Our picks for second quarter 2014 MIP (most important QG paper)

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

33.3%

33.3%

41.7%

41.7%

16.7%

8.3%

8.3%

0 vote(s)
0.0%

33.3%

33.3%

25.0%

16.7%

8.3%

25.0%

8.3%

8.3%

16.7%

25.0%

16.7%
20. Purity is not eternal at the Planck scale

8.3%
1. Jun 30, 2014

marcus

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

http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space

http://arxiv.org/abs/1406.0369
Viability of the matter bounce scenario in Loop Quantum Cosmology for general potentials
Jaume Haro, Jaume Amorós

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano

Last edited: Jun 30, 2014
2. Jun 30, 2014

marcus

http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly
(Submitted on 27 Jun 2014)
Entanglement entropy has proven to be an extremely useful concept in quantum field theory. Gauge theories are of particular interest, but for these systems the entanglement entropy is not clearly defined because the physical Hilbert space does not factor as a tensor product according to regions of space. Here we review a definition of entanglement entropy that applies to abelian and nonabelian lattice gauge theories. This entanglement entropy is obtained by embedding the physical Hilbert space into a product of Hilbert spaces associated to regions with boundary. The latter Hilbert spaces include degrees of freedom on the entangling surface that transform like surface charges under the gauge symmetry. These degrees of freedom are shown to contribute to the entanglement entropy, and the form of this contribution is determined by the gauge symmetry. We test our definition using the example of two-dimensional Yang-Mills theory, and find that it agrees with the thermal entropy in de Sitter space, and with the results of the Euclidean replica trick. We discuss the possible implications of this result for more complicated gauge theories, including quantum gravity.
12 pages. Invited article for Classical and Quantum Gravity special issue on Entanglement and Quantum Gravity

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui
(Submitted on 23 Jun 2014)
We define the analytic continuation of the number of black hole microstates in Loop Quantum Gravity to complex values of the Barbero-Immirzi parameter γ. This construction deeply relies on the link between black holes and Chern-Simons theory. Technically, the key point consists in writing the number of microstates as an integral in the complex plane of a holomorphic function, and to make use of complex analysis techniques to perform the analytic continuation. Then, we study the thermodynamical properties of the corresponding system (the black hole is viewed as a gas of indistinguishable punctures) in the framework of the grand canonical ensemble where the energy is defined à la Frodden-Gosh-Perez from the point of view of an observer located close to the horizon. The semi-classical limit occurs at the Unruh temperature TU associated to this local observer. When γ=±i, the entropy reproduces at the semi-classical limit the area law with quantum corrections. Furthermore, the quantum corrections are logarithmic provided that the chemical potential is fixed to the simple value μ=2TU.
31 pages, 2 figures.

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors
(Submitted on 14 Jun 2014)
In this article, we consider a bouncing Universe, as described for example by Loop Quantum Cosmology. If the current acceleration is due to a true cosmological constant, this constant is naturally conserved through the bounce and the Universe should also be in a (contracting) de Sitter phase in the remote past. We investigate here the possibility that the de Sitter temperature in the contracting branch fills the Universe with radiation and causes the bounce and the subsequent inflation and reheating. We also consider the possibility that this gives rise to a cyclic model of the Universe and suggest some possible tests.
5 pages.

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin
(Submitted on 10 Jun 2014)
This paper addresses the question of whether Witten's proof of positive ADM energy for classical general relativity can be extended to give a proof of positive energy for a non-perturbative quantization of general relativity. To address this question, a set of conditions is shown to be sufficient for showing the positivity of a Hamiltonian operator corresponding to the ADM energy. One of these conditions is a particular factor ordering for the constraints of general relativity, in a representation where the states are functionals of the Ashtekar connection, and the auxiliary, Witten spinor.
These developments are partly based on results derived with Artem Starodubtsev (unpublished notes, 2004).
14 pages.

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin
(Submitted on 10 Jun 2014)
We consider a quantum field theory on a spherically symmetric quantum space time described by loop quantum gravity. The spin network description of space time in such a theory leads to equations for the quantum field that are discrete. We show that to avoid significant violations of Lorentz invariance one needs to consider specific non-local interactions in the quantum field theory similar to those that appear in string theory. This is the first sign that loop quantum gravity places restrictions on the type of matter considered, and points to a connection with string theory physics.
7 pages. Honorable mention Gravity Research Foundation 2014.

http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh
(Submitted on 5 Jun 2014)
In recent years, numerical simulations with Gaussian initial states have demonstrated the existence of a quantum bounce in loop quantum cosmology in various models. A key issue pertaining to the robustness of the bounce and the associated physics is to understand the quantum evolution for more general initial states which may depart significantly from Gaussianity and may have no well defined peakedness properties. The analysis of such states, including squeezed and highly non-Gaussian states, has been computationally challenging until now. In this manuscript, we overcome these challenges by using the Chimera scheme for the spatially flat, homogeneous and isotropic model sourced with a massless scalar field. We demonstrate that the quantum bounce in loop quantum cosmology occurs even for states which are highly squeezed or are non-Gaussian with multiple peaks and with little resemblance to semi-classical states. The existence of the bounce is found to be robust, and does not depend on the properties of the states. The evolution of squeezed and non-Gaussian states turns out to be qualitatively similar to the Gaussian states and satisfies strong constraints on the growth of the relative fluctuations across the bounce. We also compare the results from the effective dynamics and find that, though it captures the qualitative aspects of the evolution for squeezed and highly non-Gaussian states, it always underestimates the bounce volume. We show that various properties of the evolution, such as the energy density at the bounce, are in excellent agreement with the predictions from an exactly solvable loop quantum cosmological model for arbitrary states.
26 pages, 16 figures.

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space
(Submitted on 3 Jun 2014)
The Koslowski-Sahlmann (KS) representation is a generalization of the representation underlying the discrete spatial geometry of Loop Quantum Gravity (LQG), to accommodate states labelled by smooth spatial geometries. As shown recently, the KS representation supports, in addition to the action of the holonomy and flux operators, the action of operators which are the quantum counterparts of certain connection dependent functions known as "background exponentials".
Here we show that the KS representation displays the following properties which are the exact counterparts of LQG ones:
(i) the abelian ∗ algebra of SU(2) holonomies and 'U(1)' background exponentials can be completed to a C* algebra
(ii) the space of semianalytic SU(2) connections is topologically dense in the spectrum of this algebra
(iii) there exists a measure on this spectrum for which the KS Hilbert space is realised as the space of square integrable functions on the spectrum
(iv) the spectrum admits a characterization as a projective limit of finite numbers of copies of SU(2) and U(1)
(v) the algebra underlying the KS representation is constructed from cylindrical functions and their derivations in exactly the same way as the LQG (holonomy-flux) algebra except that the KS cylindrical functions depend on the holonomies and the background exponentials, this extra dependence being responsible for the differences between the KS and LQG algebras.
While these results are obtained for compact spaces, they are expected to be of use for the construction of the KS representation in the asymptotically flat case.
33 pages.

http://arxiv.org/abs/1406.0369
Viability of the matter bounce scenario in Loop Quantum Cosmology for general potentials
Jaume Haro, Jaume Amorós
(Submitted on 2 Jun 2014)
We consider the matter bounce scenario in Loop Quantum Cosmology (LQC) for physical potentials that at early times provide a nearly matter dominated Universe in the contracting phase, having a reheating mechanism in the expanding phase, i.e., being able to release the energy of the scalar field creating particles that thermalize in order to match with the hot Friedmann Universe, and finally at late times leading to the current cosmic acceleration. For these models, numerically solving the dynamical equations we have seen that the teleparallel version of LQC leads to theoretical results that fit well with current observational data. More precisely, in teleparallel LQC there is a set of solutions which leads to theoretical results that match correctly with last BICEP2 data, and there is another set whose theoretical results fit well with Planck's experimental data. On the other hand, in holonomy corrected LQC the theoretical value of the tensor/scalar ratio is smaller than in teleparallel LQC, which means that there is always a set of solutions that matches with Planck's data, but for some potentials BICEP2 experimental results disfavours holonomy corrected LQC.
29 pages, 8 figures.

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez
(Submitted on 28 May 2014)
We analyze the relationship between entanglement (or geometric) entropy with statistical mechanical entropy of horizon degrees of freedom when described in the framework of isolated horizons in loop quantum gravity. We show that, once the relevant degrees of freedom are identified, the two notions coincide. The key ingredient linking the two notions is the structure of quantum geometry at Planck scale implied by loop quantum gravity, where correlations between the inside and outside of the black hole are mediated by eigenstates of the horizon area operator.
11 pages, 3 figures.

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
(Submitted on 16 May 2014)
We show within the usual two-dimensional approximation that unitarity and the restoration of Minkowski vacuum correlations at the end of black hole evaporation impose unexpected constraints on its mass loss rate: before disappearing the black hole emits one or more negative energy burst, leading to a temporary increase of its mass.
8 pages; nontechnical version of http://arxiv.org/abs/1404.0602 E. Bianchi, M.Smerlak
"Entanglement entropy and negative-energy fluxes in two-dimensional space times"

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura
(Submitted on 19 May 2014)
We study the Einstein gravity equations projected on a timelike surface, which represents the time evolution of what we call a gravitational screen. We show that such a screen possesses a surface tension and an internal energy, and that the Einstein equations reduce to the thermodynamic equations of a viscous bubble. We also provide a complete dictionary between gravitational and thermodynamical variables. In the non-viscous cases there are three thermodynamic equations which characterise a bubble dynamics: These are the first law, the Marangoni flow equation and the Young-Laplace equation. In all three equations the surface tension plays a central role: In the first law it appears as a work term per unit area, in the Marangoni flow its gradient drives a force, and in the Young-Laplace equation it contributes to a pressure proportional to the surface curvature. The gravity equations appear as a natural generalization of these bubble equations when the bubble itself is viscous and dynamical. In particular, it shows that the mechanism of entropy production for the viscous bubble is mapped onto the production of gravitational waves. We also review the relationship between surface tension and temperature, and discuss the usual black-hole thermodynamics from this point of view.
27 pages, 3 figures.

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll
(Submitted on 19 May 2014)
We perform a first investigation of the coupling constant flow of the nonperturbative lattice model of four-dimensional quantum gravity given in terms of Causal Dynamical Triangulations (CDT). After explaining how standard concepts of lattice field theory can be adapted to the case of this background-independent theory, we define a notion of "lines of constant physics" in coupling constant space in terms of certain semiclassical properties of the dynamically generated quantum universe. Determining flow lines with the help of Monte Carlo simulations, we find that the second-order phase transition line present in this theory can be interpreted as a UV phase transition line if we allow for an anisotropic scaling of space and time.
20 pages

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 May 2014)
This article addresses the issue of estimating the duration in inflation in bouncing cosmology when anisotropies, inevitably playing and important role, are taken into account. It is shown that in Bianchi-I loop quantum cosmology, the higher the shear, the shorter the period of inflation. For a wide range of parameters, the probability distribution function of the duration of inflation is however peaked at values compatible with data, but not much higher. This makes the whole bounce/inflationary scenario consistent and phenomenologically appealing as all the information from the bounce might then not have been fully washed out.
7pages, 5 figures.

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli
(Submitted on 23 Apr 2014)
It is possible that black holes hide a core of Planckian density, sustained by quantum-gravitational pressure. As a black hole evaporates, the core remembers the initial mass and the final explosion occurs at macroscopic scale. We investigate possible phenomenological consequences of this idea. Under several rough assumptions, we estimate that up to several short gamma-ray bursts per day, around 10 MeV, with isotropic distribution, can be expected coming from a region of a few hundred light years around us.
5 pages, 4 figures.

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale
(Submitted on 16 Apr 2014)
I give a brief introduction to the relation between loop quantum gravity and twistor theory, and comment on some perspectives on the problem of time.
10 pages, invited lecture to the "2nd International Conference on New Frontiers in Physics 2013" (ICNFP 2013), to be published in EPJ Web of Conferences vol. 71

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing
(Submitted on 15 Apr 2014)
We study the loop quantum cosmology of a flat Friedmann-Lemaitre-Robertson-Walker space-time with a Maxwell field. We show that many of the qualitative properties derived for the case of a massless scalar field also hold for a Maxwell field. In particular, the big-bang singularity is replaced by a quantum bounce, and the operator corresponding to the matter energy density is bounded above by the same critical energy density. We also numerically study the evolution of wave functions that are sharply peaked in the low energy regime, and derive effective equations which very closely approximate the full quantum dynamics of sharply peaked states at all times, including the near-bounce epoch.
27 pages, 6 figures.

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen
(Submitted on 10 Apr 2014)
Spatially homogeneous universes can be described in (loop) quantum gravity as condensates of elementary excitations of space. Their treatment is easiest in the second-quantised group field theory formalism which allows the adaptation of techniques from the description of Bose-Einstein condensates in condensed matter physics. Dynamical equations for the states can be derived directly from the underlying quantum gravity dynamics. The analogue of the Gross-Pitaevskii equation defines an anisotropic quantum cosmology model, in which the condensate wavefunction becomes a quantum cosmology wavefunction on minisuperspace. To illustrate this general formalism, we give a mapping of the gauge-invariant geometric data for a tetrahedron to a minisuperspace of homogeneous anisotropic 3-metrics. We then study an example for which we give the resulting quantum cosmology model in the general anisotropic case and derive the general analytical solution for isotropic universes. We discuss the interpretation of these solutions and comment on the validity of the WKB approximation used in previous studies.
20 pages, 2 figures.

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
(Submitted on 8 Apr 2014)
Observations indicate that our universe is characterized by a late-time accelerating phase, possibly driven by a cosmological constant Λ, with the dimensionless parameter ΛLP2 ≃ 10−122, where LP=(Gℏ/c3)1/2 is the Planck length. In this review, we describe how the emergent gravity paradigm provides a new insight and a possible solution to the cosmological constant problem. After reviewing the necessary background material, we identify the necessary and sufficient conditions for solving the cosmological constant problem. We show that these conditions are naturally satisfied in the emergent gravity paradigm in which
(i) the field equations of gravity are invariant under the addition of a constant to the matter Lagrangian and
(ii) the cosmological constant appears as an integration constant in the solution.
The numerical value of this integration constant can be related to another dimensionless number (called CosMIn) that counts the number of modes inside a Hubble volume that cross the Hubble radius during the radiation and the matter dominated epochs of the universe. The emergent gravity paradigm suggests that CosMIn has the numerical value 4π, which, in turn, leads to the correct, observed value of the cosmological constant. Further, the emergent gravity paradigm provides an alternative perspective on cosmology and interprets the expansion of the universe itself as a quest towards holographic equipartition. We discuss the implications of this novel and alternate description of cosmology.
48 pages; 5 figures. Invited review to appear in Int. Jour. Mod. Phys. D

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen
(Submitted on 7 Apr 2014)
Following earlier insights by Livine and Terno, we develop a technique for describing quantum states of the gravitational field in terms of coarse grained spin networks. We show that the number of nodes and links and the values of the spin depend on the observables chosen for the description of the state. Hence the question in the title of this paper is ill posed, unless further information about what is being measured is given.
16 pages, 9 figures.

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano
(Submitted on 25 Mar 2014)
Theories with Planck-scale deformed symmetries exhibit quantum time evolution in which purity of the density matrix is not preserved. In particular we show that the non-trivial structure of momentum space of these models is reflected in a deformed action of translation generators on operators. Such action in the case of time translation generators leads to a Lindblad-like evolution equation for density matrices when expanded at leading order in the Planckian deformation parameter. This evolution equation is covariant under the deformed realization of Lorentz symmetries characterizing these models.
6 pages.

Last edited: Jun 30, 2014
3. Jul 1, 2014

4. Jul 4, 2014

marcus

We are just beginning to pick up responses to the second quarter poll. Thanks David Horgan, Nonlinearity, and Twistor for getting us off to a good start! Here are how the votes tally up so far.

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli

One vote:
http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

5. Jul 5, 2014

6. Jul 5, 2014

marcus

Thanks for mentioning these.
The first one is on this quarter's poll, you may not have noticed it when you voted. It is #11, nine from the bottom of the list of candidates. No one has voted for it yet, but it is definitely interesting.

The last one you mentioned is actually a third quarter paper and I'm pretty sure it will be on the 3rd quarter MIP poll! It is an application of important results in a new paper by Wolfgang Wieland which we have already been discussing and will definitely be on the 3rd quarter poll. It introduces a new kind of conserved geometric quantity a kind of geometric "momentum" that makes its way thru a 4D lattice. It arrives at a new QG action functional, and a new Hamiltonian for spin foam QG.
The name Wieland (you probably know) is pronounced "vee land". He's somebody to remember. Fairly new on the scene. Cortès and Smolin are using his results in an interesting way.

We have a new respondent who just joined the poll. Welcome PhysicoRaj!
Here's how the votes stack up as of now.

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

One vote:
http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano

Last edited: Jul 5, 2014
7. Jul 6, 2014

marcus

Seven of us have registered our picks so far! Thanks Chronos, David Horgan, Julcab, Nonlinearity, PhysicoRaj, and Twistor! There've been a number of changes, since yesterday's tally. The Smolin paper has moved from two votes to three. Papers by Freidel et al (describing gravity in bulk by physics of an hypothetical surrounding bubble) and by Padmanabhans (suggestive derivation of the cosmological constant) each now have two votes. Two others (a numerical study extending Loop cosmology bounce to unpeaked and multi-peaked states of geometry and another on renormalization in Causal Dynamical Triangulations) have placed.

Several of this quarter's papers address technical issues. Smolin (http://arxiv.org/abs/1406.2611) argues that the stability of spacetime geometry itself requires a positive definite Hamiltonian (IOW positive energy support). This, he finds, requires loop quantum gravity be generalized to include smooth geometries (e.g. as was proposed by Koslowski-Sahlmann and discussed in the Campiglia-Varadarajan paper on this quarter's poll, or alternatively as was proposed by Dittrich in a paper on last quarter's poll.)

Another technical achievement figures in the Ashtekar variables "come-back" noted last quarter. http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui
The Immirzi parameter γ in LQG was originally ±i, and then later made a real number. Real γ then showed up in formulas for area operator eigenvalues and for BH entropy. These authors use complex variable techniques to extend the entropy formula to imaginary γ.

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors
Strange result, puts together already accepted physical elements so as to imply that the U may actually be cyclic, in an unexpected manner.

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
Assuming the complete and unitary evaporation of a BH via Hawking radiation leads to a kind of hiccup. A bizarre technical result that might raise suspicions that complete unitary evaporation à la Hawking is not quite the right picture.

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin
The authors find that Lqg geometry is restricted as to the kinds of matter it can accept. This might be expected to be the case with Loop (and with other frameworks as well) but it had not been made specific. It seems a notable advance to begin finding limitations of this sort.

8. Jul 11, 2014

marcus

We aren't even two weeks into the third quarter of 2014 and already 5 papers have appeared that look like interesting prospects for the September 3rd quarter poll. At this rate it's going to be a crowded field:

http://arxiv.org/abs/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
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
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
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
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: Jul 11, 2014
9. Jul 12, 2014

marcus

It's been just a week since the last time I tallied the votes. Here's how they stack up as of now,with brief editorial comment in some instances.

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui
Original Ashtekar variables reappearing.

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors
One-ups Penrose cyclic: U eventually develops pure de S. patches which can recollapse and bounce.

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin
Stability argument supporting, among other things, the Koslowski-Sahlmann extension of Lqg.

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez
Equivalence of entropy concepts.

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
Another in list of curious/paradoxical behavior implied by conventional (unitary) BH evaporation. Is the original Hawking BH evaporation story wrong?

http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

One vote:
http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano

Last edited: Jul 12, 2014
10. Jul 15, 2014

marcus

The maximum allowed by the poll format is 20, so one expects each of the three months in a quarter to yield, on average, about 7 papers interesting enough to include on the poll. So far July has been somewhat unusual in this regard: eight (at least at first sight) noteworthy papers appearing in the first two weeks.

http://arxiv.org/abs/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/1407.3384
Why do we remember the past and not the future? The 'time oriented coarse graining' hypothesis
Carlo Rovelli
(Submitted on 12 Jul 2014)
Phenomenological arrows of time can be traced to a past low-entropy state. Does this imply the universe was in an improbable state in the past? I suggest a different possibility: past low-entropy depends on the coarse-graining implicit in our definition of entropy. This, in turn depends on our physical coupling to the rest of the world. I conjecture that any generic motion of a sufficiently rich system satisfies the second law of thermodynamics, in either direction of time, for some choice of macroscopic observables. The low entropy of the past could then be due to the way we couple to the universe (a way needed for us doing what we do), hence to our natural macroscopic variables, rather than to a strange past microstate of the world at large.
5 pages. Few equations. An idea

http://arxiv.org/abs/1407.3027
Convergent Y ̃-Map for a new covariant Loop Quantum Gravity formulation and Implicit Reality Condition
Leonid Perlov
(Submitted on 11 Jul 2014)
One of the most important elements in a new spin-foam loop quantum gravity formulation is the map Y: HSU(2)→HSL(2,C). In this paper we provide an alternative improved map Y ̃ . The image of a new map Y ̃ contains the weighted infinite sums of SL(2,C) matrix coefficients. The sums are convergent and their limits are the square integrable functions of SL(2,C) with the measure L2(g,e−|Y|2/ℏη(g)dudY) according to the recently proved Holomorphic Peter-Weyl theorem [2]. We also discuss the consequence of a choice of a unitary principal series instead of the general principal series (sometimes called non-unitary) in EPRL model. The general principal series contains the unitary principal series as a sub-representation and becomes unitary when its parameter ν is real rather than complex. The solution of the simplicity constraint then implicitly makes the Barbero-Immirizi parameter real instead of complex. We call this - an implicit reality condition.
7 pages

http://arxiv.org/abs/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
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
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
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
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: Jul 15, 2014
11. Jul 16, 2014

marcus

Last time I tallied the votes, seven of us had indicated our choices for the quarter's most important QG papers. Now that number is up to nine. Thanks to Erkokite and Atyy for joining the poll! Here's how the candidates stand as of now,with brief comments in a few cases.

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez
Equivalence of entropy concepts.

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
Another in list of curious/paradoxical behavior implied by conventional (unitary) BH evaporation. Is the original Hawking BH evaporation story wrong?

http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly
Extends definition of entanglement entropy to gauge quantum field theories by enlarging the Hilbert space to include degrees of freedom on the surface dividing the two entangled regions.

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui
Continues the revival of original (complex Immirzi) Ashtekar variables.

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors
One-ups Penrose cyclic: U eventually develops pure de Sitter patches which can recollapse and bounce.

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin
Stability argument supporting, among other things, the Koslowski-Sahlmann extension of Lqg.

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

One vote:
http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano

Last edited: Jul 16, 2014
12. Jul 20, 2014

marcus

Thanks to Atyy, Chronos, David Horgan, Erkokite, John86, Julcab, Nonlinearity, PhysicoRaj, and Twistor for sharing your assessment of the future importance of these papers! I definitely I learn from other peoples' viewpoints and judgment about where research progress is being made---and I'm probably not alone in this. I see that since the last tally, two papers have moved up from two to three votes, namely:
Planck star phenomenology (Barrau, Rovelli) and Cosmological Constant from the Emergent Gravity Perspective (Padmanabhans, father&daughter)
Also a couple of others, namely Our Universe from the cosmological constant ( Barrau and Linsefors) and Positive energy in quantum gravity (Smolin) have risen from three to four votes.

Here's the current standing,with a few comments.

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors
One-ups Penrose cyclic: U eventually develops pure de Sitter patches which can recollapse and bounce.

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin
Stability argument supporting, among other things, the Koslowski-Sahlmann extension of Lqg.

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez
Equivalence of entropy concepts.

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
Another in list of curious/paradoxical behavior implied by conventional (unitary) BH evaporation. Is the original Hawking BH evaporation story wrong?

http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly
Extends definition of entanglement entropy to gauge quantum field theories by enlarging the Hilbert space to include degrees of freedom on the surface dividing the two entangled regions.

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui
Continues the revival of original (complex Immirzi) Ashtekar variables.

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli
BH collapse immediately bounces and explodes, but because time is slow in deep gravity this can take on the order of the age of the universe, or longer, seen from outside. Observational predictions.

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
Calculates the correct observed value of the cosmological constant from novel assumptions and quantities which, if confirmed valid, could be considered "first principles" for gravity and cosmology.

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

One vote:
http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano

BTW the two Padmanabhans, Thanu (b. 1957) and Hamsa (b. 1990) make an interesting family team.

13. Jul 30, 2014

marcus

As mentioned earlier, the format allows a maximum of 20 on the quarterly poll. July has seen an unusually large crop of interesting/noteworthy papers. We'll have to do some pruning later on. Here's a provisional collection for the month.

http://arxiv.org/abs/1407.8143
Realization of DSR-relativistic symmetries in Finsler geometries
Giovanni Amelino-Camelia, Leonardo Barcaroli, Giulia Gubitosi, Stefano Liberati, Niccoló Loret
(Submitted on 30 Jul 2014)
Finsler geometry is a well known generalization of Riemannian geometry which allows to account for a possibly non trivial structure of the space of configurations of relativistic particles. We here establish a link between Finsler geometry and the sort of models with curved momentum space and DSR-relativistic symmetries which have been recently of interest in the quantum-gravity literature. We use as case study the much-studied scenario which is inspired by the κ-Poincaré quantum group, and show that the relevant deformation of relativistic symmetries can be implemented within a Finsler geometry.
26 pages.

http://arxiv.org/abs/1407.8167
Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics
Steffen Gielen, Daniele Oriti
(Submitted on 30 Jul 2014)
In the context of group field theory condensate cosmology, we clarify the extraction of cosmological variables from the microscopic quantum gravity degrees of freedom. We show that an important implication of the second quantized formalism is the dependence of cosmological variables and equations on the quantum gravitational atomic number N (number of spin network vertices/elementary simplices). We clarify the relation of the effective cosmological equations with loop quantum cosmology, understood as an effective (hydrodynamic-like) approximation of a more fundamental quantum gravity theory. By doing so, we provide a fundamental basis to the idea of lattice refinement, showing the dependence of the effective cosmological connection on N, and hence indirectly on the scale factor. Our results open a new arena for exploring effective cosmological dynamics, as this depends crucially on the new observable N, which is entirely of quantum gravitational origin.
6 pages

http://arxiv.org/abs/1407.8166
Loop quantum cosmology from group field theory
Gianluca Calcagni
(Submitted on 30 Jul 2014)
We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory (GFT) with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology (LQC) in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute.
5 pages

http://arxiv.org/abs/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/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/1407.3384
Why do we remember the past and not the future? The 'time oriented coarse graining' hypothesis
Carlo Rovelli
(Submitted on 12 Jul 2014)
Phenomenological arrows of time can be traced to a past low-entropy state. Does this imply the universe was in an improbable state in the past? I suggest a different possibility: past low-entropy depends on the coarse-graining implicit in our definition of entropy. This, in turn depends on our physical coupling to the rest of the world. I conjecture that any generic motion of a sufficiently rich system satisfies the second law of thermodynamics, in either direction of time, for some choice of macroscopic observables. The low entropy of the past could then be due to the way we couple to the universe (a way needed for us doing what we do), hence to our natural macroscopic variables, rather than to a strange past microstate of the world at large.
5 pages. Few equations. An idea

http://arxiv.org/abs/1407.3027
Convergent Y ̃-Map for a new covariant Loop Quantum Gravity formulation and Implicit Reality Condition
Leonid Perlov
(Submitted on 11 Jul 2014)
One of the most important elements in a new spin-foam loop quantum gravity formulation is the map Y: HSU(2)→HSL(2,C). In this paper we provide an alternative improved map Y ̃ . The image of a new map Y ̃ contains the weighted infinite sums of SL(2,C) matrix coefficients. The sums are convergent and their limits are the square integrable functions of SL(2,C) with the measure L2(g,e−|Y|2/ℏη(g)dudY) according to the recently proved Holomorphic Peter-Weyl theorem [2]. We also discuss the consequence of a choice of a unitary principal series instead of the general principal series (sometimes called non-unitary) in EPRL model. The general principal series contains the unitary principal series as a sub-representation and becomes unitary when its parameter ν is real rather than complex. The solution of the simplicity constraint then implicitly makes the Barbero-Immirizi parameter real instead of complex. We call this - an implicit reality condition.
7 pages

http://arxiv.org/abs/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
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
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
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
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.

14. Aug 7, 2014

marcus

Update on the list of prospects for the third quarter MIP poll:

http://arxiv.org/abs/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/1407.8167
Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics
Steffen Gielen, Daniele Oriti
(Submitted on 30 Jul 2014)
In the context of group field theory condensate cosmology, we clarify the extraction of cosmological variables from the microscopic quantum gravity degrees of freedom. We show that an important implication of the second quantized formalism is the dependence of cosmological variables and equations on the quantum gravitational atomic number N (number of spin network vertices/elementary simplices). We clarify the relation of the effective cosmological equations with loop quantum cosmology, understood as an effective (hydrodynamic-like) approximation of a more fundamental quantum gravity theory. By doing so, we provide a fundamental basis to the idea of lattice refinement, showing the dependence of the effective cosmological connection on N, and hence indirectly on the scale factor. Our results open a new arena for exploring effective cosmological dynamics, as this depends crucially on the new observable N, which is entirely of quantum gravitational origin.
6 pages

http://arxiv.org/abs/1407.8166
Loop quantum cosmology from group field theory
Gianluca Calcagni
(Submitted on 30 Jul 2014)
We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory (GFT) with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology (LQC) in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute.
5 pages

http://arxiv.org/abs/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/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/1407.3384
Why do we remember the past and not the future? The 'time oriented coarse graining' hypothesis
Carlo Rovelli
(Submitted on 12 Jul 2014)
Phenomenological arrows of time can be traced to a past low-entropy state. Does this imply the universe was in an improbable state in the past? I suggest a different possibility: past low-entropy depends on the coarse-graining implicit in our definition of entropy. This, in turn depends on our physical coupling to the rest of the world. I conjecture that any generic motion of a sufficiently rich system satisfies the second law of thermodynamics, in either direction of time, for some choice of macroscopic observables. The low entropy of the past could then be due to the way we couple to the universe (a way needed for us doing what we do), hence to our natural macroscopic variables, rather than to a strange past microstate of the world at large.
5 pages. Few equations. An idea

http://arxiv.org/abs/1407.3027
Convergent Y ̃-Map for a new covariant Loop Quantum Gravity formulation and Implicit Reality Condition
Leonid Perlov
(Submitted on 11 Jul 2014)
One of the most important elements in a new spin-foam loop quantum gravity formulation is the map Y: HSU(2)→HSL(2,C). In this paper we provide an alternative improved map Y ̃ . The image of a new map Y ̃ contains the weighted infinite sums of SL(2,C) matrix coefficients. The sums are convergent and their limits are the square integrable functions of SL(2,C) with the measure L2(g,e−|Y|2/ℏη(g)dudY) according to the recently proved Holomorphic Peter-Weyl theorem [2]. We also discuss the consequence of a choice of a unitary principal series instead of the general principal series (sometimes called non-unitary) in EPRL model. The general principal series contains the unitary principal series as a sub-representation and becomes unitary when its parameter ν is real rather than complex. The solution of the simplicity constraint then implicitly makes the Barbero-Immirizi parameter real instead of complex. We call this - an implicit reality condition.
7 pages

http://arxiv.org/abs/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
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
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
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
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.

15. Aug 14, 2014

marcus

Prospects for the third quarter MIP poll:

http://arxiv.org/abs/1408.3260
Lorentzian Connes Distance, Spectral Graph Distance and Loop Gravity
Carlo Rovelli
(Submitted on 14 Aug 2014)
Connes' formula defines a distance in loop quantum gravity, via the spinfoam Dirac operator. A simple notion of spectral distance on a graph can be extended do the discrete Lorentzian context, providing a physically natural example of Lorentzian spectral geometry, with a neat space of Dirac operators. The Hilbert structure of the fermion space is Lorentz covariant rather than invariant.
4 pages.

http://arxiv.org/abs/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
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/1407.8167
Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics
Steffen Gielen, Daniele Oriti
(Submitted on 30 Jul 2014)
In the context of group field theory condensate cosmology, we clarify the extraction of cosmological variables from the microscopic quantum gravity degrees of freedom. We show that an important implication of the second quantized formalism is the dependence of cosmological variables and equations on the quantum gravitational atomic number N (number of spin network vertices/elementary simplices). We clarify the relation of the effective cosmological equations with loop quantum cosmology, understood as an effective (hydrodynamic-like) approximation of a more fundamental quantum gravity theory. By doing so, we provide a fundamental basis to the idea of lattice refinement, showing the dependence of the effective cosmological connection on N, and hence indirectly on the scale factor. Our results open a new arena for exploring effective cosmological dynamics, as this depends crucially on the new observable N, which is entirely of quantum gravitational origin.
6 pages

http://arxiv.org/abs/1407.8166
Loop quantum cosmology from group field theory
Gianluca Calcagni
(Submitted on 30 Jul 2014)
We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory (GFT) with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology (LQC) in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute.
5 pages

http://arxiv.org/abs/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/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/1407.3027
Convergent Y ̃-Map for a new covariant Loop Quantum Gravity formulation and Implicit Reality Condition
Leonid Perlov
(Submitted on 11 Jul 2014)
One of the most important elements in a new spin-foam loop quantum gravity formulation is the map Y: HSU(2)→HSL(2,C). In this paper we provide an alternative improved map Y ̃ . The image of a new map Y ̃ contains the weighted infinite sums of SL(2,C) matrix coefficients. The sums are convergent and their limits are the square integrable functions of SL(2,C) with the measure L2(g,e−|Y|2/ℏη(g)dudY) according to the recently proved Holomorphic Peter-Weyl theorem [2]. We also discuss the consequence of a choice of a unitary principal series instead of the general principal series (sometimes called non-unitary) in EPRL model. The general principal series contains the unitary principal series as a sub-representation and becomes unitary when its parameter ν is real rather than complex. The solution of the simplicity constraint then implicitly makes the Barbero-Immirizi parameter real instead of complex. We call this - an implicit reality condition.
7 pages

http://arxiv.org/abs/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
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
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
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
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.[/QUOTE]

Last edited: Aug 14, 2014
16. Aug 19, 2014

marcus

Updated list of prospects for the third quarter MIP poll:

http://arxiv.org/abs/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/1408.4127
Towards phase transitions between discrete and continuum quantum spacetime from the Renormalization Group
Astrid Eichhorn, Tim Koslowski
(Submitted on 18 Aug 2014)
We establish the functional Renormalization Group as an exploratory tool to investigate a possible phase transition between a pre-geometric discrete phase and a geometric continuum phase in quantum gravity. In this paper, based on the analysis of [1], we study three new aspects of the double-scaling limit of matrix models as Renormalization Group fixed points: Firstly, we investigate multicritical fixed points, which are associated with quantum gravity coupled to conformal matter. Secondly, we discuss an approximation that reduces the scheme dependence of our results as well as computational effort while giving good numerical results. This is a consequence of the approximation being a solution to the unitary Ward-identity associated to the U(N) symmetry of the hermitian matrix model. Thirdly, we discuss a scenario that relates the double scaling limit to fixed points of continuum quantum gravity.
14 pages, 1 figure

http://arxiv.org/abs/1408.3260
Lorentzian Connes Distance, Spectral Graph Distance and Loop Gravity
Carlo Rovelli
(Submitted on 14 Aug 2014)
Connes' formula defines a distance in loop quantum gravity, via the spinfoam Dirac operator. A simple notion of spectral distance on a graph can be extended do the discrete Lorentzian context, providing a physically natural example of Lorentzian spectral geometry, with a neat space of Dirac operators. The Hilbert structure of the fermion space is Lorentz covariant rather than invariant.
4 pages.

http://arxiv.org/abs/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
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/1407.8167
Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics
Steffen Gielen, Daniele Oriti
(Submitted on 30 Jul 2014)
In the context of group field theory condensate cosmology, we clarify the extraction of cosmological variables from the microscopic quantum gravity degrees of freedom. We show that an important implication of the second quantized formalism is the dependence of cosmological variables and equations on the quantum gravitational atomic number N (number of spin network vertices/elementary simplices). We clarify the relation of the effective cosmological equations with loop quantum cosmology, understood as an effective (hydrodynamic-like) approximation of a more fundamental quantum gravity theory. By doing so, we provide a fundamental basis to the idea of lattice refinement, showing the dependence of the effective cosmological connection on N, and hence indirectly on the scale factor. Our results open a new arena for exploring effective cosmological dynamics, as this depends crucially on the new observable N, which is entirely of quantum gravitational origin.
6 pages

http://arxiv.org/abs/1407.8166
Loop quantum cosmology from group field theory
Gianluca Calcagni
(Submitted on 30 Jul 2014)
We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory (GFT) with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology (LQC) in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute.
5 pages

http://arxiv.org/abs/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/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/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
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
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
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
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.

17. Aug 31, 2014

marcus

Prospects for the third quarter MIP poll. Thanks to atyy for spotting several of these. I've lost track how many:

http://arxiv.org/abs/1408.7112
Group Field Theory and Loop Quantum Gravity
Daniele Oriti
(Submitted on 29 Aug 2014)
We introduce the group field theory formalism for quantum gravity, mainly from the point of view of loop quantum gravity, stressing its promising aspects. We outline the foundations of the formalism, survey recent results and offer a perspective on future developments.
23 pages.

http://arxiv.org/abs/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
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/1408.3260
Lorentzian Connes Distance, Spectral Graph Distance and Loop Gravity
Carlo Rovelli
(Submitted on 14 Aug 2014)
Connes' formula defines a distance in loop quantum gravity, via the spinfoam Dirac operator. A simple notion of spectral distance on a graph can be extended do the discrete Lorentzian context, providing a physically natural example of Lorentzian spectral geometry, with a neat space of Dirac operators. The Hilbert structure of the fermion space is Lorentz covariant rather than invariant.
4 pages.

http://arxiv.org/abs/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
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/1407.8166
Loop quantum cosmology from group field theory
Gianluca Calcagni
(Submitted on 30 Jul 2014)
We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory (GFT) with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology (LQC) in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute.
5 pages

http://arxiv.org/abs/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/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/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
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
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
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
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.

18. Sep 6, 2014

marcus

Another hat-tip to atyy, who just spotted another potentially important prospect for the third quarter MIP poll. I'd guess the Dittrich paper will turn out to be one of the most important for the year.

http://arxiv.org/abs/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
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
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
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

http://arxiv.org/abs/1408.7112
Group Field Theory and Loop Quantum Gravity
Daniele Oriti
(Submitted on 29 Aug 2014)
We introduce the group field theory formalism for quantum gravity, mainly from the point of view of loop quantum gravity, stressing its promising aspects. We outline the foundations of the formalism, survey recent results and offer a perspective on future developments.
23 pages, contribution to the volume: "Loop Quantum Gravity", edited by A. Ashtekar and J. Pullin, to be published in the World Scientific series "100 Years of General Relativity"

http://arxiv.org/abs/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
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/1408.3260
Lorentzian Connes Distance, Spectral Graph Distance and Loop Gravity
Carlo Rovelli
(Submitted on 14 Aug 2014)
Connes' formula defines a distance in loop quantum gravity, via the spinfoam Dirac operator. A simple notion of spectral distance on a graph can be extended do the discrete Lorentzian context, providing a physically natural example of Lorentzian spectral geometry, with a neat space of Dirac operators. The Hilbert structure of the fermion space is Lorentz covariant rather than invariant.
4 pages.

http://arxiv.org/abs/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
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/1407.8166
Loop quantum cosmology from group field theory
Gianluca Calcagni
(Submitted on 30 Jul 2014)
We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory (GFT) with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology (LQC) in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute.
5 pages

http://arxiv.org/abs/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/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/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
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
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
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
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.

19. Sep 10, 2014

marcus

Twelve of us have made our views known on the second quarter papers so far. Thanks to Atyy, Chronos, David Horgan, Erkokite, John86, Julcab, Nonlinearity, Ohwilleke, PhysicoRaj, TomStoer, and Twistor for making this an interesting poll. Since the last tally, two papers have moved out in front with five votes each, namely Our Universe from the cosmological constant (Barrau and Linsefors) and Positive energy in quantum gravity (Smolin).

Here's the current standing,with a few comments.

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors
One-ups Penrose cyclic: U eventually develops pure de Sitter patches which can recollapse and bounce.

http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin
Stability argument supporting, among other things, the Koslowski-Sahlmann extension of Lqg.

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez
Equivalence of entropy concepts.

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
Another in list of curious/paradoxical behavior implied by conventional (unitary) BH evaporation. Is the original Hawking BH evaporation story wrong?

http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly
Extends definition of entanglement entropy to gauge quantum field theories by enlarging the Hilbert space to include degrees of freedom on the surface dividing the two entangled regions.

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour, Amaury Mouchet, Karim Noui
Continues the revival of original (complex Immirzi) Ashtekar variables.

http://arxiv.org/abs/1404.5821
Planck star phenomenology
Aurelien Barrau, Carlo Rovelli
BH collapse immediately bounces and explodes, but because time is slow in deep gravity this can take on the order of the age of the universe, or longer, seen from outside. Observational predictions.

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura

http://arxiv.org/abs/1404.2284
Cosmological Constant from the Emergent Gravity Perspective
Calculates the correct observed value of the cosmological constant from novel assumptions and quantities which, if confirmed valid, could be considered "first principles" for gravity and cosmology.

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll

http://arxiv.org/abs/1404.2944
Quantum cosmology of (loop) quantum gravity condensates: An example
Steffen Gielen

http://arxiv.org/abs/1404.1750
How many quanta are there in a quantum spacetime?
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen

One vote:
http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano

20. Sep 11, 2014

marcus

Three more to add to the list of prospective candidates. As with the last additions, two of these are thanks to atty. Included is a second Dittrich paper which could (like the previous) turn out to be one of the most important of the year. We now have 20 prospects which is the maximum accepted by the poll format, so any more September additions will have to displace papers now on the list.

http://arxiv.org/abs/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/1409.2407
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/1409.2471
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/1409.1450
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
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
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
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

http://arxiv.org/abs/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
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/1408.3260
Lorentzian Connes Distance, Spectral Graph Distance and Loop Gravity
Carlo Rovelli
(Submitted on 14 Aug 2014)
Connes' formula defines a distance in loop quantum gravity, via the spinfoam Dirac operator. A simple notion of spectral distance on a graph can be extended do the discrete Lorentzian context, providing a physically natural example of Lorentzian spectral geometry, with a neat space of Dirac operators. The Hilbert structure of the fermion space is Lorentz covariant rather than invariant.
4 pages.

http://arxiv.org/abs/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
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/1407.8166
Loop quantum cosmology from group field theory
Gianluca Calcagni
(Submitted on 30 Jul 2014)
We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory (GFT) with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology (LQC) in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute.
5 pages

http://arxiv.org/abs/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/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/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
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
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
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
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: Sep 11, 2014