# Our picks for third quarter 2014 MIP (most important QG paper) Part II

Tags:

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

0 vote(s)
0.0%

44.4%

55.6%

22.2%

11.1%

33.3%

33.3%

11.1%

22.2%
10. ### New action for simplicial gravity in four dimensions

55.6%
1. Sep 30, 2014

### marcus

This is the second half of the poll. Of the ten candidates in Part II, please indicate the ones you think will prove most significant for future Loop-and-allied QG research. The poll is multiple choice, and it's possible to vote for several papers. Abstracts follow in the next post. Here's a link to the other half of the poll:
https://www.physicsforums.com/threa...14-mip-most-important-qg-paper-part-i.773589/

http://arxiv.org/abs/1408.5367
http://inspirehep.net/search?p=find eprint 1408.5367
Rethinking Connes' approach to the standard model of particle physics via non-commutative geometry
Shane Farnsworth, Latham Boyle

http://arxiv.org/abs/1408.4336
http://inspirehep.net/search?p=find eprint 1408.4336
From General Relativity to Quantum Gravity
Abhay Ashtekar, Martin Reuter, Carlo Rovelli

http://arxiv.org/abs/1408.3050
http://inspirehep.net/search?p=find eprint 1408.3050
A scenario for black hole evaporation on a quantum geometry
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1408.0121
http://inspirehep.net/search?p=find eprint 1408.0121
Thermally correlated states in Loop Quantum Gravity
Goffredo Chirco, Carlo Rovelli, Paola Ruggiero

http://arxiv.org/abs/1407.7746
http://inspirehep.net/search?p=find eprint 1407.7746
On background-independent renormalization of spin foam models
Benjamin Bahr

http://arxiv.org/abs/1407.2909
http://inspirehep.net/search?p=find eprint 1407.2909
Linking shape dynamics and loop quantum gravity
Lee Smolin

http://arxiv.org/abs/1407.0989
http://inspirehep.net/search?p=find eprint 1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1407.0284
http://inspirehep.net/search?p=find eprint 1407.0284
The Tensor Theory Space
Vincent Rivasseau

http://arxiv.org/abs/1407.0032
http://inspirehep.net/search?p=find eprint 1407.0032
Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin

http://arxiv.org/abs/1407.0025
http://inspirehep.net/search?p=find eprint 1407.0025
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland
==================================
The 20 candidate papers are divided into two sets because the poll format only accepts up to 10 options. Notice that because out of habit I list the papers in reverse chronological order it happens that this second batch consists of interesting papers that appeared EARLY in the quarter. We've had longer to think about them and see how they've been received. ...
Here are the papers on Part I (please check out both parts of the poll!) :

http://arxiv.org/abs/1409.7455
http://inspirehep.net/search?p=find eprint 1409.7455
Point particles in 2+1 dimensions: general relativity and loop gravity descriptions
Jonathan Ziprick

http://arxiv.org/abs/1409.4031
http://inspirehep.net/search?p=find eprint 1409.4031
Fast Radio Bursts and White Hole Signals
Aurélien Barrau, Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/1409.3526
http://inspirehep.net/search?p=find eprint 1409.3526
A 2-categorical state sum model
Aristide Baratin, Laurent Freidel

http://arxiv.org/abs/1409.3150
http://inspirehep.net/search?p=find eprint 1409.3150
Group field theories for all loop quantum gravity
Daniele Oriti, James P. Ryan, Johannes Thürigen

http://arxiv.org/abs/1409.2471
http://inspirehep.net/search?p=find eprint 1409.2471
Quanta of Geometry
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov

http://arxiv.org/abs/1409.2407
http://inspirehep.net/search?p=find eprint 1409.2407
Decorated tensor network renormalization for lattice gauge theories and spin foam models
Bianca Dittrich, Sebastian Mizera, Sebastian Steinhaus

http://arxiv.org/abs/1409.1450
http://inspirehep.net/search?p=find eprint 1409.1450
The continuum limit of loop quantum gravity - a framework for solving the theory
Bianca Dittrich

http://arxiv.org/abs/1409.0917
http://inspirehep.net/search?p=find eprint 1409.0917
Identification of a gravitational arrow of time
Julian Barbour, Tim Koslowski, Flavio Mercati

http://arxiv.org/abs/1409.0836
http://inspirehep.net/search?p=find eprint 1409.0836
A note on the secondary simplicity constraints in loop quantum gravity
Fabio Anzà, Simone Speziale

http://arxiv.org/abs/1409.0144
http://inspirehep.net/search?p=find eprint 1409.0144
Entanglement entropy production in gravitational collapse: covariant regularization and solvable models
Eugenio Bianchi, Tommaso De Lorenzo, Matteo Smerlak
=========================

Last edited: Oct 20, 2014
2. Sep 30, 2014

### marcus

Here are the abstracts for the second half of the poll: the ten papers in Part II.

http://arxiv.org/abs/1408.5367
http://inspirehep.net/search?p=find eprint 1408.5367
Rethinking Connes' approach to the standard model of particle physics via non-commutative geometry
Shane Farnsworth, Latham Boyle
(Submitted on 22 Aug 2014)
Connes' non-commutative geometry (NCG) is a generalization of Riemannian geometry that is particularly apt for expressing the standard model of particle physics coupled to Einstein gravity. In a previous paper, we suggested a reformulation of this framework that is: (i) simpler and more unified in its axioms, and (ii) allows the Lagrangian for the standard model of particle physics (coupled to Einstein gravity) to be specified in a way that is tighter and more explanatory than the traditional algorithm based on effective field theory. Here we explain how this same reformulation yields a new perspective on the symmetries of a given NCG. Applying this perspective to the NCG traditionally used to describe the standard model we find, instead, an extension of the standard model by an extra U(1)B−L gauge symmetry, and a single extra complex scalar field σ, which is a singlet under SU(3)C×SU(2)L×U(1)Y, but has B−L=2. This field has cosmological implications, and offers a new solution to the discrepancy between the observed Higgs mass and the NCG prediction.
5 pages

http://arxiv.org/abs/1408.4336
http://inspirehep.net/search?p=find eprint 1408.4336
From General Relativity to Quantum Gravity
Abhay Ashtekar, Martin Reuter, Carlo Rovelli
(Submitted on 19 Aug 2014)
In general relativity (GR), spacetime geometry is no longer just a background arena but a physical and dynamical entity with its own degrees of freedom. We present an overview of approaches to quantum gravity in which this central feature of GR is at the forefront. However, the short distance dynamics in the quantum theory are quite different from those of GR and classical spacetimes and gravitons emerge only in a suitable limit. Our emphasis is on communicating the key strategies, the main results and open issues. In the spirit of this volume, we focus on a few avenues that have led to the most significant advances over the past 2-3 decades.
54 pages, 5 figures. To appear in General Relativity and Gravitation: A Centennial Survey, commissioned by the International Society for General Relativity and Gravitation and to be published by Cambridge University Press. Abhay Ashtekar served as the 'coordinating author' and combined the three contributions.

http://arxiv.org/abs/1408.3050
http://inspirehep.net/search?p=find eprint 1408.3050
A scenario for black hole evaporation on a quantum geometry
Rodolfo Gambini, Jorge Pullin
(Submitted on 13 Aug 2014)
We incorporate elements of the recently discovered exact solutions of the quantum constraints of loop quantum gravity for vacuum spherically symmetric space-times into the paradigm of black hole evaporation due to Ashtekar and Bojowald. The quantization of the area of the surfaces of symmetry of the solutions implies that the number of nice slices that can be fit inside the black hole is finite. The foliation eventually moves through the region where the singularity in the classical theory used to be and all the particles that fell into the black hole due to Hawking radiation emerge finally as a white hole. This yields a variant of a scenario advocated by Arkani-Hamed et al. Fluctuations in the horizon that naturally arise in the quantum space time allow radiation to emerge during the evaporation process due to stimulated emission allowing evaporation to proceed beyond Page time without reaching the maximum entanglement limit until the formation of the white hole. No firewalls nor remnants arise in this scenario.
5 pages.

http://arxiv.org/abs/1408.0121
http://inspirehep.net/search?p=find eprint 1408.0121
Thermally correlated states in Loop Quantum Gravity
Goffredo Chirco, Carlo Rovelli, Paola Ruggiero
(Submitted on 1 Aug 2014)
We study a class of loop-quantum-gravity states characterized by (ultra-local) thermal correlations that reproduce some features of the ultraviolet structure of the perturbative quantum field theory vacuum. In particular, they satisfy an analog of the Bisognano-Wichmann theorem. These states are peaked on the intrinsic geometry and admit a semiclassical interpretation. We study how the correlations extend on the spin-network beyond the ultra local limit.
11 pages, 4 figures

http://arxiv.org/abs/1407.7746
http://inspirehep.net/search?p=find eprint 1407.7746
On background-independent renormalization of spin foam models
Benjamin Bahr
(Submitted on 29 Jul 2014)
In this article we discuss an implementation of renormalization group ideas to spin foam models, where there is no a priori length scale with which to define the flow. In the context of the continuum limit of these models, we show how the notion of cylindrical consistency of path integral measures gives a natural analogue of Wilson's RG flow equations for background-independent systems. We discuss the conditions for the continuum measures to be diffeomorphism-invariant, and consider both exact and approximate examples.
23 pages, 12 figures

http://arxiv.org/abs/1407.2909
http://inspirehep.net/search?p=find eprint 1407.2909
Linking shape dynamics and loop quantum gravity
Lee Smolin
(Submitted on 10 Jul 2014)
Shape dynamics is a reformulation of general relativity, locally equivalent to Einstein's theory, in which the refoliation invariance of the older theory is traded for local scale invariance. Shape dynamics is here derived in a formulation related to the Ashtekar variables by beginning with a modification of the Plebanski action. The constraints of shape dynamics and their algebra are reproduced in terms of these new variables.
12 pages.

http://arxiv.org/abs/1407.0989
http://inspirehep.net/search?p=find eprint 1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli
(Submitted on 3 Jul 2014)
We show that there is a classical metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole. We compute this metric explicitly. We show how quantum theory determines the (long) time for the process to happen. A black hole can thus quantum-tunnel into a white hole. For this to happen, quantum gravity should affect the metric also in a small region outside the horizon: we show that contrary to what is commonly assumed, this is not forbidden by causality or by the semiclassical approximation, because quantum effects can pile up over a long time. This scenario alters radically the discussion on the black hole information puzzle.
10 pages, 5 figures.

http://arxiv.org/abs/1407.0284
http://inspirehep.net/search?p=find eprint 1407.0284
The Tensor Theory Space
Vincent Rivasseau
(Submitted on 1 Jul 2014)
The tensor track is a background-independent discretization of quantum gravity which includes a sum over all topologies. We discuss how to define a functional renormalization group flow and the Wetterich equation in the corresponding theory space. This space is different from the Einsteinian theory space of asymptotic safety. It includes all fixed-rank tensor-invariant interactions, hence generalizes matrix models and the (Moyal) non-commutative field theory space.
12 pages. This short note is intended as a complement to arXiv:1311.1461, to appear in the Proceedings of the Workshop on Noncommutative Field Theory and Gravity in Corfu September 2013, Fortshritt. Phys. 2014

http://arxiv.org/abs/1407.0032
http://inspirehep.net/search?p=find eprint 1407.0032
Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin
(Submitted on 30 Jun 2014)
Energetic causal sets are causal sets endowed by a flow of energy-momentum between causally related events. These incorporate a novel mechanism for the emergence of space-time from causal relations. Here we construct a spin foam model which is also an energetic causal set model. This model is closely related to the model introduced by Wieland, and this construction makes use of results used there. What makes a spin foam model also an energetic causal set is Wieland's identification of new momenta, conserved at events (or four-simplices), whose norms are not mass, but the volume of tetrahedra. This realizes the torsion constraints, which are missing in previous spin foam models, and are needed to relate the connection dynamics to those of the metric, as in general relativity. This identification makes it possible to apply the new mechanism for the emergence of space-time to a spin foam model.
16 pages, 4 figures.

http://arxiv.org/abs/1407.0025
http://inspirehep.net/search?p=find eprint 1407.0025
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland
(Submitted on 30 Jun 2014)
We develop a proposal for a theory of simplicial gravity with spinors as the fundamental configuration variables. The underlying action describes a mechanical system with finitely many degrees of freedom, the system has a Hamiltonian and local gauge symmetries. We will close with some comments on the resulting quantum theory, and explain the relation to loop quantum gravity and twisted geometries. The paper appears in parallel with an article by Cortês and Smolin, who study the relevance of the model for energetic causal sets and various other approaches to quantum gravity.
26 pages, 2 figures.

Last edited: Oct 20, 2014
3. Oct 2, 2014

### marcus

Thanks David! I appreciate having your views. As I recall you do QG research and also comment on Loop papers on your blog, so your picks on the poll probably reflect your own professional interest and activity. People may be curious why I split the poll into two ten-item segments this time. In the past I've ordinarily had 20 choices on the quarterly poll--it seems to be about the right number. It's hard to narrow the field down much further because there tend to be about that many really interesting papers that could turn out to play a significant role in future developments. And people's perspectives and assessments differ so the votes tend to spread out and cover the range of candidates.

At the moment though, with the new PF setup, only ten options are allowed by the poll format. When you are making a poll you keep adding options and when you reach ten the system will not allow you to add any more. That is why I presented this quarter's poll in two segments: Part I and Part II.
Had to make a quick decision: I hadn't learned about the limitation ahead of time and was prepared with 20 candidates as usual.

I'm hoping people will respond on both parts. We'll see how it goes.

4. Oct 2, 2014

### marcus

In a sense it is easier to choose among the options in Part II because they have been OUT longer. A quarter, three months, is about 90 days and these papers are ones that appeared in roughly the first half of the quarter, the first 50 days or so.

I'd always listed the papers in reverse chronological order, most recent first. So when I divided the list into two segments Part I was naturally the ten most recent. Maybe that's the wrong way to do it. Maybe Part I of the poll should consist of papers that have been out the longest, so one has had more time to consider their implications and possible longterm value. Also one begins to see which ones get cited by other researchers. Which new directions get followed up and gain momentum, and so on.

5. Oct 17, 2014

### marcus

Thanks to Atyy, Chronos, and David Horgan for getting the third quarter poll started! Because of format restriction I divided the 20 candidates into two lists of 10. So the poll was split into parts I and II. To look at the voting I've combined the two, and marked the papers to be found in the other list (part I) with an asterisk. Here's a link to Part I of the poll:
https://www.physicsforums.com/threa...14-mip-most-important-qg-paper-part-i.773589/ The abstracts for the starred titles are there if you wish to refer to them.
Let's see how the votes look so far.:

http://arxiv.org/abs/1409.2407
http://inspirehep.net/search?p=find eprint 1409.2407
Decorated tensor network renormalization for lattice gauge theories and spin foam models*
Bianca Dittrich, Sebastian Mizera, Sebastian Steinhaus333

http://arxiv.org/abs/1407.0025
http://inspirehep.net/search?p=find eprint 1407.0025
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland

http://arxiv.org/abs/1409.2471
http://inspirehep.net/search?p=find eprint 1409.2471
Quanta of Geometry*
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov222

http://arxiv.org/abs/1409.1450
http://inspirehep.net/search?p=find eprint 1409.1450
The continuum limit of loop quantum gravity - a framework for solving the theory*
Bianca Dittrich222

Linking shape dynamics and loop quantum gravity
Lee Smolin

Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin

One vote:
http://arxiv.org/abs/1409.4031
http://inspirehep.net/search?p=find eprint 1409.4031
Fast Radio Bursts and White Hole Signals*
Aurélien Barrau, Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/1409.3526
http://inspirehep.net/search?p=find eprint 1409.3526
A 2-categorical state sum model*
Aristide Baratin, Laurent Freidel

http://arxiv.org/abs/1408.4336
http://inspirehep.net/search?p=find eprint 1408.4336
From General Relativity to Quantum Gravity
Abhay Ashtekar, Martin Reuter, Carlo Rovelli

http://arxiv.org/abs/1408.3050
http://inspirehep.net/search?p=find eprint 1408.3050
A scenario for black hole evaporation on a quantum geometry
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1408.0121
http://inspirehep.net/search?p=find eprint 1408.0121
Thermally correlated states in Loop Quantum Gravity
Goffredo Chirco, Carlo Rovelli, Paola Ruggiero

http://arxiv.org/abs/1407.7746
http://inspirehep.net/search?p=find eprint 1407.7746
On background-independent renormalization of spin foam models
Benjamin Bahr

http://arxiv.org/abs/1407.0989
http://inspirehep.net/search?p=find eprint 1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli

http://arxiv.org/abs/1407.0284
http://inspirehep.net/search?p=find eprint 1407.0284
The Tensor Theory Space
Vincent Rivasseau

Last edited: Oct 20, 2014
6. Oct 18, 2014

### Chronos

I've been following Rivasseau with interest for the past couple years and his concepts appear to be gaining traction. My tensor calculus skills are, however, essentially nonexistent, so I am unable to offer more than a GUT feeling.

7. Oct 20, 2014

### marcus

Thanks to Atyy, Chronos, David Horgan, Julcab, Martinbn, and Ohwilleke for getting the third quarter poll off to a good start! Because of format restriction I divided the 20 candidates into two lists of 10. So the poll was split into parts I and II. To look at the voting I've combined the two, and marked the papers to be found in the other list (part I) with an asterisk. Here's a link to Part I of the poll:
https://www.physicsforums.com/threa...14-mip-most-important-qg-paper-part-i.773589/ The abstracts for the starred titles are there if you wish to refer to them.
Let's see how the votes look so far.:

http://arxiv.org/abs/1409.2407
http://inspirehep.net/search?p=find eprint 1409.2407
Decorated tensor network renormalization for lattice gauge theories and spin foam models*
Bianca Dittrich, Sebastian Mizera, Sebastian Steinhaus

http://arxiv.org/abs/1409.1450
http://inspirehep.net/search?p=find eprint 1409.1450
The continuum limit of loop quantum gravity - a framework for solving the theory*
Bianca Dittrich

http://arxiv.org/abs/1408.3050
http://inspirehep.net/search?p=find eprint 1408.3050
A scenario for black hole evaporation on a quantum geometry
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1407.0025
http://inspirehep.net/search?p=find eprint 1407.0025
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland

http://arxiv.org/abs/1409.2471
http://inspirehep.net/search?p=find eprint 1409.2471
Quanta of Geometry*
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov

http://arxiv.org/abs/1409.0917
http://inspirehep.net/search?p=find eprint 1409.0917
Identification of a gravitational arrow of time*
Julian Barbour, Tim Koslowski, Flavio Mercati

http://arxiv.org/abs/1408.4336
http://inspirehep.net/search?p=find eprint 1408.4336
From General Relativity to Quantum Gravity
Abhay Ashtekar, Martin Reuter, Carlo Rovelli

Linking shape dynamics and loop quantum gravity
Lee Smolin

http://arxiv.org/abs/1407.0989
http://inspirehep.net/search?p=find eprint 1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli

Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin

Point particles in 2+1 dimensions: general relativity and loop gravity descriptions*
Jonathan Ziprick

http://arxiv.org/abs/1409.4031
http://inspirehep.net/search?p=find eprint 1409.4031
Fast Radio Bursts and White Hole Signals*
Aurélien Barrau, Carlo Rovelli, Francesca Vidotto

http://arxiv.org/abs/1409.3526
http://inspirehep.net/search?p=find eprint 1409.3526
A 2-categorical state sum model*
Aristide Baratin, Laurent Freidel

http://arxiv.org/abs/1408.0121
http://inspirehep.net/search?p=find eprint 1408.0121
Thermally correlated states in Loop Quantum Gravity
Goffredo Chirco, Carlo Rovelli, Paola Ruggiero

http://arxiv.org/abs/1407.7746
http://inspirehep.net/search?p=find eprint 1407.7746
On background-independent renormalization of spin foam models
Benjamin Bahr

http://arxiv.org/abs/1407.0284
http://inspirehep.net/search?p=find eprint 1407.0284
The Tensor Theory Space
Vincent Rivasseau
=====================

Last edited: Oct 21, 2014
8. Nov 5, 2014

### marcus

Thirteen of us have taken part in the third quarter poll so far. Thanks to all, including the most recent arrivals, PhantomJay and Nonlinearity! The current tally of the votes is in the Part I thread.
https://www.physicsforums.com/threa...mportant-qg-paper-part-i.773589/#post-4901759

I'll use this Part II thread to accumulate fourth quarter prospects for the next poll.

http://arxiv.org/abs/1411.3592
Projective Loop Quantum Gravity I. State Space
Suzanne Lanéry, Thomas Thiemann
(Submitted on 11 Nov 2014)
Instead of formulating the state space of a quantum field theory over one big Hilbert space, it has been proposed by Kijowski to describe quantum states as projective families of density matrices over a collection of smaller, simpler Hilbert spaces. Beside the physical motivations for this approach, it could help designing a quantum state space holding the states we need. In [Okolów 2013, arXiv:1304.6330] the description of a theory of Abelian connections within this framework was developed, an important insight being to use building blocks labeled by combinations of edges and surfaces. The present work generalizes this construction to an arbitrary gauge group G (in particular, G is neither assumed to be Abelian nor compact). This involves refining the definition of the label set, as well as deriving explicit formulas to relate the Hilbert spaces attached to different labels.
If the gauge group happens to be compact, we also have at our disposal the well-established Ashtekar-Lewandowski Hilbert space, which is defined as an inductive limit using building blocks labeled by edges only. We then show that the quantum state space presented here can be thought as a natural extension of the space of density matrices over this Hilbert space. In addition, it is manifest from the classical counterparts of both formalisms that the projective approach allows for a more balanced treatment of the holonomy and flux variables, so it might pave the way for the development of more satisfactory coherent states.
81 pages, many figures

http://arxiv.org/abs/1411.3180
Functional Renormalisation Group Approach for Tensorial Group Field Theory: a Rank-3 Model
Dario Benedetti, Joseph Ben Geloun, Daniele Oriti
(Submitted on 12 Nov 2014)
We set up the Functional Renormalisation Group formalism for Tensorial Group Field Theory in full generality. We then apply it to a rank-3 model over U(1) x U(1) x U(1), endowed with a linear kinetic term and nonlocal interactions. The system of FRG equations turns out to be non-autonomous in the RG flow parameter. This feature is explained by the existence of a hidden scale, the radius of the group manifold. We investigate in detail the opposite regimes of large cut-off (UV) and small cut-off (IR) of the FRG equations, where the system becomes autonomous, and we find, in both case, Gaussian and non-Gaussian fixed points. We derive and interpret the critical exponents and flow diagrams associated with these fixed points, and discuss how the UV and IR regimes are matched at finite N. Finally, we discuss the evidence for a phase transition from a symmetric phase to a broken or condensed phase, from an RG perspective, finding that this seems to exist only in the approximate regime of very large radius of the group manifold, as to be expected for systems on compact manifolds.
28 pages, 14 figures

http://arxiv.org/abs/1411.0977
Geometry and the Quantum: Basics
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov
(Submitted on 4 Nov 2014)
Motivated by the construction of spectral manifolds in noncommutative geometry, we introduce a higher degree Heisenberg commutation relation involving the Dirac operator and the Feynman slash of scalar fields. This commutation relation appears in two versions, one sided and two sided. It implies the quantization of the volume. In the one-sided case it implies that the manifold decomposes into a disconnected sum of spheres which will represent quanta of geometry. The two sided version in dimension 4 predicts the two algebras M2(H) and M4(C) which are the algebraic constituents of the Standard Model of particle physics. This taken together with the non-commutative algebra of functions allows one to reconstruct, using the spectral action, the Lagrangian of gravity coupled with the Standard Model. We show that any connected Riemannian Spin 4-manifold with quantized volume >4 (in suitable units) appears as an irreducible representation of the two-sided commutation relations in dimension 4 and that these representations give a seductive model of the "particle picture" for a theory of quantum gravity in which both the Einstein geometric standpoint and the Standard Model emerge from Quantum Mechanics. Physical applications of this quantization scheme will follow in a separate publication.
33 pages, 2 figures

http://arxiv.org/abs/1411.0272
Silent initial conditions for cosmological perturbations with a change of space-time signature
Jakub Mielczarek, Linda Linsefors, Aurelien Barrau
(Submitted on 2 Nov 2014)
Recent calculations in loop quantum cosmology suggest that a transition from a Lorentzian to an Euclidean space-time might take place in the very early Universe. The transition point leads to a state of silence, characterized by a vanishing speed of light. This behavior can be interpreted as a decoupling of different space points, similar to the one characterizing the BKL phase.
In this study, we address the issue of imposing initial conditions for the cosmological perturbations at the transition point between the Lorentzian and Euclidean phases. Motivated by the decoupling of space points, initial conditions characterized by a lack of correlations are investigated. We show that the "white noise" initial conditions are supported by the analysis of the vacuum state in the Euclidean regime adjacent to the state of silence.
Furthermore, the possibility of imposing the silent initial conditions at the trans-Planckian surface, characterized by a vanishing speed for the propagation of modes with wavelengths of the order of the Planck length, is studied. Such initial conditions might result from a loop-deformations of the Poincaré algebra. The conversion of the silent initial power spectrum to a scale-invariant one is also examined.
12 pages, 8 figures.

http://arxiv.org/abs/1410.7062
No firewalls in quantum gravity: the role of discreteness of quantum geometry in resolving the information loss paradox
Alejandro Perez
(Submitted on 26 Oct 2014)
In an approach to quantum gravity where space-time arises from coarse graining of fundamentally discrete structures, black hole formation and subsequent evaporation can be described by a unitary evolution without the problems encountered by the standard remnant scenario or the schemes where information is assumed to come out with the radiation while evaporation (firewalls and complementarity). The final state is purified by correlations with the fundamental pre-geometric structures (in the sense of Wheeler) which are available in such approaches, and, like defects in the underlying space-time weave, can carry zero energy.
12 pages, 7 figures.

http://arxiv.org/abs/1410.5609
A quantum reduction to spherical symmetry in loop quantum gravity
Norbert Bodendorfer, Jerzy Lewandowski, Jedrzej Świeżewski
(Submitted on 21 Oct 2014)
Based on a recent purely geometric construction of observables for the spatial diffeomorphism constraint, we propose two distinct quantum reductions to spherical symmetry within full 3+1-dimensional loop quantum gravity. The construction of observables corresponds to using the radial gauge for the spatial metric and allows to identify rotations around a central observer as unitary transformations in the quantum theory. Group averaging over these rotations yields our first proposal for spherical symmetry. Hamiltonians of the full theory with angle-independent lapse preserve this spherically symmetric subsector of the full Hilbert space. A second proposal consists in implementing the vanishing of a certain vector field in spherical symmetry as a constraint on the full Hilbert space, leading to a close analogue of diffeomorphisms invariant states. While this second set of spherically symmetric states does not allow for using the full Hamiltonian, it is naturally suited to implement the spherically symmetric midisuperspace Hamiltonian, as an operator in the full theory, on it. Due to the canonical structure of the reduced variables, the holonomy-flux algebra behaves effectively as a one parameter family of 2+1-dimensional algebras along the radial coordinate, leading to a diagonal non-vanishing volume operator on 3-valent vertices. The quantum dynamics thus becomes tractable, including scenarios like spherically symmetric dust collapse.
5 pages

http://arxiv.org/abs/1410.4815
Further evidence for asymptotic safety of quantum gravity
Kevin Falls, Daniel F. Litim, Konstantinos Nikolakopoulos, Christoph Rahmede
(Submitted on 17 Oct 2014)
The asymptotic safety conjecture is examined for quantum gravity in four dimensions. Using the renormalisation group, we find evidence for an interacting UV fixed point for polynomial actions up to the 34th power in the Ricci scalar. The extrapolation to infinite polynomial order is given, and the self-consistency of the fixed point is established using a bootstrap test. All details of our analysis are provided. We also clarify further aspects such as stability, convergence, the role of boundary conditions, and a partial degeneracy of eigenvalues. Within this setting we find strong support for the conjecture.
43 pages, 17 figures

http://arxiv.org/abs/1410.4788
Loop Quantum Cosmology from Loop Quantum Gravity
Emanuele Alesci, Francesco Cianfrani
(Submitted on 17 Oct 2014)
We show how Loop Quantum Cosmology can be derived as an effective semiclassical description of Loop Quantum Gravity. Using the tools of QRLG, a gauge fixed version of LQG, we take the coherent states of the fundamental microscopic theory suitable to describe a Bianchi I Universe and we find a mapping between the expectation value of the Hamiltonian and the dynamics of LQC. Our results are in agreement with a lattice refinement framework for LQC, thus the so called old'' and improved-dynamics'' regularization schemes can be reproduced. These amount to different choices of relations between local variables and the smeared ones entering the definition of the coherent states. The leading order of the fundamental theory corresponds to LQC, but we also find different inverse volume corrections, that depend on a purely quantum observable, namely the number of nodes of the states.
5 pages

http://arxiv.org/abs/1410.4411
Consistency of matter models with asymptotically safe quantum gravity
P. Donà, Astrid Eichhorn, Roberto Percacci
(Submitted on 16 Oct 2014)
We discuss the compatibility of quantum gravity with dynamical matter degrees of freedom. Specifically, we present bounds we obtained in [1] on the allowed number and type of matter fields within asymptotically safe quantum gravity. As a novel result, we show bounds on the allowed number of spin-3/2 (Rarita-Schwinger) fields, e.g., the gravitino. These bounds, obtained within truncated Renormalization Group flows, indicate the compatibility of asymptotic safety with the matter fields of the standard model. Further, they suggest that extensions of the matter content of the standard model are severely restricted in asymptotic safety. This means that searches for new particles at colliders could provide experimental tests for this particular approach to quantum gravity.
8 pages, 1 figure, 1 table. Proceedings of Theory Canada 9; new results on the gravitino,

http://arxiv.org/abs/1410.1714
Loop quantum gravity and observations
A. Barrau, J. Grain
(Submitted on 7 Oct 2014)
Quantum gravity has long been thought to be completely decoupled from experiments or observations. Although it is true that smoking guns are still missing, there are now serious hopes that quantum gravity phenomena might be tested. We review here some possible ways to observe loop quantum gravity effects either in the framework of cosmology or in astroparticle physics.

Last edited: Nov 16, 2014