# Combined fourth quarter MIP poll (for most important QG papers)

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

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14. ### Loop quantum gravity and observations

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1. Dec 29, 2014

### marcus

Indicate the papers you think will prove most significant for future Loop-and-allied QG research. The poll is multiple choice, so it's possible to vote for several. Abstracts follow in the next post.
The poll format has been expanded so as to allow more candidates. I brought forward three that more than one person voted for earlier this quarter, and include them here in a combined poll covering the whole quarter.

http://arxiv.org/abs/1412.8247
Pachner moves in a 4d Riemannian holomorphic Spin Foam model
Andrzej Banburski, Lin-Qing Chen, Laurent Freidel, Jeff Hnybida

http://arxiv.org/abs/1412.7546
SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry
Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello

http://arxiv.org/abs/1412.7435
Horizon entropy with loop quantum gravity methods
Daniele Pranzetti, Hanno Sahlmann

http://arxiv.org/abs/1412.6015
On the Effective Metric of a Planck Star
Tommaso De Lorenzo, Costantino Pacilio, Carlo Rovelli, Simone Speziale

http://arxiv.org/abs/1412.5851
Black holes as gases of punctures with a chemical potential: Bose-Einstein condensation and logarithmic corrections to the entropy
Olivier Asin, Jibril Ben Achour, Marc Geiller, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1412.3752
Flux formulation of loop quantum gravity: Classical framework
Bianca Dittrich, Marc Geiller

http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing

http://arxiv.org/abs/1411.5672
Canonical linearized Regge Calculus: counting lattice gravitons with Pachner moves
Philipp A. Hoehn

http://arxiv.org/abs/1411.3589
Projective Limits of State Spaces I. Classical Formalism
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1411.3590
Projective Limits of State Spaces II. Quantum Formalism
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1411.3591
Projective Limits of State Spaces III. Toy-Models
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1411.3592
Projective Loop Quantum Gravity I. State Space
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1411.0977
Geometry and the Quantum: Basics
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov

http://arxiv.org/abs/1410.1714
Loop quantum gravity and observations
A. Barrau, J. Grain

2. Dec 29, 2014

### marcus

Here are the brief summaries and links to the papers on this quarter's poll:

http://arxiv.org/abs/1412.8247
Pachner moves in a 4d Riemannian holomorphic Spin Foam model
Andrzej Banburski, Lin-Qing Chen, Laurent Freidel, Jeff Hnybida
(Submitted on 29 Dec 2014)
In this work we study a Spin Foam model for 4d Riemannian gravity, and propose a new way of imposing the simplicity constraints that uses the recently developed holomorphic representation. Using the power of the holomorphic integration techniques, and with the introduction of two new tools: the homogeneity map and the loop identity, for the first time we give the analytic expressions for the behaviour of the Spin Foam amplitudes under 4-dimensional Pachner moves. It turns out that this behaviour is controlled by an insertion of nonlocal mixing operators. In the case of the 5-1 move, the expression governing the change of the amplitude can be interpreted as a vertex renormalisation equation. We find a natural truncation scheme that allows us to get an invariance up to an overall factor for the 4-2 and 5-1 moves, but not for the 3-3 move. The study of the divergences shows that there is a range of parameter space for which the 4-2 move is finite while the 5-1 move diverges. This opens up the possibility to recover diffeomorphism invariance in the continuum limit of Spin Foam models for 4D Quantum Gravity.
48 pages, 30 figures

http://arxiv.org/abs/1412.7546
SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry
Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello
(Submitted on 23 Dec 2014)
We study the expectation value of a nonplanar Wilson graph operator in SL(2,C) Chern-Simons theory on S3. In particular we analyze its asymptotic behaviour in the double-scaling limit in which both the representation labels and the Chern-Simons coupling are taken to be large, but with fixed ratio. When the Wilson graph operator has a specific form, motivated by loop quantum gravity, the critical point equations obtained in this double-scaling limit describe a very specific class of flat connection on the graph complement manifold. We find that flat connections in this class are in correspondence with the geometries of constant curvature 4-simplices. The result is fully non-perturbative from the perspective of the reconstructed geometry. We also show that the asymptotic behavior of the amplitude contains at the leading order an oscillatory part proportional to the Regge action for the single 4-simplex in the presence of a cosmological constant. In particular, the cosmological term contains the full-fledged curved volume of the 4-simplex. Interestingly, the volume term stems from the asymptotics of the Chern-Simons action. This can be understood as arising from the relation between Chern-Simons theory on the boundary of a region, and a theory defined by an F2 action in the bulk. Another peculiarity of our approach is that the sign of the curvature of the reconstructed geometry, and hence of the cosmological constant in the Regge action, is not fixed a priori, but rather emerges semiclassically and dynamically from the solution of the equations of motion. In other words, this work suggests a relation between 4-dimensional loop quantum gravity with a cosmological constant and SL(2,C) Chern-Simons theory in 3-dimensions with knotted graph defects.
54+11 pages, 9 figures
search key [SL(2,C) Chern-Simons LQG]

http://arxiv.org/abs/1412.7435
Horizon entropy with loop quantum gravity methods
Daniele Pranzetti, Hanno Sahlmann
(Submitted on 23 Dec 2014)
We show that the spherically symmetric isolated horizon can be described in terms of an SU(2) connection and a su(2) valued one form, obeying certain constraints. The horizon symplectic structure is precisely the one of 3d gravity in a first order formulation. We quantize the horizon degrees of freedom in the framework of loop quantum gravity, with methods recently developed for 3d gravity with non-vanishing cosmological constant. Bulk excitations ending on the horizon act very similar to particles in 3d gravity. The Bekenstein-Hawking law is recovered in the limit of imaginary Barbero-Immirzi parameter. Alternative methods of quantization are also discussed.
17 pages, 2 figures
search key [horizon entropy loop methods]

http://arxiv.org/abs/1412.6015
On the Effective Metric of a Planck Star
Tommaso De Lorenzo, Costantino Pacilio, Carlo Rovelli, Simone Speziale
(Submitted on 18 Dec 2014)
Spacetime metrics describing non-singular' black holes are commonly studied in the literature as effective modification to the Schwarzschild solution that mimic quantum gravity effects removing the central singularity. Here we point out that to be physically plausible, such metrics should also incorporate the 1-loop quantum corrections to the Newton potential and a non-trivial time delay between an observer at infinity and an observer in the regular center. We present a modification of the well-known Hayward metric that features these two properties. We discuss bounds on the maximal time delay imposed by conditions on the curvature, and the consequences for the weak energy condition, in general violated by the large transversal pressures introduced by the time delay.
10 pages, many figures
search key [metric planck star]

http://arxiv.org/abs/1412.5851
Black holes as gases of punctures with a chemical potential: Bose-Einstein condensation and logarithmic corrections to the entropy
Olivier Asin, Jibril Ben Achour, Marc Geiller, Karim Noui, Alejandro Perez
(Submitted on 18 Dec 2014)
We study the thermodynamical properties of black holes when described as gases of indistinguishable punctures with a chemical potential. In this picture, which arises from loop quantum gravity, the black hole microstates are defined by finite families of half-integers spins coloring the punctures, and the near-horizon energy measured by quasi-local stationary observers defines the various thermodynamical ensembles. The punctures carry excitations of quantum geometry in the form of quanta of area, and the total horizon area aH is given by the sum of these microscopic contributions. We assume here that the system satisfies the Bose-Einstein statistics, and that each microstate is degenerate with a holographic degeneracy given by exp(λaH/ℓPl2) and λ>0.
We analyze in detail the thermodynamical properties resulting from these inputs, and in particular compute the grand canonical entropy. We explain why the requirements that the temperature be fixed to the Unruh temperature and that the chemical potential vanishes do not specify completely the semi-classical regime of large horizon area, and classify in turn what the various regimes can be. When the degeneracy saturates the holographic bound (λ=1/4), there exists a semi-classical regime in which the subleading corrections to the entropy are logarithmic. Furthermore, this regime corresponds to a Bose-Einstein condensation, in the sense that it is dominated by punctures carrying the minimal (or ground state) spin value 1/2.
22 pages
search key [black hole gas punctures]

http://arxiv.org/abs/1412.3752
Flux formulation of loop quantum gravity: Classical framework
Bianca Dittrich, Marc Geiller
(Submitted on 11 Dec 2014)
We recently introduced a new representation for loop quantum gravity, which is based on the BF vacuum and is in this sense much nearer to the spirit of spin foam dynamics. In the present paper we lay out the classical framework underlying this new formulation. The central objects in our construction are the so-called integrated fluxes, which are defined as the integral of the electric field variable over surfaces of codimension one, and related in turn to Wilson surface operators. These integrated flux observables will play an important role in the coarse graining of states in loop quantum gravity, and can be used to encode in this context the notion of curvature-induced torsion. We furthermore define a continuum phase space as the modified projective limit of a family of discrete phase spaces based on triangulations. This continuum phase space yields a continuum (holonomy-flux) algebra of observables. We show that the corresponding Poisson algebra is closed by computing the Poisson brackets between the integrated fluxes, which have the novel property of being allowed to intersect each other.
60 pages, 13 figures
search key [flux formulation LQG]

http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 9 Dec 2014)
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
14 pages, 8 figures
search key [LambdaCDM bounce]

http://arxiv.org/abs/1411.5672
Canonical linearized Regge Calculus: counting lattice gravitons with Pachner moves
Philipp A. Hoehn
(Submitted on 20 Nov 2014)
We afford a systematic and comprehensive account of the canonical dynamics of 4D Regge Calculus perturbatively expanded to linear order around a flat background. To this end, we consider the Pachner moves which generate the most basic and general simplicial evolution scheme. The linearized regime features a vertex displacement (diffeomorphism') symmetry for which we derive an abelian constraint algebra. This permits to identify gauge invariant lattice gravitons' as propagating curvature degrees of freedom. The Pachner moves admit a simple method to explicitly count the gauge and graviton' degrees of freedom on an evolving triangulated hypersurface and we clarify the distinct role of each move in the dynamics. It is shown that the 1-4 move generates four lapse and shift' variables and four conjugate vertex displacement generators; the 2-3 move generates a graviton'; the 3-2 move removes one graviton' and produces the only non-trivial equation of motion; and the 4-1 move removes four lapse and shift' variables and trivializes the four conjugate symmetry generators. It is further shown that the Pachner moves preserve the vertex displacement generators. These results may provide new impetus for exploring `graviton dynamics' in discrete quantum gravity models.
search key [graviton Pachner moves]

http://arxiv.org/abs/1411.3589
Projective Limits of State Spaces I. Classical Formalism
Suzanne Lanéry, Thomas Thiemann
(Submitted on 11 Nov 2014)
In this series of papers, we investigate the projective framework initiated by Jerzy Kijowski and Andrzej Okolów, which describes the states of a quantum (field) theory as projective families of density matrices. The present first paper aims at clarifying the classical structures that underlies this formalism, namely projective limits of symplectic manifolds. In particular, this allows us to discuss accurately the issues hindering an easy implementation of the dynamics in this context, and to formulate a strategy for overcoming them.
51 pages, many figures
search key [projective limit state classical]

http://arxiv.org/abs/1411.3590
Projective Limits of State Spaces II. Quantum Formalism
Suzanne Lanéry, Thomas Thiemann
(Submitted on 11 Nov 2014)
In this series of papers, we investigate the projective framework initiated by Jerzy Kijowski and Andrzej Okolów, which describes the states of a quantum theory as projective families of density matrices. After discussing the formalism at the classical level in a first paper, the present second paper is devoted to the quantum theory. In particular, we inspect in detail how such quantum projective state spaces relate to inductive limit Hilbert spaces and to infinite tensor product constructions. Regarding the quantization of classical projective structures into quantum ones, we extend the results by Okolów [arXiv:1304.6330], that were set up in the context of linear configuration spaces, to configuration spaces given by simply-connected Lie groups, and to holomorphic quantization of complex phase spaces.
56 pages, 2 figures
search key [projective limit state quantum]

http://arxiv.org/abs/1411.3591
Projective Limits of State Spaces III. Toy-Models
Suzanne Lanéry, Thomas Thiemann
(Submitted on 11 Nov 2014)
In this series of papers, we investigate the projective framework initiated by Jerzy Kijowski and Andrzej Okolów, which describes the states of a quantum theory as projective families of density matrices. A strategy to implement the dynamics in this formalism was presented in our first paper, which we now test in two simple toy-models. The first one is a very basic linear model, meant as an illustration of the general procedure, and we will only discuss it at the classical level. In the second one, we reformulate the Schrödinger equation, treated as a classical field theory, within this projective framework, and proceed to its (non-relativistic) second quantization. We are then able to reproduce the physical content of the usual Fock quantization.
40 pages
search key [projective limit state model]

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

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

http://arxiv.org/abs/1410.1714
Loop quantum gravity and observations
A. Barrau, J. Grain
(Submitted on 7 Oct 2014)
Quantum gravity has long been thought to be completely decoupled from experiments or observations. Although it is true that smoking guns are still missing, there are now serious hopes that quantum gravity phenomena might be tested. We review here some possible ways to observe loop quantum gravity effects either in the framework of cosmology or in astroparticle physics.
25 pages, 8 figures. To be published in the World Scientific series "100 Years of General Relativity" as a chapter in the Loop Quantum Gravity volume, edited by A. Ashtekar and J. Pullin.
[barrau LQG observations]

Last edited: Dec 29, 2014
3. Dec 30, 2014

### marcus

Hi David! I was glad to see your estimates of the significance of current work. They count extra for me, knowing that you do research and have a professional interest in QG.
I strongly agree with you on choices which reflect the prime importance of connecting QG with observations and a sense that the Lanery Thiemann venture into projective limit LQG is potentially consequential. It could facilitate dealing with semiclassical states, and the GR limit, and it could lead to a generalization beyond spin networks to include other ways of describing and measuring states of geometry. I'm a little concerned that their gambit might turn out to be too general---lack the practical specificity of graphs and two-complexes. Your judgment on that score could balance/buttress mine.

Last edited: Dec 30, 2014
4. Dec 30, 2014

### marcus

David, recalling that you and I both voted for the CCM paper (along with Julcab) on the initial halfway version of this quarter's poll, it occurs to me to remark that if I were in Rovelli's shoes, or those of one or two of the other Spinfoam QG researchers, I would like if history would work out that I could visit with Ali Chamseddine or Alain Connes and discuss this from page 25 of their [geometry quantum basics] paper. In the following passage think of n = 4:
==quote Chamsedding Connes Mukhanov==
A tentative particle picture in Quantum Gravity

One of the basic conceptual ingredients of Quantum Field Theory is the notion of particle which Wigner formulated as irreducible representations of the Poincaré group. When dealing with general relativity we shall see that (in the Euclidean = imaginary time formulation) there is a natural corresponding particle picture in which the irreducible representations of the two-sided higher Heisenberg relation play the role of “particles”. Thus the role of the Poincaré group is now played by the algebra of relations existing between the line element and the slash of scalar fields.

We shall first explain why it is natural from the point of view of differential geometry also, to consider the two sets of Γ-matrices and then take the
operators Y and Y′ as being the correct variables for a first shot at a theory of quantum gravity. Once we have the Y and Y′ we can use them and get a map (Y,Y′) : M → Sn ×Sn from the manifold M to the product of two n- spheres. The first question which comes in this respect is if, given a compact n-dimensional manifold M one can find a map (Y, Yç) : M → Sn × Sn which embeds M as a submanifold of Sn × Sn. Fortunately this is a known result, the strong embedding theorem of Whitney, [24], which asserts that any smooth real n-dimensional manifold (required also to be Hausdorff and second-countable) can be smoothly embedded in the real 2n-space. Of course R2n = Rn × Rn ⊂ Sn × Sn so that one gets the required embedding. This result shows that there is no restriction by viewing the pair (Y,Y′) as the correct “coordinate” variables. ...
==endquote==
On page 27 they have the result that in their new version of Quantized GR Geometry the volume is quantized which would seem very good to a LQG researcher and something one would like to understand more about. And then on page 29 there are the conclusions:
==quote==
Conclusions
In this paper we have uncovered a higher analogue of the Heisenberg commutation relation whose irreducible representations provide a tentative picture for quanta of geometry. We have shown that 4-dimensional Spin geometries with quantized volume give such irreducible representations of the two-sided relation involving the Dirac operator and the Feynman slash of scalar fields and the two possibilities for the Clifford algebras which provide the gamma matrices with which the scalar fields are contracted. These instantonic fields provide maps Y, Y ′ from the four-dimensional manifold M4 to S4. The intuitive picture using the two maps from M4 to S4 is that the four-manifold is built out of a very large number of the two kinds of spheres of Planckian volume. The volume of space-time is quantized in terms of the sum of the two winding numbers of the two maps. More suggestively the Euclidean space-time history unfolds to macroscopic dimension from the product of two 4-spheres of Planckian volume as a butterfly unfolds from its chrysalis. Moreover, amazingly, in dimension 4 the algebras of Clifford valued functions which appear naturally from the Feynman slash of scalar fields coincide exactly with the algebras that were singled out in our algebraic understanding of the standard model using noncommutative geometry thus yielding the natural guess that the spectral action will give the unification of gravity with the Standard Model (more precisely of its asymptotically free extension as a Pati-Salam model as explained in [5]).
Having established the mathematical foundation for the quantization of geometry, we shall present consequences and physical applications of these results in a forthcoming publication [6].
==endquote==

Publication [6] is something that Chamseddine and Connes have in the works with Slava Mukhanov.
He just posted a paper which downplays "multiverse" thinking by showing that you can have inflation on a oneshot moderate basis for this universe without getting trapped into an eternal proliferation. Interesting guy.

Last edited: Dec 31, 2014
5. Dec 30, 2014

### marcus

Looking back over the year, I'm impressed by how many major developments showed up in the fourth quarter.
Should notice though that this CCM thing was foreshadowed by a short (4 page) paper by Chamseddine Connes Mukhanov that we had on the THIRD quarter MIP poll
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

BTW this paper already has 5 citations.
https://www.physicsforums.com/threa...14-mip-most-important-qg-paper-part-i.773589/
kudos to Atyy and Chronos for spotting it as significant.
Some more links in case anyone wants to look back at earlier quarters:
I: https://www.physicsforums.com/threa...portant-qg-paper-poll-great-selection.746462/
II: https://www.physicsforums.com/threa...rter-2014-mip-most-important-qg-paper.760106/
IIIB: https://www.physicsforums.com/threa...4-mip-most-important-qg-paper-part-ii.773590/

Last edited: Dec 30, 2014
6. Jan 1, 2015

7. Jan 1, 2015

### marcus

Thanks to Atyy, David, and Shyan for getting the 4th quarter poll off to a good start! It's only the first of the year and already four of us have registered our picks.

Thanks also to Greg Bernhardt for adjusting the new poll format so it is no longer limited to 10 candidates! It's now much more convenient to use. We can wrap up the whole three months' output of significant papers in a single MIP poll.

Have to say it: Loop&spinfoam QG and the whole QG field seems ripe for change over the next couple of years. Loops 15 is apt to be quite interesting!
What if Ali Chamseddine showed up? Or some other person associated with the CC&M gambit.
And there is the Projective LQG initiative by Suzanne Lanery and Thomas Thiemann.
Plus Dittrich team's "Flux formulation of LQG"
Plus Wolfgang Wieland's "New Action" for spin foam QG, which bridges over towards Causal Sets QG. It makes a connection with the "energetic causal sets" of Cortes and Smolin.
Also in the Loop Cosmology area there is a connection with the McGill group's "matter bounce" where you even have a collaborative paper by Yi-fu Cai (on the McGill side) and Ed Wilson-Ewing (on the Loop side). there is a general convergence happening in ( bounce) quantum cosmology, as I see it. It also involves Sergei Odintsov and some Barcelona people.

Loops 15 conference is not until July 2015, but here's the link to the embryonic website in case anyone wants to book mark it and keep an eye on how the program develops:
http://www.gravity.physik.fau.de/events/loops15/loops15.shtml

Last edited: Jan 1, 2015
8. Feb 27, 2015

### marcus

Since we're two thirds into the quarter I should start lining up candidates for the first quarter 2015 MIP poll.
Here are a few of the potentially important January and February papers:
http://arxiv.org/abs/1502.06125
ΛCDM Bounce Cosmology without ΛCDM: the case of modified gravity
S.D. Odintsov, V.K. Oikonomou
(Submitted on 21 Feb 2015)
We provide an F(R) gravity description of a ΛCDM bouncing model, without the need for matter fluids or for cosmological constant. As we explicitly demonstrate, the two cosmological eras that constitute the ΛCDM bouncing model, can be generated by F(R) gravity which can lead to accelerating cosmologies. The resulting F(R) gravity has Einstein frame inflationary properties that have concordance to the latest Planck observational data. Both the F(R) gravity stability properties are thoroughly investigated and also, the gravitational particle production, a feature necessary for the viability of the ΛCDM bounce scenario, is also addressed. As we will show, the ΛCDM bounce model can be successfully described by pure F(R) gravity, with appealing phenomenological attributes, which we extensively discuss.
31 pages, accepted by PRD

http://arxiv.org/abs/1502.04640
The Lorentzian proper vertex amplitude: Classical analysis and quantum derivation
Jonathan Engle, Antonia Zipfel
(Submitted on 16 Feb 2015)
Spin foam models, an approach to defining the dynamics of loop quantum gravity, make use of the Plebanski formulation of gravity, in which gravity is recovered from a topological field theory via certain constraints called simplicity constraints. However, the simplicity constraints in their usual form select more than just one gravitational sector as well as a degenerate sector. This was shown, in previous work, to be the reason for the "extra" terms appearing in the semiclassical limit of the Euclidean EPRL amplitude. In this previous work, a way to eliminate the extra sectors, and hence terms, was developed, leading to the what was called the Euclidean proper vertex amplitude. In the present work, these results are extended to the Lorentzian signature, establishing what is called the Lorentzian proper vertex amplitude. This extension is non-trivial and involves a number of new elements since, for Lorentzian bivectors, the split into self-dual and anti-self-dual parts, on which the Euclidean derivation was based, is no longer available. In fact, the classical parts of the present derivation provide not only an extension to the Lorentzian case, but also, with minor modifications, provide a new, more four dimensionally covariant derivation for the Euclidean case. The new elements in the quantum part of the derivation are due to the different structure of unitary representations of the Lorentz group.
36 pages

http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
Rodolfo Gambini, Jorge Pullin
(Submitted on 11 Feb 2015)
The Montevideo interpretation of quantum mechanics, which consists in supplementing environmental decoherence with fundamental limitations in measurement stemming from gravity, has been described in several publications. However, some of them appeared before the full picture provided by the interpretation was developed. As such it can be difficult to get a good understanding via the published literature. Here we summarize it in a self contained brief presentation including all its principal elements.
10 pages

http://arxiv.org/abs/1502.03230
An Extended Matter Bounce Scenario: current status and challenges
Jaume de Haro, Yi-Fu Cai
(Submitted on 11 Feb 2015)
As an alternative to the paradigm of slow roll inflation, we propose an extended scenario of the matter bounce cosmology in which the Universe has experienced a quasi-matter contracting phase with a variable background equation of state parameter. This extended matter bounce scenario can be realized by considering a single scalar field evolving along an approximately exponential potential. Our result reveals that the rolling of the scalar field in general leads to a running behavior on the spectral index of primordial cosmological perturbations and a negative running can be realized in this model. We constrain the corresponding parameter space by using the newly released Planck data. To apply this scenario, we revisit bouncing cosmologies within the context of modified gravity theories, in particular, the holonomy corrected loop quantum cosmology and teleparallel F(T) gravity. A gravitational process of reheating is presented in such a matter bounce scenario to demonstrate the condition of satisfying current observations. We also comment on several unresolved issues that often appear in matter bounce models.
31 pages, 2 figures.

http://arxiv.org/abs/1502.02431
Comparison of primordial tensor power spectra from the deformed algebra and dressed metric approaches in loop quantum cosmology
B. Bolliet, J. Grain, C. Stahl, L. Linsefors, A. Barrau
(Submitted on 9 Feb 2015)
Loop quantum cosmology tries to capture the main ideas of loop quantum gravity and to apply them to the Universe as a whole. Two main approaches within this framework have been considered to date for the study of cosmological perturbations: the dressed metric approach and the deformed algebra approach. They both have advantages and drawbacks. In this article, we accurately compare their predictions. In particular, we compute the associated primordial tensor power spectra. We show -- numerically and analytically -- that the large scale behavior is similar for both approaches and compatible with the usual prediction of general relativity. The small scale behavior is, the other way round, drastically different. Most importantly, we show that in a range of wavenumbers explicitly calculated, both approaches do agree on predictions that, in addition, differ from standard general relativity and do not depend on unknown parameters. These features of the power spectrum at intermediate scales might constitute a universal loop quantum cosmology prediction that can hopefully lead to observational tests and constraints. We also present a complete analytical study of the background evolution for the bouncing universe that can be used for other purposes.
14 pages, 4 figures

http://arxiv.org/abs/1502.00278
Compact phase space, cosmological constant, discrete time
Carlo Rovelli, Francesca Vidotto
(Submitted on 1 Feb 2015)
We study the quantization of geometry in the presence of a cosmological constant, using a discretization with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.
6 pages

http://arxiv.org/abs/1501.06591
Superbounce and Loop Quantum Ekpyrotic Cosmologies from Modified Gravity: F(R), F(G) and F(T) Theories
S.D. Odintsov, V.K. Oikonomou, Emmanuel N. Saridakis
(Submitted on 26 Jan 2015)
We investigate the realization of two bouncing paradigms, namely of the superbounce and the loop quantum cosmological ekpyrosis, in the framework of various modified gravities. In particular, we focus on the F(R), F(G) and F(T) gravities, and we reconstruct their specific subclasses which lead to such universe evolutions. These subclasses constitute from power laws, polynomials, or hypergeometric ansatzes, which can be approximated by power laws. The qualitative similarity of different effective gravities which realize the above two bouncing cosmologies, indicates to some universality lying behind such a bounce. Finally, performing a linear perturbation analysis, we show that the obtained solutions are conditionally or fully stable.
31 pages.

http://arxiv.org/abs/1501.06270
Matter Bounce Scenario in F(T) gravity
Jaume Haro, Jaume Amorós
(Submitted on 26 Jan 2015)
It is shown that teleparallel F(T) theories of gravity combined with holonomy corrected Loop Quantum Cosmology (LQC) support a Matter Bounce Scenario (MBS) which is a potential alternative to the inflationary paradigm. The Matter Bounce Scenario is reviewed and, according to the current observational data provided by PLANCK's team, we have summarized all the conditions that it has to satisfy in order to be a viable alternative to inflation, such as to provide a theoretical value of the spectral index and its running compatible with the latest PLANCK data, to have a reheating process via gravitational particle production, or to predict some signatures in the non-gaussianities of the power spectrum. The calculation of the power spectrum for scalar perturbations and the ratio of tensor to scalar perturbations has been done, in the simplest case of an exact matter dominated background, for both holonomy corrected LQC and teleparallel F(T) gravity. Finally, we have discussed the challenges (essentially, dealing with non-gaussianities, the calculation of the 3-point function in flat spatial geometries for theories beyond General Relativity) and problems (Jeans instabilities in the case of holonomy corrected LQC or local Lorentz dependence in teleparallelism) that arise in either bouncing scenario.
6 pages. Communication to the FFP2014 (Frontiers in Fundamental Physics, Marseille 2014). To appear in Proceedings of Science

http://arxiv.org/abs/1501.03007
The shape dynamics description of gravity
Tim Koslowski
(Submitted on 13 Jan 2015)
Classical gravity can be described as a relational dynamical system without ever appealing to spacetime or its geometry. This description is the so-called shape dynamics description of gravity. The existence of relational first principles from which the shape dynamics description of gravity can be derived is a motivation to consider shape dynamics (rather than GR) as the fundamental description of gravity. Adopting this point of view leads to the question: What is the role of spacetime in the shape dynamics description of gravity? This question contains many aspects: Compatibility of shape dynamics with the description of gravity in terms of spacetime geometry, the role of local Minkowski space, universality of spacetime geometry and the nature of quantum particles, which can no longer be assumed to be irreducible representations of the Poincare group. In this contribution I derive effective spacetime structures by considering how matter fluctuations evolve along with shape dynamics. This evolution reveals an "experienced spacetime geometry." This leads (in an idealized approximation) to local Minkowski space and causal relations. The small scale structure of the emergent geometric picture depends on the specific probes used to experience spacetime, which limits the applicability of effective spacetime to describe shape dynamics. I conclude with discussing the nature of quantum fluctuations (particles) in shape dynamics and how local Minkowski spacetime emerges from the evolution of quantum particles.
16 pages, a submission to the proceedings of Theory Canada 9

http://arxiv.org/abs/1501.02963
Quantum Geometry and Black Holes
J. Fernando Barbero G., Alejandro Perez
(Submitted on 13 Jan 2015)
We present an overall picture of the advances in the description of black hole physics from the perspective of loop quantum gravity. After an introduction that discusses the main conceptual issues we present some details about the classical and quantum geometry of isolated horizons and their quantum geometry and then use this scheme to give a natural definition of the entropy of black holes. The entropy computations can be neatly expressed in the form of combinatorial problems solvable with the help of methods based on number theory and the use of generating functions. The recovery of the Bekenstein-Hawking law and corrections to it is explained in some detail. After this, due attention is paid to the discussion of semiclassical issues. An important point in this respect is the proper interpretation of the horizon area as the energy that should appear in the statistical-mechanical treatment of the black hole model presented here. The chapter ends with a comparison between the microscopic and semiclassical approaches to the computation of the entropy and discusses a number of issues regarding the relation between entanglement and statistical entropy and the possibility of comparing the subdominant (logarithmic) corrections to the entropy obtained with the help of the Euclidean path integral with the ones obtained in the present framework.
39 pages. Contribution to appear in the World Scientific series "100 Years of General Relativity" edited by A. Ashtekar and J. Pullin

http://arxiv.org/abs/1501.00855
Closure constraints for hyperbolic tetrahedra
Christoph Charles, Etera R. Livine
(Submitted on 5 Jan 2015)
We investigate the generalization of loop gravity's twisted geometries to a q-deformed gauge group. In the standard undeformed case, loop gravity is a formulation of general relativity as a diffeomorphism-invariant SU(2) gauge theory. Its classical states are graphs provided with algebraic data. In particular closure constraints at every node of the graph ensure their interpretation as twisted geometries. Dual to each node, one has a polyhedron embedded in flat space R3. One then glues them allowing for both curvature and torsion. It was recently conjectured that q-deforming the gauge group SU(2) would allow to account for a non-vanishing cosmological constant Lambda, and in particular that deforming the loop gravity phase space with real parameter q>0 would lead to a generalization of twisted geometries to a hyperbolic curvature. Following this insight, we look for generalization of the closure constraints to the hyperbolic case. In particular, we introduce two new closure constraints for hyperbolic tetrahedra. One is compact and expressed in terms of normal rotations (group elements in SU(2) associated to the triangles) and the second is non-compact and expressed in terms of triangular matrices (group elements in SB(2,C)). We show that these closure constraints both define a unique dual tetrahedron (up to global translations on the three-dimensional one-sheet hyperboloid) and are thus ultimately equivalent.
24 pages

Last edited: Feb 27, 2015
9. Mar 7, 2015

### marcus

Nine of us have responded to the poll so far. Thanks to Atyy, Breo, Chronos, David Horgan, Francesca, Ghostcrown, Nonlinearity, and Shyan for helping form a collective assessment of the recent quarter's research! It helps me (and may you) to learn and adjust perspective when I see how others rate the importance of different research. It would be great to hear from more, so please join in responding and make your assessments of current research known.

Meanwhile, it's time to put together the list of candidates for the first quarter 2015 poll. I'll check to see what needs to be included for consideration with the above list.

http://arxiv.org/abs/1503.02981
Four-Dimensional Entropy from Three-Dimensional Gravity
S. Carlip
(Submitted on 10 Mar 2015)
At the horizon of a black hole, the action of (3+1)-dimensional loop quantum gravity acquires a boundary term that is formally identical to an action for three-dimensional gravity. I show how to use this correspondence to obtain the entropy of the (3+1)-dimensional black hole from well-understood conformal field theory computations of the entropy in (2+1)-dimensional de Sitter space.
8 pages

http://arxiv.org/abs/1503.01671
Aspects of the Bosonic Spectral Action
Mairi Sakellariadou (King's College London)
(Submitted on 5 Mar 2015)
A brief description of the elements of noncommutative spectral geometry as an approach to unification is presented. The physical implications of the doubling of the algebra are discussed. Some high energy phenomenological as well as various cosmological consequences are presented. A constraint in one of the three free parameters, namely the one related to the coupling constants at unification, is obtained, and the possible role of scalar fields is highlighted. A novel spectral action approach based upon zeta function regularisation, in order to address some of the issues of the traditional bosonic spectral action based on a cutoff function and a cutoff scale, is discussed.
16 pages, Invited talk in the Fourth Symposium on Prospects in the Physics of Discrete Symmetries, DISCRETE 2014, King's College London,2-6 December 2014

http://arxiv.org/abs/1503.01636
The microscopic structure of 2D CDT coupled to matter
J. Ambjorn, A. Goerlich, J. Jurkiewicz, H. Zhang
(Submitted on 5 Mar 2015)
We show that for 1+1 dimensional Causal Dynamical Triangulations (CDT) coupled to 4 massive scalar fields one can construct an effective transfer matrix if the masses squared is larger than or equal to 0.05. The properties of this transfer matrix can explain why CDT coupled to matter can behave completely different from "pure" CDT. We identify the important critical exponent in the effective action, which may determine the universality class of the model.
14 pages,lot of figures

http://arxiv.org/abs/1503.00442
Inflationary cosmology in modified gravity theories
Kazuharu Bamba, Sergei D. Odintsov
(Submitted on 2 Mar 2015)
We review inflationary cosmology in modified gravity such as R2 gravity with its extensions in order to generalize the Starobinsky inflation model. In particular, we explore inflation realized by three kinds of effects: modification of gravity, the quantum anomaly, and the R2 term in loop quantum cosmology. It is explicitly demonstrated that in these inflationary models, the spectral index of scalar modes of the density perturbations and the tensor-to-scalar ratio can be consistent with the Planck results. Bounce cosmology in F(R) gravity is also explained.
24 pages, invited review to appear in Symmetry

http://arxiv.org/abs/1502.06770
Quantum Transitions Between Classical Histories: Bouncing Cosmologies
James Hartle, Thomas Hertog
(Submitted on 24 Feb 2015)
In a quantum theory of gravity spacetime behaves classically when quantum probabilities are high for histories of geometry and field that are correlated in time by the Einstein equation. Probabilities follow from the quantum state. This quantum perspective on classicality has important implications:
(a) Classical histories are generally available only in limited patches of the configuration space on which the state lives.
(b) In a given patch states generally predict relative probabilities for an ensemble of possible classical histories.
(c) In between patches classical predictability breaks down and is replaced by quantum evolution connecting classical histories in different patches.
(d) Classical predictability can break down on scales well below the Planck scale, and with no breakdown in the classical equations of motion.
We support and illustrate (a)-(d) by calculating the quantum transition across the de Sitter like throat connecting asymptotically classical, inflating histories in the no-boundary quantum state. This supplies probabilities for how a classical history on one side transitions and branches into a range of classical histories on the opposite side. We also comment on the implications of (a)-(d) for the dynamics of black holes and eternal inflation.
36 pages, 6 figures

http://arxiv.org/abs/1502.06125
ΛCDM Bounce Cosmology without ΛCDM: the case of modified gravity
S.D. Odintsov, V.K. Oikonomou
(Submitted on 21 Feb 2015)
We provide an F(R) gravity description of a ΛCDM bouncing model, without the need for matter fluids or for cosmological constant. As we explicitly demonstrate, the two cosmological eras that constitute the ΛCDM bouncing model, can be generated by F(R) gravity which can lead to accelerating cosmologies. The resulting F(R) gravity has Einstein frame inflationary properties that have concordance to the latest Planck observational data. Both the F(R) gravity stability properties are thoroughly investigated and also, the gravitational particle production, a feature necessary for the viability of the ΛCDM bounce scenario, is also addressed. As we will show, the ΛCDM bounce model can be successfully described by pure F(R) gravity, with appealing phenomenological attributes, which we extensively discuss.
31 pages, accepted by PRD

http://arxiv.org/abs/1502.04640
The Lorentzian proper vertex amplitude: Classical analysis and quantum derivation
Jonathan Engle, Antonia Zipfel
(Submitted on 16 Feb 2015)
Spin foam models, an approach to defining the dynamics of loop quantum gravity, make use of the Plebanski formulation of gravity, in which gravity is recovered from a topological field theory via certain constraints called simplicity constraints. However, the simplicity constraints in their usual form select more than just one gravitational sector as well as a degenerate sector. This was shown, in previous work, to be the reason for the "extra" terms appearing in the semiclassical limit of the Euclidean EPRL amplitude. In this previous work, a way to eliminate the extra sectors, and hence terms, was developed, leading to the what was called the Euclidean proper vertex amplitude. In the present work, these results are extended to the Lorentzian signature, establishing what is called the Lorentzian proper vertex amplitude. This extension is non-trivial and involves a number of new elements since, for Lorentzian bivectors, the split into self-dual and anti-self-dual parts, on which the Euclidean derivation was based, is no longer available. In fact, the classical parts of the present derivation provide not only an extension to the Lorentzian case, but also, with minor modifications, provide a new, more four dimensionally covariant derivation for the Euclidean case. The new elements in the quantum part of the derivation are due to the different structure of unitary representations of the Lorentz group.
36 pages

http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
Rodolfo Gambini, Jorge Pullin
(Submitted on 11 Feb 2015)
The Montevideo interpretation of quantum mechanics, which consists in supplementing environmental decoherence with fundamental limitations in measurement stemming from gravity, has been described in several publications. However, some of them appeared before the full picture provided by the interpretation was developed. As such it can be difficult to get a good understanding via the published literature. Here we summarize it in a self contained brief presentation including all its principal elements.
10 pages

http://arxiv.org/abs/1502.03230
An Extended Matter Bounce Scenario: current status and challenges
Jaume de Haro, Yi-Fu Cai
(Submitted on 11 Feb 2015)
As an alternative to the paradigm of slow roll inflation, we propose an extended scenario of the matter bounce cosmology in which the Universe has experienced a quasi-matter contracting phase with a variable background equation of state parameter. This extended matter bounce scenario can be realized by considering a single scalar field evolving along an approximately exponential potential. Our result reveals that the rolling of the scalar field in general leads to a running behavior on the spectral index of primordial cosmological perturbations and a negative running can be realized in this model. We constrain the corresponding parameter space by using the newly released Planck data. To apply this scenario, we revisit bouncing cosmologies within the context of modified gravity theories, in particular, the holonomy corrected loop quantum cosmology and teleparallel F(T) gravity. A gravitational process of reheating is presented in such a matter bounce scenario to demonstrate the condition of satisfying current observations. We also comment on several unresolved issues that often appear in matter bounce models.
31 pages, 2 figures.

http://arxiv.org/abs/1502.02431
Comparison of primordial tensor power spectra from the deformed algebra and dressed metric approaches in loop quantum cosmology
B. Bolliet, J. Grain, C. Stahl, L. Linsefors, A. Barrau
(Submitted on 9 Feb 2015)
Loop quantum cosmology tries to capture the main ideas of loop quantum gravity and to apply them to the Universe as a whole. Two main approaches within this framework have been considered to date for the study of cosmological perturbations: the dressed metric approach and the deformed algebra approach. They both have advantages and drawbacks. In this article, we accurately compare their predictions. In particular, we compute the associated primordial tensor power spectra. We show -- numerically and analytically -- that the large scale behavior is similar for both approaches and compatible with the usual prediction of general relativity. The small scale behavior is, the other way round, drastically different. Most importantly, we show that in a range of wavenumbers explicitly calculated, both approaches do agree on predictions that, in addition, differ from standard general relativity and do not depend on unknown parameters. These features of the power spectrum at intermediate scales might constitute a universal loop quantum cosmology prediction that can hopefully lead to observational tests and constraints. We also present a complete analytical study of the background evolution for the bouncing universe that can be used for other purposes.
14 pages, 4 figures

http://arxiv.org/abs/1502.00278
Compact phase space, cosmological constant, discrete time
Carlo Rovelli, Francesca Vidotto
(Submitted on 1 Feb 2015)
We study the quantization of geometry in the presence of a cosmological constant, using a discretization with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.
6 pages

http://arxiv.org/abs/1501.06591
Superbounce and Loop Quantum Ekpyrotic Cosmologies from Modified Gravity: F(R), F(G) and F(T) Theories
S.D. Odintsov, V.K. Oikonomou, Emmanuel N. Saridakis
(Submitted on 26 Jan 2015)
We investigate the realization of two bouncing paradigms, namely of the superbounce and the loop quantum cosmological ekpyrosis, in the framework of various modified gravities. In particular, we focus on the F(R), F(G) and F(T) gravities, and we reconstruct their specific subclasses which lead to such universe evolutions. These subclasses constitute from power laws, polynomials, or hypergeometric ansatzes, which can be approximated by power laws. The qualitative similarity of different effective gravities which realize the above two bouncing cosmologies, indicates to some universality lying behind such a bounce. Finally, performing a linear perturbation analysis, we show that the obtained solutions are conditionally or fully stable.
31 pages.

http://arxiv.org/abs/1501.06270
Matter Bounce Scenario in F(T) gravity
Jaume Haro, Jaume Amorós
(Submitted on 26 Jan 2015)
It is shown that teleparallel F(T) theories of gravity combined with holonomy corrected Loop Quantum Cosmology (LQC) support a Matter Bounce Scenario (MBS) which is a potential alternative to the inflationary paradigm. The Matter Bounce Scenario is reviewed and, according to the current observational data provided by PLANCK's team, we have summarized all the conditions that it has to satisfy in order to be a viable alternative to inflation, such as to provide a theoretical value of the spectral index and its running compatible with the latest PLANCK data, to have a reheating process via gravitational particle production, or to predict some signatures in the non-gaussianities of the power spectrum. The calculation of the power spectrum for scalar perturbations and the ratio of tensor to scalar perturbations has been done, in the simplest case of an exact matter dominated background, for both holonomy corrected LQC and teleparallel F(T) gravity. Finally, we have discussed the challenges (essentially, dealing with non-gaussianities, the calculation of the 3-point function in flat spatial geometries for theories beyond General Relativity) and problems (Jeans instabilities in the case of holonomy corrected LQC or local Lorentz dependence in teleparallelism) that arise in either bouncing scenario.
6 pages. Communication to the FFP2014 (Frontiers in Fundamental Physics, Marseille 2014). To appear in Proceedings of Science

http://arxiv.org/abs/1501.03007
The shape dynamics description of gravity
Tim Koslowski
(Submitted on 13 Jan 2015)
Classical gravity can be described as a relational dynamical system without ever appealing to spacetime or its geometry. This description is the so-called shape dynamics description of gravity. The existence of relational first principles from which the shape dynamics description of gravity can be derived is a motivation to consider shape dynamics (rather than GR) as the fundamental description of gravity. Adopting this point of view leads to the question: What is the role of spacetime in the shape dynamics description of gravity? This question contains many aspects: Compatibility of shape dynamics with the description of gravity in terms of spacetime geometry, the role of local Minkowski space, universality of spacetime geometry and the nature of quantum particles, which can no longer be assumed to be irreducible representations of the Poincare group. In this contribution I derive effective spacetime structures by considering how matter fluctuations evolve along with shape dynamics. This evolution reveals an "experienced spacetime geometry." This leads (in an idealized approximation) to local Minkowski space and causal relations. The small scale structure of the emergent geometric picture depends on the specific probes used to experience spacetime, which limits the applicability of effective spacetime to describe shape dynamics. I conclude with discussing the nature of quantum fluctuations (particles) in shape dynamics and how local Minkowski spacetime emerges from the evolution of quantum particles.
16 pages, a submission to the proceedings of Theory Canada 9

http://arxiv.org/abs/1501.02963
Quantum Geometry and Black Holes
J. Fernando Barbero G., Alejandro Perez
(Submitted on 13 Jan 2015)
We present an overall picture of the advances in the description of black hole physics from the perspective of loop quantum gravity. After an introduction that discusses the main conceptual issues we present some details about the classical and quantum geometry of isolated horizons and their quantum geometry and then use this scheme to give a natural definition of the entropy of black holes. The entropy computations can be neatly expressed in the form of combinatorial problems solvable with the help of methods based on number theory and the use of generating functions. The recovery of the Bekenstein-Hawking law and corrections to it is explained in some detail. After this, due attention is paid to the discussion of semiclassical issues. An important point in this respect is the proper interpretation of the horizon area as the energy that should appear in the statistical-mechanical treatment of the black hole model presented here. The chapter ends with a comparison between the microscopic and semiclassical approaches to the computation of the entropy and discusses a number of issues regarding the relation between entanglement and statistical entropy and the possibility of comparing the subdominant (logarithmic) corrections to the entropy obtained with the help of the Euclidean path integral with the ones obtained in the present framework.
39 pages. Contribution to appear in the World Scientific series "100 Years of General Relativity" edited by A. Ashtekar and J. Pullin

http://arxiv.org/abs/1501.00855
Closure constraints for hyperbolic tetrahedra
Christoph Charles, Etera R. Livine
(Submitted on 5 Jan 2015)
We investigate the generalization of loop gravity's twisted geometries to a q-deformed gauge group. In the standard undeformed case, loop gravity is a formulation of general relativity as a diffeomorphism-invariant SU(2) gauge theory. Its classical states are graphs provided with algebraic data. In particular closure constraints at every node of the graph ensure their interpretation as twisted geometries. Dual to each node, one has a polyhedron embedded in flat space R3. One then glues them allowing for both curvature and torsion. It was recently conjectured that q-deforming the gauge group SU(2) would allow to account for a non-vanishing cosmological constant Lambda, and in particular that deforming the loop gravity phase space with real parameter q>0 would lead to a generalization of twisted geometries to a hyperbolic curvature. Following this insight, we look for generalization of the closure constraints to the hyperbolic case. In particular, we introduce two new closure constraints for hyperbolic tetrahedra. One is compact and expressed in terms of normal rotations (group elements in SU(2) associated to the triangles) and the second is non-compact and expressed in terms of triangular matrices (group elements in SB(2,C)). We show that these closure constraints both define a unique dual tetrahedron (up to global translations on the three-dimensional one-sheet hyperboloid) and are thus ultimately equivalent.
24 pages

Last edited: Mar 11, 2015
10. Mar 11, 2015

### marcus

How the votes tally up so far, on the fourth quarter 2014 poll:
http://arxiv.org/abs/1411.0977
Geometry and the Quantum: Basics
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov

http://arxiv.org/abs/1412.7435
Horizon entropy with loop quantum gravity methods
Daniele Pranzetti, Hanno Sahlmann

http://arxiv.org/abs/1411.3590
Projective Limits of State Spaces II. Quantum Formalism
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1411.3592
Projective Loop Quantum Gravity I. State Space
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1410.1714
Loop quantum gravity and observations
A. Barrau, J. Grain

http://arxiv.org/abs/1412.7546
SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry
Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello

http://arxiv.org/abs/1412.6015
On the Effective Metric of a Planck Star
Tommaso De Lorenzo, Costantino Pacilio, Carlo Rovelli, Simone Speziale

http://arxiv.org/abs/1412.3752
Flux formulation of loop quantum gravity: Classical framework
Bianca Dittrich, Marc Geiller

http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing

1 vote
http://arxiv.org/abs/1412.8247
Pachner moves in a 4d Riemannian holomorphic Spin Foam model
Andrzej Banburski, Lin-Qing Chen, Laurent Freidel, Jeff Hnybida

http://arxiv.org/abs/1412.5851
Black holes as gases of punctures with a chemical potential: Bose-Einstein condensation and logarithmic corrections to the entropy
Olivier Asin, Jibril Ben Achour, Marc Geiller, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1411.3591
Projective Limits of State Spaces III. Toy-Models
Suzanne Lanéry, Thomas Thiemann
========================

Last edited: Mar 11, 2015
11. Mar 30, 2015

### marcus

Tentative list of candidates for the first quarter 2015 poll.

http://arxiv.org/abs/1503.09154
Some implications of signature-change in cosmological models of loop quantum gravity
Martin Bojowald, Jakub Mielczarek
(Submitted on 31 Mar 2015)
Signature change at high density has been obtained as a possible consequence of deformed space-time structures in models of loop quantum gravity. This article provides a conceptual discussion of implications for cosmological scenarios, based on an application of mathematical results for mixed-type partial differential equations (the Tricomi problem). While the effective equations from which signature change has been derived are shown to be locally regular and therefore reliable, the underlying theory of loop quantum gravity may face several global problems in its semiclassical solutions.
35 pages, 5 figures

http://arxiv.org/abs/1503.07855
Loop quantum cosmology with self-dual variables
Edward Wilson-Ewing
(Submitted on 26 Mar 2015)
Using the complex-valued self-dual connection variables, the loop quantum cosmology of a closed Friedmann universe coupled to a massless scalar field is studied. It is shown how the reality conditions can be imposed in the quantum theory by choosing a particular measure for the inner product in the kinematical Hilbert space. While holonomies of the self-dual Ashtekar connection are not well-defined in the kinematical Hilbert space, it is possible to introduce a family of generalized holonomy-like operators, some of which are well-defined; these operators in turn are used in the definition of a Hamiltonian constraint operator where the scalar field can be used as a relational clock. The resulting quantum dynamics are similar, although not identical, to standard loop quantum cosmology constructed from the Ashtekar-Barbero variables with a real Immirzi parameter. Effective Friedmann equations are derived, which provide a good approximation to the full quantum dynamics for sharply-peaked states whose volume remains much larger than the Planck volume, and they show that for these states quantum gravity effects resolve the big-bang and big-crunch singularities and replace them by a non-singular bounce. Finally, the loop quantization in self-dual variables of a flat Friedmann space-time is recovered in the limit of zero spatial curvature and is identical to the standard loop quantization in terms of the real-valued Ashtekar-Barbero variables.
10 pages http://inspirehep.net/record/1356275

http://arxiv.org/abs/1503.06472
Black holes in Asymptotically Safe Gravity
Frank Saueressig, Natalia Alkofer, Giulio D'Odorico, Francesca Vidotto
(Submitted on 22 Mar 2015)
Black holes are among the most fascinating objects populating our universe. Their characteristic features, encompassing spacetime singularities, event horizons, and black hole thermodynamics, provide a rich testing ground for quantum gravity ideas. In this note we observe that the renormalization group improved Schwarzschild black holes constructed by Bonanno and Reuter within Weinberg's asymptotic safety program constitute a prototypical example of a Hayward geometry used to model non-singular black holes within quantum gravity phenomenology. Moreover, they share many features of a Planck star: their effective geometry naturally incorporates the one-loop corrections found in the effective field theory framework, their Kretschmann scalar is bounded, and the black hole singularity is replaced by a regular de Sitter patch. The role of the cosmological constant in the renormalization group improvement process is briefly discussed.
6 pages, 3 figures; prepared for the proceedings of the conference "Frontiers of Fundamental Physics 14"

http://arxiv.org/abs/1503.02981
Four-Dimensional Entropy from Three-Dimensional Gravity
S. Carlip
(Submitted on 10 Mar 2015)
At the horizon of a black hole, the action of (3+1)-dimensional loop quantum gravity acquires a boundary term that is formally identical to an action for three-dimensional gravity. I show how to use this correspondence to obtain the entropy of the (3+1)-dimensional black hole from well-understood conformal field theory computations of the entropy in (2+1)-dimensional de Sitter space.
8 pages

http://arxiv.org/abs/1502.06770
Quantum Transitions Between Classical Histories: Bouncing Cosmologies
James Hartle, Thomas Hertog
(Submitted on 24 Feb 2015)
In a quantum theory of gravity spacetime behaves classically when quantum probabilities are high for histories of geometry and field that are correlated in time by the Einstein equation. Probabilities follow from the quantum state. This quantum perspective on classicality has important implications:
(a) Classical histories are generally available only in limited patches of the configuration space on which the state lives.
(b) In a given patch states generally predict relative probabilities for an ensemble of possible classical histories.
(c) In between patches classical predictability breaks down and is replaced by quantum evolution connecting classical histories in different patches.
(d) Classical predictability can break down on scales well below the Planck scale, and with no breakdown in the classical equations of motion.
We support and illustrate (a)-(d) by calculating the quantum transition across the de Sitter like throat connecting asymptotically classical, inflating histories in the no-boundary quantum state. This supplies probabilities for how a classical history on one side transitions and branches into a range of classical histories on the opposite side. We also comment on the implications of (a)-(d) for the dynamics of black holes and eternal inflation.
36 pages, 6 figures

http://arxiv.org/abs/1502.06125
ΛCDM Bounce Cosmology without ΛCDM: the case of modified gravity
S.D. Odintsov, V.K. Oikonomou
(Submitted on 21 Feb 2015)
We provide an F(R) gravity description of a ΛCDM bouncing model, without the need for matter fluids or for cosmological constant. As we explicitly demonstrate, the two cosmological eras that constitute the ΛCDM bouncing model, can be generated by F(R) gravity which can lead to accelerating cosmologies. The resulting F(R) gravity has Einstein frame inflationary properties that have concordance to the latest Planck observational data. Both the F(R) gravity stability properties are thoroughly investigated and also, the gravitational particle production, a feature necessary for the viability of the ΛCDM bounce scenario, is also addressed. As we will show, the ΛCDM bounce model can be successfully described by pure F(R) gravity, with appealing phenomenological attributes, which we extensively discuss.
31 pages, accepted by PRD

http://arxiv.org/abs/1502.04640
The Lorentzian proper vertex amplitude: Classical analysis and quantum derivation
Jonathan Engle, Antonia Zipfel
(Submitted on 16 Feb 2015)
Spin foam models, an approach to defining the dynamics of loop quantum gravity, make use of the Plebanski formulation of gravity, in which gravity is recovered from a topological field theory via certain constraints called simplicity constraints. However, the simplicity constraints in their usual form select more than just one gravitational sector as well as a degenerate sector. This was shown, in previous work, to be the reason for the "extra" terms appearing in the semiclassical limit of the Euclidean EPRL amplitude. In this previous work, a way to eliminate the extra sectors, and hence terms, was developed, leading to the what was called the Euclidean proper vertex amplitude. In the present work, these results are extended to the Lorentzian signature, establishing what is called the Lorentzian proper vertex amplitude. This extension is non-trivial and involves a number of new elements since, for Lorentzian bivectors, the split into self-dual and anti-self-dual parts, on which the Euclidean derivation was based, is no longer available. In fact, the classical parts of the present derivation provide not only an extension to the Lorentzian case, but also, with minor modifications, provide a new, more four dimensionally covariant derivation for the Euclidean case. The new elements in the quantum part of the derivation are due to the different structure of unitary representations of the Lorentz group.
36 pages

http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
Rodolfo Gambini, Jorge Pullin
(Submitted on 11 Feb 2015)
The Montevideo interpretation of quantum mechanics, which consists in supplementing environmental decoherence with fundamental limitations in measurement stemming from gravity, has been described in several publications. However, some of them appeared before the full picture provided by the interpretation was developed. As such it can be difficult to get a good understanding via the published literature. Here we summarize it in a self contained brief presentation including all its principal elements.
10 pages

http://arxiv.org/abs/1502.00278
Compact phase space, cosmological constant, discrete time
Carlo Rovelli, Francesca Vidotto
(Submitted on 1 Feb 2015)
We study the quantization of geometry in the presence of a cosmological constant, using a discretization with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.
6 pages

http://arxiv.org/abs/1501.06270
Matter Bounce Scenario in F(T) gravity
Jaume Haro, Jaume Amorós
(Submitted on 26 Jan 2015)
It is shown that teleparallel F(T) theories of gravity combined with holonomy corrected Loop Quantum Cosmology (LQC) support a Matter Bounce Scenario (MBS) which is a potential alternative to the inflationary paradigm. The Matter Bounce Scenario is reviewed and, according to the current observational data provided by PLANCK's team, we have summarized all the conditions that it has to satisfy in order to be a viable alternative to inflation, such as to provide a theoretical value of the spectral index and its running compatible with the latest PLANCK data, to have a reheating process via gravitational particle production, or to predict some signatures in the non-gaussianities of the power spectrum. The calculation of the power spectrum for scalar perturbations and the ratio of tensor to scalar perturbations has been done, in the simplest case of an exact matter dominated background, for both holonomy corrected LQC and teleparallel F(T) gravity. Finally, we have discussed the challenges (essentially, dealing with non-gaussianities, the calculation of the 3-point function in flat spatial geometries for theories beyond General Relativity) and problems (Jeans instabilities in the case of holonomy corrected LQC or local Lorentz dependence in teleparallelism) that arise in either bouncing scenario.
6 pages. Communication to the FFP2014 (Frontiers in Fundamental Physics, Marseille 2014). To appear in Proceedings of Science

http://arxiv.org/abs/1501.03007
The shape dynamics description of gravity
Tim Koslowski
(Submitted on 13 Jan 2015)
Classical gravity can be described as a relational dynamical system without ever appealing to spacetime or its geometry. This description is the so-called shape dynamics description of gravity. The existence of relational first principles from which the shape dynamics description of gravity can be derived is a motivation to consider shape dynamics (rather than GR) as the fundamental description of gravity. Adopting this point of view leads to the question: What is the role of spacetime in the shape dynamics description of gravity? This question contains many aspects: Compatibility of shape dynamics with the description of gravity in terms of spacetime geometry, the role of local Minkowski space, universality of spacetime geometry and the nature of quantum particles, which can no longer be assumed to be irreducible representations of the Poincare group. In this contribution I derive effective spacetime structures by considering how matter fluctuations evolve along with shape dynamics. This evolution reveals an "experienced spacetime geometry." This leads (in an idealized approximation) to local Minkowski space and causal relations. The small scale structure of the emergent geometric picture depends on the specific probes used to experience spacetime, which limits the applicability of effective spacetime to describe shape dynamics. I conclude with discussing the nature of quantum fluctuations (particles) in shape dynamics and how local Minkowski spacetime emerges from the evolution of quantum particles.
16 pages, a submission to the proceedings of Theory Canada 9

http://arxiv.org/abs/1501.02963
Quantum Geometry and Black Holes
J. Fernando Barbero G., Alejandro Perez
(Submitted on 13 Jan 2015)
We present an overall picture of the advances in the description of black hole physics from the perspective of loop quantum gravity. After an introduction that discusses the main conceptual issues we present some details about the classical and quantum geometry of isolated horizons and their quantum geometry and then use this scheme to give a natural definition of the entropy of black holes. The entropy computations can be neatly expressed in the form of combinatorial problems solvable with the help of methods based on number theory and the use of generating functions. The recovery of the Bekenstein-Hawking law and corrections to it is explained in some detail. After this, due attention is paid to the discussion of semiclassical issues. An important point in this respect is the proper interpretation of the horizon area as the energy that should appear in the statistical-mechanical treatment of the black hole model presented here. The chapter ends with a comparison between the microscopic and semiclassical approaches to the computation of the entropy and discusses a number of issues regarding the relation between entanglement and statistical entropy and the possibility of comparing the subdominant (logarithmic) corrections to the entropy obtained with the help of the Euclidean path integral with the ones obtained in the present framework.
39 pages. Contribution to appear in the World Scientific series "100 Years of General Relativity" edited by A. Ashtekar and J. Pullin

http://arxiv.org/abs/1501.00855
Closure constraints for hyperbolic tetrahedra
Christoph Charles, Etera R. Livine
(Submitted on 5 Jan 2015)
We investigate the generalization of loop gravity's twisted geometries to a q-deformed gauge group. In the standard undeformed case, loop gravity is a formulation of general relativity as a diffeomorphism-invariant SU(2) gauge theory. Its classical states are graphs provided with algebraic data. In particular closure constraints at every node of the graph ensure their interpretation as twisted geometries. Dual to each node, one has a polyhedron embedded in flat space R3. One then glues them allowing for both curvature and torsion. It was recently conjectured that q-deforming the gauge group SU(2) would allow to account for a non-vanishing cosmological constant Lambda, and in particular that deforming the loop gravity phase space with real parameter q>0 would lead to a generalization of twisted geometries to a hyperbolic curvature. Following this insight, we look for generalization of the closure constraints to the hyperbolic case. In particular, we introduce two new closure constraints for hyperbolic tetrahedra. One is compact and expressed in terms of normal rotations (group elements in SU(2) associated to the triangles) and the second is non-compact and expressed in terms of triangular matrices (group elements in SB(2,C)). We show that these closure constraints both define a unique dual tetrahedron (up to global translations on the three-dimensional one-sheet hyperboloid) and are thus ultimately equivalent.
24 pages

Last edited: Apr 1, 2015
12. Apr 2, 2015

### marcus

Ten of us have rated the fourth quarter papers so far. Thanks to Atyy, Breo, Chronos, David Horgan, Francesca, Ghostcrown, MTd2, Nonlinearity, and Shyan! Here's how the votes tally up at present:

http://arxiv.org/abs/1411.0977
Geometry and the Quantum: Basics
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov

http://arxiv.org/abs/1412.7435
Horizon entropy with loop quantum gravity methods
Daniele Pranzetti, Hanno Sahlmann

http://arxiv.org/abs/1412.6015
On the Effective Metric of a Planck Star
Tommaso De Lorenzo, Costantino Pacilio, Carlo Rovelli, Simone Speziale

http://arxiv.org/abs/1411.3590
Projective Limits of State Spaces II. Quantum Formalism
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1411.3592
Projective Loop Quantum Gravity I. State Space
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1410.1714
Loop quantum gravity and observations
A. Barrau, J. Grain

http://arxiv.org/abs/1412.7546
SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry
Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello

http://arxiv.org/abs/1412.3752
Flux formulation of loop quantum gravity: Classical framework
Bianca Dittrich, Marc Geiller

http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing

http://arxiv.org/abs/1411.3589
Projective Limits of State Spaces I. Classical Formalism
Suzanne Lanéry, Thomas Thiemann

1 vote
http://arxiv.org/abs/1412.8247
Pachner moves in a 4d Riemannian holomorphic Spin Foam model
Andrzej Banburski, Lin-Qing Chen, Laurent Freidel, Jeff Hnybida

http://arxiv.org/abs/1412.5851
Black holes as gases of punctures with a chemical potential: Bose-Einstein condensation and logarithmic corrections to the entropy
Olivier Asin, Jibril Ben Achour, Marc Geiller, Karim Noui, Alejandro Perez

http://arxiv.org/abs/1411.3591
Projective Limits of State Spaces III. Toy-Models
Suzanne Lanéry, Thomas Thiemann