physicsforums.com/showthread.php?t=535170
fourth quarter 2011:
http://physicsforums.com/showthread.php?t=563724
second quarter 2012:
http://physicsforums.com/showthread.php?t=617250
third quarter 2012:
http://physicsforums.com/showthread.php?t=640181
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marcus
#2
Dec29-12, 12:30 AM
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Here are the abstracts for the candidate papers.

http://arxiv.org/abs/1212.5571
A positive formalism for quantum theory in the generalized boundary formulation
Robert Oeckl
(Submitted on 21 Dec 2012)
We introduce a new "positive formalism" for encoding quantum theories in the general boundary formulation, somewhat analogous to the mixed state formalism of the standard formulation. This makes the probability interpretation more natural and elegant, eliminates operationally irrelevant structure and opens the general boundary formulation to quantum information theory.
28 pages

http://arxiv.org/abs/1212.5246
Gravitational origin of the weak interaction's chirality
Stephon Alexander, Antonino Marciano, Lee Smolin
(Submitted on 20 Dec 2012)
We present a new unification of the electro-weak and gravitational interactions based on the joining the weak SU(2) gauge fields with the left handed part of the space-time connection, into a single gauge field valued in the complexification of the local Lorentz group. Hence, the weak interactions emerge as the right handed chiral half of the space-time connection, which explains the chirality of the weak interaction. This is possible, because, as shown by Plebanski, Ashtekar, and others, the other chiral half of the space-time connection is enough to code the dynamics of the gravitational degrees of freedom.
This unification is achieved within an extension of the Plebanski action previously proposed by one of us. The theory has two phases. A parity symmetric phase yields, as shown by Speziale, a bi-metric theory with eight degrees of freedom: the massless graviton, a massive spin two field and a scalar ghost. Because of the latter this phase is unstable. Parity is broken in a stable phase where the eight degrees of freedom arrange themselves as the massless graviton coupled to an SU(2) triplet of chirally coupled Yang-Mills fields. It is also shown that under this breaking a Dirac fermion expresses itself as a chiral neutrino paired with a scalar field with the quantum numbers of the Higgs.
21 pages

http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers
(Submitted on 20 Dec 2012)
We propose entanglement entropy as a probe of the architecture of spacetime in quantum gravity. We argue that the leading contribution to this entropy satisfies an area law for any sufficiently large region in a smooth spacetime, which, in fact, is given by the Bekenstein-Hawking formula. This conjecture is supported by various lines of evidence from perturbative quantum gravity, simplified models of induced gravity and loop quantum gravity, as well as the AdS/CFT correspondence.
8 pages, 1 figure

http://arxiv.org/abs/1212.4060
Black Hole Entropy from complex Ashtekar variables
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez
(Submitted on 17 Dec 2012)
In loop quantum gravity, the number NΓ(aH, γ) of microstates of a black hole for a given discrete geometry Γ depends on the so-called Barbero-Immirzi parameter γ. Using a suitable analytic continuation of γ to complex values, we show that the number NΓ(aH, ±i) of microstates behaves as exp(aH/(4 lp2)) for large area aH in the large spin semiclassical limit. Such a correspondence with the semiclassical Bekenstein-Hawking entropy law points towards an unanticipated and remarkable feature of the original complex Ashtekar variables for quantum gravity.
5 pages

http://arxiv.org/abs/1212.3527
Asymptotic silence in loop quantum cosmology
Jakub Mielczarek
(Submitted on 14 Dec 2012)
The state of asymptotic silence, characterized by causal disconnection of the space points, emerges from various approaches aiming to describe gravitational phenomena in the limit of large curvatures. In particular, such behavior was anticipated by Belinsky, Khalatnikov and Lifgarbagez (BKL) in their famous conjecture put forward in the early seventies of the last century. While the BKL conjecture is based on purely classical considerations, one can expect that asymptotic silence should have its quantum counterpart at the level of a more fundamental theory of quantum gravity, which is the relevant description of gravitational phenomena in the limit of large energy densities. Here, we summarize some recent results which give support to such a possibility. More precisely, we discuss occurrence of the asymptotic silence due to polymerization of space at the Planck scale, in the framework of loop quantum cosmology. In the discussed model, the state of asymptotic silence is realized at the energy density ρ = ρc/2, where ρc is the maximal allowed energy density, being of the order of the Planck energy density. At energy densities ρ > ρc/2, the universe becomes 4D Euclidean space without causal structure. Therefore, the asymptotic silence appears to be an intermediate state of space between the Lorentzian and Euclidean phases.
4 pages, 3 figures, talk presented at the Multiverse and Fundamental Cosmology Conference, 10-14 September, 2012, Szczecin, Poland

http://arxiv.org/abs/1212.2852
Primordial tensor power spectrum in holonomy corrected Omega-LQC
Linda Linsefors, Thomas Cailleteau, Aurelien Barrau, Julien Grain
(Submitted on 12 Dec 2012)
The holonomy correction is one of the main terms arising when implementing loop quantum gravity ideas at an effective level in cosmology. The recent construction of an anomaly free algebra has shown that the formalism used, up to now, to derive the primordial spectrum of fluctuations was not correct. This article aims at computing the tensor spectrum in a fully consistent way within this deformed and closed algebra.
5 pages, 6 figures

http://lanl.arxiv.org/abs/1212.1930
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron
Christopher E. Coleman-Smith, Berndt Müller
(Submitted on 9 Dec 2012)
We present an analysis of the dynamics of the equifacial pentahedron on the Kapovich-Millson phase space under a volume preserving Hamiltonian. The classical dynamics of polyhedra under such a Hamiltonian may arise from the classical limit of the node volume operators in loop quantum gravity. The pentahedron is the simplest nontrivial polyhedron for which the dynamics may be chaotic. We consider the distribution of polyhedral configurations throughout the space and find indications that the borders between certain configurations act as separatrices. We examine the local stability of trajectories within this phase space and find that locally unstable regions dominate although extended stable regions are present. Canonical and microcanonical estimates of the Kolmogorov-Sinai entropy suggest that the pentahedron is a strongly chaotic system. The presence of chaos is further suggested by calculations of intermediate time Lyapunov exponents which saturate to non zero values.
20 Pages, 19 Figures

http://arxiv.org/abs/1211.7311
Pentahedral volume, chaos, and quantum gravity
Hal M. Haggard
(Submitted on 30 Nov 2012)
We show that chaotic classical dynamics associated to the volume of discrete grains of space leads to quantal spectra that are gapped between zero and nonzero volume. This strengthens the connection between spectral discreteness in the quantum geometry of gravity and tame ultraviolet behavior. We complete a detailed analysis of the geometry of a pentahedron, providing new insights into the volume operator and evidence of classical chaos in the dynamics it generates. These results reveal an unexplored realm of application for chaos in quantum gravity.
5 pages, 4 figures

http://arxiv.org/abs/1211.6269
The Matter Bounce Scenario in Loop Quantum Cosmology
Edward Wilson-Ewing
(Submitted on 27 Nov 2012)
In the matter bounce scenario, a dust-dominated contracting space-time generates scale-invariant perturbations that, assuming a nonsingular bouncing cosmology, propagate to the expanding branch and set appropriate initial conditions for the radiation-dominated era. Since this scenario depends on the presence of a bounce, it seems appropriate to consider it in the context of loop quantum cosmology where a bouncing universe naturally arises. It turns out that quantum gravity effects play an important role beyond simply providing the bounce. Indeed, quantum gravity corrections to the Mukhanov-Sasaki equations significantly modify some of the results obtained in a purely classical setting: while the predicted spectra of scalar and tensor perturbations are both almost scale-invariant with identical small red tilts in agreement with previous results, the tensor to scalar ratio is now expected to be r≈ 9 x 10-4, which is much smaller than the original classical prediction. Finally, for the predicted amplitude of the scalar perturbations to agree with observations, the critical density in loop quantum cosmology must be of the order ρcrit ~ 10-9 ρPlanck.
8 pages

http://arxiv.org/abs/1211.4807
Holonomy-flux spinfoam amplitude
Claudio Perini
(Submitted on 20 Nov 2012)
We introduce a holomorphic representation for the Lorentzian EPRL spinfoam on arbitrary 2-complexes. The representation is obtained via the Ashtekar-Lewandowski-Marolf-Mourao-Thiemann heat kernel coherent state transform. The new variables are classical holonomy-flux phase space variables (h,X) ≈ T*SU(2) of Hamiltonian loop quantum gravity prescribing the holonomies of the Ashtekar connection A = Γ + γK, and their conjugate gravitational fluxes. For small heat kernel 'time' the spinfoam amplitude is peaked on classical space-time geometries, where at most countably many curvatures are allowed for non-zero Barbero-Immirzi parameter. We briefly comment on the possibility to use the alternative flipped classical limit.
33 pages

http://arxiv.org/abs/1211.2166
The spin connection of twisted geometry
Hal M. Haggard, Carlo Rovelli, Francesca Vidotto, Wolfgang Wieland
(Submitted on 9 Nov 2012)
Twisted geometry is a piecewise-flat geometry less rigid than Regge geometry. In Loop Gravity, it provides the classical limit for each step of the truncation utilized in the definition of the quantum theory. We define the torsionless spin-connection of a twisted geometry. The difficulty given by the discontinuity of the triad is addressed by interpolating between triads. The curvature of the resulting spin connection reduces to the Regge curvature in the case of a Regge geometry.
5 pages, 2 figures

http://arxiv.org/abs/1211.1354
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 6 Nov 2012)
Cosmological perturbations are generally described by quantum fields on (curved but) classical space-times. While this strategy has a large domain of validity, it can not be justified in the quantum gravity era where curvature and matter densities are of Planck scale. Using techniques from loop quantum gravity, the standard theory of cosmological perturbations is extended to overcome this limitation. The new framework sharpens conceptual issues by distinguishing between the true and apparent trans-Planckian difficulties and provides sufficient conditions under which the true difficulties can be overcome within a quantum gravity theory. In a companion paper, this framework is applied to the standard inflationary model, with interesting implications to theory as well as observations.
50 pages. This is first of the two detailed papers on which arXiv 1209.1609 (PRL at press) is based

http://arxiv.org/abs/1211.1244
Schwinger-Dyson Equations in Group Field Theories of Quantum Gravity
Thomas Krajewski
(Submitted on 6 Nov 2012)
In this talk, we elaborate on the operation of graph contraction introduced by Gurau in his study of the Schwinger-Dyson equations. After a brief review of colored tensor models, we identify the Lie algebra appearing in the Schwinger-Dyson equations as a Lie algebra associated to a Hopf algebra of the Connes-Kreimer type. Then, we show how this operation also leads to an analogue of the Wilsonian flow for the effective action. Finally, we sketch how this formalism may be adapted to group field theories.
6 pages. Talk given at "The XXIX International Colloquium on Group-Theoretical Methods in Physics", Chern Institute of Mathematics August 2012, submitted to the conference proceedings

http://arxiv.org/abs/1211.0522
Horizon entanglement entropy and universality of the graviton coupling
Eugenio Bianchi
(Submitted on 2 Nov 2012)
We compute the low-energy variation of the horizon entanglement entropy for matter fields and gravitons in Minkowski space. While the entropy is divergent, the variation under a perturbation of the vacuum state is finite and proportional to the energy flux through the Rindler horizon. Due to the universal coupling of gravitons to the energy-momentum tensor, the variation of the entanglement entropy is universal and equal to the change in area of the event horizon divided by 4 times Newton's constant - independently from the number and type of matter fields. The physical mechanism presented provides an explanation of the microscopic origin of the Bekenstein-Hawking entropy in terms of entanglement entropy.
7 pages

http://arxiv.org/abs/1211.0161
Emergent Isotropy-Breaking in Quantum Cosmology
Andrea Dapor, Jerzy Lewandowski
(Submitted on 1 Nov 2012)
We consider a massive quantum test Klein-Gordon field probing an isotropic quantum cosmological space-time in the background. The result obtained is surprising. It turns out, that despite the isotropy of the quantum gravitational field, the semi-classical metric experienced by a mode of the K-G field is non-isotropic. The anisotropy depends on the direction of the momentum of the mode. Specifically, what we do is to derive a semi-classical space-time which emerges to a mode of the field. The method amounts to a comparison between QFT on a quantum background and QFT on a classical curved space-time, giving rise to an emergent metric tensor. The components of the semi-classical metric tensor are calculated from the equation of propagation of the quantum K-G field in the test field approximation. The anisotropies are of a quantum nature: they are proportional to Planck constant and "dress" the isotropic classical space-time obtained in the classical limit.
6 pages

http://arxiv.org/abs/1210.6215
Pure connection formalism for gravity: Feynman rules and the graviton-graviton scattering
Gianluca Delfino, Kirill Krasnov, Carlos Scarinci
(Submitted on 23 Oct 2012)
We continue to develop the pure connection formalism for gravity. We derive the Feynman rules for computing the connection correlation functions, as well as the prescription for obtaining the Minkowski space graviton scattering amplitudes from the latter. The present formalism turns out to be significantly simpler than the one based on the metric in many aspects. The most drastic difference with the usual approach is that the conformal factor of the metric, which is a source of difficulties in the metric treatment, does not propagate in the connection formulation even off-shell. This simplifies both the linearized theory and the interactions. For comparison, in our approach the complete off-shell cubic GR interaction contains just 3 terms, with only a single term relevant at tree level. This should be compared to at least a dozen terms in the metric formalism. We put the technology developed to use and compute the simplest graviton-graviton scattering amplitudes....
... This serves as a good illustration of the type of parity violation present in these theories. We find that the parity-violating amplitudes are important at high energies, and that a general parity-violating member of our class of theories "likes" one helicity (negative in our conventions) more than the other in the sense that at high energies it tends to convert all present gravitons into those of negative helicity.
46 pages.

http://arxiv.org/abs/1210.5276
Geometric asymptotics for spin foam lattice gauge gravity on arbitrary triangulations
Frank Hellmann, Wojciech Kaminski
(Submitted on 18 Oct 2012)
We study the behavior of holonomy spin foam partition functions, a form of lattice gauge gravity, on generic 4d-triangulations using micro local analysis. To do so we adapt tools from the renormalization theory of quantum field theory on curved space times. This allows us, for the first time, to study the partition function without taking any limits on the interior of the triangulation.
We establish that for many of the most widely used models the geometricity constraints, which reduce the gauge theory to a geometric one, introduce strong accidental curvature constraints. These limit the curvature around each triangle of the triangulation to a finite set of values. We demonstrate how to modify the partition function to avoid this problem. Finally the new methods introduced provide a starting point for studying the regularization ambiguities and renormalization of the partition function.
4+6 pages, 1 figure

http://arxiv.org/abs/1210.4504
A new perspective on cosmology in Loop Quantum Gravity
Emanuele Alesci, Francesco Cianfrani
(Submitted on 16 Oct 2012)
We present a new cosmological model derived from Loop Quantum Gravity. The formulation is based on a projection of the kinematical Hilbert space of the full theory down to a subspace representing the proper arena for an inhomogeneous Bianchi I model. This procedure gives a direct link between the full theory and its cosmological sector. The emerging quantum cosmological model represents a simplified arena on which the complete canonical quantization program can be tested. The achievements of this analysis could also shed light on Loop Quantum Cosmology and its relation with the full theory.
5 pages

http://arxiv.org/abs/1210.0418
Interpretation of the triad orientations in loop quantum cosmology
Claus Kiefer, Christian Schell
(Submitted on 1 Oct 2012)
Loop quantum cosmology allows for arbitrary superpositions of the triad variable. We show here how these superpositions can become indistinguishable from a classical mixture by the interaction with fermions. We calculate the reduced density matrix for a locally rotationally symmetric Bianchi I model and show that the purity factor for the triads decreases by decoherence. In this way, the Universe assumes a definite orientation.
12 pages, 1 figure
atyy
#3
Dec29-12, 10:10 AM
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Well, the Mayans were right. The world has ended. marcus voted for a string theory paper;)

marcus
#4
Dec29-12, 12:48 PM
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Our picks for fourth quarter 2012 MIP (most important QG paper)

Quote Quote by atyy View Post
Well, the Mayans were right. The world has ended. marcus voted for a string theory paper;)

But I take David Gross' words seriously: for years when he gives the intro or the summation talks at string cons he says we don't know what "string theory" is, and "there is still some missing piece".

Theories do not exist in nature otherwise than as human constructs in a human language (like eg math). So if you cannot say what it is, a theory does not exist.
Since "string theory" is meaningless phrase, how can there be such thing as a "string theory paper" for me to be interested in?
There are lots and lots of stringy mathematics papers and stringy philosophy papers, but no one distinguishable physical theory has gelled yet. And one may never gel, people get tired and move on.

Or one might yet gel! that would be great! then we would have another detailed model of what was going on right around the start of expansion, making predictions that we could check against high resolution maps of the ancient light.

BTW Atyy: EBEX antarctic balloon-borne CMB mapper was launched yesterday. I posted about it in Cosmo forum. More detail--B-mode polarization--more constraints on models of the start of expansion. This is "where it's at" QG-wise.

For me, the exciting motivation for moving to a Tomita-time C*-algebra formulation of QM and GR is that this might allow modeling of the start of expansion to be perfected e.g. in Loop cosmology context. A star-algebra formulation replacing the differential manifold picture of space-time (that e.g. AdSCFT is based on) could also be David's missing piece.

Anyway thanks for your remark, giving me a chance to talk about personal PoV.
atyy
#5
Dec29-12, 04:00 PM
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Quote Quote by marcus View Post
BTW Atyy: EBEX antarctic balloon-borne CMB mapper was launched yesterday. I posted about it in Cosmo forum. More detail--B-mode polarization--more constraints on models of the start of expansion. This is "where it's at" QG-wise.
Balloon? Weren't the previous mappers outside the atmosphere? But one from inside the atmosphere can put more constraints?

Hmm, yes indeed balloon borne:
http://groups.physics.umn.edu/cosmology/ebex/
http://arxiv.org/abs/astro-ph/0501111
marcus
#6
Dec30-12, 12:26 PM
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Quote Quote by atyy View Post
Balloon? Weren't the previous mappers outside the atmosphere? But one from inside the atmosphere can put more constraints?

Hmm, yes indeed balloon borne:
http://groups.physics.umn.edu/cosmology/ebex/
http://arxiv.org/abs/astro-ph/0501111
Early mapping was from U2 airplane, and also balloon in 1990s, then the well known COBE satellite. But balloon still makes sense. Here's how I think of it: The INSTRUMENTS keep getting better, or more specific to the questions being asked, and one cannot launch a new spacecraft every time one redesigns the instrument package. Plus the antarctic air is very clear and transparent to microwaves. Plus one does not need to map the WHOLE sky to learn stuff. The section of sky visible from antarctic is an adequate SAMPLE.
The angular power spectrum is much the same in whatever general direction as long as the Milkyway galaxy is not in the way.
===========================

But more on topic. You referred to the Bianchi Myers paper. That paper is extremely interesting. I find more and more in it each time I go back.

It is not simply that, by being simultaneously both a loop paper and a string paper, it is part of the breakdown of division between camps that is going on.
marcus
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Jan4-13, 06:36 PM
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Cordial thanks to Nonlinearity and Atyy for helping get this quarter's poll started! Some front runners have emerged:

3 votes
http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers
(Submitted on 20 Dec 2012)
We propose entanglement entropy as a probe of the architecture of spacetime in quantum gravity. We argue that the leading contribution to this entropy satisfies an area law for any sufficiently large region in a smooth spacetime, which, in fact, is given by the Bekenstein-Hawking formula. This conjecture is supported by various lines of evidence from perturbative quantum gravity, simplified models of induced gravity and loop quantum gravity, as well as the AdS/CFT correspondence.
8 pages, 1 figure


2 votes
http://lanl.arxiv.org/abs/1212.1930
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron
Christopher E. Coleman-Smith, Berndt Müller
(Submitted on 9 Dec 2012)
We present an analysis of the dynamics of the equifacial pentahedron on the Kapovich-Millson phase space under a volume preserving Hamiltonian. The classical dynamics of polyhedra under such a Hamiltonian may arise from the classical limit of the node volume operators in loop quantum gravity. The pentahedron is the simplest nontrivial polyhedron for which the dynamics may be chaotic. We consider the distribution of polyhedral configurations throughout the space and find indications that the borders between certain configurations act as separatrices. We examine the local stability of trajectories within this phase space and find that locally unstable regions dominate although extended stable regions are present. Canonical and microcanonical estimates of the Kolmogorov-Sinai entropy suggest that the pentahedron is a strongly chaotic system. The presence of chaos is further suggested by calculations of intermediate time Lyapunov exponents which saturate to non zero values.
20 Pages, 19 Figures

http://arxiv.org/abs/1211.7311
Pentahedral volume, chaos, and quantum gravity
Hal M. Haggard
(Submitted on 30 Nov 2012)
We show that chaotic classical dynamics associated to the volume of discrete grains of space leads to quantal spectra that are gapped between zero and nonzero volume. This strengthens the connection between spectral discreteness in the quantum geometry of gravity and tame ultraviolet behavior. We complete a detailed analysis of the geometry of a pentahedron, providing new insights into the volume operator and evidence of classical chaos in the dynamics it generates. These results reveal an unexplored realm of application for chaos in quantum gravity.
5 pages, 4 figures

http://arxiv.org/abs/1211.2166
The spin connection of twisted geometry
Hal M. Haggard, Carlo Rovelli, Francesca Vidotto, Wolfgang Wieland
(Submitted on 9 Nov 2012)
Twisted geometry is a piecewise-flat geometry less rigid than Regge geometry. In Loop Gravity, it provides the classical limit for each step of the truncation utilized in the definition of the quantum theory. We define the torsionless spin-connection of a twisted geometry. The difficulty given by the discontinuity of the triad is addressed by interpolating between triads. The curvature of the resulting spin connection reduces to the Regge curvature in the case of a Regge geometry.
5 pages, 2 figures

http://arxiv.org/abs/1211.0522
Horizon entanglement entropy and universality of the graviton coupling
Eugenio Bianchi
(Submitted on 2 Nov 2012)
We compute the low-energy variation of the horizon entanglement entropy for matter fields and gravitons in Minkowski space. While the entropy is divergent, the variation under a perturbation of the vacuum state is finite and proportional to the energy flux through the Rindler horizon. Due to the universal coupling of gravitons to the energy-momentum tensor, the variation of the entanglement entropy is universal and equal to the change in area of the event horizon divided by 4 times Newton's constant - independently from the number and type of matter fields. The physical mechanism presented provides an explanation of the microscopic origin of the Bekenstein-Hawking entropy in terms of entanglement entropy.
7 pages
marcus
#8
Jan4-13, 08:23 PM
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Some things about the current frontrunners, possibly coincidental and not significant, stand out. The poll with 19 papers covered a broad range of authors and topics. But if you look at the 5 frontrunners, two topics stand out:
entanglement entropy in quantum gravity (2 out of 5)
classical support for the discreteness of the volume operator spectrum (2 out of 5)

Another feature that happens to show up is CROSSOVER from string theory and from hep-th (high energy physics, theoretical)

Haggard has previous papers and talks about how classical and semiclassical analysis supports the geometric operator discreteness. But now he is joined in studying the pentahedron, in particular, by Berndt Müller. Here's Müller's profile. 300-some citable papers (average # cites per paper = 30-some). Divided up by subject as follows:

Subject categories
Theory-Nucl (113)
Phenomenology-HEP (87)
Theory-HEP (17)
Experiment-Nucl (15)
Astrophysics (6)
...
...
So here is someone who is not usually part of Loop community, or attending Loop conferences etc, but has gotten interested in Haggard's work and in this question of chaotic classical system leading to discretenss of quantum operator spectrum.
http://inspirehep.net/author/B.Muller.1/
==================

Likewise with Bob Myers, who collaborated with Eugenio Bianchi on an entanglement entropy paper that is currently one of the frontrunners in our poll. He's not usually doing Loop research either.
http://inspirehep.net/author/R.C.Myers.1/
179 citable papers (average about 80 cites per) tagged with these keywords:
Field theory: conformal (38)
string model (34)
bibliography (31)
black hole (31)
holography (30)
supersymmetry (29)
field equations: solution (27)
membrane model: D-brane (27)
membrane model: p-brane (25)
duality (21)
...
...
So that's the kind of crossover I see that just happens to be exemplified in the lead papers in poll. It's not a big deal but it is nice to see little signs of it here and there. Another aspect of this: the International Advisory Committee for Loops 2013 is amazingly diverse. http://www.perimeterinstitute.ca/conferences/loops-13 Names like:
Giddings
Bjorken
Brandenberger
Dowker
Jacobson
Marcolli
Minic
Percacci
Unruh
and several others you wouldn't normally think of as being scientific advisors to the Loops conference.
Maybe this is one of the main thing that is happening this years, forming ties with other research areas.
Or the formation of a broad QG mainstream. I don't know anything definite about it, just noticing a few curious details like this which may mean nothing.
marcus
#9
Jan29-13, 04:02 PM
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Here's a short list of what I think are the most interesting papers that appeared this month. It gives an idea of what the first quarter 2013 is apt to look like.

It is important that the relation between LQG and the cosmology application LQC has been clarified by the Alesci Cianfrani and the Engle papers. One can do the symmetry reduction AFTER quantization. So there is no obstacle to viewing LQC as a straightforward application of the full theory. In fact Engle shows that one can EMBED LQC in full theory without ever invoking the piecewise linear category, or fixing on some particular graph structure.

This opens the way to testing full LQG theory by confronting LQC predictions with early universe observation. So it's a 2013 milestone.

The Marcolli Suijlekom paper opens a possible path to building the standard matter field model into LQG. It lets the NODES of the network be SPECTRAL GEOMETRY CHUNKS instead of ordinary geometry chunks. Thus the standard model can live there. It remains to be seen if this exciting gambit can accomplish its goal.

I have included the Gielen paper because I suspect that
1) merging with LQC will show LQG predicts a bounce cosmology
2) in view of observer-independent Tomita flow, a bounce cosmology has a preferred foliation
3) if the full LQG theory has a preferred foliation then Gielen's paper applies.

Wolfgang Wieland's paper puts the whole business of secondary constraints, reality conditions etc ON A NEW FOOTING. We should recognize that it changes the terms of the discussion. So it is a major paper.

http://arxiv.org/abs/1301.1264
Inflation as a prediction of loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 Jan 2013)
Loop quantum cosmology is known to be closely linked with an inflationary phase. In this article, we study quantitatively the probability for a long enough stage of slow-roll inflation to occur, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "pre-bounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is indeed more than enough to solve all the standard cosmological problems. In this precise sense, a satisfactory inflation is therefore a clear prediction of loop gravity. In addition, we derive an original and stringent upper limit on the Barbero-Immirzi parameter. The general picture about inflation, super-inflation, deflation and super-deflation is also much clarified in the framework of bouncing cosmologies.
6 pages, 4 figures

http://arxiv.org/abs/1301.1692
Contracted Lorentz Invariance for Gravity with a Preferred Foliation
Steffen Gielen
(Submitted on 8 Jan 2013)
In canonical gravity, the choice of a local time direction is not obviously compatible with local Lorentz invariance. One way to address this issue is to view gravity as a gauge theory on observer space, rather than spacetime. In a Lorentz covariant theory observer space is the space of unit future-directed timelike vectors; picking such a vector locally breaks the symmetry to a subgroup SO(D) of SO(D,1), so that on observer space the local symmetry group is SO(D).
Observer space geometries naturally describe any gravitational theory that only assumes local invariance under SO(D). Here we construct non-relativistic observer spaces for gravity with a fixed foliation,...This observer space geometry provides a novel geometric framework for the study of preferred foliation theories.
15 pages

http://arxiv.org/abs/1301.2245
Quantum-Reduced Loop Gravity: Cosmology
Emanuele Alesci, Francesco Cianfrani
(Submitted on 10 Jan 2013)
We introduce a new framework for loop quantum gravity: mimicking the spinfoam quantization procedure we propose to study the symmetric sectors of the theory imposing the reduction weakly on the full kinematical Hilbert space of the canonical theory. As a first application of Quantum-Reduced Loop Gravity we study the inhomogeneous Bianchi I model. The emerging quantum cosmological model represents a simplified arena on which the complete canonical quantization program can be tested. The achievements of this analysis could elucidate the relationship between Loop Quantum Cosmology and the full theory.
26 pages.

http://arxiv.org/abs/1301.3480
Gauge networks in noncommutative geometry
Matilde Marcolli, Walter D. van Suijlekom
(Submitted on 15 Jan 2013)
We introduce gauge networks as generalizations of spin networks and lattice gauge fields to almost-commutative manifolds. The configuration space of quiver representations (modulo equivalence) in the category of finite spectral triples is studied; gauge networks appear as an orthonormal basis in a corresponding Hilbert space. We give many examples of gauge networks, also beyond the well-known spin network examples. We find a Hamiltonian operator on this Hilbert space, inducing a time evolution on the C*-algebra of gauge network correspondences.
Given a representation in the category of spectral triples of a quiver embedded in a spin manifold, we define a discretized Dirac operator on the quiver. We compute the spectral action of this Dirac operator on a four-dimensional lattice, and find that it reduces to the Wilson action for lattice gauge theories and a Higgs field lattice system. As such, in the continuum limit it reduces to the Yang-Mills-Higgs system. For the three-dimensional case, we relate the spectral action functional to the Kogut-Susskind Hamiltonian.
30 pages

http://arxiv.org/abs/1301.5859
Hamiltonian spinfoam gravity
Wolfgang M. Wieland
(Submitted on 24 Jan 2013)
This paper presents a Hamiltonian formulation of spinfoam-gravity, which leads to a straight-forward canonical quantisation. To begin with, we derive a continuum action adapted to the simplicial decomposition. The equations of motion admit a Hamiltonian formulation, allowing us to perform the constraint analysis. We do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. This comes as a surprise. In the continuum theory, the reality conditions are preserved in time, only if the torsionless condition (a secondary constraint) holds true. Studying an additional conservation law for each spinfoam vertex, we discuss the issue of torsion and argue that spinfoam gravity may indeed miss an additional constraint. Next, we canonically quantise. Transition amplitudes match the EPRL (Engle--Pereira--Rovelli--Livine) model, the only difference being the additional torsional constraint affecting the vertex amplitude.
28 pages, 2 figures

http://arxiv.org/abs/1301.6210
Embedding loop quantum cosmology without piecewise linearity
Jonathan Engle
(Submitted on 26 Jan 2013)
An important goal is to understand better the relation between full loop quantum gravity (LQG) and the simplified, reduced theory known as loop quantum cosmology (LQC), directly at the quantum level. Such a firmer understanding would increase confidence in the reduced theory as a tool for formulating predictions of the full theory, as well as permitting lessons from the reduced theory to guide further development in the full theory. The present paper constructs an embedding of the usual state space of LQC into that of standard LQG, that is, LQG based on piecewise analytic paths. The embedding is well-defined even prior to solving the diffeomorphism constraint, at no point is a graph fixed, and at no point is the piecewise linear category used. This motivates for the first time a definition of operators in LQC corresponding to holonomies along non-piecewise-linear paths, without changing the usual kinematics of LQC in any way. The new embedding intertwines all operators corresponding to such holonomies, and all elements in its image satisfy an operator equation which classically implies homogeneity and isotropy. The construction is made possible by a recent result proven by Fleischhack.
18 pages

It will be fascinating to see what February and March add to this short list. This seems to be a time of creative ferment in several LQG-related areas of research.
Drakkith
#10
Jan29-13, 08:23 PM
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I'm afraid my head may explode if I click on any of the links...the knowledge contained within these tomes is overwhelming.
marcus
#11
Jan29-13, 08:57 PM
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Quote Quote by Drakkith View Post
I'm afraid my head may explode if I click on any of the links...the knowledge contained within these tomes is overwhelming.
Well you put it in a comical light, the way you say it, but it points to a serious bottleneck problem. You are a regular in cosmo forum so you are familiar with the fact that geometry is dynamic. There's gravitational lensing and dramatic amounts of expansion and collapse to extreme density etc.

So we humans collectively share this historical moment when we come to realize that the Greek geometry (of distances and lines of sight and angles) that we always believed in, and kinda still believe in, is wrong. Distances can increase without things moving in any ordinary sense. The gravity of a cluster can act as a giant magnifying glass and allow us to see farther. Triangles don't add to 180.

And moreover much of the physics we rely on from photosynthesis down to cell-phones is quantum—we are gradually becoming aware of that.

So for us to have a coherent picture of the world going back to the start of expansion (when densities were so extremely high that quantum effects must have been predominant) and coming up to the present we have to have, somebody must struggle to attain for us, a quantum conception of geometry.

And these people are, very gamely I think, struggling to do that. But there is a communication bottleneck. Nobody boils the struggle for quantum geometry down to bare comprehensible essentials.
Drakkith
#12
Jan29-13, 09:12 PM
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Quote Quote by marcus View Post
Well you put it in a comical light, the way you say it, but it points to a serious bottleneck problem. You are a regular in cosmo forum so you are familiar with the fact that geometry is dynamic. There's gravitational lensing and dramatic amounts of expansion and collapse to extreme density etc.

So we humans collectively share this historical moment when we come to realize that the Greek geometry (of distances and lines of sight and angles) that we always believed in, and kinda still believe in, is wrong. Distances can increase without things moving in any ordinary sense. The gravity of a cluster can act as a giant magnifying glass and allow us to see farther. Triangles don't add to 180.
Son of a...Marcus, you just made my day. I knew that geometry was...weird, when it comes to GR and such, but the way you put it was like a lightbulb blazing to life over my head like a protostar in the throws of birth. The simple statement of "the geometry we always believed in is wrong". I never thought about it being, well, wrong before. Feels like the time I finally realized that QM makes a buttload of sense if you just stop thinking that everything you expect coming from our scale is the "right" way.
marcus
#13
Jan31-13, 02:20 PM
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Hi Drakkith, it really helps get me thinking how to explain the different ways these papers are important when you nudge like that (in a nice way). The research going on now in QG is at a fairly deep level and it's not always easy to describe and connect the dots. It's encouraging to know of other people's interest.

A big thanks to Atyy, Chronos, Demystifier, Nonlinear, and Skydive Phil!

Six of us have responded to the poll and indicated our choices for the most significant papers of 4th quarter 2012. Here's how the votes stack up:

Four votes:

http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers


Two votes:

http://arxiv.org/abs/1212.4060
Black Hole Entropy from complex Ashtekar variables
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez

http://lanl.arxiv.org/abs/1212.1930
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron
Christopher E. Coleman-Smith, Berndt Müller

http://arxiv.org/abs/1211.7311
Pentahedral volume, chaos, and quantum gravity
Hal M. Haggard

http://arxiv.org/abs/1211.6269
The Matter Bounce Scenario in Loop Quantum Cosmology
Edward Wilson-Ewing

http://arxiv.org/abs/1211.2166
The spin connection of twisted geometry
Hal M. Haggard, Carlo Rovelli, Francesca Vidotto, Wolfgang Wieland

http://arxiv.org/abs/1211.1354
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1211.0522
Horizon entanglement entropy and universality of the graviton coupling
Eugenio Bianchi

Isochoric means "constant volume". That is how a pentahedron doing jazzercise can have something analogous to a Hamiltonian and a dynamical system. With a tetrahedron, a small change in how it starts will only result in a small change in where it goes. But chop one of the corners off the tetrahedron to make it have 5 faces, and be a pentahedron, and the flow becomes chaotic. The faces won't even always be triangular---a small change at the start can lead to radically different development later on. In the Berndt Müller paper one can think of isochoric, or constant volume, as analogous to an energy conservation restriction. So the pentahedron has a phase space, largely determined by the variable areas of its 5 faces, and you give it a shove to start it and it does this stretchy-squooshy modern dance, but always staying the same volume.

The special relevance to quantum geometry (LQG) is that it validates the "volume gap". Here's part of the abstract of Hal Haggard's paper:
"We show that chaotic classical dynamics associated to the volume of discrete grains of space leads to quantal spectra that are gapped between zero and nonzero volume. This strengthens the connection between spectral discreteness in the quantum geometry of gravity and tame ultraviolet behavior."
marcus
#14
Feb5-13, 05:01 PM
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I'll update the short list of especially interesting papers that have appeared so far in first quarter 2013.

http://arxiv.org/abs/1302.0724
Death and resurrection of the zeroth principle of thermodynamics
Hal M. Haggard, Carlo Rovelli
(Submitted on 4 Feb 2013)
The zeroth principle of thermodynamics in the form "temperature is uniform at equilibrium" is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.
5 pages, 2 figures

]http://arxiv.org/abs/1302.0254
The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 1 Feb 2013)
Using techniques from loop quantum gravity, the standard theory of cosmological perturbations was recently generalized to encompass the Planck era. We now apply this framework to explore pre-inflationary dynamics. The framework enables us to isolate and resolve the true trans-Planckian difficulties, with interesting lessons both for theory and observations. Specifically, for a large class of initial conditions at the bounce, we are led to a self consistent extension of the inflationary paradigm over the 11 orders of magnitude in density and curvature, from the big bounce to the onset of slow roll. In addition, for a narrow window of initial conditions, there are departures from the standard paradigm, with novel effects ---such as a modification of the consistency relation between the ratio of the tensor to scalar power spectrum and the tensor spectral index, as well as a new source for non-Gaussianities--- which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
64 pages, 15 figures

http://arxiv.org/abs/1301.6210
Embedding loop quantum cosmology without piecewise linearity
Jonathan Engle
(Submitted on 26 Jan 2013)
An important goal is to understand better the relation between full loop quantum gravity (LQG) and the simplified, reduced theory known as loop quantum cosmology (LQC), directly at the quantum level. Such a firmer understanding would increase confidence in the reduced theory as a tool for formulating predictions of the full theory, as well as permitting lessons from the reduced theory to guide further development in the full theory. The present paper constructs an embedding of the usual state space of LQC into that of standard LQG, that is, LQG based on piecewise analytic paths. The embedding is well-defined even prior to solving the diffeomorphism constraint, at no point is a graph fixed, and at no point is the piecewise linear category used. This motivates for the first time a definition of operators in LQC corresponding to holonomies along non-piecewise-linear paths, without changing the usual kinematics of LQC in any way. The new embedding intertwines all operators corresponding to such holonomies, and all elements in its image satisfy an operator equation which classically implies homogeneity and isotropy. The construction is made possible by a recent result proven by Fleischhack.
18 pages

http://arxiv.org/abs/1301.5859
Hamiltonian spinfoam gravity
Wolfgang M. Wieland
(Submitted on 24 Jan 2013)
This paper presents a Hamiltonian formulation of spinfoam-gravity, which leads to a straight-forward canonical quantisation. To begin with, we derive a continuum action adapted to the simplicial decomposition. The equations of motion admit a Hamiltonian formulation, allowing us to perform the constraint analysis. We do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. This comes as a surprise. In the continuum theory, the reality conditions are preserved in time, only if the torsionless condition (a secondary constraint) holds true. Studying an additional conservation law for each spinfoam vertex, we discuss the issue of torsion and argue that spinfoam gravity may indeed miss an additional constraint. Next, we canonically quantise. Transition amplitudes match the EPRL (Engle--Pereira--Rovelli--Livine) model, the only difference being the additional torsional constraint affecting the vertex amplitude.
28 pages, 2 figures

http://arxiv.org/abs/1301.3480
Gauge networks in noncommutative geometry
Matilde Marcolli, Walter D. van Suijlekom
(Submitted on 15 Jan 2013)
We introduce gauge networks as generalizations of spin networks and lattice gauge fields to almost-commutative manifolds. The configuration space of quiver representations (modulo equivalence) in the category of finite spectral triples is studied; gauge networks appear as an orthonormal basis in a corresponding Hilbert space. We give many examples of gauge networks, also beyond the well-known spin network examples. We find a Hamiltonian operator on this Hilbert space, inducing a time evolution on the C*-algebra of gauge network correspondences.
Given a representation in the category of spectral triples of a quiver embedded in a spin manifold, we define a discretized Dirac operator on the quiver. We compute the spectral action of this Dirac operator on a four-dimensional lattice, and find that it reduces to the Wilson action for lattice gauge theories and a Higgs field lattice system. As such, in the continuum limit it reduces to the Yang-Mills-Higgs system. For the three-dimensional case, we relate the spectral action functional to the Kogut-Susskind Hamiltonian.
30 pages

http://arxiv.org/abs/1301.2245
Quantum-Reduced Loop Gravity: Cosmology
Emanuele Alesci, Francesco Cianfrani
(Submitted on 10 Jan 2013)
We introduce a new framework for loop quantum gravity: mimicking the spinfoam quantization procedure we propose to study the symmetric sectors of the theory imposing the reduction weakly on the full kinematical Hilbert space of the canonical theory. As a first application of Quantum-Reduced Loop Gravity we study the inhomogeneous Bianchi I model. The emerging quantum cosmological model represents a simplified arena on which the complete canonical quantization program can be tested. The achievements of this analysis could elucidate the relationship between Loop Quantum Cosmology and the full theory.
26 pages.

http://arxiv.org/abs/1301.1264
Inflation as a prediction of loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 Jan 2013)
Loop quantum cosmology is known to be closely linked with an inflationary phase. In this article, we study quantitatively the probability for a long enough stage of slow-roll inflation to occur, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "pre-bounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is indeed more than enough to solve all the standard cosmological problems. In this precise sense, a satisfactory inflation is therefore a clear prediction of loop gravity. In addition, we derive an original and stringent upper limit on the Barbero-Immirzi parameter. The general picture about inflation, super-inflation, deflation and super-deflation is also much clarified in the framework of bouncing cosmologies.
6 pages, 4 figures

A few comments:

http://arxiv.org/abs/1212.5226
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
was significantly revised on 30 January 2013 in a way that strongly suggests the existence of a fourth neutrino, which might constitute the extra matter in the standard LCDM cosmology.

The Haggard Rovelli will, I think, turn out to be basic. It gives a reasonable idea both of (1) time and the physical meaning of the passage of time, and (2) the meaning of equilibrium and temperature in a general covariant context.

The Alesci Cianfrani and the Engle papers are important because they clarify the relation between LQG and the cosmology application LQC. One can do the symmetry reduction AFTER quantization. So there is no obstacle to viewing LQC as a straightforward application of the full theory. Predictions stemming from the LQC bounce at the start of expansion (prior to conventional inflation) can therefore serve to test the full LQG theory.

The Marcolli Suijlekom paper opens a possible way to build the standard matter field model into LQG. It lets the NODES of the network be SPECTRAL GEOMETRY CHUNKS instead of ordinary geometry chunks. Thus the standard model can live there.

Wolfgang Wieland's paper puts the business of simplicity constraints and reality conditions on a new footing. It changes the terms of the discussion and so is a major paper.
marcus
#15
Feb10-13, 03:01 PM
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The first quarter is approaching the halfway mark and we seem to have a bunch of potentially quite significant Lqg papers--over half being relevant to cosmology. Aldo Riello's indicates that the dependence of the spin foam model on the vacuum curvature Lambda is logarithmic. That's encouraging and hopefully it will be born out by future work.
Agullo et al extend LQC analysis into the pre-inflation epoch--modeling matter fields and inhomogeneities near the bounce. Engle and Alesci et al show that LQC is in fact a valid application of the full Lqg theory. Haggard and Rovelli have extended the idea of thermodynamic/informational equilibrium to covariant systems.

I'll comment further on the current crop of papers after listing what I think are 8 especially interesting ones that have appeared so far this year. For conciseness some of the abstracts (which appear in the previous post in full) have been cut.

http://arxiv.org/abs/1302.1781
Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity
Aldo Riello
(Submitted on 7 Feb 2013)
We calculate the most divergent contribution to the self-energy (or "melonic") graph in the context of the Lorentzian EPRL-FK Spin Foam model of Quantum Gravity. We find that such a contribution is logarithmically divergent in the cut-off over the SU(2)-representation spins when one chooses the face amplitude guaranteeing the face-splitting invariance of the foam. We also find that the dependence on the boundary data is different from that of the bare propagator. This fact has its origin in the non-commutativity of the EPRL-FK Y-map with the projector onto SL(2,C)-invariant states. In the course of the paper, we discuss in detail the approximations used during the calculations, its geometrical interpretation as well as the physical consequences of our result.
55 pages, 8 figures

http://arxiv.org/abs/1302.0724
Death and resurrection of the zeroth principle of thermodynamics
Hal M. Haggard, Carlo Rovelli
(Submitted on 4 Feb 2013)
The zeroth principle of thermodynamics in the form "temperature is uniform at equilibrium" is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.
5 pages, 2 figures

http://arxiv.org/abs/1302.0254
The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 1 Feb 2013)
Using techniques from loop quantum gravity, the standard theory of cosmological perturbations was recently generalized to encompass the Planck era. We now apply this framework to explore pre-inflationary dynamics. The framework enables us to isolate and resolve the true trans-Planckian difficulties, with interesting lessons both for theory and observations. ... departures from the standard paradigm, with novel effects ---such as a modification of the consistency relation between the ratio of the tensor to scalar power spectrum and the tensor spectral index, as well as a new source for non-Gaussianities--- which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
64 pages, 15 figures

http://arxiv.org/abs/1301.6210
Embedding loop quantum cosmology without piecewise linearity
Jonathan Engle
(Submitted on 26 Jan 2013)
An important goal is to understand better the relation between full loop quantum gravity (LQG) and the simplified, reduced theory known as loop quantum cosmology (LQC), directly at the quantum level. Such a firmer understanding would increase confidence in the reduced theory as a tool for formulating predictions of the full theory, ...The present paper constructs an embedding of the usual state space of LQC into that of standard LQG, that is, LQG based on piecewise analytic paths. The embedding is well-defined even prior to solving the diffeomorphism constraint, at no point is a graph fixed, and at no point is the piecewise linear category used. ... The construction is made possible by a recent result proven by Fleischhack.
18 pages

http://arxiv.org/abs/1301.5859
Hamiltonian spinfoam gravity
Wolfgang M. Wieland
(Submitted on 24 Jan 2013)
This paper presents a Hamiltonian formulation of spinfoam-gravity, which leads to a straight-forward canonical quantisation. To begin with, we derive a continuum action adapted to the simplicial decomposition. The equations of motion admit a Hamiltonian formulation, allowing us to perform the constraint analysis. We do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. ... Transition amplitudes match the EPRL (Engle--Pereira--Rovelli--Livine) model, the only difference being the additional torsional constraint affecting the vertex amplitude.
28 pages, 2 figures

http://arxiv.org/abs/1301.3480
Gauge networks in noncommutative geometry
Matilde Marcolli, Walter D. van Suijlekom
(Submitted on 15 Jan 2013)
We introduce gauge networks as generalizations of spin networks and lattice gauge fields to almost-commutative manifolds. ... gauge networks appear as an orthonormal basis in a corresponding Hilbert space. We give many examples of gauge networks, also beyond the well-known spin network examples. We find a Hamiltonian operator on this Hilbert space, inducing a time evolution on the C*-algebra of gauge network correspondences.
... we define a discretized Dirac operator on the quiver. We compute the spectral action of this Dirac operator on a four-dimensional lattice, and find that it reduces to the Wilson action for lattice gauge theories and a Higgs field lattice system. As such, in the continuum limit it reduces to the Yang-Mills-Higgs system. ...
30 pages

http://arxiv.org/abs/1301.2245
Quantum-Reduced Loop Gravity: Cosmology
Emanuele Alesci, Francesco Cianfrani
(Submitted on 10 Jan 2013)
We introduce a new framework for loop quantum gravity: mimicking the spinfoam quantization procedure we propose to study the symmetric sectors of the theory imposing the reduction weakly on the full kinematical Hilbert space of the canonical theory...The achievements of this analysis could elucidate the relationship between Loop Quantum Cosmology and the full theory.
26 pages.

http://arxiv.org/abs/1301.1264
Inflation as a prediction of loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 Jan 2013)
Loop quantum cosmology is known to be closely linked with an inflationary phase. In this article, we study quantitatively the probability for a long enough stage of slow-roll inflation to occur, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "pre-bounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is indeed more than enough to solve all the standard cosmological problems...
6 pages, 4 figures



The Marcolli Suijlekom paper provides a way to put standard model matter into Lqg. The notes of the network (called a "quiver") become chunks of spectral geometry (in which the standard particle model is endemic) rather than chunks of ordinary geometry. Thus the standard model becomes native, in a certain sense, to this new version of Lqg.

Wolfgang Wieland's paper puts the theory's simplicity constraints and reality conditions on a new footing.
marcus
#16
Feb14-13, 09:24 PM
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Thanks to all who have responded so far!
I've often learned from considering other people's perspectives on the most interesting and significant recent QG research papers. Here's how the votes stack up:

Four votes:

http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers

http://arxiv.org/abs/1211.2166
The spin connection of twisted geometry
Hal M. Haggard, Carlo Rovelli, Francesca Vidotto, Wolfgang Wieland

Three votes:

http://arxiv.org/abs/1212.4060
Black Hole Entropy from complex Ashtekar variables
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez

Two votes:

http://lanl.arxiv.org/abs/1212.1930
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron
Christopher E. Coleman-Smith, Berndt Müller

http://arxiv.org/abs/1211.7311
Pentahedral volume, chaos, and quantum gravity
Hal M. Haggard

http://arxiv.org/abs/1211.6269
The Matter Bounce Scenario in Loop Quantum Cosmology
Edward Wilson-Ewing

http://arxiv.org/abs/1211.1354
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1211.0522
Horizon entanglement entropy and universality of the graviton coupling
Eugenio Bianchi

http://arxiv.org/abs/1210.0418
Interpretation of the triad orientations in loop quantum cosmology
Claus Kiefer, Christian Schell
marcus
#17
Mar5-13, 09:58 PM
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In view of the upcoming MIP poll, here are a few Loop and allied papers of special interest that have appeared so far in the first quarter of 2013.

http://arxiv.org/abs/1303.0195
Living in Curved Momentum Space
J. Kowalski-Glikman
(Submitted on 1 Mar 2013)
In this paper we review some aspects of relativistic particles' mechanics in the case of a non-trivial geometry of momentum space. We start with showing how the curved momentum space arises in the theory of gravity in 2+1 dimensions coupled to particles, when (topological) degrees of freedom of gravity are solved for. We argue that there might exist a similar topological phase of quantum gravity in 3+1 dimensions. Then we characterize the main properties of the theory of interacting particles with curved momentum space and the symmetries of the action. We discuss the spacetime picture and the emergence of the principle of relative locality, according to which locality of events is not absolute but becomes observer dependent, in the controllable, relativistic way. We conclude with the detailed review of the most studied kappa-Poincare framework, which corresponds to the de Sitter momentum space.
23 pages

http://arxiv.org/abs/1302.7142
Holonomy Operator and Quantization Ambiguities on Spinor Space
Etera R. Livine
(Submitted on 28 Feb 2013)
We construct the holonomy-flux operator algebra in the recently developed spinor formulation of loop gravity. We show that, when restricting to SU(2)-gauge invariant operators, the familiar grasping and Wilson loop operators are written as composite operators built from the gauge-invariant 'generalized ladder operators' recently introduced in the U(N) approach to intertwiners and spin networks. We comment on quantization ambiguities that appear in the definition of the holonomy operator and use these ambiguities as a toy model to test a class of quantization ambiguities which is present in the standard regularization and definition of the Hamiltonian constraint operator in loop quantum gravity.
14 pages

http://arxiv.org/abs/1302.7037
Loop Quantization of Shape Dynamics
Tim Koslowski
(Submitted on 28 Feb 2013)
Loop Quantum Gravity (LQG) is a promising approach to quantum gravity, in particular because it is based on a rigorous quantization of the kinematics of gravity. A difficult and still open problem in the LQG program is the construction of the physical Hilbert space for pure quantum gravity. This is due to the complicated nature of the Hamilton constraints. The Shape Dynamics description of General Relativity (GR) replaces the Hamilton constraints with spatial Weyl constraints, so the problem of finding the physical Hilbert space reduces to the problem of quantizing the Weyl constraints. Unfortunately, it turns out that a loop quantization of Weyl constraints is far from trivial despite their intuitive physical interpretation. A tentative quantization proposal and interpretation proposal is given in this contribution.
3 pages

http://arxiv.org/abs/1302.5265
The loop quantum gravity black hole
Rodolfo Gambini, Jorge Pullin
(Submitted on 21 Feb 2013)
We quantize spherically symmetric vacuum gravity without gauge fixing the diffeomorphism constraint. Through a rescaling, we make the algebra of Hamiltonian constraints Abelian and therefore the constraint algebra is a true Lie algebra. This allows the completion of the Dirac quantization procedure using loop quantum gravity techniques. We can construct explicitly the exact solutions of the physical Hilbert space annihilated by all constraints. New observables living in the bulk appear at the quantum level (analogous to spin in quantum mechanics) that are not present at the classical level and are associated with the discrete nature of the spin network states of loop quantum gravity. The resulting quantum space-times resolve the singularity present in the classical theory inside black holes. The new observables that arise suggest a possible resolution for the "firewall" problem of evaporating black holes.
4 pages

http://arxiv.org/abs/1302.3833
Loop Quantum Cosmology
Ivan Agullo, Alejandro Corichi
(Submitted on 15 Feb 2013)
This Chapter provides an up to date, pedagogical review of some of the most relevant advances in loop quantum cosmology. We review the quantization of homogeneous cosmological models, their singularity resolution and the formulation of effective equations that incorporate the main quantum corrections to the dynamics. We also summarize the theory of quantized metric perturbations propagating in those quantum backgrounds. Finally, we describe how this framework can be applied to obtain a self-consistent extension of the inflationary scenario to incorporate quantum aspects of gravity, and to explore possible phenomenological consequences.
52 pages, 5 figures. To appear as a Chapter of "The Springer Handbook of Spacetime," edited by A. Ashtekar and V. Petkov. (Springer-Verlag, at Press).

http://arxiv.org/abs/1302.1781
Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity
Aldo Riello
(Submitted on 7 Feb 2013)
We calculate the most divergent contribution to the self-energy (or "melonic") graph in the context of the Lorentzian EPRL-FK Spin Foam model of Quantum Gravity. We find that such a contribution is logarithmically divergent in the cut-off over the SU(2)-representation spins when one chooses the face amplitude guaranteeing the face-splitting invariance of the foam. We also find that the dependence on the boundary data is different from that of the bare propagator. This fact has its origin in the non-commutativity of the EPRL-FK Y-map with the projector onto SL(2,C)-invariant states. In the course of the paper, we discuss in detail the approximations used during the calculations, its geometrical interpretation as well as the physical consequences of our result.
55 pages, 8 figures

http://arxiv.org/abs/1302.0724
Death and resurrection of the zeroth principle of thermodynamics
Hal M. Haggard, Carlo Rovelli
(Submitted on 4 Feb 2013)
The zeroth principle of thermodynamics in the form "temperature is uniform at equilibrium" is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.
5 pages, 2 figures

http://arxiv.org/abs/1302.0254
The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 1 Feb 2013)
Using techniques from loop quantum gravity, the standard theory of cosmological perturbations was recently generalized to encompass the Planck era. We now apply this framework to explore pre-inflationary dynamics. The framework enables us to isolate and resolve the true trans-Planckian difficulties, with interesting lessons both for theory and observations. ... departures from the standard paradigm, with novel effects ---such as a modification of the consistency relation between the ratio of the tensor to scalar power spectrum and the tensor spectral index, as well as a new source for non-Gaussianities--- which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
64 pages, 15 figures

http://arxiv.org/abs/1301.6210
Embedding loop quantum cosmology without piecewise linearity
Jonathan Engle
(Submitted on 26 Jan 2013)
An important goal is to understand better the relation between full loop quantum gravity (LQG) and the simplified, reduced theory known as loop quantum cosmology (LQC), directly at the quantum level. Such a firmer understanding would increase confidence in the reduced theory as a tool for formulating predictions of the full theory, ...The present paper constructs an embedding of the usual state space of LQC into that of standard LQG, that is, LQG based on piecewise analytic paths. The embedding is well-defined even prior to solving the diffeomorphism constraint, at no point is a graph fixed, and at no point is the piecewise linear category used. ... The construction is made possible by a recent result proven by Fleischhack.
18 pages

http://arxiv.org/abs/1301.5859
Hamiltonian spinfoam gravity
Wolfgang M. Wieland
(Submitted on 24 Jan 2013)
This paper presents a Hamiltonian formulation of spinfoam-gravity, which leads to a straight-forward canonical quantisation. To begin with, we derive a continuum action adapted to the simplicial decomposition. The equations of motion admit a Hamiltonian formulation, allowing us to perform the constraint analysis. We do not find any secondary constraints, but only get restrictions on the Lagrange multipliers enforcing the reality conditions. ... Transition amplitudes match the EPRL (Engle--Pereira--Rovelli--Livine) model, the only difference being the additional torsional constraint affecting the vertex amplitude.
28 pages, 2 figures

http://arxiv.org/abs/1301.3480
Gauge networks in noncommutative geometry
Matilde Marcolli, Walter D. van Suijlekom
(Submitted on 15 Jan 2013)
We introduce gauge networks as generalizations of spin networks and lattice gauge fields to almost-commutative manifolds. ... gauge networks appear as an orthonormal basis in a corresponding Hilbert space. We give many examples of gauge networks, also beyond the well-known spin network examples. We find a Hamiltonian operator on this Hilbert space, inducing a time evolution on the C*-algebra of gauge network correspondences.
... we define a discretized Dirac operator on the quiver. We compute the spectral action of this Dirac operator on a four-dimensional lattice, and find that it reduces to the Wilson action for lattice gauge theories and a Higgs field lattice system. As such, in the continuum limit it reduces to the Yang-Mills-Higgs system. ...
30 pages

http://arxiv.org/abs/1301.2245
Quantum-Reduced Loop Gravity: Cosmology
Emanuele Alesci, Francesco Cianfrani
(Submitted on 10 Jan 2013)
We introduce a new framework for loop quantum gravity: mimicking the spinfoam quantization procedure we propose to study the symmetric sectors of the theory imposing the reduction weakly on the full kinematical Hilbert space of the canonical theory...The achievements of this analysis could elucidate the relationship between Loop Quantum Cosmology and the full theory.
26 pages.

http://arxiv.org/abs/1301.1264
Inflation as a prediction of loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 Jan 2013)
Loop quantum cosmology is known to be closely linked with an inflationary phase. In this article, we study quantitatively the probability for a long enough stage of slow-roll inflation to occur, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "pre-bounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is indeed more than enough to solve all the standard cosmological problems...
6 pages, 4 figures
marcus
#18
Mar6-13, 04:59 PM
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PF Gold
marcus's Avatar
P: 23,274
Cordial thanks! to Adpc, Atyy, Chronos, Demystifier, Devils, Erkant, Julcab12, Nonlinear, and Skydive Phil. It can be instructive to see what others have identified as especially significant research. I often find myself taking a second look at a paper others of us pick, to better understand its potential importance.

Here's how the votes currently stack up:

Four votes:

http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers

http://arxiv.org/abs/1212.4060
Black Hole Entropy from complex Ashtekar variables
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1211.2166
The spin connection of twisted geometry
Hal M. Haggard, Carlo Rovelli, Francesca Vidotto, Wolfgang Wieland

Two votes:

http://lanl.arxiv.org/abs/1212.1930
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron
Christopher E. Coleman-Smith, Berndt Müller

http://arxiv.org/abs/1211.7311
Pentahedral volume, chaos, and quantum gravity
Hal M. Haggard

http://arxiv.org/abs/1211.6269
The Matter Bounce Scenario in Loop Quantum Cosmology
Edward Wilson-Ewing

http://arxiv.org/abs/1211.1354
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1211.0522
Horizon entanglement entropy and universality of the graviton coupling
Eugenio Bianchi

http://arxiv.org/abs/1210.4504
A new perspective on cosmology in Loop Quantum Gravity
Emanuele Alesci, Francesco Cianfrani

http://arxiv.org/abs/1210.0418
Interpretation of the triad orientations in loop quantum cosmology
Claus Kiefer, Christian Schell

Plus six that I didn't list here, each of which only got ONE vote, so far.


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