# Our picks for second quarter 2015 MIP (most important QG paper)

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

20. ### No fermion doubling in quantum geometry

1. Jun 30, 2015

### marcus

Please indicate the papers you think will prove most significant for future Loop-and-allied QG research. The poll is multiple choice, so it's possible to vote for several. Abstracts follow in the next post.

http://arxiv.org/abs/1504.01065
Wilson loops in CDT quantum gravity
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll

http://arxiv.org/abs/1504.02169
Coherent states, quantum gravity and the Born-Oppenheimer approximation, I: General considerations
Alexander Stottmeister, Thomas Thiemann

http://arxiv.org/abs/1504.02170
Coherent states, quantum gravity and the Born-Oppenheimer approximation, II: Compact Lie Groups
Alexander Stottmeister, Thomas Thiemann

http://arxiv.org/abs/1504.02171
Coherent states, quantum gravity and the Born-Oppenheimer approximation, III: Applications to loop quantum gravity
Alexander Stottmeister, Thomas Thiemann

http://arxiv.org/abs/1504.02822
Duality between Spin networks and the 2D Ising model
Valentin Bonzom, Francesco Costantino, Etera R. Livine

http://arxiv.org/abs/1504.05352
Black hole spectroscopy from Loop Quantum Gravity models
Aurelien Barrau, Xiangyu Cao, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1504.07559
Loop quantum cosmology: From pre-inflationary dynamics to observations
Abhay Ashtekar, Aurelien Barrau

http://arxiv.org/abs/1504.07100
Quantum Holonomy Theory
Johannes Aastrup, Jesper M. Grimstrup

http://arxiv.org/abs/1505.00223
Graphical method in loop quantum gravity: I. Derivation of the closed formula for the matrix element of the volume operator
Jinsong Yang, Yongge Ma

http://arxiv.org/abs/1505.00225
Graphical method in loop quantum gravity: II. The Hamiltonian constraint and inverse volume operators
Jinsong Yang, Yongge Ma

http://arxiv.org/abs/1505.03119
Computing the Effective Action with the Functional Renormalization Group
Alessandro Codello, Roberto Percacci, Leslaw Rachwal, Alberto Tonero

http://arxiv.org/abs/1505.04088
Gravitational crystal inside the black hole
H. Nikolic

http://arxiv.org/abs/1505.04753
Entanglement equilibrium and the Einstein equation
Ted Jacobson

http://arxiv.org/abs/1506.00299
New scalar constraint operator for loop quantum gravity
Mehdi Assanioussi, Jerzy Lewandowski, Ilkka Mäkinen

http://arxiv.org/abs/1506.01018
Hawking Radiation Energy and Entropy from a Bianchi-Smerlak Semiclassical Black Hole
Shohreh Abdolrahimi, Don N. Page

http://arxiv.org/abs/1506.03053
Encoding Curved Tetrahedra in Face Holonomies: a Phase Space of Shapes from Group-Valued Moment Maps
Hal M. Haggard, Muxin Han, Aldo Riello

http://arxiv.org/abs/1506.04749
Coupled intertwiner dynamics - a toy model for coupling matter to spin foam models
Sebastian Steinhaus

http://arxiv.org/abs/1506.08073
Lie Group Cosmology
A. Garrett Lisi

http://arxiv.org/abs/1506.08571
A new realization of quantum geometry
Benjamin Bahr, Bianca Dittrich, Marc Geiller

http://arxiv.org/abs/1506.08794
No fermion doubling in quantum geometry
Rodolfo Gambini, Jorge Pullin

2. Jun 30, 2015

### marcus

http://arxiv.org/abs/1504.01065
Wilson loops in CDT quantum gravity
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 4 Apr 2015)
By explicit construction, we show that one can in a simple way introduce and measure gravitational holonomies and Wilson loops in lattice formulations of nonperturbative quantum gravity based on (Causal) Dynamical Triangulations. We use this set-up to investigate a class of Wilson line observables associated with the world line of a point particle coupled to quantum gravity, and deduce from their expectation values that the underlying holonomies cover the group manifold of SO(4) uniformly.
30 pages, 5 figures

http://arxiv.org/abs/1504.02169
Coherent states, quantum gravity and the Born-Oppenheimer approximation, I: General considerations
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
This article, as the first of three, aims at establishing the (time-dependent) Born-Oppenheimer approximation, in the sense of space adiabatic perturbation theory, for quantum systems constructed by techniques of the loop quantum gravity framework, especially the canonical formulation of the latter. The analysis presented here fits into a rather general framework, and offers a solution to the problem of applying the usual Born-Oppenheimer ansatz for molecular (or structurally analogous) systems to more general quantum systems (e.g. spin-orbit models) by means of space adiabatic perturbation theory. The proposed solution is applied to a simple, finite dimensional model of interacting spin systems, which serves as a non-trivial, minimal model of the aforesaid problem. Furthermore, it is explained how the content of this article, and its companion, affect the possible extraction of quantum field theory on curved spacetime from loop quantum gravity (including matter fields).

http://arxiv.org/abs/1504.02170
Coherent states, quantum gravity and the Born-Oppenheimer approximation, II: Compact Lie Groups
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
In this article, the second of three, we discuss and develop the basis of a Weyl quantisation for compact Lie groups aiming at loop quantum gravity-type models. This Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity. Additionally, we conjecture the existence of a new form of the Segal-Bargmann-Hall "coherent state" transform for compact Lie groups G, which we prove for G=U(1)n and support by numerical evidence for G=SU(2). The reason for conjoining this conjecture with the main topic of this article originates in the observation, that the coherent state transform can be used as a basic building block of a coherent state quantisation (Berezin quantisation) for compact Lie groups G. But, as Weyl and Berezin quantisation for ℝ2d are intimately related by heat kernel evolution, it is natural to ask, whether a similar connection exists for compact Lie groups, as well. Moreover, since the formulation of space adiabatic perturbation theory requires a (deformation) quantisation as minimal input, we analyse the question to what extent the coherent state quantisation, defined by the Segal-Bargmann-Hall transform, can serve as basis of the former.

http://arxiv.org/abs/1504.02171
Coherent states, quantum gravity and the Born-Oppenheimer approximation, III: Applications to loop quantum gravity
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
In this article, the third of three, we analyse how the Weyl quantisation for compact Lie groups presented in the second article of this series fits with the projective-phase space structure of loop quantum gravity-type models. Thus, the proposed Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity.

http://arxiv.org/abs/1504.02822
Duality between Spin networks and the 2D Ising model
Valentin Bonzom, Francesco Costantino, Etera R. Livine
(Submitted on 11 Apr 2015)
The goal of this paper is to exhibit a deep relation between the partition function of the Ising model on a planar trivalent graph and the generating series of the spin network evaluations on the same graph. We provide respectively a fermionic and a bosonic Gaussian integral formulation for each of these functions and we show that they are the inverse of each other (up to some explicit constants) by exhibiting a supersymmetry relating the two formulations. We investigate three aspects and applications of this duality. First, we propose higher order supersymmetric theories which couple the geometry of the spin networks to the Ising model and for which supersymmetric localization still holds. Secondly, after interpreting the generating function of spin network evaluations as the projection of a coherent state of loop quantum gravity onto the flat connection state, we find the probability distribution induced by that coherent state on the edge spins and study its stationary phase approximation. It is found that the stationary points correspond to the critical values of the couplings of the 2D Ising model, at least for isoradial graphs. Third, we analyze the mapping of the correlations of the Ising model to spin network observables, and describe the phase transition on those observables on the hexagonal lattice. This opens the door to many new possibilities, especially for the study of the coarse-graining and continuum limit of spin networks in the context of quantum gravity.
35 pages

http://arxiv.org/abs/1504.05352
Black hole spectroscopy from Loop Quantum Gravity models
Aurelien Barrau, Xiangyu Cao, Karim Noui, Alejandro Perez
(Submitted on 21 Apr 2015)
Using Monte Carlo simulations, we compute the integrated emission spectra of black holes in the framework of Loop Quantum Gravity (LQG). The black hole emission rates are governed by the entropy whose value, in recent holographic loop quantum gravity models, was shown to agree at leading order with the Bekenstein-Hawking entropy. Quantum corrections depend on the Barbero-Immirzi parameter γ. Starting with black holes of initial horizon area A∼102 in Planck units, we present the spectra for different values of γ. Each spectrum clearly decomposes in two distinct parts: a continuous background which corresponds to the semi-classical stages of the evaporation and a series of discrete peaks which constitutes a signature of the deep quantum structure of the black hole. We show that γ has an effect on both parts that we analyze in details. Finally, we estimate the number of black holes and the instrumental resolution required to experimentally distinguish between the considered models.
11 pages, 9 figures

http://arxiv.org/abs/1504.07559
Loop quantum cosmology: From pre-inflationary dynamics to observations
Abhay Ashtekar, Aurelien Barrau
(Submitted on 28 Apr 2015)
The Planck collaboration has provided us rich information about the early universe, and a host of new observational missions will soon shed further light on the anomalies' that appear to exist on the largest angular scales. From a quantum gravity perspective, it is natural to inquire if one can trace back the origin of such puzzling features to Planck scale physics. Loop quantum cosmology provides a promising avenue to explore this issue because of its natural resolution of the big bang singularity. Thanks to advances over the last decade, the theory has matured sufficiently to allow concrete calculations of the phenomenological consequences of its pre-inflationary dynamics. In this article we summarize the current status of the ensuing two-way dialog between quantum gravity and observations.
20 pages, 5 figures. Invited review article for the "focus issue" of Classical and Quantum Gravity : "Planck and the fundamentals of cosmology"

http://arxiv.org/abs/1504.07100
Quantum Holonomy Theory
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 27 Apr 2015)
We present quantum holonomy theory, which is a non-perturbative theory of quantum gravity coupled to fermionic degrees of freedom. The theory is based on a C*-algebra that involves holonomy-diffeomorphisms on a 3-dimensional manifold and which encodes the canonical commutation relations of canonical quantum gravity formulated in terms of Ashtekar variables. Employing a Dirac type operator on the configuration space of Ashtekar connections we obtain a semi-classical state and a kinematical Hilbert space via its GNS construction. We use the Dirac type operator, which provides a metric structure over the space of Ashtekar connections, to define a scalar curvature operator, from which we obtain a candidate for a Hamilton operator. We show that the classical Hamilton constraint of general relativity emerges from this in a semi-classical limit and we then compute the operator constraint algebra. Also, we find states in the kinematical Hilbert space on which the expectation value of the Dirac type operator gives the Dirac Hamiltonian in a semi-classical limit and thus provides a connection to fermionic quantum field theory. Finally, an almost-commutative algebra emerges from the holonomy-diffeomorphism algebra in the same limit.
76 pages, 6 figures

http://arxiv.org/abs/1505.00223
Graphical method in loop quantum gravity: I. Derivation of the closed formula for the matrix element of the volume operator
Jinsong Yang, Yongge Ma
(Submitted on 1 May 2015)
To adopt a practical method to calculate the action of geometrical operators on quantum states is a crucial task in loop quantum gravity. In the series of papers, we will introduce a graphical method, developed by Yutsis and Brink, to loop quantum gravity. The graphical method provides a very powerful technique for simplifying complicated calculations. In this first paper, the closed formula of volume operator is derived via the graphical method. By employing suitable and non-ambiguous graphs to represent the acting of operators as well as the spin network states, we use the simple rules for transforming graphs to yield the resulting formula. Comparing with the complicated algebraic derivation in some literatures, our procedure is more concise, intuitive and visual. The resulting matrix elements of volume operator is compact and uniform, fitting for both gauge-invariant and gauge-variant spin network states.
40 pages

http://arxiv.org/abs/1505.00225
Graphical method in loop quantum gravity: II. The Hamiltonian constraint and inverse volume operators
Jinsong Yang, Yongge Ma
(Submitted on 1 May 2015)
This is the second paper in the series to introduce a graphical method to loop quantum gravity. We employ the graphical method as a powerful tool to calculate the actions of the Hamiltonian constraint operator and the so-called inverse volume operator on spin network states with trivalent vertices. Both of the operators involve the co-triad operator which contains holonomies by construction. The non-ambiguous, concise and visual characters of our graphical method ensure the rigour for our calculations. Our results indicate some corrections to the existing results in literatures for both operators.
19 pages

http://arxiv.org/abs/1505.03119
Computing the Effective Action with the Functional Renormalization Group
Alessandro Codello, Roberto Percacci, Leslaw Rachwal, Alberto Tonero
(Submitted on 12 May 2015)
The "exact" or "functional" renormalization group equation describes the renormalization group flow of the effective average action Γk. The ordinary effective action Γ0 can be obtained by integrating the flow equation from an ultraviolet scale k=Λ down to k=0. We give several examples of such calculations at one-loop, both in renormalizable and in effective field theories. We use the results of Barvinsky, Vilkovisky and Avramidi on the non-local heat kernel coefficients to reproduce the four point scattering amplitude in the case of a real scalar field theory with quartic potential and in the case of the pion chiral lagrangian. In the case of gauge theories, we reproduce the vacuum polarization of QED and of Yang-Mills theory. We also compute the two point functions for scalars and gravitons in the effective field theory of scalar fields minimally coupled to gravity.
40 pages

http://arxiv.org/abs/1505.04088
Gravitational crystal inside the black hole
H. Nikolic
(Submitted on 15 May 2015)
Crystals, as quantum objects typically much larger than their lattice spacing, are a counterexample to a frequent prejudice that quantum effects should not be pronounced at macroscopic distances. We propose that the Einstein theory of gravity only describes a fluid phase and that a phase transition of crystallization can occur under extreme conditions such as those inside the black hole. Such a crystal phase with lattice spacing of the order of the Planck length offers a natural mechanism for pronounced quantum-gravity effects at distances much larger than the Planck length. A resolution of the black-hole information paradox is proposed, according to which all information is stored in a crystal-phase remnant with size and mass much above the Planck scale.
6 pages

http://arxiv.org/abs/1505.04753
Entanglement equilibrium and the Einstein equation
Ted Jacobson
(Submitted on 18 May 2015)
We show that the semiclassical Einstein equation holds if and only if the entanglement entropy in small causal diamonds is stationary at constant volume, when varied from a maximally symmetric vacuum state of geometry and quantum fields. The argument hinges on a conjecture about the variation of the conformal boost energy of quantum fields in small diamonds.
7 pages

http://arxiv.org/abs/1506.00299
New scalar constraint operator for loop quantum gravity
Mehdi Assanioussi, Jerzy Lewandowski, Ilkka Mäkinen
(Submitted on 31 May 2015)
We present a concrete and explicit construction of a new scalar constraint operator for loop quantum gravity. The operator is defined on the recently introduced space of partially diffeomorphism invariant states, and this space is preserved by the action of the operator. To define the Euclidean part of the scalar constraint operator, we propose a specific regularization based on the idea of so-called "special" loops. The Lorentzian part of the quantum scalar constraint is merely the curvature operator that has been introduced in an earlier work. Due to the properties of the special loops assignment, the adjoint operator of the non-symmetric constraint operator is densely defined on the partially diffeomorphism invariant Hilbert space. This fact opens up the possibility of defining a symmetric scalar constraint operator as a suitable combination of the original operator and its adjoint. We also show that the algebra of the scalar constraint operators is anomaly free, and describe the structure of the kernel of these operators on a general level.
14 pages.

http://arxiv.org/abs/1506.01018
Hawking Radiation Energy and Entropy from a Bianchi-Smerlak Semiclassical Black Hole
Shohreh Abdolrahimi, Don N. Page
(Submitted on 2 Jun 2015)
Eugenio Bianchi and Matteo Smerlak have found a relationship between the Hawking radiation energy and von Neumann entropy in a conformal field emitted by a semiclassical two-dimensional black hole. We compare this relationship with what might be expected for unitary evolution of a quantum black hole in four and higher dimensions. If one neglects the expected increase in the radiation entropy over the decrease in the black hole Bekenstein-Hawking A/4 entropy that arises from the scattering of the radiation by the barrier near the black hole, the relation works very well, except near the peak of the radiation von Neumann entropy and near the final evaporation. These discrepancies are calculated and discussed as tiny differences between a semiclassical treatment and a quantum gravity treatment.
17 pages.

http://arxiv.org/abs/1506.03053
Encoding Curved Tetrahedra in Face Holonomies: a Phase Space of Shapes from Group-Valued Moment Maps
Hal M. Haggard, Muxin Han, Aldo Riello
(Submitted on 9 Jun 2015)
We present a generalization of Minkowski's classic theorem on the reconstruction of tetrahedra from algebraic data to homogeneously curved spaces. Euclidean notions such as the normal vector to a face are replaced by Levi-Civita holonomies around each of the tetrahedron's faces. This allows the reconstruction of both spherical and hyperbolic tetrahedra within a unified framework. A new type of hyperbolic simplex is introduced in order for all the sectors encoded in the algebraic data to be covered. Generalizing the phase space of shapes associated to flat tetrahedra leads to group valued moment maps and quasi-Poisson spaces. These discrete geometries provide a natural arena for considering the quantization of gravity including a cosmological constant. A concrete realization of this is provided by the relation with the spin-network states of loop quantum gravity. This work therefore provides a bottom-up justification for the emergence of deformed gauge symmetries and quantum groups in 3+1 dimensional covariant loop quantum gravity in the presence of a cosmological constant.
38 pages and 9 figures

http://arxiv.org/abs/1506.08073
Lie Group Cosmology
A. Garrett Lisi
(Submitted on 24 Jun 2015)
Our universe is a deforming Lie group.
42 pages, 1 figure

http://arxiv.org/abs/1506.04749
Coupled intertwiner dynamics - a toy model for coupling matter to spin foam models
Sebastian Steinhaus
The universal coupling of matter and gravity is one of the most important features of general relativity. In quantum gravity, in particular spin foams, matter couplings have been defined in the past, yet the mutual dynamics, in particular if matter and gravity are strongly coupled, are hardly explored, which is related to the definition of both matter and gravitational degrees of freedom on the discretisation. However extracting this mutual dynamics is crucial in testing the viability of the spin foam approach and also establishing connections to other discrete approaches such as lattice gauge theories.
Therefore, we introduce a simple 2D toy model for Yang--Mills coupled to spin foams, namely an Ising model coupled to so--called intertwiner models defined for SU(2)k. The two systems are coupled by choosing the Ising coupling constant to depend on spin labels of the background, as these are interpreted as the edge lengths of the discretisation. We coarse grain this toy model via tensor network renormalization and uncover an interesting dynamics: the Ising phase transition temperature turns out to be sensitive to the background configurations and conversely, the Ising model can induce phase transitions in the background. Moreover, we observe a strong coupling of both systems if close to both phase transitions.
31 + 6 pages, 8 figures, 7 tables

http://arxiv.org/abs/1506.08571
A new realization of quantum geometry
Benjamin Bahr, Bianca Dittrich, Marc Geiller
(Submitted on 29 Jun 2015)
We construct in this article a new realization of quantum geometry, which is obtained by quantizing the recently-introduced flux formulation of loop quantum gravity. In this framework, the vacuum is peaked on flat connections, and states are built upon it by creating local curvature excitations. The inner product induces a discrete topology on the gauge group, which turns out to be an essential ingredient for the construction of a continuum limit Hilbert space. This leads to a representation of the full holonomy-flux algebra of loop quantum gravity which is unitarily-inequivalent to the one based on the Ashtekar-Isham-Lewandowski vacuum. It therefore provides a new notion of quantum geometry. We discuss how the spectra of geometric operators, including holonomy and area operators, are affected by this new quantization. In particular, we find that the area operator is bounded, and that there are two different ways in which the Barbero-Immirzi parameter can be taken into account. The methods introduced in this work open up new possibilities for investigating further realizations of quantum geometry based on different vacua.
72 pages, 6 figures

http://arxiv.org/abs/1506.08794
No fermion doubling in quantum geometry
Rodolfo Gambini, Jorge Pullin
(Submitted on 29 Jun 2015)
In loop quantum gravity the discrete nature of quantum geometry acts as a natural regulator for matter theories. Studies of quantum field theory in quantum space-times in spherical symmetry in the canonical approach have shown that the main effect of the quantum geometry is to discretize the equations of matter fields. This raises the possibility that in the case of fermion fields one could confront the usual fermion doubling problem that arises in lattice gauge theories. We suggest, again based on recent results on spherical symmetry, that since the background space-times will generically involve superpositions of states associated with different discretizations the phenomenon may not arise. This opens a possibility of incorporating chiral fermions in the framework of loop quantum gravity.
2 pages.

3. Jul 1, 2015

### strangerep

Marcus,

I'm curious about these regular polls which have been going for a while now. Have you also compiled any correlations (or lack thereof) between these polls, and actual impact (years later) on real world physics?

(For that matter, how would one even construct a measure of such correlation??)

4. Jul 1, 2015

### marcus

Hi Strangerep, thanks for your interest. Frankly I'm interested in keeping up and if possible improving my own modest ability to forecast impact and judge the potential of new developments. I think it is one way to tell how well I understand the field if I do OK at gauging the importance of new research---if I don't miss too many papers that later prove significant and garner a lot of cites, get used and built on in later research, and so on.

So for me, choosing which candidates go on the quarterly poll and then voting is a kind of regular "exercise machine" or "obstacle course" that I set for myself. I go on record, so to speak, and as a matter of course recall and look back later. I also learn from seeing what other people vote for. I am not so interested in group performance---for one thing the group of respondents changes quite a lot and it is hardly a representative sample of PF or BtSM. But I get a dim but helpful impression of some savvy individuals by what they pick to vote for, and I learn from them. BTW I'd be interested to see your assessment of the current QG research. : ^)

Since I'm taking the trouble to train my own insight and adjust my own perspective on the field, and going on record so I can look back later and say "duh!" or give myself a pat on the back it seems sociable to share the opportunity with anyone else who wants to do the same. And there are also people whose interests I've come to know, whose choices I pay special attention to, and whose viewpoints I try to understand.

Last edited: Jul 1, 2015
5. Jul 1, 2015

### strangerep

Oh, OK. I didn't understand that this was the purpose of these threads.

TBH, I'm underwhelmed by most of it, since there seems to be precious little plausible contact with real world physics. Occasionally a paper will pop up offering a potential (though maybe implausible) experimental avenue, but I get the feeling most QG researchers are not really interested in that side of things.

In this sense, I feel similarly to how I felt about string theory unification efforts 20+ yrs ago -- when I perceived that it was too divorced from real world physics (even way back then). I found it easy to decide not to invest much time in it. I'm feeling quietly pleased with myself now.

6. Jul 1, 2015

### marcus

This paper is #7 on the second quarter MIP poll. I would urge anyone who seriously believes in the importance of LQG connecting with real world observations to take a look:
http://arxiv.org/abs/1504.07559
Loop quantum cosmology: From pre-inflationary dynamics to observations
Abhay Ashtekar, Aurelien Barrau
(Submitted on 28 Apr 2015)
The Planck collaboration has provided us rich information about the early universe, and a host of new observational missions will soon shed further light on the anomalies' that appear to exist on the largest angular scales. From a quantum gravity perspective, it is natural to inquire if one can trace back the origin of such puzzling features to Planck scale physics. Loop quantum cosmology provides a promising avenue to explore this issue because of its natural resolution of the big bang singularity. Thanks to advances over the last decade, the theory has matured sufficiently to allow concrete calculations of the phenomenological consequences of its pre-inflationary dynamics. In this article we summarize the current status of the ensuing two-way dialog between quantum gravity and observations.
20 pages, 5 figures. Invited review article for the "focus issue" of Classical and Quantum Gravity : "Planck and the fundamentals of cosmology"

7. Jul 1, 2015

### marcus

Here's another Loop cosmology paper concerned with real world physics (i.e. astrophysical observations. ) It happens to be #7 on the fourth quarter 2014 poll:
https://www.physicsforums.com/threa...mip-poll-for-most-important-qg-papers.789671/

http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 9 Dec 2014)
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. ... Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
14 pages, 8 figures

Again to help objectively gauge the temperature of LQG interest in real world physics (primarily through astrophysical observation) here is a section of the Loops 2015 Plenary Session schedule of talks July 6 thru July 10, so next week:
==quote==

Plenary Session
Tuesday, 09:00 - 10:45
Early-Universe Cosmology: Issues and Opportunities
09:00 - 09:45, Anna Ijjas (Princeton University, USA)
Inflationary cosmology is commonly considered as the “standard model" of the early-universe, though, it has several open issues. Some of the problems have been known since the introduction of inflationary theory in the early 80s but some of the problems have been realized first later as we learnt to better understand the theory and gained more experimental data. I will present the main problems with inflation, discuss alternative approaches and point to the opportunities the current situation gives us.

Numerical loop quantum cosmology: overview and recent results
10:00 - 10:45, Parampreet Singh (Louisiana State Universitty, USA)
In this talk, we will discuss some of the recent developments in the investigations on singularity resolution using numerical simulations in loop quantum cosmology. Thanks to the introduction of new techniques, the bounce can now be established for a wide variety of states and the reliability of the effective dynamics can be checked rigorously. These results will be discussed in isotropic and anisotropic models.

Plenary Session
Tuesday, 11:15 - 13:00
Hybrid Loop Quantum Cosmology
11:15 - 12:00, Mercedes Martin-Benito (Radboud University Nijmegen, Netherlands)
I will present an overview of the hybrid approach to quantize inhomogeneous cosmologies in the framework of loop quantum cosmology. Though I will keep the discussion as general as possible, for definiteness I will focus on two particular examples largely studied so far: linearly polarized Gowdy cosmologies with spatial three-torus topology, which is the simplest inhomogeneous cosmological model, and the flat homogeneous and isotropic model minimally coupled to a scalar field and with cosmological perturbations. After explaining the hybrid quantization of these models, I will discuss some aspects of the resulting quantum dynamics, and also comment on approximations that one can introduce to extract physical results.

Loop quantum cosmology and alternatives to inflation
12:15 - 13:00, Edward Wilson-Ewing (Albert Einstein Institute, Germany)
High precision observations of the cosmic microwave background provide strong constraints on the dynamics of the early universe and raise the hope that it may be possible to detect quantum gravity effects. In this talk, I will focus on realizations of the ekpyrotic and matter bounce scenarios in loop quantum cosmology. These are alternatives to inflation where scale-invariant perturbations are generated in a contracting background Friedmann space-time which later bounces due to loop quantum cosmology effects. I will show how it is possible to explicitly calculate the evolution of the perturbations through the non-singular bounce and explain under what conditions scale-invariance is preserved. I will also discuss how loop quantum cosmology can affect observational quantities; one such effect is a damping of the amplitude of tensor modes.

Plenary Session
Wednesday, 09:00 - 10:45
Pathways in Quantum Gravity Phenomenology
09:00 - 09:45, Stefano Liberati (SISSA, Trieste, Italy)
In this talk I will offer a panoramic view on the lessons and the achievements gathered so far in the quest for probing the fabric of spacetime. In particular, I will discuss possible scenarios for the mesoscopic physics between our classical world and full quantum gravity regimes. In doing so I shall focus on the perspectives to test them so to gain some guidance in the development of quantum gravity.
==endquote==

Last edited: Jul 1, 2015
8. Jul 3, 2015

### marcus

Thanks to all who have made your assessments of the past quarter's research known! and gotten the poll off to a good start. What I see already taking shape is a sense that what matters most now are papers with a new vision and a new ontology.
By that I mean they present a new idea of what quantum states the interactive geometric reality consists of. A different Hilbert space, a different algebra of observables defined on it. Garrett Lisi's paper is clearly a favorite but there are three others three runners-up and they all do this in one way or another.

I started a thread about the Bahr-Dittrich-Geiller paper, which achieves a new version of LQG which is unitarily inequivalent to the one based on Ashtekar-Lewandowski vacuum, and which is more closely compatible with the spinfoam QG side of Loop gravity, the path integral development of the theory. It's good to have the two sides mutually supportive and consistent. BDG states are built of local curvature excitations on an uncurved vacuum.

Another of the runners-up is a paper by Jesper Grimstrup and Johannes Aastrup that presents a really new basis for quantum geometry.
http://arxiv.org/abs/1504.07100
Quantum Holonomy Theory
Johannes Aastrup, Jesper M. Grimstrup
I'll quote some from the introduction at the beginning and also briefly from the conclusions (page 60) near the end.
======quote======
The search for fundamental principles in Nature is the leitmotiv of modern physics. ...The dream is to uncover an ultimate principle behind it all – the fundamental theory – which will end the reductionist ladder of descent and leave no door to peek behind.

The framework of quantum holonomy theory proposes such a first principle. The theory is built over an algebra that encodes how diffeomorphisms act on spinors. Thus, the fundamental building blocks are ”moving stuff in space” and as such seem immune to further reduction: the question ”what are diffeomorphisms made of?” makes little sense.
==endquote==
The idea is you cannot get more basic, in geometry, than how stuff is moved around, IOW than the diffeomorphisms themselves

==quote==
What we find is a non-perturbative and background independent quantum mechanics of diffeomorphisms, where we on the one hand have an algebra generated by...
==endquote==

Then in the conclusions, on page 60, they re-emphasize this and list the 4 main objectives which the Quantum Holonomy model achieves:

==quote page 60==
In this paper we have presented quantum holonomy theory, which is a nonperturbative and background independent theory of quantum gravity coupled to quantized degrees of matter.

Four central objectives are met in this paper. The first is the formulation of a first principle – namely the QHD(M) algebra –, which serves as the foundation for this approach to a theory of quantum gravity. It is the conceptual simplicity of the QHD(M) algebra, which makes it attractive. What could be more natural, more poetic, as a foundation for a theory of quantum gravity than an algebra that simply encodes how tensorial degrees of freedom – i.e. stuff – are moved in space? We find it surprising that this algebra, which encodes the mathematical setup of canonical quantum gravity, has not, to the best of our knowledge, been studied before.

The second central objective met in this paper is the finding that semiclassical states exist on ... the algebra obtained from QHD(M) by forming a canonical Dirac type operator and considering its commutators ..
..., we find that semi-classical states appear extremely natural in our approach. ..

The third central objective met in this paper is the formulation of a geometrical principle, which provides us with a Hamilton constraint operator from which the classical Hamilton constraint emerges in a semi-classical limit. This means that we do obtain general relativity in a semi-classical limit from our construction.

The fourth central objective met in this paper is the identification of elements and mechanisms of unification. This point is essentially an adaptation of results already published. First of all, the HD(M) algebra is inherently non-commutative, which immediately places our construction well within the domain of [getting the Standard Model of particle theory as in Connes' Non-commutative Geometry]
==endquote==

Last edited: Jul 3, 2015
9. Jul 7, 2015

### marcus

Papers which might be candidates for the 3rd quarter poll:

http://arxiv.org/abs/1507.08112
Running of the scalar spectral index in bouncing cosmologies
Jean-Luc Lehners, Edward Wilson-Ewing
(Submitted on 29 Jul 2015)
We calculate the running of the scalar index in the ekpyrotic and matter bounce cosmological scenarios, and find that it is typically negative for ekpyrotic models, while it is typically positive for realizations of the matter bounce where multiple fields are present. This can be compared to inflation, where the observationally preferred models typically predict a negative running. The magnitude of the running is expected to be between 10−4 and up to 10−2, leading in some cases to interesting expectations for near-future observations.
6 pages

http://arxiv.org/abs/1507.07591
Discrete Hamiltonian for General Relativity
Jonathan Ziprick, Jack Gegenberg
(Submitted on 27 Jul 2015)
Beginning from canonical general relativity written in terms of Ashtekar variables, we derive a discrete phase space with a physical Hamiltonian for gravity. The key idea is to define the gravitational fields within a complex of three-dimensional cells such that the dynamics is completely described by discrete boundary variables, and the full theory is recovered in the continuum limit. Canonical quantization is attainable within the loop quantum gravity framework, and we believe this will lead to a promising candidate for quantum gravity.
6 pages

http://arxiv.org/abs/1507.05424
Phenomenology of bouncing black holes in quantum gravity: a closer look
Aurelien Barrau, Boris Bolliet, Francesca Vidotto, Celine Weimer
(Submitted on 20 Jul 2015)
It was recently shown that black holes could be bouncing stars as a consequence of quantum gravity. We investigate the astrophysical signals implied by this hypothesis, focusing on primordial black holes. We consider different possible bounce times and study the integrated diffuse emission.
8 pages, 8 figures

http://arxiv.org/abs/1507.04703
Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
Ivan Agullo
(Submitted on 16 Jul 2015)
We argue that the anomalous power asymmetry observed in the cosmic microwave background (CMB) may have originated in a cosmic bounce preceding inflation. In loop quantum cosmology (LQC) the big bang singularity is generically replaced by a bounce due to quantum gravitational effects. We compute the spectrum of inflationary non-Gaussianity and show that strong correlation between observable scales and modes with longer (super-horizon) wavelength arise as a consequence of the evolution of perturbations across the LQC bounce. These correlations are strongly scale dependent and induce a dipole-dominated modulation on large angular scales in the CMB, in agreement with observations.
7 pages, 3 figure

http://arxiv.org/abs/1507.02573
Quantum gravity at the corner
Laurent Freidel, Alejandro Perez
(Submitted on 9 Jul 2015)
We investigate the quantum geometry of 2d surface S bounding the Cauchy slices of 4d gravitational system. We investigate in detail and for the first time the symplectic current that naturally arises boundary term in the first order formulation of general relativity in terms of the Ashtekar-Barbero connection. This current is proportional to the simplest quadratic form constructed out of the triad field, pulled back on S. We show that the would-be-gauge degrees of freedom---arising from SU(2) gauge transformations plus diffeomorphisms tangent to the boundary, are entirely described by the boundary 2-dimensional symplectic form and give rise to a representation at each point of S of SL(2,ℝ)×SU(2). Independently of the connection with gravity, this system is very simple and rich at the quantum level with possible connections with conformal field theory in 2d. A direct application of the quantum theory is modelling of the black horizons in quantum gravity.
16 pages, 3 figures.

http://arxiv.org/abs/1507.01153
Coherent State Operators in Loop Quantum Gravity
Emanuele Alesci, Andrea Dapor, Jerzy Lewandowski, Ilkka Makinen, Jan Sikorski
(Submitted on 4 Jul 2015)
We present a new method for constructing operators in loop quantum gravity. The construction is an application of the general idea of "coherent state quantization", which allows one to associate a unique quantum operator to every function on a classical phase space. Using the heat kernel coherent states of Hall and Thiemann, we show how to construct operators corresponding to functions depending on holonomies and fluxes associated to a fixed graph. We construct the coherent state versions of the fundamental holonomy and flux operators, as well as the basic geometric operators of area, angle and volume. Our calculations show that the corresponding canonical operators are recovered from the coherent state operators in the limit of large spins.
34 pages, 4 figures

http://arxiv.org/abs/1507.01149
Loop quantum gravity coupled to a scalar field
Jerzy Lewandowski, Hanno Sahlmann
(Submitted on 4 Jul 2015)
We reconsider the Rovelli-Smolin model of gravity coupled to the Klein-Gordon time field with an eye towards capturing the degrees of freedom of the scalar field lost in the framework in which time is deparametrized by the scalar field. Several new results for loop quantum gravity are obtained:
(i) a Hilbert space for the gravity-matter system and a non-standard representation of the scalar field thereon is constructed,
(ii) a new operator for the scalar constraint of the coupled system is defined and investigated,
(iii) methods for solving the constraint are developed.
Commutators of the new constraint do not vanish, but seem to reproduce a part of the Dirac algebra. This, however, poses problems for finding solutions. Hence the states we consider -- and perhaps the whole setup -- still needs some improvement.
As a side result we describe a representation of the gravitational degrees of freedom in which the flux is diagonal. This representation bears a strong resemblance to the BF vacuum of Dittrich and Geiller.
18 pages

http://arxiv.org/abs/1507.00986
New Hamiltonian constraint operator for loop quantum gravity
Jinsong Yang, Yongge Ma
(Submitted on 3 Jul 2015)
A new symmetric Hamiltonian constraint operator is proposed for loop quantum gravity, which is well defined in the Hilbert space of diffeomorphism invariant states up to non-planar vertices. On one hand, it inherits the advantage of the original regularization method, so that its regulated version in the kinematical Hilbert space is diffeomorphism covariant and creates new vertices to the spin networks. On the other hand, it overcomes the problem in the original treatment, so that there is less ambiguity in its construction and its quantum algebra is anomaly-free in a suitable sense. The regularization procedure for the Hamiltonian constraint operator can also be applied to the symmetric model of loop quantum cosmology, which leads to a new quantum dynamics of the cosmological model.
5 pages

http://arxiv.org/abs/1507.00851
Ashtekar-Barbero holonomy on the hyperboloid: Immirzi parameter as a Cut-off for Quantum Gravity
Christoph Charles, Etera R. Livine
(Submitted on 3 Jul 2015)
In the context of the geometrical interpretation of the spin network states of Loop Quantum Gravity, we look at the holonomies of the Ashtekar-Barbero connection on loops embedded in space-like hyperboloids. We use this simple setting to illustrate two points. First, the Ashtekar-Barbero connection is not a space-time connection, its holonomies depend on the spacetime embedding of the canonical hypersurface. This fact is usually interpreted as an inconvenience, but we use it to extract the extrinsic curvature from the holonomy and separate it from the 3d intrinsic curvature. Second, we show the limitations of this reconstruction procedure, due to a periodicity of the holonomy in the Immirzi parameter, which underlines the role of a real Immirzi parameter as a cut-off for general relativity at the quantum level in contrast with its role of a mere coupling constant at the classical level.
8 pages

http://arxiv.org/abs/1507.00226
Cyclic universe from Loop Quantum Gravity
Francesco Cianfrani, Jerzy Kowalski-Glikman, Giacomo Rosati
(Submitted on 1 Jul 2015 (v1), last revised 9 Jul 2015 (this version, v2))
We discuss how a cyclic model for the flat universe can be constructively derived from Loop Quantum Gravity. This model has a lower bounce, at small values of the scale factor, which shares many similarities with that of Loop Quantum Cosmology. We find that quantum gravity corrections can be also relevant at energy densities much smaller than the Planckian one and that they can induce an upper bounce at large values of the scale factor.
4 pages

Last edited: Aug 11, 2015
10. Jul 8, 2015

### marcus

Atyy, Chronos, David Horgan, Garrett, and Lariko, it's good to see what you think about the relative importance of the April-June QG research papers. Comparing others perspectives and choices to my own always gets me to see different angles and I learn from it. Thanks to everyone who has voted!

So far the paper by Garrett Lisi is in first place.

Two papers are tied for second place: One is the Quantum Holonomy paper by Aastrup and Grimstrup. It too is an approach to unifying gravity (GR) with the standard model of particle physics--in a way by taking a fresh look at what reality is "made of", e.g. spacetime is seen as the different ways stuff can move.
The other, by Ashtekar and Barrau, maps out the way to convert Loop cosmology's pre-inflation dynamics (what it says was going on BEFORE inflation and even before the start of expansion) into predictions about do-able observations. Cosmo phenomenology is Aurelien Barrau's specialty: what can be seen in the sky that enables us to test theories.

There are also, at this point, two papers tied for third place: a remarkable one by Bianca Dittrich, Marc Geiller, and Benjamin Bahr which is also about re-envisioning quantum gravity, a new realization of what geometry is "made of",
and one by Haggard, Han, and Riello which further develops the exciting idea of implementing the cosmological curvature constant Lambda in simplicial QG by using curved simplexes embodying that slight residual curvature--that intrinsic "vacuum curvature"--instead of flat ones.

Last edited: Jul 10, 2015
11. Aug 1, 2015

### marcus

Nonlinearity has joined us! We are seven who have taken part in the poll, thus far. Here's how the votes stack up at this point:

First place:
http://arxiv.org/abs/1506.08073
Lie Group Cosmology
A. Garrett Lisi

Tied for second place:
http://arxiv.org/abs/1504.07559
Loop quantum cosmology: From pre-inflationary dynamics to observations
Abhay Ashtekar, Aurelien Barrau

http://arxiv.org/abs/1504.07100
Quantum Holonomy Theory
Johannes Aastrup, Jesper M. Grimstrup

Tied for third place:
http://arxiv.org/abs/1505.04753
Entanglement equilibrium and the Einstein equation
Ted Jacobson

http://arxiv.org/abs/1506.03053
Encoding Curved Tetrahedra in Face Holonomies: a Phase Space of Shapes from Group-Valued Moment Maps
Hal M. Haggard, Muxin Han, Aldo Riello

http://arxiv.org/abs/1506.08571
A new realization of quantum geometry
Benjamin Bahr, Bianca Dittrich, Marc Geiller

=====================
Votes recently registered for these, 4 of which until then no one had picked, called my attention to them and made me want to better understand their possible significance.
http://arxiv.org/abs/1504.02822
Duality between Spin networks and the 2D Ising model
Valentin Bonzom, Francesco Costantino, Etera R. Livine

http://arxiv.org/abs/1504.05352
Black hole spectroscopy from Loop Quantum Gravity models
Aurelien Barrau, Xiangyu Cao, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1505.04088
Gravitational crystal inside the black hole
H. Nikolic

http://arxiv.org/abs/1505.04753
Entanglement equilibrium and the Einstein equation
Ted Jacobson

http://arxiv.org/abs/1506.01018
Hawking Radiation Energy and Entropy from a Bianchi-Smerlak Semiclassical Black Hole
Shohreh Abdolrahimi, Don N. Page

Last edited: Aug 1, 2015
12. Aug 11, 2015

### marcus

Potential candidates for the 3rd quarter poll:

http://arxiv.org/abs/1508.01947
Chaos, Dirac observables and constraint quantization
Bianca Dittrich, Philipp A. Hoehn, Tim A. Koslowski, Mike I. Nelson
(Submitted on 8 Aug 2015)
There is good evidence that full general relativity is non-integrable or even chaotic. We point out the severe repercussions: differentiable Dirac observables and a reduced phase space do not exist in non-integrable constrained systems and are thus unlikely to occur in a generic general relativistic context. Instead, gauge invariant quantities generally become discontinuous, thus not admitting Poisson-algebraic structures and posing serious challenges to a quantization. Non-integrability also renders the paradigm of relational dynamics cumbersome, thereby straining common interpretations of the dynamics. We illustrate these conceptual and technical challenges with simple toy models. In particular, we exhibit reparametrization invariant models which fail to be integrable and, as a consequence, can either not be quantized with standard methods or lead to sick quantum theories without a semiclassical limit. These troubles are qualitatively distinct from semiclassical subtleties in unconstrained quantum chaos and can be directly traced back to the scarcity of Dirac observables. As a possible resolution, we propose to change the method of quantization by refining the configuration space topology until the generalized observables become continuous in the new topology and can acquire a quantum representation. This leads to the polymer quantization method underlying loop quantum cosmology and gravity. Remarkably, the polymer quantum theory circumvents the problems of the quantization with smooth topology, indicating that non-integrability and chaos, while a challenge, may not be a fundamental obstruction for quantum gravity.
48 pages, 9 figures, lots of discussion

http://arxiv.org/abs/1508.01416
Spin Foams Without Spins
Jeff Hnybida
(Submitted on 6 Aug 2015)
We formulate the spin foam representation of discrete SU(2) gauge theory as a product of vertex amplitudes each of which is the spin network generating function of the boundary graph dual to the vertex. Thus the sums over spins have been carried out. We focus on the character expansion of Yang-Mills theory which is an approximate heat kernel regularization of BF theory. The boundary data of each n-valent node is an element of the Grassmannian Gr(2,n) which carries a coherent representation of U(n) and a geometrical interpretation as a framed polyhedron of fixed total area. Ultimately, sums over spins are traded for contour integrals over simple poles and recoupling theory is avoided using generating functions.
21 pages, 2 figures

http://arxiv.org/abs/1507.08112
Running of the scalar spectral index in bouncing cosmologies
Jean-Luc Lehners, Edward Wilson-Ewing
(Submitted on 29 Jul 2015)
We calculate the running of the scalar index in the ekpyrotic and matter bounce cosmological scenarios, and find that it is typically negative for ekpyrotic models, while it is typically positive for realizations of the matter bounce where multiple fields are present. This can be compared to inflation, where the observationally preferred models typically predict a negative running. The magnitude of the running is expected to be between 10−4 and up to 10−2, leading in some cases to interesting expectations for near-future observations.
6 pages

http://arxiv.org/abs/1507.07591
Discrete Hamiltonian for General Relativity
Jonathan Ziprick, Jack Gegenberg
(Submitted on 27 Jul 2015)
Beginning from canonical general relativity written in terms of Ashtekar variables, we derive a discrete phase space with a physical Hamiltonian for gravity. The key idea is to define the gravitational fields within a complex of three-dimensional cells such that the dynamics is completely described by discrete boundary variables, and the full theory is recovered in the continuum limit. Canonical quantization is attainable within the loop quantum gravity framework, and we believe this will lead to a promising candidate for quantum gravity.
6 pages

http://arxiv.org/abs/1507.05424
Phenomenology of bouncing black holes in quantum gravity: a closer look
Aurelien Barrau, Boris Bolliet, Francesca Vidotto, Celine Weimer
(Submitted on 20 Jul 2015)
It was recently shown that black holes could be bouncing stars as a consequence of quantum gravity. We investigate the astrophysical signals implied by this hypothesis, focusing on primordial black holes. We consider different possible bounce times and study the integrated diffuse emission.
8 pages, 8 figures

http://arxiv.org/abs/1507.04703
Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
Ivan Agullo
(Submitted on 16 Jul 2015)
We argue that the anomalous power asymmetry observed in the cosmic microwave background (CMB) may have originated in a cosmic bounce preceding inflation. In loop quantum cosmology (LQC) the big bang singularity is generically replaced by a bounce due to quantum gravitational effects. We compute the spectrum of inflationary non-Gaussianity and show that strong correlation between observable scales and modes with longer (super-horizon) wavelength arise as a consequence of the evolution of perturbations across the LQC bounce. These correlations are strongly scale dependent and induce a dipole-dominated modulation on large angular scales in the CMB, in agreement with observations.
7 pages, 3 figure

http://arxiv.org/abs/1507.02573
Quantum gravity at the corner
Laurent Freidel, Alejandro Perez
(Submitted on 9 Jul 2015)
We investigate the quantum geometry of 2d surface S bounding the Cauchy slices of 4d gravitational system. We investigate in detail and for the first time the symplectic current that naturally arises boundary term in the first order formulation of general relativity in terms of the Ashtekar-Barbero connection. This current is proportional to the simplest quadratic form constructed out of the triad field, pulled back on S. We show that the would-be-gauge degrees of freedom---arising from SU(2) gauge transformations plus diffeomorphisms tangent to the boundary, are entirely described by the boundary 2-dimensional symplectic form and give rise to a representation at each point of S of SL(2,ℝ)×SU(2). Independently of the connection with gravity, this system is very simple and rich at the quantum level with possible connections with conformal field theory in 2d. A direct application of the quantum theory is modelling of the black horizons in quantum gravity.
16 pages, 3 figures.

http://arxiv.org/abs/1507.01153
Coherent State Operators in Loop Quantum Gravity
Emanuele Alesci, Andrea Dapor, Jerzy Lewandowski, Ilkka Makinen, Jan Sikorski
(Submitted on 4 Jul 2015)
We present a new method for constructing operators in loop quantum gravity. The construction is an application of the general idea of "coherent state quantization", which allows one to associate a unique quantum operator to every function on a classical phase space. Using the heat kernel coherent states of Hall and Thiemann, we show how to construct operators corresponding to functions depending on holonomies and fluxes associated to a fixed graph. We construct the coherent state versions of the fundamental holonomy and flux operators, as well as the basic geometric operators of area, angle and volume. Our calculations show that the corresponding canonical operators are recovered from the coherent state operators in the limit of large spins.
34 pages, 4 figures

http://arxiv.org/abs/1507.01149
Loop quantum gravity coupled to a scalar field
Jerzy Lewandowski, Hanno Sahlmann
(Submitted on 4 Jul 2015)
We reconsider the Rovelli-Smolin model of gravity coupled to the Klein-Gordon time field with an eye towards capturing the degrees of freedom of the scalar field lost in the framework in which time is deparametrized by the scalar field. Several new results for loop quantum gravity are obtained:
(i) a Hilbert space for the gravity-matter system and a non-standard representation of the scalar field thereon is constructed,
(ii) a new operator for the scalar constraint of the coupled system is defined and investigated,
(iii) methods for solving the constraint are developed.
Commutators of the new constraint do not vanish, but seem to reproduce a part of the Dirac algebra. This, however, poses problems for finding solutions. Hence the states we consider -- and perhaps the whole setup -- still needs some improvement.
As a side result we describe a representation of the gravitational degrees of freedom in which the flux is diagonal. This representation bears a strong resemblance to the BF vacuum of Dittrich and Geiller.
18 pages

http://arxiv.org/abs/1507.00986
New Hamiltonian constraint operator for loop quantum gravity
Jinsong Yang, Yongge Ma
(Submitted on 3 Jul 2015)
A new symmetric Hamiltonian constraint operator is proposed for loop quantum gravity, which is well defined in the Hilbert space of diffeomorphism invariant states up to non-planar vertices. On one hand, it inherits the advantage of the original regularization method, so that its regulated version in the kinematical Hilbert space is diffeomorphism covariant and creates new vertices to the spin networks. On the other hand, it overcomes the problem in the original treatment, so that there is less ambiguity in its construction and its quantum algebra is anomaly-free in a suitable sense. The regularization procedure for the Hamiltonian constraint operator can also be applied to the symmetric model of loop quantum cosmology, which leads to a new quantum dynamics of the cosmological model.
5 pages

http://arxiv.org/abs/1507.00851
Ashtekar-Barbero holonomy on the hyperboloid: Immirzi parameter as a Cut-off for Quantum Gravity
Christoph Charles, Etera R. Livine
(Submitted on 3 Jul 2015)
In the context of the geometrical interpretation of the spin network states of Loop Quantum Gravity, we look at the holonomies of the Ashtekar-Barbero connection on loops embedded in space-like hyperboloids. We use this simple setting to illustrate two points. First, the Ashtekar-Barbero connection is not a space-time connection, its holonomies depend on the spacetime embedding of the canonical hypersurface. This fact is usually interpreted as an inconvenience, but we use it to extract the extrinsic curvature from the holonomy and separate it from the 3d intrinsic curvature. Second, we show the limitations of this reconstruction procedure, due to a periodicity of the holonomy in the Immirzi parameter, which underlines the role of a real Immirzi parameter as a cut-off for general relativity at the quantum level in contrast with its role of a mere coupling constant at the classical level.
8 pages

http://arxiv.org/abs/1507.00226
Cyclic universe from Loop Quantum Gravity
Francesco Cianfrani, Jerzy Kowalski-Glikman, Giacomo Rosati
(Submitted on 1 Jul 2015 (v1), last revised 9 Jul 2015 (this version, v2))
We discuss how a cyclic model for the flat universe can be constructively derived from Loop Quantum Gravity. This model has a lower bounce, at small values of the scale factor, which shares many similarities with that of Loop Quantum Cosmology. We find that quantum gravity corrections can be also relevant at energy densities much smaller than the Planckian one and that they can induce an upper bounce at large values of the scale factor.
4 pages

13. Aug 11, 2015

### atyy

The paper on Dirac observables and chaos looks very interesting - rubi has just been discussing this in the QM forum (some tangent off a wave function collapse thread!

14. Aug 12, 2015

### marcus

I looked for it! I saw your post with a Bicep Blaster poster
https://www.physicsforums.com/threa...-function-collapse.826089/page-7#post-5195804
but I couldn't find the discussion of the Dittrich paper. Could you give me a link?
Or is it that Rubi has been discussing the general topic---not the Dittrich paper?

I agree it looks very interesting! And it dovetails closely their June paper about a "new realization" of quantum geometry---that was on the 2nd quarter poll. As I recall it got two votes. You and I both voted for it : ^)

http://arxiv.org/abs/1506.08571
A new realization of quantum geometry
Benjamin Bahr, Bianca Dittrich, Marc Geiller
(Submitted on 29 Jun 2015)
We construct in this article a new realization of quantum geometry, which is obtained by quantizing the recently-introduced flux formulation of loop quantum gravity. In this framework, the vacuum is peaked on flat connections, and states are built upon it by creating local curvature excitations. The inner product induces a discrete topology on the gauge group, which turns out to be an essential ingredient for the construction of a continuum limit Hilbert space. This leads to a representation of the full holonomy-flux algebra of loop quantum gravity which is unitarily-inequivalent to the one based on the Ashtekar-Isham-Lewandowski vacuum. It therefore provides a new notion of quantum geometry. We discuss how the spectra of geometric operators, including holonomy and area operators, are affected by this new quantization. In particular, we find that the area operator is bounded, and that there are two different ways in which the Barbero-Immirzi parameter can be taken into account. The methods introduced in this work open up new possibilities for investigating further realizations of quantum geometry based on different vacua.
72 pages, 6 figures

Last edited: Aug 12, 2015
15. Aug 28, 2015

### marcus

Part of this Seurat 1888 painting La Seine a la Grande Jatte concludes Rovelli's recent 4 page paper. The paper makes a concise cogent argument for what you could call a pointillist interpretation of quantum mechanics: all we really know for real consists of interactions, events, relations among discrete observations.
Maybe that is all reality consists of, and it is a mistake to try to impose heavy connecting lines between events. It leads us to imagine monstrosities like the multitudinous branching or collapse of wave functions--- both of which fail to have the simple time reversibility of the basic laws relating observations.

Marking up Nature's pointillist painting with heavy continuous trajectories is something we can do as a book-keeping device, and then it's natural to violate time-reversal symmetry because we are recording our past observations and making projections about the future. It is our book-keeping that breaks the T-symmetry that is otherwise inherent in the web of events. It's fitting that it should, given its function, but our book-keeping shouldn't be allowed to intrude into our conception of reality

http://arxiv.org/abs/1508.05533
An argument against the realistic interpretation of the wave function
Carlo Rovelli
(Submitted on 22 Aug 2015)
Testable predictions of quantum mechanics are invariant under time reversal. But the change of the quantum state in time is not so, neither in the collapse nor in the no-collapse interpretations of the theory. This fact challenges the realistic interpretation of the quantum state. On the other hand, this fact follows easily if we interpret the quantum state as a mere calculation device, bookkeeping past real quantum events. The same conclusion follows from the analysis of the meaning of the wave function in the semiclassical regime.
4 pages, 3 figures

Last edited: Aug 28, 2015
16. Sep 3, 2015

### marcus

The third quarter poll covers thru the end of September--I'll update the list of potential candidates. It is currently 20 papers. Poll question:
Which paper(s) will contribute most significantly to future research?

http://arxiv.org/abs/1509.08899
Generalized effective description of loop quantum cosmology
Abhay Ashtekar, Brajesh Gupt
(Submitted on 29 Sep 2015)
The effective description of loop quantum cosmology (LQC) has proved to be a convenient platform to study phenomenological implications of the quantum bounce that resolves the classical big-bang singularity. Originally, this description was derived using Gaussian quantum states with small dispersions. In this paper we present a generalization to incorporate states with large dispersions. Specifically, we derive the \emph{generalized} effective Friedmann and Raychaudhuri equations and propose a generalized effective Hamiltonian which are being used in an ongoing study of the phenomenological consequences of a broad class of quantum geometries. We also discuss an interesting interplay between the physics of states with larger dispersions in standard LQC, and of sharply peaked states in (hypothetical) LQC theories with larger area gap.
21 pages, 4 figures

http://arxiv.org/abs/1509.08788
Recent results in CDT quantum gravity
Jan Ambjorn, Daniel Coumbe, Jakub Gizbert-Studnicki, Jerzy Jurkiewicz
(Submitted on 29 Sep 2015)
We review some recent results from the causal dynamical triangulation (CDT) approach to quantum gravity. We review recent observations of dimensional reduction at a number of previously undetermined points in the parameter space of CDT, and discuss their possible relevance to the asymptotic safety scenario. We also present an updated phase diagram of CDT, discussing properties of a newly discovered phase and its possible relation to a signature change of the metric.
6 pages, 3 figures, 1 table. Contribution to the proceedings of MG14, Rome July 2015

http://arxiv.org/abs/1509.05693
Detailed analysis of the predictions of loop quantum cosmology for the primordial power spectra
Ivan Agullo, Noah A. Morris
(Submitted on 18 Sep 2015)
We provide an exhaustive numerical exploration of the predictions of loop quantum cosmology (LQC) with a post-bounce phase of inflation for the primordial power spectrum of scalar and tensor perturbations. We extend previous analysis by characterizing the phenomenologically relevant parameter space and by constraining it using observations. Furthermore, we characterize the shape of LQC-corrections to observable quantities across this parameter space. Our analysis provides a framework to contrast more accurately the theory with forthcoming polarization data, and it also paves the road for the computation of other observables beyond the power spectra, such as non-Gaussianity.
24 pages, 5 figures

http://arxiv.org/abs/1509.02036
A note on quantum supergravity and AdS/CFT
Norbert Bodendorfer
(Submitted on 7 Sep 2015)
We note that the non-perturbative quantisation of supergravity as recently investigated using loop quantum gravity techniques provides an opportunity to probe an interesting sector of the AdS/CFT correspondence, which is usually not considered in conventional treatments. In particular, assuming a certain amount of convergence between the quantum supergravity sector of string theory and quantum supergravity constructed via loop quantum gravity techniques, we argue that the large quantum number expansion in loop quantum supergravity corresponds to the 1/Nc2 expansion in the corresponding gauge theory. In order to argue that we are indeed dealing with an appropriate quantum supergravity sector of string theory, high energy (α′) corrections are being neglected, leading to a gauge theory at strong coupling, yet finite Nc. The arguments given in this paper are mainly of qualitative nature, with the aim of serving as a starting point for a more in depth interaction between the string theory and loop quantum gravity communities.
8 pages.

http://arxiv.org/abs/1509.01312
Analog of the Peter-Weyl Expansion for Lorentz Group
Leonid Perlov
(Submitted on 4 Sep 2015)

19pages.

http://arxiv.org/abs/1509.00466
4d Quantum Geometry from 3d Supersymmetric Gauge Theory and Holomorphic Block
Muxin Han
(Submitted on 31 Aug 2015)
A class of 3d N=2 supersymmetric gauge theories are constructed and shown to encode the simplicial geometries in 4-dimensions. The gauge theories are defined by applying the Dimofte-Gaiotto-Gukov construction in 3d/3d correspondence to certain graph complement 3-manifolds. Given a gauge theory in this class, the massive supersymmetric vacua of the theory contain the classical geometries on a 4d simplicial complex. The corresponding 4d simplicial geometries are locally constant curvature (either dS or AdS), in the sense that they are made by gluing geometrical 4-simplices of the same constant curvature. When the simplicial complex is sufficiently refined, the simplicial geometries can approximate all possible smooth geometries on 4-manifold. At the quantum level, we propose that a class of holomorphic blocks defined in arXiv:1211.1986 from the 3d N=2 gauge theories are wave functions of quantum 4d simplicial geometries. In the semiclassical limit, the asymptotic behavior of holomorphic block reproduces the classical action of 4d Einstein-Hilbert gravity in the simplicial context.
40+7 pages, 9 figures

http://arxiv.org/abs/1509.00458
Four-dimensional Quantum Gravity with a Cosmological Constant from Three-dimensional Holomorphic Blocks
Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello
(Submitted on 1 Sep 2015)
Prominent approaches to quantum gravity struggle when it comes to incorporating a positive cosmological constant in their models. Using quantization of a complex SL(2,ℂ) Chern-Simons theory we include a cosmological constant, of either sign, into a model of quantum gravity.
5 pages and 2 figures

http://arxiv.org/abs/1508.07961
Quantum Cuboids and the EPRL-FK path integral for quantum gravity
Benjamin Bahr, Sebastian Steinhaus
(Submitted on 31 Aug 2015)
In this work, we investigate the 4d path integral for Euclidean quantum gravity on a hypercubic lattice, as given by the EPRL-FK model. To tackle the problem, we restrict to a set of quantum geometries that reflects the large amount of lattice symmetries. In particular, the sum over intertwiners is restricted to quantum cuboids, i.e. coherent intertwiners which describe a cuboidal geometry in the large-j limit.
Using asymptotic expressions for the vertex amplitude, we find several interesting properties of the state sum. First of all, the value of coupling constants in the amplitude functions determines whether geometric or non-geometric configurations dominate the path integral. Secondly, there is a critical value of the coupling constant α, which separates two phases. In one, the main contribution comes from very irregular and crumpled states. In the other, the dominant contribution comes from a highly regular configuration, which can be interpreted as flat Euclidean space, with small non-geometric perturbations around it.
Thirdly, we use the state sum to compute the physical norm of kinematical states, i.e. their norm in the physical Hilbert space. We find that states which describe boundary geometry with high torsion have exponentially suppressed physical norm. We argue that this allows one to exclude them from the state sum in calculations.
15 pages, 15 figures

http://arxiv.org/abs/1508.06786
Primordial scalar power spectrum from the Euclidean bounce of loop quantum cosmology
Susanne Schander, Aurélien Barrau, Boris Bolliet, Linda Linsefors, Julien Grain
(Submitted on 27 Aug 2015)
In effective models of loop quantum cosmology, the holonomy corrections lead to a deformed algebra of constraints. Among other consequences of this new spacetime structure is the emergence of an Euclidean phase around the bounce. In this article, we explicitly compute the resulting primordial power spectrum for scalar modes by setting initial conditions in the contracting phase.
10 pages, 4 figures

http://arxiv.org/abs/1508.05953
Locality and entanglement in bandlimited quantum field theory
Jason Pye, William Donnelly, Achim Kempf
(Submitted on 24 Aug 2015)
We consider a model for a Planck scale ultraviolet cutoff which is based on Shannon sampling. Shannon sampling originated in information theory, where it expresses the equivalence of continuous and discrete representations of information. When applied to quantum field theory, Shannon sampling expresses a hard ultraviolet cutoff in the form of a bandlimitation. This introduces nonlocality at the cutoff scale in a way that is more subtle than a simple discretization of space: quantum fields can then be represented as either living on continuous space or, entirely equivalently, as living on any one lattice whose average spacing is sufficiently small. We explicitly calculate vacuum entanglement entropies in 1+1 dimension and we find a transition between logarithmic and linear scaling of the entropy, which is the expected 1+1 dimensional analog of the transition from an area to a volume law. We also use entanglement entropy and mutual information as measures to probe in detail the localizability of the field degrees of freedom. We find that, even though neither translation nor rotation invariance are broken, each field degree of freedom occupies an incompressible volume of space, indicating a finite information density.
23 pages, 13 figures.

http://arxiv.org/abs/1508.05543
Relational Quantum Cosmology
Francesca Vidotto
(Submitted on 22 Aug 2015)
The application of quantum theory to cosmology raises a number of conceptual questions, such as the role of the quantum-mechanical notion of "observer" or the absence of a time variable in the Wheeler-DeWitt equation. I point out that a relational formulation of quantum mechanics, and more in general the observation that evolution is always relational, provides a coherent solution to this tangle of problems.
20 pages, 4 figures. Contribution to the forthcoming book on Philosophy of Cosmology edited by K. Chamcham, J. Barrow, J. Silk and S. Saunders for Cambridge University Press

http://arxiv.org/abs/1508.01947
Chaos, Dirac observables and constraint quantization
Bianca Dittrich, Philipp A. Hoehn, Tim A. Koslowski, Mike I. Nelson
(Submitted on 8 Aug 2015)
There is good evidence that full general relativity is non-integrable or even chaotic. We point out the severe repercussions: differentiable Dirac observables and a reduced phase space do not exist in non-integrable constrained systems and are thus unlikely to occur in a generic general relativistic context. Instead, gauge invariant quantities generally become discontinuous, thus not admitting Poisson-algebraic structures and posing serious challenges to a quantization. Non-integrability also renders the paradigm of relational dynamics cumbersome, thereby straining common interpretations of the dynamics. We illustrate these conceptual and technical challenges with simple toy models. In particular, we exhibit reparametrization invariant models which fail to be integrable and, as a consequence, can either not be quantized with standard methods or lead to sick quantum theories without a semiclassical limit. These troubles are qualitatively distinct from semiclassical subtleties in unconstrained quantum chaos and can be directly traced back to the scarcity of Dirac observables. As a possible resolution, we propose to change the method of quantization by refining the configuration space topology until the generalized observables become continuous in the new topology and can acquire a quantum representation. This leads to the polymer quantization method underlying loop quantum cosmology and gravity. Remarkably, the polymer quantum theory circumvents the problems of the quantization with smooth topology, indicating that non-integrability and chaos, while a challenge, may not be a fundamental obstruction for quantum gravity.
48 pages, 9 figures, lots of discussion

http://arxiv.org/abs/1508.01416
Spin Foams Without Spins
Jeff Hnybida
(Submitted on 6 Aug 2015)
We formulate the spin foam representation of discrete SU(2) gauge theory as a product of vertex amplitudes each of which is the spin network generating function of the boundary graph dual to the vertex. Thus the sums over spins have been carried out. We focus on the character expansion of Yang-Mills theory which is an approximate heat kernel regularization of BF theory. The boundary data of each n-valent node is an element of the Grassmannian Gr(2,n) which carries a coherent representation of U(n) and a geometrical interpretation as a framed polyhedron of fixed total area. Ultimately, sums over spins are traded for contour integrals over simple poles and recoupling theory is avoided using generating functions.
21 pages, 2 figures

http://arxiv.org/abs/1507.08112
Running of the scalar spectral index in bouncing cosmologies
Jean-Luc Lehners, Edward Wilson-Ewing
(Submitted on 29 Jul 2015)
We calculate the running of the scalar index in the ekpyrotic and matter bounce cosmological scenarios, and find that it is typically negative for ekpyrotic models, while it is typically positive for realizations of the matter bounce where multiple fields are present. This can be compared to inflation, where the observationally preferred models typically predict a negative running. The magnitude of the running is expected to be between 10−4 and up to 10−2, leading in some cases to interesting expectations for near-future observations.
6 pages

http://arxiv.org/abs/1507.07591
Discrete Hamiltonian for General Relativity
Jonathan Ziprick, Jack Gegenberg
(Submitted on 27 Jul 2015)
Beginning from canonical general relativity written in terms of Ashtekar variables, we derive a discrete phase space with a physical Hamiltonian for gravity. The key idea is to define the gravitational fields within a complex of three-dimensional cells such that the dynamics is completely described by discrete boundary variables, and the full theory is recovered in the continuum limit. Canonical quantization is attainable within the loop quantum gravity framework, and we believe this will lead to a promising candidate for quantum gravity.
6 pages

http://arxiv.org/abs/1507.05424
Phenomenology of bouncing black holes in quantum gravity: a closer look
Aurelien Barrau, Boris Bolliet, Francesca Vidotto, Celine Weimer
(Submitted on 20 Jul 2015)
It was recently shown that black holes could be bouncing stars as a consequence of quantum gravity. We investigate the astrophysical signals implied by this hypothesis, focusing on primordial black holes. We consider different possible bounce times and study the integrated diffuse emission.
8 pages, 8 figures

http://arxiv.org/abs/1507.04703
Loop quantum cosmology, non-Gaussianity, and CMB power asymmetry
Ivan Agullo
(Submitted on 16 Jul 2015)
We argue that the anomalous power asymmetry observed in the cosmic microwave background (CMB) may have originated in a cosmic bounce preceding inflation. In loop quantum cosmology (LQC) the big bang singularity is generically replaced by a bounce due to quantum gravitational effects. We compute the spectrum of inflationary non-Gaussianity and show that strong correlation between observable scales and modes with longer (super-horizon) wavelength arise as a consequence of the evolution of perturbations across the LQC bounce. These correlations are strongly scale dependent and induce a dipole-dominated modulation on large angular scales in the CMB, in agreement with observations.
7 pages, 3 figure

http://arxiv.org/abs/1507.01153
Coherent State Operators in Loop Quantum Gravity
Emanuele Alesci, Andrea Dapor, Jerzy Lewandowski, Ilkka Makinen, Jan Sikorski
(Submitted on 4 Jul 2015)
We present a new method for constructing operators in loop quantum gravity. The construction is an application of the general idea of "coherent state quantization", which allows one to associate a unique quantum operator to every function on a classical phase space. Using the heat kernel coherent states of Hall and Thiemann, we show how to construct operators corresponding to functions depending on holonomies and fluxes associated to a fixed graph. We construct the coherent state versions of the fundamental holonomy and flux operators, as well as the basic geometric operators of area, angle and volume. Our calculations show that the corresponding canonical operators are recovered from the coherent state operators in the limit of large spins.
34 pages, 4 figures

http://arxiv.org/abs/1507.00986
New Hamiltonian constraint operator for loop quantum gravity
Jinsong Yang, Yongge Ma
(Submitted on 3 Jul 2015)
A new symmetric Hamiltonian constraint operator is proposed for loop quantum gravity, which is well defined in the Hilbert space of diffeomorphism invariant states up to non-planar vertices. On one hand, it inherits the advantage of the original regularization method, so that its regulated version in the kinematical Hilbert space is diffeomorphism covariant and creates new vertices to the spin networks. On the other hand, it overcomes the problem in the original treatment, so that there is less ambiguity in its construction and its quantum algebra is anomaly-free in a suitable sense. The regularization procedure for the Hamiltonian constraint operator can also be applied to the symmetric model of loop quantum cosmology, which leads to a new quantum dynamics of the cosmological model.
5 pages

http://arxiv.org/abs/1507.00851
Ashtekar-Barbero holonomy on the hyperboloid: Immirzi parameter as a Cut-off for Quantum Gravity
Christoph Charles, Etera R. Livine
(Submitted on 3 Jul 2015)
In the context of the geometrical interpretation of the spin network states of Loop Quantum Gravity, we look at the holonomies of the Ashtekar-Barbero connection on loops embedded in space-like hyperboloids. We use this simple setting to illustrate two points. First, the Ashtekar-Barbero connection is not a space-time connection, its holonomies depend on the spacetime embedding of the canonical hypersurface. This fact is usually interpreted as an inconvenience, but we use it to extract the extrinsic curvature from the holonomy and separate it from the 3d intrinsic curvature. Second, we show the limitations of this reconstruction procedure, due to a periodicity of the holonomy in the Immirzi parameter, which underlines the role of a real Immirzi parameter as a cut-off for general relativity at the quantum level in contrast with its role of a mere coupling constant at the classical level.
8 pages

Last edited: Oct 3, 2015