# A Poll: Fourth quarter 2015 MIP (most important QG papers)

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

1. Jan 6, 2016

### marcus

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

http://arxiv.org/abs/1512.09010
Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies
Anna Ijjas, Paul J. Steinhardt

http://arxiv.org/abs/1512.08959
Entanglement time in the primordial universe
Eugenio Bianchi, Lucas Hackl, Nelson Yokomizo

http://arxiv.org/abs/1512.08346
Quantum black hole without singularity
Claus Kiefer

http://arxiv.org/abs/1512.07690
SL(2,C) Chern-Simons Theory, Flat Connections, and Four-dimensional Quantum Geometry
Hal M. Haggard, Muxin Han, Wojciech Kaminski, Aldo Riello

http://arxiv.org/abs/1512.05743
Evolution of the tensor-to-scalar ratio across the loop quantum cosmology bounce
Edward Wilson-Ewing

http://arxiv.org/abs/1512.05331
A simpler way of imposing simplicity constraints
Andrzej Banburski, Lin-Qing Chen

http://arxiv.org/abs/1512.04566
Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
Tommaso De Lorenzo, Alejandro Perez

http://arxiv.org/abs/1512.03684
Anisotropic loop quantum cosmology with self-dual variables
Edward Wilson-Ewing

http://arxiv.org/abs/1512.02083
Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan

http://arxiv.org/abs/1512.01589
Asymptotic safety in an interacting system of gravity and scalar matter
Pietro Donà, Astrid Eichhorn, Peter Labus, Roberto Percacci

http://arxiv.org/abs/1512.00713
An embedding of loop quantum cosmology in (b, v) variables into a full theory context
Norbert Bodendorfer

http://arxiv.org/abs/1512.00221
On the relation between reduced quantisation and quantum reduction for spherical symmetry in loop quantum gravity
Norbert Bodendorfer, Antonia Zipfel

http://arxiv.org/abs/1511.05441
3D holography: from discretum to continuum
Valentin Bonzom, Bianca Dittrich

http://arxiv.org/abs/1511.03644
Hessian and graviton propagator of the proper vertex
Atousa Chaharsough Shirazi, Jonathan Engle, Ilya Vilensky

http://arxiv.org/abs/1510.04896
Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential
Béatrice Bonga, Brajesh Gupt

http://arxiv.org/abs/1510.04701
Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez

http://arxiv.org/abs/1510.03858
The thermodynamics of quantum spacetime histories
Lee Smolin

http://arxiv.org/abs/1510.03855
Inflationary spectra with inverse-volume corrections in loop quantum cosmology and their observational constraints from Planck 2015 data
Tao Zhu, Anzhong Wang, Klaus Kirsten, Gerald Cleaver, Qin Sheng, Qiang Wu

http://arxiv.org/abs/1510.01925
Projective Loop Quantum Gravity II. Searching for Semi-Classical States
Suzanne Lanéry, Thomas Thiemann

http://arxiv.org/abs/1510.00699
A Perfect Bounce
Steffen Gielen, Neil Turok

2. Jan 6, 2016

### marcus

http://arxiv.org/abs/1512.09010
Implications of Planck2015 for inflationary, ekpyrotic and anamorphic bouncing cosmologies
Anna Ijjas, Paul J. Steinhardt
(Submitted on 30 Dec 2015)
The results from Planck2015, when combined with earlier observations from WMAP, ACT, SPT and other experiments, were the first observations to disfavor the "classic" inflationary paradigm. To satisfy the observational constraints, inflationary theorists have been forced to consider plateau-like inflaton potentials that introduce more parameters and more fine-tuning, problematic initial conditions, multiverse-unpredictability issues, and a new 'unlikeliness problem.' Some propose turning instead to a "postmodern" inflationary paradigm in which the cosmological properties in our observable universe are only locally valid and set randomly, with completely different properties (and perhaps even different physical laws) existing in most regions outside our horizon. By contrast, the new results are consistent with the simplest versions of ekpyrotic cyclic models in which the universe is smoothed and flattened during a period of slow contraction followed by a bounce, and another promising bouncing theory, anamorphic cosmology, has been proposed that can produce distinctive predictions.
9 pages and 2 figures. Invited review article for the focus issue "Planck and the fundamentals of cosmology" in Classical and Quantum Gravity. Accepted for publication.

http://arxiv.org/abs/1512.08959
Entanglement time in the primordial universe
Eugenio Bianchi, Lucas Hackl, Nelson Yokomizo
(Submitted on 30 Dec 2015)
We investigate the behavior of the entanglement entropy of space in the primordial phase of the universe before the beginning of cosmic inflation. We argue that in this phase the entanglement entropy of a region of space grows from a zero-law to an area-law. This behavior provides a quantum version of the classical BKL conjecture that spatially separated points decouple in the approach to a cosmological singularity. We show that the relational growth of the entanglement entropy with the scale factor provides a new statistical notion of arrow of time in quantum gravity. The growth of entanglement in the pre-inflationary phase provides a mechanism for the production of the quantum correlations present at the beginning of inflation and imprinted in the CMB sky.
12 pages, 3 figures, Honorable Mention in the Gravity Research Foundation 2015 Essay Competition

http://arxiv.org/abs/1512.08346
Quantum black hole without singularity
Claus Kiefer
(Submitted on 28 Dec 2015)
We discuss the quantization of a spherical dust shell in a rigorous manner. Classically, the shell can collapse to form a black hole with a singularity. In the quantum theory, we construct a well-defined self-adjoint extension for the Hamilton operator. As a result, the evolution is unitary and the singularity is avoided. If we represent the shell initially by a narrow wave packet, it will first contract until it reaches the region where classically a black hole would form, but then re-expands to infinity. In a way, the state can be interpreted as a superposition of a black hole with a white hole.
5 pages, invited contribution to the BH6 session at the Marcel Grossmann Conference MG14

http://arxiv.org/abs/1512.07690
SL(2,C) Chern-Simons Theory, Flat Connections, and Four-dimensional Quantum Geometry
Hal M. Haggard, Muxin Han, Wojciech Kaminski, Aldo Riello
(Submitted on 24 Dec 2015)
The present paper analyze SL(2,ℂ) Chern-Simons theory on a class of graph complement 3-manifolds, and its relation with classical and quantum geometries on 4-dimensional manifolds. In classical theory, we explain the correspondence between a class of SL(2,ℂ) flat connections on 3-manifold and the Lorentzian simplicial geometries in 4 dimensions. The class of flat connections on the graph complement 3-manifold is specified by a certain boundary condition. The corresponding simplicial 4-dimensional geometries are made by constant curvature 4-simplices. The quantization of 4d simplicial geometry can be carried out via the quantization of flat connection on 3-manifold in Chern-Simons theory. In quantum SL(2,ℂ) Chern-Simons theory, a basis of physical wave functions is the class of (holomorphic) 3d block, defined by analytically continued Chern-Simons path integral over Lefschetz thimbles. Here we propose that the (holomorphic) 3d block with the proper boundary condition imposed gives the quantization of simplicial 4-dimensional geometry. Interestingly in the semiclassical asymptotic expansion of (holomorphic) 3d block, the leading contribution gives the classical action of simplicial Einstein-Hilbert gravity in 4 dimensions, i.e. Lorentzian 4d Regge action on constant curvature 4-simplices with a cosmological constant. Such a result suggests a relation between SL(2,ℂ) Chern-Simons theory on a class of 3-manifolds and simplicial quantum gravity on 4-dimensional manifolds. This paper presents the details for the results reported in arXiv:1509.00458.
43+11 pages, 9 figures

http://arxiv.org/abs/1512.05743
Evolution of the tensor-to-scalar ratio across the loop quantum cosmology bounce
Edward Wilson-Ewing
(Submitted on 17 Dec 2015)
I review how the separate universe approach to cosmological perturbation theory can be used to study the dynamics of long wavelength perturbations in loop quantum cosmology, and use it to calculate how the long wavelength perturbations evolve across the bounce assuming a constant equation of state. An interesting result is that the tensor-to-scalar ratio can be damped or amplified by quantum gravity effects during the bounce, depending on the equation of state of the matter field dominating the dynamics. In particular, if the equation of state is greater than -1/3 the value of the tensor-to-scalar ratio will be damped during the bounce, in some cases significantly.
12 pages. Prepared for submission to International Journal of Modern Physics series D, special issue on Loop Quantum Cosmology

http://arxiv.org/abs/1512.05331
A simpler way of imposing simplicity constraints
Andrzej Banburski, Lin-Qing Chen
(Submitted on 16 Dec 2015)
We investigate a way of imposing simplicity constraints in a holomorphic Spin Foam model that we recently introduced. Rather than imposing the constraints on the boundary spin network, as is usually done, one can impose the constraints directly on the Spin Foam propagator. We find that the two approaches have the same leading asymptotic behaviour, with differences appearing at higher order. This allows us to obtain a model that greatly simplifies calculations, but still has Regge Calculus as its semi-classical limit.
16 pages

http://arxiv.org/abs/1512.04566
Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
Tommaso De Lorenzo, Alejandro Perez
(Submitted on 14 Dec 2015)
A new scenario for gravitational collapse has been recently proposed by Haggard and Rovelli. Presenting the model under the name of black hole fireworks, they claimed that the accumulation of quantum gravitational effects outside the horizon can cause the tunneling of geometry from a black hole to a white hole, allowing a bounce of the collapsing star which can eventually go back to infinity. In this paper we discuss the instabilities of this model and propose a simple minimal modification which eliminates them, as well as other related instabilities discussed in the literature. The new scenario is a time-asymmetric version of the original model with a time-scale for the final explosion that is shorter than m log m in Planck units. Our analysis highlights the importance of irreversibility in gravitational collapse which, in turn, uncovers important issues that cannot be addressed in detail without a full quantum gravity treatment.
18 Pages, 6 Figures

http://arxiv.org/abs/1512.03684
Anisotropic loop quantum cosmology with self-dual variables
Edward Wilson-Ewing
(Submitted on 11 Dec 2015)
A loop quantization of the diagonal class A Bianchi models starting from the complex-valued self-dual connection variables is presented in this paper. The basic operators in the quantum theory correspond to areas and generalized holonomies of the Ashtekar connection and the reality conditions are implemented via the choice of the inner product on the kinematical Hilbert space. The action of the Hamiltonian constraint operator is given explicitly for the case when the matter content is a massless scalar field (in which case the scalar field can be used as a relational clock), and it is shown that the big-bang and big-crunch singularities are resolved in the sense that singular and non-singular states decouple under the action of the Hamiltonian constraint operator.
16 pages

http://arxiv.org/abs/1512.02083
Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan
(Submitted on 7 Dec 2015)
Several quantum gravity scenarios lead to physics below the Planck scale characterised by nonlocal, Lorentz invariant equations of motion. We show that such non-local effective field theories lead to a modified Schrödinger evolution in the nonrelativistic limit. In particular, the nonlocal evolution of opto-mechanical quantum oscillators is characterised by a spontaneous periodic squeezing that cannot be generated by environmental effects. We discuss constraints on the nonlocality obtained by past experiments, and show how future experiments (already under construction) will either see such effects or otherwise cast severe bounds on the non-locality scale (well beyond the current limits set by the Large Hadron Collider). This paves the way for table top, high precision experiments on massive quantum objects as a promising new avenue for testing some quantum gravity phenomenology.
5 pages, 1 figure

http://arxiv.org/abs/1512.01589
Asymptotic safety in an interacting system of gravity and scalar matter
Pietro Donà, Astrid Eichhorn, Peter Labus, Roberto Percacci
(Submitted on 4 Dec 2015)
Asymptotic safety is an attractive scenario for the dynamics of quantum spacetime. Here, we work from a phenomenologically motivated point of view and emphasize that a viable dynamics for quantum gravity in our universe must account for the existence of matter. In particular, we explore the scale-dependence of a scalar matter-gravity-vertex, and investigate whether an interacting fixed point exists for the so-defined Newton coupling. We find a viable fixed point in the pure-gravity system, disregarding scalar quantum fluctuations. We explore its extensions to the case with dynamical scalars, and find indications of asymptotic safety in the matter-gravity system. We moreover examine the anomalous dimensions for different components of the metric fluctuations, and find significant differences between the transverse traceless and scalar component.
15 pages, 14 figures

http://arxiv.org/abs/1512.00713
An embedding of loop quantum cosmology in (b, v) variables into a full theory context
Norbert Bodendorfer
(Submitted on 2 Dec 2015)
Loop quantum cosmology in (b, v) variables, which is governed by a unit step size difference equation, is embedded into a full theory context based on similar variables. From the full theory perspective, the symmetry reduction is characterised by the vanishing of certain phase space functions which are implemented as operator equations in the quantum theory. The loop quantum cosmology dynamics arise as the action of the full theory Hamiltonian on maximally coarse states in the kernel of the reduction constraints. An application of this reduction procedure to spherical symmetry is also sketched, with similar results, but only one canonical pair in (b, v) form.
17 pages

http://arxiv.org/abs/1512.00221
On the relation between reduced quantisation and quantum reduction for spherical symmetry in loop quantum gravity
Norbert Bodendorfer, Antonia Zipfel
(Submitted on 1 Dec 2015)
Building on a recent proposal for a quantum reduction to spherical symmetry from full loop quantum gravity, we investigate the relation between a quantisation of spherically symmetric general relativity and a reduction at the quantum level. To this end, we generalise the previously proposed quantum reduction by dropping the gauge fixing condition on the radial diffeomorphisms, thus allowing to make direct contact between previous work on reduced quantisation. A dictionary between spherically symmetric variables and observables with respect to the reduction constraints in the full theory is discussed, as well as an embedding of reduced quantum states to a sub sector of the quantum symmetry reduced full theory states. On this full theory sub sector, the quantum algebra of the mentioned observables is computed and shown to qualitatively reproduce the quantum algebra of the reduced variables in the large quantum number limit for a specific choice of regularisation. Insufficiencies in recovering the reduced algebra quantitatively from the full theory are attributed to the oversimplified full theory quantum states we use.
34 pages

http://arxiv.org/abs/1511.05441
3D holography: from discretum to continuum
Valentin Bonzom, Bianca Dittrich
(Submitted on 17 Nov 2015)
We study the one-loop partition function of 3D gravity without cosmological constant on the solid torus with arbitrary metric fluctuations on the boundary. To this end we employ the discrete approach of (quantum) Regge calculus. In contrast with similar calculations performed directly in the continuum, we work with a boundary at finite distance from the torus axis. We show that after taking the continuum limit on the boundary - but still keeping finite distance from the torus axis - the one-loop correction is the same as the one recently found in the continuum in Barnich et al. for an asymptotically flat boundary. The discrete approach taken here allows to identify the boundary degrees of freedom which are responsible for the non-trivial structure of the one-loop correction. We therefore calculate also the Hamilton-Jacobi function to quadratic order in the boundary fluctuations both in the discrete set-up and directly in the continuum theory. We identify a dual boundary field theory with a Liouville type coupling to the boundary metric. The discrete set-up allows again to identify the dual field with degrees of freedom associated to radial bulk edges attached to the boundary. Integrating out this dual field reproduces the (boundary diffeomorphism invariant part of the) quadratic order of the Hamilton-Jacobi functional. The considerations here show that bulk boundary dualities might also emerge at finite boundaries and moreover that discrete approaches are helpful in identifying such dualities.
42 pages

http://arxiv.org/abs/1511.03644
Hessian and graviton propagator of the proper vertex
Atousa Chaharsough Shirazi, Jonathan Engle, Ilya Vilensky
(Submitted on 11 Nov 2015)
The proper spin-foam vertex amplitude is obtained from the EPRL vertex by projecting out all but a single gravitational sector, in order to achieve correct semi-classical behavior. In this paper we calculate the gravitational two-point function predicted by the proper spin-foam vertex to lowest order in the vertex expansion. We find the same answer as in the EPRL case in the continuum spectrum' limit, so that the theory is consistent with the predictions of linearized gravity in the regime of small curvature. The method for calculating the two-point function is similar to that used in prior works: we cast it in terms of an action integral and to use stationary phase methods. Thus, the calculation of the Hessian matrix plays a key role. Once the Hessian is calculated, it is used not only to calculate the two-point function, but also to calculate the coefficient appearing in the semi-classical limit of the proper vertex amplitude itself. This coefficient is the effective discrete "measure factor" encoded in the spin-foam model. Through a non-trivial cancellation of different factors, we find that this coefficient is the same as the coefficient in front of the term in the asymptotics of the EPRL vertex corresponding to the selected gravitational sector.
20 pages

http://arxiv.org/abs/1510.04896
Phenomenological investigation of a quantum gravity extension of inflation with the Starobinsky potential
Béatrice Bonga, Brajesh Gupt
(Submitted on 16 Oct 2015)
We investigate the pre-inflationary dynamics of inflation with the Starobinsky potential, favored by recent data from the Planck mission, using techniques developed to study cosmological perturbations on quantum spacetimes in the framework of loop quantum gravity. We find that for a large part of the initial data, inflation compatible with observations occurs. There exists a subset of this initial data that leads to quantum gravity signatures that are potentially observable. Interestingly, despite the different inflationary dynamics, these quantum gravity corrections to the powerspectra are similar to those obtained for inflation with a quadratic potential, including suppression of power at large scales. Furthermore, for super horizon modes the tensor modes show deviations from the standard inflationary paradigm that are unique to the Starobinsky potential and could be important for non-Gaussian modulation and tensor fossils.
30 pages, 11 figures

http://arxiv.org/abs/1510.04701
Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
(Submitted on 15 Oct 2015)
We study the transmission of information and correlations through quantum fields in cosmological backgrounds. With this aim, we make use of quantum information tools to quantify the classical and quantum correlations induced by a quantum massless scalar field in two particle detectors, one located in the early universe (Alice's) and the other located at a later time (Bob's). In particular, we focus on two phenomena: a) the consequences on the transmission of information of the violations of the strong Huygens principle for quantum fields, and b) the analysis of the field vacuum correlations via correlation harvesting from Alice to Bob. We will study a standard cosmological model first and then assess whether these results also hold if we use other than the general relativistic dynamics. As a particular example, we will study the transmission of information through the Big Bounce, that replaces the Big Bang, in the effective dynamics of Loop Quantum Cosmology.
16 pages, 10 figures.

http://arxiv.org/abs/1510.03858
The thermodynamics of quantum spacetime histories
Lee Smolin
(Submitted on 13 Oct 2015)
We show that the simplicity constraints, which define the dynamics of spin foam models, imply, and are implied by, the first law of thermodynamics, when the latter is applied to causal diamonds in the quantum spacetime. This result reveals an intimate connection between the holographic nature of gravity, as reflected by the Bekenstein entropy, and the fact that general relativity and other gravitational theories can be understood as constrained topological field theories.
To state and derive this correspondence we describe causal diamonds in the causal structure of spin foam histories and generalize arguments given for the near horizon region of black holes by Frodden, Gosh and Perez and Bianchi. This allows us to apply a recent argument of Jacobson to show that if a spin foam history has a semiclassical limit described in terms of a smooth metric geometry, that geometry satisfies the Einstein equations.
These results suggest also a proposal for a quantum equivalence principle.
39 pages, 6 figures

http://arxiv.org/abs/1510.03855
Inflationary spectra with inverse-volume corrections in loop quantum cosmology and their observational constraints from Planck 2015 data
Tao Zhu, Anzhong Wang, Klaus Kirsten, Gerald Cleaver, Qin Sheng, Qiang Wu
(Submitted on 13 Oct 2015)
We derive the primordial power spectra, spectral indices and runnings of both cosmological scalar perturbations and gravitational waves in the framework of loop quantum cosmology with the inverse-volume quantum corrections. This represents an extension of our previous treatment for σ being integers to the case with any given value of σ. For this purpose, we adopt a new calculational strategy in the uniform asymptotic approximation, by expanding the involved integrals first in terms of the inverse-volume correction parameter to its first-order, a consistent requirement of the approximation of the inverse-volume corrections. In this way, we calculate explicitly the quantum gravitational corrections to the standard inflationary spectra and spectral indices to the second-order of the slow-roll parameters, and obtain the observational constraints on the inverse-volume corrections from Planck 2015 data for various values of σ. Using these constraints we discuss whether these quantum gravitational corrections lead to measurable signatures in the cosmological observations. We show that the scale-dependent contributions to inflationary spectra from the inverse-volume corrections could be well within the range of the detectability of the forthcoming generation of experiments.
19 pages, 4 figures, and 2 tables

http://arxiv.org/abs/1510.01925
Projective Loop Quantum Gravity II. Searching for Semi-Classical States
Suzanne Lanéry, Thomas Thiemann
(Submitted on 7 Oct 2015)
In [arXiv:1411.3592] an extension of the Ashtekar-Lewandowski (AL) state space of Loop Quantum Gravity was set up with the help a projective formalism introduced by Kijowski [Kijowski 1977; see also: arXiv:1304.6330, arXiv:1411.3590]. The motivation for this work was to achieve a more balanced treatment of the position and momentum variables (aka. holonomies and fluxes). Indeed, states in the AL Hilbert spaces describe discrete quantum excitations on top of a vacuum which is an eigenstate of the flux variables (a no-geometry' state): in such states, most holonomies are totally spread, making it difficult to approximate a smooth, classical 4-geometry. However, going beyond the AL sector does not fully resolve this difficulty: one uncovers a deeper issue hindering the construction of states semi-classical with respect to a full set of observables. In the present article, we analyze this issue in the case of real-valued holonomies (we will briefly comment on the heuristic implications for other gauge groups, eg. (2)). Specifically, we show that, in this case, there does not exist any state on the holonomy-flux algebra in which the variances of the holonomies and fluxes observables would all be finite, let alone small. It is important to note that this obstruction cannot be bypassed by further enlarging the quantum state space, for it arises from the structure of the algebra itself: as there are too many (uncountably many) non-vanishing commutators between the holonomy and flux operators, the corresponding Heisenberg inequalities force the quantum uncertainties to blow up uncontrollably. A way out would be to suitably restrict the algebra of observables. In a companion paper we take the first steps in this direction by developing a general framework to perform such a restriction without giving up the universality and diffeomorphism invariance of the theory.
51 pages, 1 figure

http://arxiv.org/abs/1510.00699
A Perfect Bounce
Steffen Gielen, Neil Turok
(Submitted on 2 Oct 2015)
We study the quantum cosmology of a universe with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. For FRW backgrounds, we are able to perform the quantum gravity path integral exactly. We find the evolution to describe a "perfect bounce," in which the universe passes smoothly through the singularity. The Feynman path integral amplitude is precisely that of a relativistic oscillator, for which the scale factor of the universe is the time and the scalar fields are the spatial coordinates. This picture provides natural, unitary quantum mechanical evolution across a bounce. We also study the quantum evolution of anisotropies and of inhomogeneous perturbations, at linear and nonlinear order. We provide evidence for a semiclassical description in which all fields pass "around" the cosmological singularity along complex classical paths.
5 pages.

3. Mar 8, 2016

### marcus

I normally use the thread of last quarter's MIP poll as a place to gather candidates for the upcoming poll. Here are some interesting LQG-and-allied research papers that have appeared this quarter (Jan-March 2016)
I was impressed by the quality and creative advances achieved in this quarter's batch of papers. Some titles from the list (in chronological order) you might want to check out:
Projective quantum states for Loop Quantum Gravity coupled to tensor fields
On Unification of Gravity and Gauge Interactions
Can chaos be observed in quantum gravity?
Emergent Friedmann dynamics with a quantum bounce from quantum gravity condensates
Bouncing cosmologies from quantum gravity condensates
The Fock Space of Loopy Spin Networks for Quantum Gravity
Volume Entropy
Short-scale Emergence of Classical Geometry, in Euclidean Loop Quantum Gravity
Accelerated expansion of the Universe without an inflaton and resolution of the initial singularity from GFT condensates
Effective cosmological constant induced by stochastic fluctuations of Newton's constant

Here is the complete list, currently 17, with abstracts:

http://arxiv.org/abs/1603.08658
**The Atoms Of Space, Gravity and the Cosmological Constant
(Submitted on 29 Mar 2016)
I describe an approach which connects classical gravity with the quantum microstructure of spacetime. The field equations arise from maximizing the density of states of matter plus geometry. The former is identified using the thermodynamics of null surfaces while the latter arises due to the existence of a zero-point length in the spacetime. The resulting field equations remain invariant when a constant is added to the matter Lagrangian, which is a symmetry of the matter sector. Therefore, the cosmological constant arises as an integration constant. A non-zero value (Λ) of the cosmological constant renders the amount of cosmic information (Ic) accessible to an eternal observer finite and hence is directly related to it. This relation allows us to determine the numerical value of (Λ)from the quantum structure of spacetime.
Invited Review; 32 pages; 3 figures

http://arxiv.org/abs/1603.07931
**Short-scale Emergence of Classical Geometry, in Euclidean Loop Quantum Gravity
Vincent Bayle, François Collet, Carlo Rovelli
(Submitted on 25 Mar 2016)
We study the euclidean covariant loop-quantum-gravity vertex numerically, using a cylindrically symmetric boundary state and a convenient value of the Barbero-Immirzi parameter. We show that a classical geometry emerges already at low spin. We also recognise the appearance of the degenerate configurations.
17 pages, 12 figures

http://arxiv.org/abs/1603.04170
**Effective cosmological constant induced by stochastic fluctuations of Newton's constant
Marco de Cesare, Fedele Lizzi, Mairi Sakellariadou
(Submitted on 14 Mar 2016)
We consider implications of the microscopic dynamics of spacetime for the evolution of cosmological models. We argue that quantum geometry effects may lead to stochastic fluctuations of the gravitational constant, which is thus considered as a macroscopic effective dynamical quantity. Consistency with Riemannian geometry entails the presence of a time-dependent dark energy term in the modified field equations, which can be expressed in terms of the dynamical gravitational constant. We suggest that the late-time accelerated expansion of the Universe may be ascribed to quantum fluctuations in the geometry of spacetime rather than the vacuum energy from the matter sector.
10 pages, 1 figure

http://arxiv.org/abs/1603.01764
**Accelerated expansion of the Universe without an inflaton and resolution of the initial singularity from GFT condensates
(Submitted on 5 Mar 2016)
We study the expansion of the Universe using an effective Friedmann equation obtained from the dynamics of GFT isotropic condensates. A promising feature of this model is the occurrence of an era of accelerated expansion, without the need to introduce an inflaton field with an appropriately chosen potential. Although the evolution equations are "classical", the cosmological model is entirely quantum and does not admit a description in terms of a classical spacetime. Consistency with Riemannian geometry holds only at late times, when standard cosmology is recovered. Hence the dynamics is given in purely relational terms. An effective gravitational constant is seen to arise from the collective behaviour of spacetime quanta, as described by GFT. The occurrence of a bounce, which resolves the initial spacetime singularity, is shown to be a general property of the model.
4 pages, 4 figures

http://arxiv.org/abs/1603.01561
Volume Entropy
Valerio Astuti, Marios Christodoulou, Carlo Rovelli
(Submitted on 4 Mar 2016)
Building on a technical result by Brunnemann and Rideout on the spectrum of the Volume operator in Loop Quantum Gravity, we show that the dimension of the space of the quadrivalent states --with finite-volume individual nodes-- describing a region with total volume smaller than V, has finite dimension, bounded by VlogV. This allows us to introduce the notion of "volume entropy": the von Neumann entropy associated to the measurement of volume.
5 pages
Note: cites http://relativity.phys.lsu.edu/ilqgs/ashtekar022316.pdf
slides from Ashtekar's talk at the ILQGS

http://arxiv.org/abs/1603.01128
**Diffeomorphism invariant cosmological symmetry in full quantum gravity
Christopher Beetle, Jonathan S. Engle, Matthew E. Hogan, Phillip Mendonca
(Submitted on 3 Mar 2016)
This paper summarizes a new proposal to define rigorously a sector of loop quantum gravity at the diffeomorphism invariant level corresponding to homogeneous and isotropic cosmologies, thereby enabling a detailed comparison of results in loop quantum gravity and loop quantum cosmology. The key technical steps we have completed are (a) to formulate conditions for homogeneity and isotropy in a diffeomorphism covariant way on the classical phase space of general relativity, and (b) to translate these conditions consistently using well-understood techniques to loop quantum gravity. Some additional steps, such as constructing a specific embedding of the Hilbert space of loop quantum cosmology into a space of (distributional) states in the full theory, remain incomplete. However, we also describe, as a proof of concept, a complete analysis of an analogous embedding of homogeneous and isotropic loop quantum cosmology into the quantum Bianchi I model of Ashtekar and Wilson-Ewing. Details will appear in a pair of forthcoming papers.
8 pages; invited submission for special issue of Int. J. Mod. Phys. D

http://arxiv.org/abs/1603.01117
**The Fock Space of Loopy Spin Networks for Quantum Gravity
Christoph Charles, Etera R. Livine
(Submitted on 3 Mar 2016)
In the context of the coarse-graining of loop quantum gravity, we introduce loopy and tagged spin networks, which generalize the standard spin network states to account explicitly for non-trivial curvature and torsion. Both structures relax the closure constraints imposed at the spin network vertices. While tagged spin networks merely carry an extra spin at every vertex encoding the overall closure defect, loopy spin networks allow for an arbitrary number of loops attached to each vertex. These little loops can be interpreted as local excitations of the quantum gravitational field and we discuss the statistics to endow them with. The resulting Fock space of loopy spin networks realizes new truncation of loop quantum gravity, allowing to formulate its graph-changing dynamics on a fixed background graph plus local degrees of freedom attached to the graph nodes. This provides a framework for re-introducing a non-trivial background quantum geometry around which we would study the effective dynamics of perturbations. We study how to implement the dynamics of topological BF theory in this framework. We realize the projection on flat connections through holonomy constraints and we pay special attention to their often overlooked non-trivial flat solutions defined by higher derivatives of the δ-distribution.
53 pages

http://arxiv.org/abs/1602.08271
**Bouncing cosmologies from quantum gravity condensates
Daniele Oriti, Lorenzo Sindoni, Edward Wilson-Ewing
(Submitted on 26 Feb 2016)
We show how the large-scale cosmological dynamics can be obtained from the hydrodynamics of isotropic group field theory condensate states in the Gross-Pitaevskii approximation. The correct Friedmann equations are recovered in the semi-classical limit for some choices of the parameters in the action for the group field theory, and quantum gravity corrections arise in the high-curvature regime causing a bounce which generically resolves the big-bang and big-crunch singularities.
4 pages

http://arxiv.org/abs/1602.08104
Quantum cosmology from group field theory condensates: a review
Steffen Gielen, Lorenzo Sindoni
(Submitted on 25 Feb 2016)
We give, in some detail, a critical overview over recent work towards deriving a cosmological phenomenology from the fundamental quantum dynamics of group field theory (GFT), based on the picture of a macroscopic universe as a "condensate" of a large number of quanta of geometry which are given by excitations of the GFT field over a "no-space" vacuum. We emphasise conceptual foundations, relations to other research programmes in GFT and the wider context of loop quantum gravity (LQG), and connections to the quantum physics of real Bose-Einstein condensates. We show how to extract an effective dynamics for GFT condensates from the microscopic GFT physics, and how to compare it with predictions of more conventional quantum cosmology models, in particular loop quantum cosmology (LQC). No detailed familiarity with the GFT formalism is assumed.
46 pages, 5 figures, invited review for SIGMA Special Issue on Tensor Models, Formalism and Applications

http://arxiv.org/abs/1602.05881
**Emergent Friedmann dynamics with a quantum bounce from quantum gravity condensates
Daniele Oriti, Lorenzo Sindoni, Edward Wilson-Ewing
(Submitted on 18 Feb 2016)
We study the effective cosmological dynamics, emerging as the hydrodynamics of simple condensate states, of a group field theory model for quantum gravity coupled to a massless scalar field and reduced to its isotropic sector. The quantum equations of motion for these group field theory condensate states are given in relational terms with respect to the scalar field, from which effective dynamics for spatially flat, homogeneous and isotropic space-times can be extracted. The result is a generalization of the Friedmann equations, including quantum gravity modifications, in a specific regime of the theory. The classical Friedmann equations of general relativity are recovered in a suitable semi-classical limit for some range of parameters of the microscopic dynamics. An important result is that the quantum geometries associated with these GFT condensate states are non-singular: a bounce generically occurs in the Planck regime. For some choices of condensate states, these modified Friedmann equations are very similar to those of loop quantum cosmology.
58 pages

http://arxiv.org/abs/1602.04452
Conceptual issues in loop quantum cosmology
Aurélien Barrau, Boris Bolliet
(Submitted on 14 Feb 2016 )
Loop quantum gravity is a mature theory. To proceed to explicit calculations in cosmology, it is necessary to make assumptions and simplifications based on the symmetries of the cosmological setting. Symmetry reduction is especially critical when dealing with cosmological perturbations. The present article reviews several approaches to the problem of building a consistent formalism that describes the dynamics of perturbations on a quantum spacetime and tries to address their respective strengths and weaknesses. We also review the main open issues in loop quantum cosmology.
12 pages. Invited article for an IJMP volume dedicated to loop quantum gravity

http://arxiv.org/abs/1602.03237
**Can chaos be observed in quantum gravity?
Bianca Dittrich, Philipp A. Hoehn, Tim A. Koslowski, Mike I. Nelson
(Submitted on 10 Feb 2016)
Full general relativity is almost certainly non-integrable and likely chaotic and therefore almost certainly possesses neither differentiable Dirac observables nor a reduced phase space. It follows that the standard notion of observable has to be extended to include non-differentiable or even discontinuous generalized observables. These cannot carry Poisson-algebraic structures and do not admit a standard quantization. This has deep consequences for a quantum theory of gravity, which we investigate in a simple model for a system with Hamiltonian constraint that fails to be completely integrable. We show that basing the quantization on standard topology precludes a semiclassical limit and can even prohibit any solutions to the quantum constraints. Our proposed solution to this problem is to refine topology such that a complete set of Dirac observables becomes continuous. In the toy model it turns out that a refinement to a polymer-type topology, as e.g. used in loop quantum cosmology, is sufficient. Basing quantization of the toy model on this finer topology, we find a complete set of quantum Dirac observables and a suitable semiclassical limit.
4 pages + references = 6 pages
http://inspirehep.net/record/1420693?ln=en

http://arxiv.org/abs/1602.02295
**On Unification of Gravity and Gauge Interactions
Ali H. Chamseddine, Viatcheslav Mukhanov
(Submitted on 6 Feb 2016)
The tangent group of the four dimensional space-time does not need to have the same number of dimensions as the base manifold. Considering a higher dimensional Lorentz group as the symmetry of the tangent space, we unify gravity and gauge interactions in a natural way. The spin connection of the gauged Lorentz group is then responsible for both gravity and gauge fields, and the action for the gauged fields becomes part of the spin curvature squared. The realistic group which unifies all known particles and interactions is the SO(1,13) Lorentz group whose gauge part leads to SO(10) grand unified theory and contains double the number of required fermions in the fundamental spinor representation. We briefly discuss the Brout-Englert-Higgs mechanism which breaks the SO(1,13) symmetry first to SO(1,3)×SU(3)×SU(2)×U(1) and further to SO(1,3)×SU(3)×U(1) and gives very heavy masses to half of the fermions leaving the others with light masses.

http://arxiv.org/abs/1602.01861
Twisted geometries, twistors and conformal transformations
Miklos Långvik, Simone Speziale
(Submitted on 4 Feb 2016)
The twisted geometries of spin network states are described by simple twistors, isomorphic to null twistors with a time-like direction singled out. The isomorphism depends on the Immirzi parameter, and reduces to the identity when the parameter goes to infinity. Using this twistorial representation we study the action of the conformal group SU(2,2) on the classical phase space of loop quantum gravity, described by twisted geometry. The generators of translations and conformal boosts do not preserve the geometric structure, whereas the dilatation generator does. It corresponds to a 1-parameter family of embeddings of T*SL(2,C) in twistor space, and its action preserves the intrinsic geometry while changing the extrinsic one - that is the boosts among polyhedra. We discuss the implication of this action from a dynamical point of view, and compare it with a discretisation of the dilatation generator of the continuum phase space, given by the Lie derivative of the group character. At leading order in the continuum limit, the latter reproduces the same transformation of the extrinsic geometry, while also rescaling the areas and volumes and preserving the angles associated with the intrinsic geometry. Away from the continuum limit its action has an interesting non-linear structure, but is in general incompatible with the closure constraint needed for the geometric interpretation. As a side result, we compute the precise relation between the extrinsic geometry used in twisted geometries and the one defined in the gauge-invariant parametrization by Dittrich and Ryan, and show that the secondary simplicity constraints they posited coincide with those dynamically derived in the toy model of [1409.0836].
20 pages

http://arxiv.org/abs/1601.05707
**Projective quantum states for Loop Quantum Gravity coupled to tensor fields
Andrzej Okolow
(Submitted on 21 Jan 2016)
We present a construction of kinematic quantum states for theories of tensor fields of an arbitrary sort. The construction is based on projective techniques by Kijowski. Applying projective quantum states for Loop Quantum Gravity obtained by Lanery and Thiemann we construct quantum states for LQG coupled to tensor fields.
23 pages.

http://arxiv.org/abs/1601.05688
**Quantum self-gravitating collapsing matter in a quantum geometry
Miguel Campiglia, Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 21 Jan 2016)
The problem of how space-time responds to gravitating quantum matter in full quantum gravity has been one of the main questions that any program of quantization of gravity should address. Here we analyze this issue by considering the quantization of a collapsing null shell coupled to spherically symmetric loop quantum gravity. We show that the constraint algebra of canonical gravity is Abelian both classically and when quantized using loop quantum gravity techniques. The Hamiltonian constraint is well defined and suitable Dirac observables characterizing the problem were identified at the quantum level. We can write the metric as a parameterized Dirac observable at the quantum level and study the physics of the collapsing shell and black hole formation. We show how the singularity inside the black hole is eliminated by loop quantum gravity and how the shell can traverse it. The construction is compatible with a scenario in which the shell tunnels into a baby universe inside the black hole or one in which it could emerge through a white hole.
4 pages

http://arxiv.org/abs/1601.01716
Primordial power spectra for scalar perturbations in loop quantum cosmology
Daniel Martín de Blas, Javier Olmedo
(Submitted on 7 Jan 2016)
We provide the power spectrum of small scalar perturbations propagating in an inflationary scenario within loop quantum cosmology. We consider the hybrid quantization approach applied to a Friedmann--Robertson--Walker spacetime with flat spatial sections coupled to a massive scalar field. We study the quantum dynamics of scalar perturbations on an effective background within this hybrid approach. We consider in our study adiabatic states of different orders. For them, we find that the hybrid quantization is in good agreement with the predictions of the dressed metric approach. We also propose an initial vacuum state for the perturbations, and compute the primordial and the anisotropy power spectrum in order to qualitatively compare with the current observations of Planck mission. We find that our vacuum state is in good agreement with them, showing a suppression of the power spectrum for large scale anisotropies. We compare with other choices already studied in the literature.
22 pages, 12 figures

Last edited: Mar 30, 2016
4. Mar 8, 2016

### marcus

The Astuti, Christodoulou, Rovelli (ACR) paper "Volume Entropy" may be a game changer in a certain sense if it erases the need for Penrose' WEYL CURVATURE HYPOTHESIS.

It offers an alternative answer to the cosmological entropy puzzle. We see that the entropy of matter and radiation (which were all spread out) was HIGH in the early universe. So how can the second law be satisfied? How can entropy have been increasing?

One answer is to look at the gravitational field. Maybe something about it tells us that its entropy was very low---overcoming matter entropy and making the total entropy low. Penrose WCH does this.

But wouldn't it be simpler to simply observe that a typical VOLUME was incredibly low compared with what it became later ? If entropy increases with volume (in a way the ACR authors discovered) this would do the trick just as well, without invoking Weyl curvature.

http://arxiv.org/abs/1603.01561
Volume Entropy
Valerio Astuti, Marios Christodoulou, Carlo Rovelli
(Submitted on 4 Mar 2016)
Building on a technical result by Brunnemann and Rideout on the spectrum of the Volume operator in Loop Quantum Gravity, we show that the dimension of the space of the quadrivalent states --with finite-volume individual nodes-- describing a region with total volume smaller than V, has finite dimension, bounded by VlogV. This allows us to introduce the notion of "volume entropy": the von Neumann entropy associated to the measurement of volume.
5 pages
http://inspirehep.net/record/1426122?ln=en
Note: cites http://relativity.phys.lsu.edu/ilqgs/ashtekar022316.pdf
slides from Ashtekar's talk at the ILQGS

==quote from "conclusions" section==
For the second principle of thermodynamics to hold, the initial state of the universe must have had low entropy. On the other hand, from cosmic background radiation observations, the initial state of matter must have been close to having maximal entropy. Pen- rose addresses this discrepancy by taking into consideration the entropy associated to gravitational degrees of freedom. His hypothesis is that the degrees of freedom which have been activated to bring the increase in entropy from the initial state are the ones associated to the Weyl curvature tensor, which in his hypothesis was null in the initial state of the universe. A definition of the bulk entropy of space, which, as would be expected, grows with the volume, could perhaps perform the same role as the Weyl curvature degrees of freedom do in Penrose’s hypothesis: the universe had a much smaller volume close to its initial state, so the total available entropy was low - regardless of the matter entropy content - and has increased since, just because for a space of larger volume we have a greater number of states describing its geometry.

We close with a very speculative remark. Does the fact that entropy is large for larger volumes imply the existence of an entropic force driving to larger volumes? That is, could there be a statistical bias for transitions to geometries of greater volume? ...
...
...
==endquote==

another possible game changer is the Padmanabhan paper listed above:

http://arxiv.org/abs/1603.08658
The Atoms Of Space, Gravity and the Cosmological Constant
(Submitted on 29 Mar 2016)
I describe an approach which connects classical gravity with the quantum microstructure of spacetime. The field equations arise from maximizing the density of states of matter plus geometry. The former is identified using the thermodynamics of null surfaces while the latter arises due to the existence of a zero-point length in the spacetime. The resulting field equations remain invariant when a constant is added to the matter Lagrangian, which is a symmetry of the matter sector. Therefore, the cosmological constant arises as an integration constant. A non-zero value (Λ) of the cosmological constant renders the amount of cosmic information (Ic) accessible to an eternal observer finite and hence is directly related to it. This relation allows us to determine the numerical value of (Λ)from the quantum structure of spacetime.
Invited Review; 32 pages; 3 figures

==quote==
Substituting this into Eq. (59), we get a remarkable formula for the cosmological constant
...
...
If we take the typical values ρin = (1.2 × 1015 GeV)4 , ρeq = (0.86 eV)4 , we get ρΛ = (2.2 × 10−3 eV)4 which agrees well with observed value! In other words, the idea that the cosmic information content accessible to an eternal observer, Ic, is equal to the basic quantum gravitational unit of information IQG = 4π, determines the numerical value of the cosmological constant correctly. ...
==endquote==

"space", "gravity" are other words for geometry
"atoms of geometry and the cosmological constant" you get this thread:
"ATOMS OF SPACE, GEOMETRY, AND THE COSMOLOGICAL CONSTANT"
atoms of space and the cosmological constant
atoms of gravity and the cosmological constant
and that thread is the first hit.
I'd like a google tag for it something like [atoms space cosmological constant]

Last edited: Mar 29, 2016
5. Mar 12, 2016

### marcus

A paper on the 4th quarter 2015 poll that just got voted for in the past couple of days was, interestingly, the subject of some especially favorable comment by Sabine Hossenfelder, here:
http://backreaction.blogspot.com/2015/12/quantum-gravity-could-be-observable-in.html
"Quantum gravity could be observable in the oscillation frequency of heavy quantum states"
Basically the prospect is for TABLETOP experiments to observe QG effects and constraint QG theories.

The paper is
http://arxiv.org/abs/1512.02083
Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
Alessio Belenchia, Dionigi M. T. Benincasa, Stefano Liberati, Francesco Marin, Francesco Marino, Antonello Ortolan
(Submitted on 7 Dec 2015)
Several quantum gravity scenarios lead to physics below the Planck scale characterised by nonlocal, Lorentz invariant equations of motion. We show that such non-local effective field theories lead to a modified Schrödinger evolution in the nonrelativistic limit. In particular, the nonlocal evolution of opto-mechanical quantum oscillators is characterised by a spontaneous periodic squeezing that cannot be generated by environmental effects. We discuss constraints on the nonlocality obtained by past experiments, and show how future experiments (already under construction) will either see such effects or otherwise cast severe bounds on the non-locality scale (well beyond the current limits set by the Large Hadron Collider). This paves the way for table top, high precision experiments on massive quantum objects as a promising new avenue for testing some quantum gravity phenomenology.
5 pages, 1 figure
http://inspirehep.net/record/1408503?ln=en

It's significant that the paper is co-authored by Stefano Liberati--a world-class authority on QG phenomenology with some 5000 citations to his work. Here's his Inspire profile:
http://inspirehep.net/author/profile/S.Liberati.1

Last edited: Mar 12, 2016
6. Mar 16, 2016

### marcus

BTW thanks to David Horgan, nolxiii, and Nonlinearity for joining me in this poll! Nolxiii voted for just one paper---I thought it was a really interesting choice, because it opens up the possibility of table top, normal lab scale experiments detecting quantum gravity effects.

Besides that one, these 4 papers recently gained votes.
Entanglement time in the primordial universe
Quantum black hole without singularity
Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
The thermodynamics of quantum spacetime histories

Entanglement time in the primordial universe
Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
3D holography: from discretum to continuum

Last edited: Mar 18, 2016
7. Mar 22, 2016

### marcus

The poll is beginning to take shape, pointing in directions I certainly did not anticipate. Thanks to Amrator, Atyy, David Horgan, Greg, Nolxii, Nonlinearity, there are 7 of us participating so far!
These five papers recently gained votes:

A simpler way of imposing simplicity constraints
Improved Black Hole Fireworks: Asymmetric Black-Hole-to-White-Hole Tunneling Scenario
Anisotropic loop quantum cosmology with self-dual variables
Tests of Quantum Gravity induced non-locality via opto-mechanical quantum oscillators
3D holography: from discretum to continuum

The last, "Tests of QG..." seems to have won the most approval so far.

Last edited: Mar 22, 2016