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

[marcus]

There are twenty candidates, and the poll is divided into Parts I and II, each with ten. The poll is multiple choice so it's possible to vote for several papers. Please indicate the ones you think will prove most significant for future Loop-and-allied QG research. Abstracts follow in the next post.

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
===================================
THE FOLLOWING ARE ON PART II, which is here:
https://www.physicsforums.com/threa...4-mip-most-important-qg-paper-part-ii.773590/

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

 

[marcus]

Here are the abstracts for the first ten candidates (Part I)
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
(Submitted on 26 Sep 2014)
We develop a Hamiltonian description of point particles in (2+1)-dimensions using connection and frame-field variables for general relativity. The topology of each spatial hypersurface is that of a punctured two-sphere with particles residing at the punctures. We describe this topology with a CW complex (a collection of two-cells glued together along edges), and use this to fix a gauge and reduce the Hamiltonian. The equations of motion for the fields describe a dynamical triangulation where each vertex moves according to the equation of motion for a free relativistic particle. The evolution is continuous except for when triangles collapse (i.e. the edges become parallel) causing discrete, topological changes in the underlying CW complex.
We then introduce the loop gravity phase space parameterized by holonomy-flux variables on a graph (a network of one-dimensional links). By embedding a graph within the CW complex, we find a description of this system in terms of loop variables. The resulting equations of motion describe the same dynamical triangulation as the connection and frame-field variables. In this framework, the collapse of a triangle causes a discrete change in the underlying graph, giving a concrete realization of the graph-changing moves that many expect to feature in full loop quantum gravity. The main result is a dynamical model of loop gravity which agrees with general relativity and is well-suited for quantization using existing methods.
31 pages, 14 figures

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
(Submitted on 14 Sep 2014)
We estimate the size of a primordial black hole exploding today via a white hole transition, and the power in the resulting explosion, using a simple model. We point out that Fast Radio Bursts, strong signals with millisecond duration, probably extragalactic and having unknown source, have wavelength not far from the expected size of the exploding hole. We also discuss the possible higher energy components of the signal.
5 pages

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
(Submitted on 11 Sep 2014)
It has long been argued that higher categories provide the proper algebraic structure underlying state sum invariants of 4-manifolds. This idea has been refined recently, by proposing to use 2-groups and their representations as specific examples of 2-categories. The challenge has been to make these proposals fully explicit. Here we give a concrete realization of this program. Building upon our earlier work with Baez and Wise on the representation theory of 2-groups, we construct a four-dimensional state sum model based on a categorified version of the Euclidean group. We define and explicitly compute the simplex weights, which may be viewed a categorified analogue of Racah-Wigner 6j-symbols. These weights solve an hexagon equation that encodes the formal invariance of the state sum under the Pachner moves of the triangulation. This result unravels the combinatorial formulation of the Feynman amplitudes of quantum field theory on flat spacetime proposed in [1], which was shown to lead after gauge-fixing to Korepanov's invariant of 4-manifolds.
13 pages

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
(Submitted on 10 Sep 2014)
Group field theories represent a 2nd quantized reformulation of the loop quantum gravity state space and a completion of the spin foam formalism. States of the canonical theory, in the traditional continuum setting, have support on graphs of arbitrary valence. On the other hand, group field theories have usually been defined in a simplicial context, thus dealing with a restricted set of graphs. In this paper, we generalize the combinatorics of group field theories to cover all the loop quantum gravity state space. As an explicit example, we describe the GFT formulation of the KKL spin foam model, as well as a particular modified version. We show that the use of tensor model tools allows for the most effective construction. In order to clarify the mathematical basis of our construction and of the formalisms with which we deal, we also give an exhaustive description of the combinatorial structures entering spin foam models and group field theories, both at the level of the boundary states and of the quantum amplitudes.
49 pages, 24 figures.

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
(Submitted on 8 Sep 2014)
In the construction of spectral manifolds in noncommutative geometry, a higher degree Heisenberg commutation relation involving the Dirac operator and the Feynman slash of real scalar fields naturally appears and implies, by equality with the index formula, the quantization of the volume. We first show that this condition implies that the manifold decomposes into disconnected spheres which will represent quanta of geometry. We then refine the condition by involving the real structure and two types of geometric quanta, and show that connected manifolds with large quantized volume are then obtained as solutions. When this condition is adopted in the gravitational action it leads to the quantization of the four volume with the cosmological constant obtained as an integration constant. Restricting the condition to a three dimensional hypersurface implies quantization of the three volume and the possible appearance of mimetic dark matter. When restricting to a two dimensional hypersurface, under appropriate boundary conditions, this results in the quantization of area and has many interesting applications to black hole physics.
4 pages.

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
(Submitted on 8 Sep 2014)
Tensor network techniques have proved to be powerful tools that can be employed to explore the large scale dynamics of lattice systems. Nonetheless, the redundancy of degrees of freedom in lattice gauge theories (and related models) poses a challenge for standard tensor network algorithms. We accommodate for such systems by introducing an additional structure decorating the tensor network. This allows to explicitly preserve the gauge symmetry of the system under coarse graining and straightforwardly interpret the fixed point tensors. Using this novel information encoded in the decoration might eventually lead to new methods incorporating both analytical and numerical techniques.
14 pages, 12 figures.

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
(Submitted on 4 Sep 2014)
The construction of a continuum limit for the dynamics of loop quantum gravity is unavoidable to complete the theory. We explain that such a construction is equivalent to obtaining the continuum physical Hilbert space, which encodes the solutions of the theory. We present iterative coarse graining methods to construct physical states in a truncation scheme and explain in which sense this scheme represents a renormalization flow. We comment on the role of diffeomorphism symmetry as an indicator for the continuum limit.
18 pages, 1 figure, for a volume edited by A. Ashtekar and J. Pullin, to be published in the World Scientific series "100 Years of General Relativity"

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
(Submitted on 2 Sep 2014)
It is widely believed that special initial conditions must be imposed on any time-symmetric law if its solutions are to exhibit behavior of any kind that defines an `arrow of time'. We show that this is not so. The simplest non-trivial time-symmetric law that can be used to model a dynamically closed universe is the Newtonian N-body problem with vanishing total energy and angular momentum. Because of special properties of this system (likely to be shared by any law of the Universe), its typical solutions all divide at a uniquely defined point into two halves. In each a well-defined measure of shape complexity fluctuates but grows irreversibly between rising bounds from that point. Structures that store dynamical information are created as the complexity grows and act as `records'. Each solution can be viewed as having a single past and two distinct futures emerging from it. Any internal observer must be in one half of the solution and will only be aware of the records of one branch and deduce a unique past and future direction from inspection of the available records.
6 pages. To appear in Physical Review Letters

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
(Submitted on 2 Sep 2014)
A debate has appeared in the literature on loop quantum gravity and spin foams, over whether secondary simplicity constraints should imply the shape matching conditions reducing twisted geometries to Regge geometries. We address the question using a model in which secondary simplicity constraints arise from a dynamical preservation of the primary ones, and answer it in the affirmative. The origin of the extra condition is to be found in the different graph localisations of the various constraints. Our results are consistent with previous claims by Dittrich and Ryan, and extend their validity to Lorentzian signature and a priori arbitrary cellular decompositions. Finally, we show how the (gauge-invariant version of the) twist angle ξ featuring in twisted geometries equals on-shell the Regge dihedral angle multiplied by the Immirzi parameter, thus recovering the discrete extrinsic geometry from the Ashtekar-Barbero holonomy.
15 pages.

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
(Submitted on 30 Aug 2014)
We study the dynamics of vacuum entanglement in the process of gravitational collapse and subsequent black hole evaporation. In the first part of the paper, we introduce a covariant regularization of entanglement entropy tailored to curved spacetimes; this regularization allows us to propose precise definitions for the concepts of black hole "exterior entropy" and "radiation entropy." For a Vaidya model of collapse we find results consistent with the standard thermodynamic properties of Hawking radiation. In the second part of the paper, we compute the vacuum entanglement entropy of various spherically-symmetric spacetimes of interest, including the nonsingular black hole model of Bardeen, Hayward, Frolov and Rovelli-Vidotto and the "black hole fireworks" model of Haggard-Rovelli. We discuss specifically the role of event and trapping horizons in connection with the behavior of the radiation entropy at future null infinity. We observe in particular that (i) in the presence of an event horizon the radiation entropy diverges at the end of the evaporation process, (ii) in models of nonsingular evaporation (with a trapped region but no event horizon) the generalized second law holds only at early times and is violated in the "purifying" phase, (iii) at late times the radiation entropy can become negative (i.e. the radiation can be less correlated than the vacuum) before going back to zero leading to an up-down-up behavior for the Page curve of a unitarily evaporating black hole.
35 pages, 14 figures
===========================
For convenient review, to have all the abstracts together in one place, here are the abstracts for the ten PAPERS ON PART II of the poll:
https://www.physicsforums.com/threa...4-mip-most-important-qg-paper-part-ii.773590/

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.

 

[marcus]

Thanks to AndrewAmmerlaa, Atyy, Chronos, Dave Horgan, Delta31415, Gluon_printer, Julcab, Maybe_Memorie, Martinbn, and Ohwilleke for your thoughtful responses on both parts of the poll! Personally I'd be interested in your comments on current developments and the thinking behind your assessment of these papers. I think a lot is going on.
Looking ahead, there's growing emphasis on the observational side of LQG.
Just this month there appeared the paper by two phenomenologists who specialize in observational astrophysics, Aurelien Barrau and Julien Grain, and an important paper by Alesci and Calcagni which, I think, completes the derivation of Loop Cosmology from the main LQG theory. This makes the main theory testable by any campaign of observation which tests LQC predictions. It's a major step forward.

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/1410.1714
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.
25 pages, 8 figures. Chapter for a volume edited by A. Ashtekar and J. Pullin, to be published in the World Scientific series "100 Years of General Relativity"

Among other tell-tale footprints of LQG, particularly traces of the bounce in the Cosmic Microwave Background, Barrau and Grain also discuss the prediction of exploding black holes and the opportunity for observational testing afforded by that.

Looking back at the third quarter poll, there were TWO exploding black hole papers one by Haggard and Rovelli and the other by Gambini and Pullin. The two use different lines of argument to conclude that black hole gravitational collapse can end up as a WHITE HOLE which spews out the matter and information which originally fell in. The analytical approaches differ but the conclusions seem supportive.

I see that as part of a general trend towards discovering QG observability. QG is so to speak coming out in the open more. C.f. also the Barrau, Rovelli, Vidotto paper about Fast Radio Bursts, on the current poll.

Another interesting development is the seeming merger of Spin Foam QG (Wieland's new version) with the Energetic Causal Set QG of Cortês and Smolin. That could turn out to be significant because ECS has some seriously nice philosophical features. I'll try, or several of us could, to be more explicit about that in a later post.

 

[ohwilleke]

FWIW, while I voted for a few, this is one of the less exciting quarters in the last couple of years.

 

[marcus]

Hi Ohwilleke : ^)
I think you realize (e.g. from my posts on your "What can we expect that's COOL from LQG" thread) that from my perspective this past quarter's list is one of the MOST exciting. As I indicated, that is in part because of the last two papers listed on the Part II poll.
They represent essentially a hybrid or partial merger of LQG with one of the philosophically most sophisticated (and I think physically well-justified) alternatives to LQG, namely Causal Sets.

In a completely rigorous sense, a spacetime geometry in GR is 90% causal structure and 10% local volume information, the local scale or size of things.
The GR metric describing the geometry is a matrix with 10 independent numbers, only one of which is needed to determine the local scale and the other 9 determine the LIGHTCONE STRUCTURE, what points (i.e.events in your past lightcone) could be influencing or informing your world, what points are in your forward lightcone so that you can influence or communicate to them.

If you know the lightcone structure you can almost reconstruct the full metric, all you need is to be told the local scale at any given point.

So Causal Sets is basically the background independent QG approach where you postulate 1. causal structure, 2. discreteness, and 3. no global time.
The quantization has to be PATH INTEGRAL style because you deny the reality of 3D slices. There is no 3D world at some instant which then changes to the next 3D world at the next instant. That would imply a global time. So the discrete grains have to be spacetime grains. Canonical quantization is not allowed, so it is a path integral approach.

To me this seems conceptually sound. On the other hand LQG-SF has made the most progress of any of the QG approaches in cosmology, observational testing, calculating operator spectra, ... It is arguably the most developed approach. Joining the two represents a breakthrough for Causal Sets, which has so far not made as much progress along those lines. Causets is conceptually appealing but maybe has not developed enough structure to work with and get traction.

The joining forces of these two approaches is represented by #9 and #10 on this list. One way to get the preprints is to google "wieland new action" for #10 and "cortes causal spin foam" for #9.
====quote from Part II poll====
Which paper(s) will contribute most significantly to future research?
https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/edit

1. 
   Rethinking Connes' approach to the standard model of particle physics
   0 vote(s)
   0.0%
2. 
   From General Relativity to Quantum Gravity
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13001
   33.3%
3. 
   A scenario for black hole evaporation on a quantum geometry
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13002
   50.0%
4. 
   Thermally correlated states in Loop Quantum Gravity
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13003
   16.7%
5. 
   On background-independent renormalization of spin foam models
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13004
   16.7%
6. 
   Linking shape dynamics and loop quantum gravity
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13005
   33.3%
7. 
   Black hole fireworks: QG effects spark black to white hole tunneling
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13006
   33.3%
8. 
   The Tensor Theory Space
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13007
   16.7%
9. 
   Spin foam models as energetic causal sets
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13008
   33.3%
10. 
    New action for simplicial gravity in four dimensions
    https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13009
    50.0%

 

[marcus]

BTW Ohwilleke! I see you have not voted on Part II of the poll. So if you change your mind and see anything on that list of 10 that especially appeals to you as potentially significant, you can still vote on Part II.
Here's the link:
https://www.physicsforums.com/threa...4-mip-most-important-qg-paper-part-ii.773590/

 

[marcus]

I've merged the lists from Parts I and II of the poll to see how our votes stack up so far.

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

1. 
   Point particles in 2+1 dimensions: GR and loop gravity descriptions
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/poll/results?poll_response_id=12990
   
   22.2%
2. 
   Fast Radio Bursts and White Hole Signals
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/poll/results?poll_response_id=12991
   11.1%
3. 
   A 2-categorical state sum model
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/poll/results?poll_response_id=12992
   11.1%
4. 
   Group field theories for all loop quantum gravity
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/poll/results?poll_response_id=12993
   11.1%
5. 
   Quanta of Geometry
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/poll/results?poll_response_id=12994
   22.2%
6. 
   Decorated tensor network renormalization
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/poll/results?poll_response_id=12995
   44.4%
7. 
   The continuum limit of loop quantum gravity - a framework
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/poll/results?poll_response_id=12996
   44.4%
8. 
   Identification of a gravitational arrow of time
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-i.773589/poll/results?poll_response_id=12997
   44.4%
9. 
   A note on the secondary simplicity constraints in loop quantum gravity
   0 vote(s)
   0.0%
10. 
    Entanglement entropy production in gravitational collapse
    0 vote(s)
    0.0%

1. 
   Rethinking Connes' approach to the standard model of particle physics
   0 vote(s)
   0.0%
2. 
   From General Relativity to Quantum Gravity
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13001
   50.0%
3. 
   A scenario for black hole evaporation on a quantum geometry
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13002
   50.0%
4. 
   Thermally correlated states in Loop Quantum Gravity
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13003
   12.5%
5. 
   On background-independent renormalization of spin foam models
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13004
   12.5%
6. 
   Linking shape dynamics and loop quantum gravity
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13005
   25.0%
7. 
   Black hole fireworks: QG effects spark black to white hole tunneling
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13006
   25.0%
8. 
   The Tensor Theory Space
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13007
   12.5%
9. 
   Spin foam models as energetic causal sets
   https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13008
   25.0%
10. 
    New action for simplicial gravity in four dimensions
    https://www.physicsforums.com/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/threads/our-picks-for-third-quarter-2014-mip-most-important-qg-paper-part-ii.773590/poll/results?poll_response_id=13009
    62.5%

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
===================================
THE FOLLOWING ARE ON PART II, which is here:
https://www.physicsforums.com/threa...4-mip-most-important-qg-paper-part-ii.773590/

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

 

[marcus]

Thanks to AndrewAmmerlaa, Atyy, Chronos, Dave Horgan, Delta31415, Gluon_printer, Julcab, Maybe_Memorie, Martinbn, and Ohwilleke for getting the poll off to a good start! Here are the votes so far:

Five votes:
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland
http://arxiv.org/abs/1407.0025

Four votes:
Decorated tensor network renormalization
Bianca Dittrich, Sebastian Mizera, Sebastian Steinhaus
http://arxiv.org/abs/1409.2407

The continuum limit of loop quantum gravity - a framework
Bianca Dittrich
http://arxiv.org/abs/1409.1450

Identification of a gravitational arrow of time
Julian Barbour, Tim Koslowski, Flavio Mercati
http://arxiv.org/abs/1409.0917

From General Relativity to Quantum Gravity
Abhay Ashtekar, Martin Reuter, Carlo Rovelli
http://arxiv.org/abs/1408.4336

A scenario for black hole evaporation on a quantum geometry
Rodolfo Gambini, Jorge Pullin
http://arxiv.org/abs/1408.3050

Two votes:
Point particles in 2+1 dimensions: GR and loop gravity descriptions
Jonathan Ziprick
http://arxiv.org/abs/1409.7455

Quanta of Geometry
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov
http://arxiv.org/abs/1409.2471

Linking shape dynamics and loop quantum gravity
Lee Smolin
http://arxiv.org/abs/1407.2909

Black hole fireworks: QG effects spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli
http://arxiv.org/abs/1407.0989

Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin
http://arxiv.org/abs/1407.0032

One vote:
Fast Radio Bursts and White Hole Signals
Aurélien Barrau, Carlo Rovelli, Francesca Vidotto
http://arxiv.org/abs/1409.4031

A 2-categorical state sum model
Aristide Baratin, Laurent Freidel
http://arxiv.org/abs/1409.3526

Group field theories for all loop quantum gravity
Daniele Oriti, James P. Ryan, Johannes Thürigen
http://arxiv.org/abs/1409.3150

On background-independent renormalization of spin foam models
Benjamin Bahr
http://arxiv.org/abs/1407.7746

Thermally correlated states in Loop Quantum Gravity
Goffredo Chirco, Carlo Rovelli, Paola Ruggiero
http://arxiv.org/abs/1408.0121

The Tensor Theory Space
Vincent Rivasseau
http://arxiv.org/abs/1407.0284

https://www.physicsforums.com/threa...4-mip-most-important-qg-paper-part-ii.773590/

 

[marcus]

Thirteen of us have voted so far. Thanks to AndrewAmmerlaa, Atyy, Chronos, Dave Horgan, Delta31415, Gluon_printer, Julcab, Maybe_Memorie, Martinbn, Nonlinearity, Ohwilleke, and PhantomJay! I've edited this post to reflect votes of new respondents. There've been substantial changes in the standings since the tally in the preceding post. Here's how it looks now:

Five votes:
The continuum limit of loop quantum gravity - a framework
Bianca Dittrich
http://arxiv.org/abs/1409.1450

Identification of a gravitational arrow of time
Julian Barbour, Tim Koslowski, Flavio Mercati
http://arxiv.org/abs/1409.0917

A scenario for black hole evaporation on a quantum geometry
Rodolfo Gambini, Jorge Pullin
http://arxiv.org/abs/1408.3050

New action for simplicial gravity in four dimensions
Wolfgang M. Wieland
http://arxiv.org/abs/1407.0025

Four votes:
Decorated tensor network renormalization
Bianca Dittrich, Sebastian Mizera, Sebastian Steinhaus
http://arxiv.org/abs/1409.2407

From General Relativity to Quantum Gravity
Abhay Ashtekar, Martin Reuter, Carlo Rovelli
http://arxiv.org/abs/1408.4336

Three votes:
Quanta of Geometry
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov
http://arxiv.org/abs/1409.2471

Linking shape dynamics and loop quantum gravity
Lee Smolin
http://arxiv.org/abs/1407.2909

Black hole fireworks: QG effects spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli
http://arxiv.org/abs/1407.0989

Two votes:
Point particles in 2+1 dimensions: GR and loop gravity descriptions
Jonathan Ziprick
http://arxiv.org/abs/1409.7455

Fast Radio Bursts and White Hole Signals
Aurélien Barrau, Carlo Rovelli, Francesca Vidotto
http://arxiv.org/abs/1409.4031

Group field theories for all loop quantum gravity
Daniele Oriti, James P. Ryan, Johannes Thürigen
http://arxiv.org/abs/1409.3150

Thermally correlated states in Loop Quantum Gravity
Goffredo Chirco, Carlo Rovelli, Paola Ruggiero
http://arxiv.org/abs/1408.0121

Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin
http://arxiv.org/abs/1407.0032

One vote:
A 2-categorical state sum model
Aristide Baratin, Laurent Freidel
http://arxiv.org/abs/1409.3526

Entanglement entropy production in gravitational collapse: covariant regularization and solvable models
Eugenio Bianchi, Tommaso De Lorenzo, Matteo Smerlak
http://arxiv.org/abs/1409.0144

On background-independent renormalization of spin foam models
Benjamin Bahr
http://arxiv.org/abs/1407.7746

The Tensor Theory Space
Vincent Rivasseau
http://arxiv.org/abs/1407.0284
========
Because of format limits, the poll is divided into two lists of ten each. If you haven't voted in the second part yet, please do! Here's the link to Part II.
https://www.physicsforums.com/threa...4-mip-most-important-qg-paper-part-ii.773590/