Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1602.05475
Quantum Reduced Loop Gravity and the foundation of Loop Quantum Cosmology
Emanuele Alesci, Francesco Cianfrani
(Submitted on 17 Feb 2016)
Quantum Reduced Loop Gravity is a promising framework for linking Loop Quantum Gravity and the effective semiclassical dynamics of Loop Quantum Cosmology. We review its basic achievements and its main perspectives, outlining how it provides a quantum description of the Universe in terms of a cuboidal graph which constitutes the proper framework for applying loop techniques in a cosmological setting.
18 pages, invited review

http://arxiv.org/abs/1602.05499
Some notes on the Kodama state, maximal symmetry, and the isolated horizon boundary condition
Norbert Bodendorfer
(Submitted on 17 Feb 2016)
We recall some well and some less known results about the Kodama state, the related θ ambiguity in defining canonical variables, and the isolated horizon boundary condition F∝Σ. Based on them, we make some comments highlighting that the Kodama state for real connection variables can be given a precise meaning and that it implements a vacuum peaked on a (in a suitable sense) maximally symmetric geometry. We also highlight the similarity of this construction with the isolated horizon boundary condition F∝Σ and stress that it is inadequate to define the notion of a quantum horizon.
6+3 pages

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

noted:
http://arxiv.org/abs/1602.06077

 

[marcus]

http://arxiv.org/abs/1602.07006
A (not so?) novel explanation for the very special initial state of the universe
Elias Okon, Daniel Sudarsky
(Submitted on 23 Feb 2016)
We put forward a proposal that combines objective collapse models, developed in connection to quantum-foundational questions, with the so-called Weyl curvature hypothesis, introduced by Roger Penrose as an attempt to account for the very special initial state of the Universe. In particular, we explain how a curvature dependence of the collapse rate of such models could offer a dynamical justification for Penrose's conjecture. The present essay constitutes an extension of a previous proposal of ours that uses collapse theories in dealing with open problems in contemporary physical theories.
7 pages; Essay written for the Gravity Research Foundation 2016 Awards for Essays on Gravitation

http://arxiv.org/abs/1602.07478
2+1 dimensional loop quantum cosmology of Bianchi I models
You Ding, Xiangdong Zhang
(Submitted on 24 Feb 2016)
We study the anisotropic Bianchi I loop quantum cosmology in 2+1 dimensions. Both the $\mubar$ and $\mubar'$schemes are considered in the present paper and the following expected results are established: (i) the massless scalar field again play the role of emergent time variables and serves as an internal clock; (ii) By imposing the fundamental discreteness of length operator, the total Hamiltonian constraint is obtained and gives rise the evolution as a difference equation; and (iii) the exact solutions of Friedmann equation are constructed rigorously for both classical and effective level. The investigation extends the domain of validity of loop quantum cosmology to beyond the four dimensions.
11 pages

http://arxiv.org/abs/1602.07653
Quantum walk on spin network
M. M. Amaral, R. Aschheim, Klee Irwin
(Submitted on 24 Feb 2016)
We apply a discrete quantum walk from a quantum particle on a discrete quantum spacetime from loop quantum gravity and show that the related Entanglement Entropy can drive a entropic force. We apply this concepts to propose a model of a walker position topologically encoded on a spin network.
7 pages, 3 figures

noted:
http://arxiv.org/abs/1602.07546

 

[marcus]

http://arxiv.org/abs/1602.07993
Turning on gravity with the Higgs mechanism
Stephon Alexander, John D. Barrow, Joao Magueijo
(Submitted on 25 Feb 2016)
We investigate how a Higgs mechanism could be responsible for the emergence of gravity in extensions of Einstein theory. In this scenario, at high energies, symmetry restoration could "turn off" gravity, with dramatic implications for cosmology and quantum gravity. The sense in which gravity is muted depends on the details of the implementation. In the most extreme case gravity's dynamical degrees of freedom would only be unleashed after the Higgs field acquires a non-trivial vacuum expectation value, with gravity reduced to a topological field theory in the symmetric phase. We might also identify the Higgs and the Brans-Dicke fields in such a way that in the unbroken phase Newton's constant vanishes, decoupling matter and gravity. We discuss the broad implications of these scenarios.
8 pages.

http://arxiv.org/abs/1602.08019
Arrows of time in unconfined systems
Julian Barbour
(Submitted on 25 Feb 2016)
Entropy and the second law of thermodynamcs were discovered through study of the behaviour of gases in confined spaces. The related techniques developed in the kinetic theory of gases have failed to resolve the apparent conflict between the time-reversal symmetry of all known laws of nature and the existence of arrows of time that at all times and everywhere in the universe all point in the same direction. I will argue that the failure may due to unconscious application to the universe of the conceptual framework developed for confined systems. If, as seems plausible, the universe is an unconfined system, new concepts are needed.
8 pages, to be published in proceedings to the Time in Physics conference (ETH Zurich 7-11 September 2015)

http://arxiv.org/abs/1602.08020
Thermal dimension of quantum spacetime
Giovanni Amelino-Camelia, Francesco Brighenti, Giulia Gubitosi, Grasiele Santos
(Submitted on 25 Feb 2016)
Recent results suggest that a crucial crossroad for quantum gravity is the characterization of the effective dimension of spacetime at short distances, where quantum properties of spacetime become significant. This is relevant in particular for various scenarios of "dynamical dimensional reduction" which have been discussed in the literature. We are here concerned with the fact that the related research effort has been based exclusively on analyses of the "spectral dimension", which involves an unphysical Euclideanization of spacetime and is highly sensitive to the off-shell properties of a theory. As here shown, different formulations of the same physical theory can have wildly different spectral dimension. We propose that dynamical dimensional reduction should be described in terms of the "thermal dimension" which we here introduce, a notion that only depends on the physical content of the theory. We analyze a few models with dynamical reduction both of the spectral dimension and of our thermal dimension, finding in particular some cases where thermal and spectral dimension agree, but also some cases where the spectral dimension has puzzling properties while the thermal dimension gives a different and meaningful picture.
5 pages, 3 figures

special interest, the GW-gammarayburst coincidence:
http://arxiv.org/abs/1602.03920
and
http://arxiv.org/abs/1602.04735
Electromagnetic Counterparts to Black Hole Mergers Detected by LIGO
Abraham Loeb (Harvard)
(Submitted on 15 Feb 2016 (v1), last revised 22 Feb 2016 (this version, v2))
Mergers of stellar-mass black holes (BHs), such as GW150914 observed by LIGO, are not expected to have electromagnetic counterparts. However, the Fermi GBM detector identified of a gamma-ray transient 0.4 s after the gravitational wave (GW) signal GW150914 with consistent sky localization. I show that the two signals might be related if the BH binary detected by LIGO originated from two clumps in a dumbbell configuration that formed when the core of a rapidly rotating massive star collapsed. In that case, the BH binary merger was followed by a gamma-ray burst (GRB) from a jet that originated in the accretion flow around the remnant BH. A future detection of a GRB afterglow could be used to determine the redshift and precise localization of the source. A population of standard GW sirens with GRB redshifts would provide a new approach for precise measurements of cosmological distances as a function of redshift.
4 pages, accepted for publication in ApJ Letters

 

[marcus]

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/1602.08271
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/1602.08688
New volume and inverse volume operators for loop quantum gravity
Jinsong Yang, Yongge Ma
(Submitted on 28 Feb 2016)
A new alternative volume operator is constructed for loop quantum gravity by using the so-called co-triad operators as building blocks. It is shown that the new volume operator shares the same qualitative properties with the standard volume operator. Moreover, a new alternative inverse volume operator is also constructed in the light of the construction of the alternative volume operator, which is possessed of the same qualitative properties as those of the alternative volume operator. The new inverse volume operator can be employed to construct the Hamiltonian operator of matter fields, which may lead to an anomaly-free on shell quantum constraint algebra without any special restriction on the regularization procedure for gravity coupled to matter fields.
13 pages

http://arxiv.org/abs/1603.00037
Unique Fock quantization of a massive fermion field in a cosmological scenario
Jerónimo Cortez, Beatriz Elizaga Navascués, Mercedes Martín-Benito, Guillermo A. Mena Marugán, José M. Velhinho
(Submitted on 29 Feb 2016)
It is well-known that the Fock quantization of field theories in general spacetimes suffers from an infinite ambiguity, owing to the inequivalent possibilities in the selection of a representation of the canonical commutation or anticommutation relations, but also owing to the freedom in the choice of variables to describe the field among all those related by linear time-dependent transformations, including the dependence through functions of the background. In this work we remove this ambiguity (up to unitary equivalence) in the case of a massive Dirac free field propagating in a spacetime with homogeneous and isotropic spatial sections of spherical topology. Two physically reasonable conditions are imposed in order to arrive to this result: a) The invariance of the vacuum under the spatial isometries of the background, and b) The unitary implementability of the dynamical evolution that dictates the Dirac equation. We characterize the class of Fock quantizations with a non-trivial fermion dynamics that satisfy these two conditions. Then, we provide a complete proof of the unitary equivalence of the representations in this class under very mild requirements on the time variation of the background, once a criterion to discern between particles and antiparticles has been set.
20 pages

http://arxiv.org/abs/1603.00172
Exact renormalisation group equations and loop equations for tensor models
Thomas Krajewski, Reiko Toriumi
(Submitted on 1 Mar 2016)
In this paper, we review some general formulations of exact renormalisation group equations and loop equations for tensors models and tensorial group field theories. We illustrate the use of these equations in the derivation of the leading order expectation values of observables in tensor models. Furthermore, we use the exact renormalisation group equations to establish a suitable scaling dimension for interactions in Abelian tensorial group field theories with a closure constraint. We also present analogues of the loop equations for tensor models.

 

[marcus]

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

One I missed, but Alejandro spotted:
arXiv: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.

 

[marcus]

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/1603.01764
Accelerated expansion of the Universe without an inflaton and resolution of the initial singularity from GFT condensates
Marco de Cesare, Mairi Sakellariadou
(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

There has recently been a rush of "GFT condensate" papers. E.g search by authors Gielen, or Oriti.

http://arxiv.org/abs/1603.01902
Flowing in group field theory space: a review
Sylvain Carrozza
(Submitted on 7 Mar 2016)
We provide a non--technical overview of recent extensions of renormalization methods and techniques to Group Field Theories (GFTs), a class of combinatorially non--local quantum field theories which generalize matrix models to dimension d≥3. More precisely, we focus on GFTs with so--called closure constraint, which are closely related to lattice gauge theories and quantum gravity spin foam models. With the help of modern tensor model tools, a rich landscape of renormalizable theories has been unravelled. We review our current understanding of their renormalization group flows, at both perturbative and non--perturbative levels.
29 pages, 12 figures. Invited review for SIGMA Special Issue on "Tensor Models, Formalism and Applications"

http://arxiv.org/abs/1603.02076
Searching for a Continuum Limit in CDT Quantum Gravity
Jan Ambjorn, Daniel Coumbe, Jakub Gizbert-Studnicki, Jerzy Jurkiewicz
(Submitted on 7 Mar 2016)
We search for a continuum limit in the causal dynamical triangulation (CDT) approach to quantum gravity by determining the change in lattice spacing using two independent methods. The two methods yield similar results that may indicate how to tune the relevant couplings in the theory in order to take a continuum limit.
19 pages, 8 figures

possible wider interest:
http://arxiv.org/abs/1603.03039
Hand-waving and Interpretive Dance: An Introductory Course on Tensor Networks
Jacob C. Bridgeman, Christopher T. Chubb
(Submitted on 9 Mar 2016)
The curse of dimensionality associated with the Hilbert space of spin systems provides a significant obstruction to the study of condensed matter systems. Tensor networks have proven an important tool in attempting to overcome this difficulty in both the numerical and analytic regimes.
These notes form the basis for a seven lecture course, introducing the basics of a range of common tensor networks and algorithms. In particular, we cover: introductory tensor network notation, applications to quantum information, basic properties of matrix product states, a classification of quantum phases using tensor networks, algorithms for finding matrix product states, basic properties of projected entangled pair states, and multiscale entanglement renormalisation ansatz states.
The lectures are intended to be generally accessible, although the relevance of many of the examples may be lost on students without a background in many-body physics/quantum information. For each lecture, several problems are given, with worked solutions in an ancillary file.
Introductory lecture notes, worked solutions to problems in ancillary file.

http://arxiv.org/abs/1603.01204
A Theory of Ambulance Chasing
Mihailo Backović
(Submitted on 3 Mar 2016)
Ambulance chasing is a common socio-scientific phenomenon in particle physics. I argue that despite the seeming complexity, it is possible to gain insight into both the qualitative and quantitative features of ambulance chasing dynamics. Compound-Poisson statistics suffices to accommodate the time evolution of the cumulative number of papers on a topic, where basic assumptions that the interest in the topic as well as the number of available ideas decrease with time appear to drive the time evolution. It follows that if the interest scales as an inverse power law in time, the cumulative number of papers on a topic is well described by a di-gamma function, with a distinct logarithmic behavior at large times. In cases where the interest decreases exponentially with time, the model predicts that the total number of papers on the topic will converge to a fixed value as time goes to infinity. I demonstrate that the two models are able to fit at least 9 specific instances of ambulance chasing in particle physics using only two free parameters. In case of the most recent ambulance chasing instance, the ATLAS γγ excess, fits to the current data predict that the total number of papers on the topic will not exceed 310 papers by the June 1. 2016, and prior to the natural cut-off for the validity of the theory.
9 pages, 4 figures

http://arxiv.org/abs/1603.03312
The critical geometry of a thermal big bang
Niayesh Afshordi, Joao Magueijo
(Submitted on 9 Mar 2016)
We explore the space of scalar-tensor theories containing two disformally related metrics, and find a discontinuity pointing to a special "critical" cosmological solution. This solution has a simple geometrical interpretation based on the action of a probe 3-brane embedded in an EAdS₂×E₃ geometry. Due to the different maximal speeds of propagation for matter and gravity, the cosmological fluctuations start off inside the horizon even without inflation, and will more naturally have a thermal origin (since there is never vacuum domination). The critical model makes an unambiguous, non-tuned prediction for the spectral index of the scalar fluctuations left outside the horizon: n_s=0.96478(64). Adding to this that no gravitational waves are produced, we have unveiled the most predictive model on offer.

 

[marcus]

http://arxiv.org/abs/1603.03570
Large N limits in tensor models: Towards more universality classes of colored triangulations in dimension d≥2
Valentin Bonzom
(Submitted on 11 Mar 2016)
We review an approach which aims at studying discrete (pseudo--)manifolds in dimension d≥2 and called random tensor models. More specifically, we insist on generalizing the two-dimensional notion of p-angulations to higher dimensions. To do so, we consider families of triangulations built out of simplices with colored faces. Those simplices can be glued to form new building blocks, called bubbles which are pseudo--manifolds with boundaries. Bubbles can in turn be glued together to form triangulations. The main challenge is to classify the triangulations built from a given set of bubbles with respect to their numbers of bubbles and simplices of codimension two. While the colored triangulations which maximize the number of simplices of codimension two at fixed number of simplices are series-parallel objects called melonic triangulations, this is not always true anymore when restricting attention to colored triangulations built from specific bubbles. This opens up the possibility of new universality classes of colored triangulations. We review and extend three existing strategies to find those universality classes. The first two strategies consist in building new bubbles from old ones for which the problem can be solved. The third strategy is a bijection between those colored triangulations and stuffed, edge-colored maps, which are some sort of hypermaps whose hyperedges are replaced with edge-colored maps. We then show that the present approach can lead to enumeration results and identification of universality classes, by working out the example of quartic tensor models. They feature a tree-like phase, a planar phase similar to two-dimensional quantum gravity and a phase transition between them which is interpreted as a proliferation of baby universes.
29 pages, 5 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

 

[marcus]

http://arxiv.org/abs/1603.05834
Bouncing Cosmologies: Progress and Problems
Robert Brandenberger, Patrick Peter
(Submitted on 18 Mar 2016)
We review the status of bouncing cosmologies as alternatives to cosmological inflation for providing a description of the very early universe, and a source for the cosmological perturbations which are observed today. We focus on the motivation for considering bouncing cosmologies, the origin of fluctuations in these models, and the challenges which various implementations face.
30 pages, 6 figures
==quote page 14==
It is also possible that bouncing cosmologies can arise from alternative approaches to quantum gravity. Loop quantum gravity is the prime example. In fact, in loop quantum cosmology it can be shown that the cosmological singularity can be avoided at the quantum level, and that bouncing cosmologies are possible [113]. For a specific construction of a “matter bounce” in the context of loop quantum cosmology see [114].
==endquote==
This is more a review of the COMPETITION to LQC in the field of bounce cosmology. Basically a wide ranging review of many different lines of investigation of bounce cosmologies, focussing basically on every kind but LQC. As it happens reference [36] is to an LQC-based paper: the Cai and Wilson-Ewing paper "LCDM bounce scenario" but this is not typical of the paper's focus.
==quote page 6==
In the case of scalar field-driven inflation, the spectrum of cosmological perturbations is not exactly scale- invariant, but it has a slight red tilt. The same is true in the matter bounce scenario: if we add a component to matter which corresponds to the current dark energy (e.g. a small cosmological constant), then a slight red tilt results [36].
==endquote==

general interest:
http://arxiv.org/abs/1603.06008
What Happens in a Measurement?
Steven Weinberg
(Submitted on 18 Mar 2016)
It is assumed that in a measurement the system under study interacts with a macroscopic measuring apparatus, in such a way that the density matrix of the measured system evolves according to the Lindblad equation. Under an assumption of non-decreasing von Neumann entropy, conditions on the operators appearing in this equation are given that are necessary and sufficient for the late-time limit of the density matrix to take the form appropriate for a measurement. Where these conditions are satisfied, the Lindblad equation can be solved explicitly. The probabilities appearing in the late-time limit of this general solution are found to agree with the Born rule, and are independent of the details of the operators in the Lindblad equation.
12 pages

This new paper harks back to a related 2014 paper by Steven Weinberg.
http://arxiv.org/abs/1405.3483
Quantum Mechanics Without State Vectors
Steven Weinberg
(Submitted on 14 May 2014)
It is proposed to give up the description of physical states in terms of ensembles of state vectors with various probabilities, relying instead solely on the density matrix as the description of reality. With this definition of a physical state, even in entangled states nothing that is done in one isolated system can instantaneously effect the physical state of a distant isolated system. This change in the description of physical states opens up a large variety of new ways that the density matrix may transform under various symmetries, different from the unitary transformations of ordinary quantum mechanics. Such new transformation properties have been explored before, but so far only for the symmetry of time translations into the future, treated as a semi-group. Here new transformation properties are studied for general symmetry transformations forming groups, rather than semi-groups. Arguments are given that such symmetries should act on the density matrix as in ordinary quantum mechanics, but loopholes are found for all of these arguments.
28 pages

 

[marcus]

http://arxiv.org/abs/1603.08658
The Atoms Of Space, Gravity and the Cosmological Constant
T. Padmanabhan
(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/1603.07830
The electromagnetic afterglows of gravitational waves as a test for Quantum Gravity
M. A. Abramowicz, T. Bulik, G. F. R. Ellis, K. A. Meissner, M. Wielgus
(Submitted on 25 Mar 2016)
We argue that if particularly powerful electromagnetic afterglows of the gravitational waves bursts will be observed in the future, this could be used as a strong observational support for some suggested quantum alternatives for black holes (e.g., firewalls and gravastars). A universal absence of powerful afterglows should be taken as a suggestive argument against such hypothetical quantum-gravity objects.
3 pages

http://arxiv.org/abs/1603.08076
Polymerization, the Problem of Access to the Saddle Point Approximation, and Thermodynamics
Hugo A. Morales-Técotl, Daniel H. Orozco-Borunda, Saeed Rastgoo
(Submitted on 26 Mar 2016)
The saddle point approximation to the partition functions is an important way of deriving the thermodynamical properties of black holes. However, there are certain black hole models and some mathematically analog mechanical models for which this method can not be applied directly. This is due to the fact that their action evaluated on a classical solution is not finite and its first variation does not vanish for all consistent boundary conditions. These problems can be dealt with by adding a counter-term to the classical action, which is a solution of the corresponding Hamilton-Jacobi equation. In this work however, we seek an alternative solution to this problem via the polymer quantization which is motivated by the loop quantum gravity.
6 pages, Contribution to the proceedings of The Fourteenth Marcel Grossmann Meeting - MG14, to appear in a related book published by World Scientific, Singapore, 2016

http://arxiv.org/abs/1603.08448
Quantum corrections to the Mukhanov-Sasaki equations
Laura Castelló Gomar, Mercedes Martín-Benito, Guillermo A. Mena Marugán
(Submitted on 28 Mar 2016)
Recently, a lot of attention has been paid to the modifications of the power spectrum of primordial fluctuations caused by quantum cosmology effects. The origin of these modifications are corrections to the Mukhanov-Sasaki equations that govern the propagation of the primeval cosmological perturbations. The specific form of these corrections depends on a series of details of the quantization approach and of the prescription followed to implement it. Generally, nonetheless, the complexity of the theoretical quantum formulation is simplified in practice appealing to a semiclassical or effective approximation, in order to perform concrete numerical computations. In this work, we introduce technical tools and design a procedure to deal with these quantum corrections beyond the most direct approximations employed so far in the literature. In particular, by introducing an interaction picture, we extract the quantum dynamics of the homogeneous geometry in absence of scalar field potential and inhomogeneities, dynamics that has been intensively studied and that can be integrated. The rest of our analysis focuses on the interaction evolution, putting forward methods to cope with it. The ultimate aim is to develop treatments that increase our ability to discriminate between the predictions of different quantization proposals for cosmological perturbations.
14 pages, version accepted for publication in Physical Review D

http://arxiv.org/abs/1603.08459
On the fate of Birkhoff's theorem in Shape Dynamics
Flavio Mercati
(Submitted on 28 Mar 2016)
Spherically symmetric, asymptotically flat solutions of Shape Dynamics were previously studied assuming standard falloff conditions for the metric and the momenta. These ensure that the spacetime is asymptotically Minkowski, and that the falloff conditions are Poincar\'e-invariant. These assumptions however are not legitimate in Shape Dynamics, which does not make assumptions on the structure or regularity of spacetime. Analyzing the same problem in full generality, I find that the system is underdetermined, as there is one function of time that is not fixed by any condition and appears to have physical relevance. This quantity can be fixed only by studying more realistic models coupled with matter, and it turns out to be related to the dilatational momentum of the matter surrounding the region under study.
13 pages, 6 figures

possible general interest:
http://arxiv.org/abs/1603.08086
A quantum Fredkin gate
Raj B. Patel, Joseph Ho, Franck Ferreyrol, Timothy C. Ralph, Geoff J. Pryde
(Submitted on 26 Mar 2016)
Key to realising quantum computers is minimising the resources required to build logic gates into useful processing circuits. While the salient features of a quantum computer have been shown in proof-of-principle experiments, difficulties in scaling quantum systems have made more complex operations intractable. This is exemplified in the classical Fredkin (controlled-SWAP) gate for which, despite theoretical proposals, no quantum analogue has been realized. By adding control to the SWAP unitary, we use photonic qubit logic to demonstrate the first quantum Fredkin gate, which promises many applications in quantum information and measurement. We implement example algorithms and generate the highest-fidelity three-photon GHZ states to-date. The technique we use allows one to add a control operation to a black-box unitary, something impossible in the standard circuit model. Our experiment represents the first use of this technique to control a two-qubit operation and paves the way for larger controlled circuits to be realized efficiently.
8 pages, 4 figures, also includes Supplementary Material. Published version available at this http URL

 

[marcus]

http://arxiv.org/abs/1603.09671
The Vertex Expansion in the Consistent Histories Formulation of Spin Foam Loop Quantum Cosmology
David Craig, Parampreet Singh
(Submitted on 31 Mar 2016)
Assignment of consistent quantum probabilities to events in a quantum universe is a fundamental challenge which every quantum cosmology/gravity framework must overcome. In loop quantum cosmology, this issue leads to a fundamental question: What is the probability that the universe undergoes a non-singular bounce? Using the consistent histories formulation, this question was successfully answered recently by the authors for a spatially flat FRW model in the canonical approach. In this manuscript, we obtain a covariant generalization of this result. Our analysis is based on expressing loop quantum cosmology in the spin foam paradigm and using histories defined via volume transitions to compute the amplitudes of transitions obtained using a vertex expansion. We show that the probability for bounce turns out to be unity.
6 pages. To appear in Proceedings of the Fourteenth Marcel Grossman Meeting on General Relativity
possible general interest:
http://vms.fnal.gov/asset/detail?recid=1940339
at the end of Nima Arkani-Hamed's recent colloquium talk at Fermilab he says:
==quote==
String theory killed QFT, then QFT killed string theory back, now QFT is king. We’re in a situation where most people think QFT is king and string theory a derivative thing in some limits.
==endquote==
this was noted by Peter Woit, http://www.math.columbia.edu/~woit/wordpress/?p=8377
he has some more info about the talk and the direction Nima's thoughts are taking.

http://arxiv.org/abs/1603.08955
Theoretical Physics Implications of the Binary Black-Hole Merger GW150914
Nicolas Yunes, Kent Yagi, Frans Pretorius
(Submitted on 29 Mar 2016)
The gravitational-wave observation GW150914 by Advanced LIGO provides the first opportunity to learn about physics in the extreme gravity of coalescing binary black holes. The LIGO/Virgo collaboration has verified that this observation is consistent with General Relativity, constraining the presence of parametric anomalies in the signal. This paper expands this analysis to a larger class of anomalies, highlighting the inferences that can be drawn on non-standard theoretical physics mechanisms that would affect the signal. We find that GW150914 constrains a plethora of mechanisms associated with the generation and propagation of gravitational waves, including the activation of scalar fields, gravitational leakage into large extra dimensions, the variability of Newton's constant, the speed of gravity, a modified dispersion relation, gravitational Lorentz violation and the strong equivalence principle. Unlike other observations that limit these mechanisms, GW150914 is a direct probe of dynamical strong-field gravity and gravitational wave propagation. We also show that GW150914 constrains the properties of exotic compact object alternatives to Kerr black holes. The true potential for GW150914 to both constrain exotic objects and physics beyond General Relativity is limited by the lack of understanding of the dynamical strong field in almost all modified gravity theories. GW150914 thus raises the bar that these theories must pass, both in terms of having a sound theoretical underpinning, and the minimal level to which the corresponding equations of motion must be solved in binary coalescences. We conclude with a discussion of additional inferences that can be drawn from smaller-confidence observations, such as the LVT151012 trigger and electromagnetic counterparts to GW150914, the latter of which would produce dramatic constraints on the speed of gravity and gravitational Lorentz violation.
39 pages, 15 figures, submitted to Phys. Rev. D

 

[marcus]

http://arxiv.org/abs/1604.00847
A new algebraic structure in the standard model of particle physics
Latham Boyle, Shane Farnsworth
(Submitted on 4 Apr 2016)
We introduce a new formulation of non-commutative geometry (NCG): we explain its mathematical advantages and its success in capturing the structure of the standard model of particle physics. The idea, in brief, is to represent A(the algebra of differential forms on some possibly-noncommutative space) on H (the Hilbert space of spinors on that space); and to reinterpret this representation as a simple super-algebra B=A⊕H with even part A and odd part H. B is the fundamental object in our approach: we show that (nearly) all of the basic axioms and assumptions of the traditional ("spectral triple") formulation of NCG are elegantly recovered from the simple requirement that Bshould be a differential graded ∗-algebra (or "∗-DGA"). But this requirement also yields other, new, geometrical constraints. When we apply our formalism to the NCG traditionally used to describe the standard model of particle physics, we find that these new constraints are physically meaningful and phenomenologically correct. This formalism is more restrictive than effective field theory, and so explains more about the observed structure of the standard model, and offers more guidance about physics beyond the standard model.
30 pages, no figures

http://arxiv.org/abs/1604.01799
Fast Radio Bursts---A Brief Review: Some Questions, Fewer Answers
J. I. Katz
(Submitted on 6 Apr 2016)
Fast Radio Bursts are millisecond bursts of radio radiation at frequencies of about 1 GHz, recently discovered in pulsar surveys. They have not yet been definitively identified with any other astronomical object or phenomenon. The bursts are strongly dispersed, indicating passage through a high column density of low density plasma. The most economical interpretation is that this is the interglactic medium, indicating that FRB are at "cosmological" distances with redshifts in the range 0.3--1.3. Their inferred brightness temperatures are as high as 10^37∘K, implying coherent emission by "bunched" charges, as in radio pulsars. I review the astronomical sites, objects and emission processes that have been proposed as the origin of FRB, with particular attention to Soft Gamma Repeaters and giant pulsar pulses.
18 pp., 3 figs
http://inspirehep.net/author/profile/J.Katz.1

 

[atyy]

http://arxiv.org/abs/1604.01798
Comments on "Searching for a continuum limit in CDT quantum gravity"
Joshua H. Cooperman
(Submitted on 6 Apr 2016)
To facilitate the search for a continuum limit of causal dynamical triangulations, Ambjorn, Coumbe, Gizbert-Studnicki, and Jurkiewicz recently reported measurements of the lattice spacing as a function of the bare couplings. Although these authors' methods are technically sound, the conclusions that they draw from their analyses rest crucially on certain unstated assumptions. I elucidate these assumptions, and I argue that our current understanding of causal dynamical triangulations does not entail their justification.

 

[marcus]

atyy, I hope you take over maintaining this bibliography as I become less able to.
thank you, and best wishes, Marcus

(partly age-related partly medical problems, I won't specify, more having to do with energy and coordination than strictly cognitive.)

 

[marcus]

http://arxiv.org/abs/1604.02375
Improved regularization from Quantum Reduced Loop Gravity
Emanuele Alesci, Francesco Cianfrani
(Submitted on 8 Apr 2016)
The choice of the regularization scheme in Loop Quantum Cosmology (LQC) is crucial for the predicted phenomenology. We outline how the improved scheme can be naturally realized in Quantum Reduced Loop Gravity, describing the Universe as an ensemble of microstates labeled by different graphs. The new effective dynamics presents corrections to LQC, which do not significantly affect the bouncing scenario for the most relevant kinds of matter fields in cosmology (w≤1).
5 pages

(From here on, atyy, you should be the one to decide what Loop-and-allied research is included.)

 

[Jimster41]

Hope it's okay to post this here as at least potentially relevant (food for thought) to this thread's raison d'etre. Isn't Wilson's take on self similarity related to fractal spaces? She mentions the subject as an application in her last paragraph.

Marcus, best wishes to you, FWIW. ✌

Fractal Topology Foundations
Helene Porchon
(Submitted on 25 Jan 2012)
In this paper, we introduce the foundation of a fractal topological space constructed via a family of nested topological spaces endowed with subspace topologies, where the number of topological spaces involved in this family is related to the appearance of new structures on it. The greater the number of topological spaces we use, the stronger the subspace topologies we obtain. The fractal manifold model is brought up as an illustration of space that is locally homeomorphic to the fractal topological space.
Comments: 20 pages
Subjects: General Mathematics (math.GM)
MSC classes: 54A05, 54A10, 54D80, 54F65, 54H20
Journal reference: Topology and Its Applications, V159, 3156-3170, 2012
Cite as: arXiv:1201.6223 [math.GM]
(or arXiv:1201.6223v1 [math.GM] for this version)

 

[*now*]

Best wishes and I hope these are ok to add here,

https://arxiv.org/abs/1604.03828
Is classical flat Kasner spacetime flat in quantum gravity?
Parampreet Singh
(Submitted on 13 Apr 2016)
Quantum nature of classical flat Kasner spacetime is studied using effective spacetime description in loop quantum cosmology. We find that even though the spacetime curvature vanishes at the classical level, non-trivial quantum gravitational effects can arise. For the standard loop quantization of Bianchi-I spacetime, which uniquely yields universal bounds on expansion and shear scalars and results in a generic resolution of strong singularities, we find that a flat Kasner metric is not a physical solution of the effective spacetime description, except in a limit. The lack of a flat Kasner metric at the quantum level results from a novel feature of the loop quantum Bianchi-I spacetime: quantum geometry induces non-vanishing spacetime curvature components, making it not Ricci flat even when no matter is present. The non-curvature singularity of the classical flat Kasner spacetime is avoided, and the effective spacetime transits from a flat Kasner spacetime in asymptotic future, to a Minkowski spacetime in asymptotic past. Interestingly, for an alternate loop quantization which does not share some of the fine features of the standard quantization, flat Kasner spacetime with expected classical features exists. In this case, even with non-trivial quantum geometric effects, the spacetime curvature vanishes. These examples show that the character of even a flat classical vacuum spacetime can alter in a fundamental way in quantum gravity and is sensitive to the quantization procedure.
Comments: 14 pages, 2 figures. Prepared for IJMPD special issue on Loop Quantum Cosmology...https://arxiv.org/abs/1604.04183
Dark energy from non-unitarity in quantum theory
Thibaut Josset, Alejandro Perez, Daniel Sudarsky
(Submitted on 14 Apr 2016)
We consider a scheme whereby it is possible to reconcile semi-classical Einstein's equation with the violation of the conservation of the expectation value of energy-momentum that is associated with dynamical reduction theories of the quantum state for matter. The very interesting out-shot of the formulation is the appearance of a nontrivial contribution to an effective cosmological constant (which is not strictly constant). This opens the possibility of using models for dynamical collapse of the wave function to compute its value. Another interesting implication of our analysis is that tiny violations of energy-momentum conservation with negligible local effects can become very important on cosmological scales at late times.
Comments: 8 pages, 2 figures...http://arxiv.org/abs/1604.05195
Quantum gravity kinematics from extended TQFTs
Bianca Dittrich, Marc Geiller
(Submitted on 18 Apr 2016)
We show how extended topological quantum field theories (TQFTs) can be used to obtain a kinematical setup for quantum gravity, i.e. a kinematical Hilbert space together with a representation of the observable algebra including operators of quantum geometry. In particular, we consider the holonomy-flux algebra of (2+1)-dimensional Euclidean loop quantum gravity, and construct a new representation of this algebra that incorporates a positive cosmological constant. The vacuum state underlying our representation is defined by the Turaev-Viro TQFT. We therefore construct here a generalization, or more precisely a quantum deformation at root of unity, of the previously-introduced SU(2) BF representation. The extended Turaev-Viro TQFT provides a description of the excitations on top of the vacuum, which are essential to allow for a representation of the holonomies and fluxes. These excitations agree with the ones induced by massive and spinning particles, and therefore the framework presented here allows automatically for a description of the coupling of such matter to (2+1)-dimensional gravity with a cosmological constant. The new representation presents a number of advantages over the representations which exist so far. It possesses a very useful finiteness property which guarantees the discreteness of spectra for a wide class of quantum (intrinsic and extrinsic) geometrical operators. The notion of basic excitations leads to a fusion basis which offers exciting possibilities for constructing states with interesting global properties. The work presented here showcases how the framework of extended TQFTs can help design new representations and understand the associated notion of basic excitations. This is essential for the construction of the dynamics of quantum gravity, and will enable the study of possible phases of spin foam models and group field theories from a new perspective.
Comments: 76 pages...http://arxiv.org/abs/1604.05629
Projective Limits of State Spaces: Quantum Field Theory without a Vacuum
Suzanne Lanéry
(Submitted on 19 Apr 2016)
Instead of formulating the states of a Quantum Field Theory (QFT) as density matrices over a single large Hilbert space, it has been proposed by Kijowski [Kijowski, 1977] to construct them as consistent families of partial density matrices, the latter being defined over small 'building block' Hilbert spaces. In this picture, each small Hilbert space can be physically interpreted as extracting from the full theory specific degrees of freedom. This allows to reduce the quantization of a classical field theory to the quantization of finite-dimensional sub-systems, thus sidestepping some of the common ambiguities (specifically, the issues revolving around the choice of a 'vacuum state'), while obtaining robust and well-controlled quantum states spaces.
The present letter provides a self-contained introduction to this formalism, detailing its motivations as well as its relations to other approaches to QFT (such as conventional Fock-like Hilbert spaces, path-integral quantization, and the algebraic formulation). At the same time, it can serve as a reading guide to the series of more in-depth articles [arXiv:1411.3589, arXiv:1411.3590, arXiv:1411.3591, arXiv:1510.01926].
Comments: 14 pages, 3 figures...http://arxiv.org/abs/1604.06023
Emergence of a Low Spin Phase in Group Field Theory Condensates
Steffen Gielen
(Submitted on 20 Apr 2016)
Recent years have seen great progress towards deriving quantum cosmology models from the effective dynamics of condensate states in group field theory (GFT), where 'cosmology is the hydrodynamics of quantum gravity'; the classical Friedmann dynamics for homogeneous, isotropic universes, as well as loop quantum cosmology (LQC) corrections to general relativity have been shown to emerge from fundamental quantum gravity. We take one further step towards strengthening the link with LQC and show, in a rather wide class of GFT models for gravity coupled to a free massless scalar field and for generic initial conditions, that GFT condensates dynamically reach a low spin phase of many quanta of geometry, in which all but an exponentially small number of quanta are characterised by a single spin j0 (i.e. by a constant volume per quantum). In one particular simple and natural case, this spin is the lowest one, j0=1/2. The type of quantum state usually assumed in the derivation of LQC is hence derived from the quantum dynamics of GFT, and shown to be generic. As the low spin regime is reached, the dynamics of the total volume follows precisely the classical Friedmann equations. The latter result confirms and extends recent results by Oriti, Sindoni and Wilson-Ewing in the same setting.
Comments:
14 pages, revtex...http://arxiv.org/abs/1604.06537
Relativistic collapse dynamics and black hole information loss
Daniel Bedingham, Sujoy K. Modak, Daniel Sudarsky
(Submitted on 22 Apr 2016)
We study a proposal for the resolution of the black hole information puzzle within the context of modified versions of quantum theory involving spontaneous reduction of the quantum state. The theories of this kind, which were developed in order to address the so called "measurement problem" in quantum theory have, in the past, been framed in a non-relativistic setting and in that form they were previously applied to the black hole information problem. Here, and for the first time, we show in a simple toy model, a treatment of the problem within a fully relativistic setting. We also discuss the issues that the present analysis leaves as open problems to be dealt with in future refinements of the present approach.
Comments:
39 pages, 3 figures…http://arxiv.org/abs/1604.07222
On the volume inside old black holes
Marios Christodoulou, Tommaso De Lorenzo
(Submitted on 25 Apr 2016)
Black holes that have nearly evaporated are often thought of as small objects, due to their tiny exterior area. However, the horizon bounds large spacelike hypersurfaces. A compelling geometric perspective on the evolution of the interior geometry was recently shown to be provided by a generally covariant definition of the volume inside a black hole using maximal surfaces. In this article, we expand on previous results and show that finding the maximal surfaces in an arbitrary spherically symmetric spacetime is equivalent to a 1+1 geodesic problem. We then study the effect of Hawking radiation on the volume by computing the volume of maximal surfaces inside the apparent horizon of an evaporating black hole as a function of time at infinity: while the area is shrinking, the volume of these surfaces grows monotonically with advanced time, up to when the horizon has reached Planckian dimensions. The physical relevance of these results for the information paradox and the remnant scenarios are discussed.
Comments: 9 pages, 5 figures…Tuesday, Apr 5th
Wolfgang Wieland, Perimeter Institute
Title: Covariant loop quantum gravity as a topological theory with defects
PDF of the talk (541K)
Audio [.wav 45MB](Also, responding to a critique by Zeh, http://arxiv.org/abs/1601.02790 to such things as a paper previously mentioned, post #2383 )
https://arxiv.org/abs/1604.03956
Janus Points and Arrows of Time
Julian Barbour, Tim Koslowski, Flavio Mercati
(Submitted on 13 Apr 2016)
We clarify and strengthen our demonstration that arrows of time necessarily arise in unconfined systems. Contrary to a recent claim, this does not require an improbable selection principle.
Comments: 3 pages...

 

[atyy]

https://www.physicsforums.com/threads/marcus-has-passed-on.870662/

http://arxiv.org/abs/1604.06319
Quantum Holonomy Theory and Hilbert Space Representations
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 21 Apr 2016 (v1), last revised 24 Apr 2016 (this version, v2))
We present a new formulation of quantum holonomy theory, which is a candidate for a non-perturbative and background independent theory of quantum gravity coupled to matter and gauge degrees of freedom. The new formulation is based on a Hilbert space representation of the QHD(M) algebra, which is generated by holonomy-diffeomorphisms on a 3-dimensional manifold and by canonical translation operators on the underlying configuration space over which the holonomy-diffeomorphisms form a non-commutative C*-algebra. A proof that the state exist is left for later publications.

http://arxiv.org/abs/1604.06584
Spacetime Quanta? : Real Discrete Spectrum of a Quantum Spacetime Four-Volume Operator in Unimodular Loop Quantum Cosmology
Joseph Bunao
(Submitted on 22 Apr 2016)
This study considers the operator T^ corresponding to the classical spacetime four-volume T of a finite patch of spacetime in the context of Unimodular Loop Quantum Cosmology for the homogeneous and isotropic model with flat spatial sections and without matter sources. Since T is canonically conjugate to the cosmological "constant" Λ, the operator T^ is constructed by solving its canonical commutation relation with Λ^ - the operator corresponding to Λ. %This is done by expanding T^ in terms of Bender-Dunne-like basis operators T^m,n and solving for the expansion coefficients. This conjugacy, along with the action of T^ on definite volume states reducing to T, allows us to interpret that T^ is indeed a quantum spacetime four-volume operator. The eigenstates Φτ are calculated and, considering τ∈R, we find that the Φτ's are normalizable suggesting that the real line R is in the discrete spectrum of T^. The real spacetime four-volume τ is then discrete or quantized.

http://arxiv.org/abs/1604.08199
Kinematical uniqueness of homogeneous isotropic LQC
Jonathan Engle, Maximilian Hanusch
(Submitted on 27 Apr 2016)
In a paper by Ashtekar and Campiglia, invariance under volume preserving residual diffeomorphisms has been used to single out the standard representation of the reduced holonomy-flux algebra in homogeneous loop quantum cosmology (LQC). In this paper, we use invariance under all residual diffeomorphisms to single out the standard kinematical Hilbert space of homogeneous isotropic LQC for both the standard configuration space RBohr, as well as for the Fleischhack one R⊔RBohr. We first determine the scale invariant Radon measures on these spaces, and then show that the Haar measure on RBohr is the only such measure for which the momentum operator is hermitian w.r.t. the corresponding inner product. In particular, the measure is forced to be identically zero on R in the Fleischhack case, so that for both approaches, the standard kinematical LQC-Hilbert space is singled out.

http://arxiv.org/abs/1604.08786
The impact of topology in CDT quantum gravity
Jan Ambjorn, Zbigniew Drogosz, Jakub Gizbert-Studnicki, Andrzej Goerlich, Jerzy Jurkiewicz, Daniel Nemeth
(Submitted on 29 Apr 2016)
We investigate the impact of spatial topology in 3+1 dimensional Causal Dynamical Triangulations (CDT) by performing numerical simulations with toroidal spatial topology instead of the previously used spherical topology. In the case of spherical spatial topology we observed in the so-called phase C an average spatial volume distribution n(t) which after a suitable time redefinition could be identified as the spatial volume distribution of the four-sphere. Imposing toroidal spatial topology we find that the average spatial volume distribution n(t) is constant. By measuring the covariance matrix of spatial volume fluctuations we determine the form of the effective action. The difference compared to the spherical case is that the effective potential has changed such that it allows a constant average n(t). This is what we observe and this is what one would expect from a minisuperspace GR action where only the scale factor is kept as dynamical variable. Although no background geometry is put in by hand, the full quantum theory of CDT is also with toroidal spatial toplogy able to identify a classical background geometry around which there are well defined quantum fluctuations.

http://arxiv.org/abs/1605.00497
Spacetime-noncommutativity regime of Loop Quantum Gravity
Giovanni Amelino-Camelia, Malú Maira da Silva, Michele Ronco, Lorenzo Cesarini, Orchidea Maria Lecian
(Submitted on 2 May 2016)
A recent study by Bojowald and Paily provided a path toward the identification of an effective quantum-spacetime picture of Loop Quantum Gravity, applicable in the "Minkowski regime", the regime where the large-scale (coarse-grained) spacetime metric is flat. A pivotal role in the analysis is played by Loop-Quantum-Gravity-based modifications to the hypersurface deformation algebra, which leave a trace in the Minkowski regime. We here show that the symmetry-algebra results reported by Bojowald and Paily are consistent with a description of spacetime in the Minkowski regime given in terms of the κ-Minkowski noncommutative spacetime, whose relevance for the study of the quantum-gravity problem had already been proposed for independent reasons.

http://arxiv.org/abs/1605.00496
IceCube and GRB neutrinos propagating in quantum spacetime
Giovanni Amelino-Camelia, Leonardo Barcaroli, Giacomo D'Amico, Niccoló Loret, Giacomo Rosati
(Submitted on 2 May 2016)
Two recent publications have reported intriguing analyses, tentatively suggesting that some aspects of IceCube data might be manifestations of quantum-gravity-modified laws of propagation for neutrinos. We here propose a strategy of data analysis which has the advantage of being applicable to several alternative possibilities for the laws of propagation of neutrinos in a quantum spacetime. In all scenarios here of interest one should find a correlation between the energy of an observed neutrino and the difference between the time of observation of that neutrino and the trigger time of a GRB. We select accordingly some GRB-neutrino candidates among IceCube events, and our data analysis finds a rather strong such correlation. This sort of studies naturally lends itself to the introduction of a "false alarm probability", which for our analysis we estimate conservatively to be of 1%. We therefore argue that our findings should motivate a vigorous program of investigation following the strategy here advocated.

http://arxiv.org/abs/1605.00969
Schrödinger-like quantum dynamics in loop quantized black holes
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 3 May 2016)
We show, following a previous quantization of a vacuum spherically symmetric spacetime carried out in Ref. [1], that this setting admits a Schr\"odinger-like picture. More precisely, the technique adopted there for the definition of parametrized Dirac observables (that codify local information of the quantum theory) can be extended in order to accommodate different pictures. In this new picture, the quantum states are parametrized in terms of suitable gauge parameters and the observables constructed out of the kinematical ones on this space of parametrized states.

http://arxiv.org/abs/1605.01383
Loop quantum cosmology of Bianchi IX: Inclusion of inverse triad corrections
Alejandro Corichi, Asieh Karami
(Submitted on 4 May 2016)
We consider the loop quantization of the (diagonal) Bianchi type IX cosmological model. We explore different quantization prescriptions that extend the work of Wilson-Ewing and Singh. In particular, we study two different ways of implementing the so-called inverse triad corrections. We construct the corresponding Hamiltonian constraint operators and show that the singularity is formally resolved. We find the effective equations associated with the different quantization prescriptions, and study the relation with the isotropic k=1 model that, classically, is contained within the Bianchi IX model. We use geometrically defined scalar observables to explore the physical implications of each of these theories. This is the first part in a series of papers analyzing different aspects of the Bianchi IX model, with inverse corrections, within loop quantum cosmology.

 

[atyy]

http://arxiv.org/abs/1605.02241
Asymptotic Safe gravity and non-singular inflationary Big Bang with vacuum birth
Georgios Kofinas, Vasilios Zarikas
(Submitted on 7 May 2016)
General non-singular accelerating cosmological solutions for an initial cosmic period of pure vacuum birth era are derived. This vacuum era is described by a varying cosmological "constant" suggested by the Renormalisation Group flow of Asymptotic Safety scenario near the ultraviolet fixed point. In this scenario, natural exit from inflation to the standard decelerating cosmology occurs when the energy scale lowers and the cosmological "constant" becomes insignificant. In the following period where matter is also present, cosmological solutions with characteristics similar to the vacuum case are generated. Remarkably the set of equations allow for particle production and entropy generation. Alternatively, in the case of non-zero bulk viscosity, entropy production and reheating is found. As for the equations of motion, they modify Einstein equations by adding covariant kinetic terms of the cosmological "constant" which respect the Bianchi identities. An advance of the proposed framework is that it ensures a consistent description of both a quantum vacuum birth of the universe and a subsequent cosmic era in the presence of matter.

http://arxiv.org/abs/1605.02573
Cosmological perturbation theory and quantum gravity
Romeo Brunetti, Klaus Fredenhagen, Thomas-Paul Hack, Nicola Pinamonti, Katarzyna Rejzner
(Submitted on 9 May 2016)
It is shown how cosmological perturbation theory arises from a fully quantized perturbative theory of quantum gravity. Central for the derivation is a non-perturbative concept of gauge-invariant local observables by means of which perturbative invariant expressions of arbitrary order are generated. In particular, in the linearised theory, first order gauge-invariant observables familiar from cosmological perturbation theory are recovered. Explicit expressions of second order quantities are presented as well.

http://arxiv.org/abs/1605.02648
Symmetry Reduced Loop Quantum Gravity: A Bird's Eye View
Abhay Ashtekar
(Submitted on 9 May 2016)
This is a brief overview of the current status of symmetry reduced models in Loop Quantum Gravity. The goal is to provide an introduction to other more specialized and detailed reviews that follow. Since most of this work is motivated by the physics of the very early universe, I will focus primarily on Loop Quantum Cosmology and discuss quantum aspects of black holes only briefly.

 

[atyy]

http://arxiv.org/abs/1605.03632
Quasi-matter bounce and inflation in the light of the CSL model
Gabriel Leon, Gabriel R. Bengochea, Susana J. Landau
(Submitted on 11 May 2016)
The Continuous Spontaneous Localization (CSL) model has been proposed as a possible solution to the quantum measurement problem by modifying the Schr\"{o}dinger equation. In this work, we apply the CSL model to two cosmological models of the early Universe: the matter bounce scenario and slow roll inflation. In particular, we focus on the generation of the classical primordial inhomogeneities and anisotropies that arise from the dynamical evolution, provided by the CSL mechanism, of the quantum state associated to the quantum fields. In each case, we obtained a prediction for the shape and the parameters characterizing the primordial spectra (scalar and tensor), i.e. the amplitude, the spectral index and the tensor-to-scalar ratio. We found that there exist CSL parameter values, allowed by other non-cosmological experiments, for which our predictions for the angular power spectrum of the CMB temperature anisotropy are consistent with the best fit canonical model to the latest data released by the Planck Collaboration.

http://arxiv.org/abs/1605.03942
Spacetime-free Approach to Quantum Theory and Effective Spacetime Structure
Matti Raasakka
(Submitted on 12 May 2016)
Motivated by hints of the effective emergent nature of spacetime structure, we develop a spacetime-free framework for quantum theory. We find that quantum states on an extended observable algebra, the free algebra generated by the observables, may give rise to effective spacetime structures. Accordingly, perturbations of the quantum state (e.g., excitations of the vacuum) lead to perturbations of the induced effective spacetime geometry. We initiate the study of these perturbations, and their relation to gravitational phenomena.

 

[atyy]

http://arxiv.org/abs/1605.04946
Quantum typicality in spin network states of quantum geometry
Fabio Anzà, Goffredo Chirco
(Submitted on 16 May 2016)
In this letter we extend the so-called typicality approach, originally formulated in statistical mechanics contexts, to SU(2) invariant spin network states. Our results do not depend on the physical interpretation of the spin-network, however they are mainly motivated by the fact that spin-network states can describe states of quantum geometry, providing a gauge-invariant basis for the kinematical Hilbert space of several background independent approaches to quantum gravity. The first result is, by itself, the existence of a regime in which we show the emergence of a typical state. We interpret this as the prove that, in that regime there are certain (local) properties of quantum geometry which are "universal". Such set of properties is heralded by the typical state, of which we give the explicit form. This is our second result. In the end, we study some interesting properties of the typical state, proving that the area-law for the entropy of a surface must be satisfied at the local level, up to logarithmic corrections which we are able to bound.

http://arxiv.org/abs/1605.05119
The Quantum Black Hole as a Hydrogen Atom: Microstates Without Strings Attached
Gerard t Hooft
(Submitted on 17 May 2016)
Applying an expansion in spherical harmonics, turns the black hole with its microstates into something about as transparent as the hydrogen atom was in the early days of quantum mechanics. It enables us to present a concise description of the evolution laws of these microstates, linking them to perturbative quantum field theory, in the background of the Schwarzschild metric. Three pieces of insight are obtained: One, we learn how the gravitational back reaction, whose dominant component can be calculated exactly, turns particles entering the hole, into particles leaving it, by exchanging the momentum- and position operators; two, we find out how this effect removes firewalls, both on the future and the past event horizon, and three, we discover that the presence of region II in the Penrose diagram forces a topological twist in the background metric, culminating in antipodal identification. Although a cut-off is required that effectively replaces the transverse coordinates by a lattice, the effect of such a cut-off minimizes when the spherical wave expansion is applied. This expansion then reveals exactly how antipodal identification restores unitarity - for each partial

http://arxiv.org/abs/1605.05311
Axion Experiments to Algebraic Geometry: Testing Quantum Gravity via the Weak Gravity Conjecture
Ben Heidenreich, Matthew Reece, Tom Rudelius
(Submitted on 17 May 2016)
Common features of known quantum gravity theories may hint at the general nature of quantum gravity. The absence of continuous global symmetries is one such feature. This inspired the Weak Gravity Conjecture, which bounds masses of charged particles. We propose the Lattice Weak Gravity Conjecture, which further requires the existence of an infinite tower of particles of all possible charges under both abelian and nonabelian gauge groups and directly implies a cutoff for quantum field theory. It holds in a wide variety of string theory examples and has testable consequences for the real world and for pure mathematics. We sketch some implications of these ideas for models of inflation, for the QCD axion (and LIGO), for conformal field theory, and for algebraic geometry.

 

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http://arxiv.org/abs/1605.05356
Squeezed vacua in loop quantum gravity
Eugenio Bianchi, Jonathan Guglielmon, Lucas Hackl, Nelson Yokomizo
(Submitted on 17 May 2016)
We introduce squeezed vacua in loop quantum gravity, a new overcomplete basis of states that contain prescribable correlations between geometric operators. We study the behavior of long-range correlations and discuss the relevance of these states for the reconstruction of a semiclassical spacetime from loop quantum gravity.

http://arxiv.org/abs/1605.05694
Spontaneous Dimensional Reduction in Quantum Gravity
S. Carlip
(Submitted on 18 May 2016)
Hints from a number of different approaches to quantum gravity point to a phenomenon of "spontaneous dimensional reduction" to two spacetime dimensions near the Planck scale. I examine the physical meaning of the term "dimension" in this context, summarize the evidence for dimensional reduction, and discuss possible physical explanations.

 

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http://arxiv.org/abs/1605.05975
Towards the fundamental spectrum of the Quantum Yang-Mills Theory
Klaus Liegener, Thomas Thiemann
(Submitted on 19 May 2016)
In this work we focus on the quantum Einstein-Yang-Mills sector quantised by the methods of Loop Quantum Gravity (LQG). We point out the improved UV behaviour of the coupled system as compared to pure quantum Yang-Mills theory on a fixed, classical background spacetime as was considered in a seminal work by Kogut and Susskind. Furthermore, we develop a calculational scheme by which the fundamental spectrum of the quantum Yang-Mills Hamiltonian can be computed in principle and by which one can make contact to the Wilsonian renormalization group, possibly purely within the Hamiltonian framework. Finally, we comment on the relation of the fundamental spectrum to that of pure Yang-Mills theory on a (flat) classical spacetime.

http://arxiv.org/abs/1605.05979
On the UV dimensions of Loop Quantum Gravity
Michele Ronco
(Submitted on 19 May 2016)
Planck-scale dynamical dimensional reduction is attracting more and more interest in the quantum- gravity literature since it seems to be a model independent effect. However different studies base their results on different concepts of spacetime dimensionality. Most of them rely on the spectral dimension, others refer to the Hausdorff dimension and, very recently, it has been introduced also the thermal dimension. We here show that all these distinct definitions of dimension give the same outcome in the case of Loop Quantum Gravity. This is achieved by deriving a modified dispersion relation from the hypersurface-deformation algebra with quantum corrections. Moreover we also observe that the number of UV dimensions can be used to constrain the ambiguities in the choice of these Loop-Quantum-Gravity modifications of the Dirac spacetime algebra. In particular, we find that the the simplest polymerization of connections i.e. K→sin(δK)δK, which is much used in the literature, cannot reproduce the shared expectation of dUV=2.

 

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http://arxiv.org/abs/1605.06613
The evaporation spectrum of black holes from a local quantum gravity perspective
Aurélien Barrau
(Submitted on 21 May 2016)
We revisit the hypothesis of a possible line structure in the Hawking evaporation spectrum of black holes, due to non-perturbative quantum gravity effects, even arbitrarily far away from the Planck mass. We show that this naive prediction might in fact hold in the specific context of loop quantum gravity, with a small departure from the ideal case for some low-spin transitions. We also show that the effect is neither washed out by the dynamics of the process, nor by existence of a mass spectrum up to a given width, nor by the secondary component induced by the decay of neutral pions emitted during the time-integrated evaporation.

 

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http://arxiv.org/abs/1605.07510
6D Interpretation of 3D Gravity
Yannick Herfray, Kirill Krasnov, Carlos Scarinci
(Submitted on 24 May 2016)
We show that 3D gravity, in its pure connection formulation, admits a natural 6D interpretation. The 3D field equations for the connection are equivalent to 6D Hitchin equations for the Chern-Simons 3-form in the total space of the principal bundle over the 3-dimensional base. Turning this construction around one gets an explanation of why the pure connection formulation of 3D gravity exists. More generally, we interpret 3D gravity as the dimensional reduction of the 6D Hitchin theory. To this end, we show that any SU(2) invariant closed 3-form in the total space of the principal SU(2) bundle can be parametrised by a connection together with a 2-form field on the base. The dimensional reduction of the 6D Hitchin theory then gives rise to 3D gravity coupled to a topological 2-form field.

 

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http://arxiv.org/abs/1605.04767
What can we really infer from GW 150914?
J. F. Rodriguez, J. A. Rueda, R. Ruffini
(Submitted on 16 May 2016 (v1), last revised 24 May 2016 (this version, v2))
We analyze the event GW 150914 announced by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) as the gravitational-wave emission of a black-hole binary merger. We show that the parameters of the coalescing system and of the newly formed Kerr black-hole can be extracted from basic results of the gravitational-wave emission during the inspiraling and merger phases without sophisticated numerical simulations. Our strikingly accurate estimates are based on textbook formulas describing two different regimes: 1) the binary inspiraling analysis treated in Landau and Lifshitz textbook, and 2) the plunge of a particle into a black-hole, treated in the Rees-Ruffini-Wheeler textbook. It is stressed that in order to infer any astrophysical information on the masses of the system both regimes have to be independently and observationally constrained by LIGO, which does not appear to be the case.

http://arxiv.org/abs/1605.07609
What can we really infer from GW 150914? (II)
J. F. Rodriguez, J. A. Rueda, R. Ruffini
(Submitted on 24 May 2016)
In a recent letter we have outlined some issues on GW 150914, we hereby give additional details. We analyze the event GW 150914 announced by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) as the gravitational-wave emission of a black-hole binary merger. We show that the parameters of the coalescing system and of the newly formed Kerr black-hole can be extracted from basic results of the gravitational-wave emission during the inspiraling and merger phases without sophisticate numerical simulations. Our strikingly accurate estimates are based on textbook formulas describing two different regimes: 1) the binary inspiraling analysis treated in Landau and Lifshitz textbook, 2) the plunge of a particle into a black-hole, treated in the Rees-Ruffini-Wheeler textbook as well as 3) the transition between these two regimes following Detweiler's treatment of a particle infalling with non-zero angular momentum onto a black-hole. It is stressed that in order to infer any astrophysical information on the masses of the system both regimes have to be independently and observationally constrained by LIGO, which does not appear to be the case.

http://arxiv.org/abs/1605.07649
Numerical evidence for a phase transition in 4d spin foam quantum gravity
Benjamin Bahr, Sebastian Steinhaus
(Submitted on 24 May 2016)
Building on recent advances in defining Wilsonian RG flows, and in particular the notion of scales, for background-independent theories, we present a first investigation of the renormalization of the 4d spin foam path integral for quantum gravity, both analytically and numerically. Focussing on a specific truncation of the model using a hypercubic lattice, we compute the RG flow and find strong indications for a phase transition, as well as an interesting interplay between the different observed phases and the (broken) diffeomorphism symmetry of the model. Most notably, it appears that the critical point between the phases, which is a fixed point of the RG flow, is precisely where broken diffeomorphism symmetry is restored, which suggests that it might allow for the definition a continuum limit of the quantum gravity theory.

http://arxiv.org/abs/1605.08015
Quantum Gravity signatures in the Unruh effect
Natalia Alkofer, Giulio D'Odorico, Frank Saueressig, Fleur Versteegen
(Submitted on 25 May 2016)
We study quantum gravity signatures emerging from phenomenologically motivated multiscale models, spectral actions, and Causal Set Theory within the detector approach to the Unruh effect. We show that while the Unruh temperature is unaffected, Lorentz-invariant corrections to the two-point function leave a characteristic fingerprint in the induced emission rate of the accelerated detector. Generically, quantum gravity models exhibiting dynamical dimensional reduction exhibit a suppression of the Unruh rate at high energy while the rate is enhanced in Kaluza-Klein theories with compact extra dimensions. We quantify this behavior by introducing the "Unruh dimension" as the effective spacetime dimension seen by the Unruh effect and show that it is related, though not identical, to the spectral dimension used to characterize spacetime in quantum gravity. We comment on the physical origins of these effects and their relevance for black hole evaporation.

 

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http://arxiv.org/abs/1605.09618
Extrinsic curvature in 2-dimensional Causal Dynamical Triangulation
Lisa Glaser, Thomas P. Sotiriou, Silke Weinfurtner
(Submitted on 31 May 2016)
Causal Dynamical Triangulations (CDT) is a non-perturbative quantisation of general relativity. Ho\v{r}ava-Lifshitz gravity on the other hand modifies general relativity to allow for perturbative quan- tisation. Past work has given rise to the speculation that Ho\v{r}ava-Lifshitz gravity might correspond to the continuum limit of CDT. In this paper we add another piece to this puzzle by applying the CDT quantisation prescription directly to Ho\v{r}ava-Lifshitz gravity in 2 dimensions. We derive the continuum Hamiltonian and we show that it matches exactly the Hamiltonian one derives from canonically quantising the Ho\v{r}ava-Lifshitz action. Unlike the standard CDT case, here the intro- duction of a foliated lattice does not impose further restriction on the configuration space and, as a result, lattice quantisation does not leave any imprint on continuum physics as expected.

 

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http://arxiv.org/abs/1606.00769
Emergent 4D gravity on covariant quantum spaces in the IKKT model
Harold C. Steinacker
(Submitted on 2 Jun 2016)
We study perturbations of the 4-dimensional fuzzy sphere as a background in the IKKT or IIB matrix model. The linearized 4-dimensional Einstein equations are shown to arise from the classical matrix model action, without adding an Einstein-Hilbert term. The excitation modes with lowest spin are identified as gauge fields, metric and connection fields. In addition to the usual gravitational waves, there are also physical "torsion" wave excitations. The quantum structure of the geometry encodes a twisted bundle of self-dual 2-forms, which leads to a covariant 4-dimensional noncommutative geometry. The formalism of string states is used to compute one-loop corrections to the effective action. This leads to a mass term for the gravitons which is significant for ##S^{4}#, but argued to be small in the Minkowski case.

 

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http://arxiv.org/abs/1601.04744
Local subsystems in gauge theory and gravity
William Donnelly, Laurent Freidel
(Submitted on 18 Jan 2016)
We consider the problem of defining localized subsystems in gauge theory and gravity. Such systems are associated to spacelike hypersurfaces with boundaries and provide the natural setting for studying entanglement entropy of regions of space. We present a general formalism to associate a gauge-invariant classical phase space to a spatial slice with boundary by introducing new degrees of freedom on the boundary. In Yang-Mills theory the new degrees of freedom are a choice of gauge on the boundary, transformations of which are generated by the normal component of the nonabelian electric field. In general relativity the new degrees of freedom are the location of a codimension-2 surface and a choice of conformal normal frame. These degrees of freedom transform under a group of surface symmetries, consisting of diffeomorphisms of the codimension-2 boundary, and position-dependent linear deformations of its normal plane. We find the observables which generate these symmetries, consisting of the conformal normal metric and curvature of the normal connection. We discuss the implications for the problem of defining entanglement entropy in quantum gravity. Our work suggests that the Bekenstein-Hawking entropy may arise from the different ways of gluing together two partial Cauchy surfaces at a cross-section of the horizon. [/PLAIN]

http://arxiv.org/abs/1606.01829
Quantum Spaces are Modular
Laurent Freidel, Robert G. Leigh, Djordje Minic
(Submitted on 6 Jun 2016)
At present, our notion of space is a classical concept. Taking the point of view that quantum theory is more fundamental than classical physics, and that space should be given a purely quantum definition, we revisit the notion of Euclidean space from the point of view of quantum mechanics. Since space appears in physics in the form of labels on relativistic fields or Schrodinger wave functionals, we propose to define Euclidean quantum space as a choice of polarization for the Heisenberg algebra of quantum theory. We show, following Mackey, that generically, such polarizations contain a fundamental length scale and that contrary to what is implied by the Schrodinger polarization, they possesses topologically distinct spectra. These are the modular spaces. We show that they naturally come equipped with additional geometrical structures usually encountered in the context of string theory or generalized geometry. Moreover, we show how modular space reconciles the presence of a fundamental scale with translation and rotation invariance. We also discuss how the usual classical notion of space comes out as a form of thermodynamical limit of modular space while the Schrodinger space is a singular limit.

http://arxiv.org/abs/1606.01429
Brief review on black hole loop quantization
Javier Olmedo
(Submitted on 4 Jun 2016)
Here we present a review about the quantization of spherically symmetric spacetimes adopting loop quantum gravity techniques. Several models that have been studied so far share similar properties: the resolution of the classical singularity, and some of them an intrinsic discretization of the geometry. We also explain the extension to Reissner-Nordstr\"om black holes. Besides, we review how quantum test fields on these quantum geometries allow us to study phenomena like the Casimir effect or Hawking radiation. Finally, we briefly describe a recent proposal that incorporates spherically symmetric matter, discussing its relevance for the understanding of black hole evolution.

 

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http://arxiv.org/abs/1606.02826
On Spinfoams Near a Classical Curvature Singularity
Muxin Han, Mingyi Zhang
(Submitted on 9 Jun 2016)
We apply the technique of spinfoam to study the space-time which, classically, contains a curvature singularity. We derive from the full covariant Loop Quantum Gravity (LQG) that the region near curvature singularity has to be of strong quantum gravity effect. We show that the spinfoam configuration describing the near-singularity region has to be of small spins j, in order that its contribution to the full spinfoam amplitude is nontrivial. The spinfoams in low and high curvature regions of the space-time may be viewed as in two different {phases} of covariant LQG. There should be a phase transition as the space-time described by spinfoam becomes more and more curved. A candidate of order parameter is proposed for understanding the phase transition. Moreover, we also analyze the spin-spin correlation function of spinfoam, and show the correlation is of long-range in the low curvature phase. This work is a first step toward understanding the physics of black hole and early universe from the full covariant LQG theory.

 

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http://arxiv.org/abs/1606.04044
Nontrivial UV behavior of rank-4 tensor field models for quantum gravity
Joseph Ben Geloun, Tim A. Koslowski
(Submitted on 13 Jun 2016)
We investigate the universality classes of rank-4 colored bipartite U(1) tensor field models near the Gaussian fixed point with the functional renormalization group. In a truncation that contains all power counting relevant and marginal operators, we find a one-dimensional UV attractor that is connected with the Gaussian fixed point. Hence this is first evidence that the model could be asymptotically safe due to a mechanism similar to the one found in the Grosse-Wulkenhaar model, whose UV behavior near the Gaussian fixed point is also described by one-dimensional attractor that contains the Gaussian fixed point. However, the cancellation mechanism that is responsible for the simultaneous vanishing of the beta functions is new to tensor models, i.e. it does not occur in vector or matrix models.

 

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http://arxiv.org/abs/1606.04932
Geodesic completeness and the lack of strong singularities in loop quantum Kantowski-Sachs spacetime
Sahil Saini, Parampreet Singh
(Submitted on 15 Jun 2016)
Resolution of singularities in the Kantowski-Sachs model due to non-perturbative quantum gravity effects is investigated. Using the effective spacetime description for the improved dynamics version of loop quantum Kantowski-Sachs spacetimes, we show that even though expansion and shear scalars are universally bounded, there can exist events where curvature invariants can diverge. However, such events occur can only for very exotic equations of states when pressure or derivatives of energy density with respect to triads become infinite at a finite energy density. In all other cases curvature invariants are proved to remain finite for any evolution in finite proper time. We find the novel result that all strong singularities are resolved for arbitrary matter. Weak singularities pertaining to above potential curvature divergence events can exist. The effective spacetime is found to be geodesically complete for particle and null geodesics in finite time evolution. Our results add to a growing evidence for generic resolution of strong singularities in loop quantum cosmology by generalizing earlier results on isotropic and Bianchi-I spacetimes.

 

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http://arxiv.org/abs/1606.07924
Inflationary spectra from near Ω-deformed space-time transition point in Loop Quantum Cosmology
Long Chen, Jian-Yang Zhu
(Submitted on 25 Jun 2016)
Anomaly-free perturbations of loop quantum cosmology with holonomy corrections reveal a Ω -deformed space-time structure, Ω:=1−2ρ/ρc, where Ω<0 means an Euclidean-like space and Ω>0 means a Lorentz-like space. It would be reasonable to give the initial value at the space-time transition point, ρ=ρc/2, but we find it is impossible to define a Minkowski-like vacuum even for large k-modes at that time. However if we loose the condition and give the initial value near after Ω=0, e.g. Ω≃0.2, the vacuum state can be well defined and furthermore the slow roll approximation also works well in that region. Both scalar and tensor spectra are considered in the framework of loop quantum cosmology with holonomy corrections. We find that if the energy density is not too small compared with ρc/2 when the considered k-mode crossing the horizon, effective theory can give a much smaller scalar power spectrum than classical theory and the spectrum of tensor perturbations could blue shift. But when compared with the observations, since the energy densities when the modes crossed the horizon were quite smaller than ρc, the results we get are the same with previous work in the literature and the classical inflation theory.

 

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http://arxiv.org/abs/1607.00910
Big-Bounce cosmology in the presence of Immirzi field
Flavio Bombacigno, Francesco Cianfrani, Giovanni Montani
(Submitted on 4 Jul 2016)
The Immirzi parameter is promoted to be a scalar field and the Hamiltonian analysis of the corresponding dynamical system is performed in the presence of gravity. We identified some SU(2) connections, generalizing Ashtekar-Barbero variables, and we rewrite the constraints in terms of them, setting the classical formulation suitable for loop quantization. Then, we consider the reduced system obtained when restricting to a flat isotropic cosmological model. By mimicking loop quantization via an effective semiclassical treatment, we outline how quantum effects are able to tame the initial singularity both in synchronous time and when the Immirzi field is taken as a relational time.

http://arxiv.org/abs/1607.00955
Thermal Time and Kepler's Second Law
Deepak Vaid
(Submitted on 4 Jul 2016)
It is shown that a recent result regarding the average rate of evolution of a dynamical system at equilibrium in combination with the quantization of geometric areas coming from LQG, implies the validity of Kepler's Second Law of planetary motion.

 

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http://arxiv.org/abs/1607.02460
Relationalism Evolves the Universe Through the Big Bang
Tim A Koslowski, Flavio Mercati, David Sloan
(Submitted on 8 Jul 2016)
We investigate the singularities of homogeneous cosmologies from the point of view of relational (and physically relevant) degrees of freedom of the gravitational field. These do not depend on absolute units of length and duration - thus they do not include the volume and extrinsic curvature. We find that the fully relational dynamical system remains well posed for all physical times, even at the point that would be described as the big bang when evolving present day data backwards in time.This result is achieved in two steps: (1) for solutions which are gravity-dominated near the singularity, we show that any extended physical clock (whose readings only depend on the relational degrees of freedom) will undergo an infinite number of ticks before reaching the big bang. The singularity is therefore pushed into the infinite physical past of any physical clock. (2) for solutions where a stiff matter component (e.g. a massless scalar field) dominates at the singularity, we show that the relational degrees of freedom reach the point that is described as the big bang in the dimensionful description of General Relativity (GR) at a finite physical time and evolve smoothly through it, because they are decoupled from the unphysical dimensional degrees of freedom, which are the only ones that turn singular. Describing the relational dynamics with the dimensionful language of GR makes the relational dynamics appear as two singular GR solutions connected at the hypersurface of the singularity in such a way that the relational degrees of freedom evolve continuously while the orientation of the spatial frame is inverted. Our analysis applies to all GR solutions which conform to the BKL conjecture, and is therefore relevant for a large class of cosmological models with inhomogeneity.

 

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http://arxiv.org/abs/1605.05268
Realistic Observable in Background-Free Quantum Gravity: the Planck-Star Tunnelling-Time
Marios Christodoulou, Carlo Rovelli, Simone Speziale, Ilya Vilensky
(Submitted on 17 May 2016)
A gravitationally collapsed object can bounce-out from its horizon via a tunnelling process that violates the classical equations in a finite region. Since tunnelling is a non-perturbative phenomenon, it cannot be described in terms of quantum fluctuations around a classical solution and a background-free formulation of quantum gravity is needed to analyze it. Here we use Loop Quantum Gravity to compute the amplitude for this process, in a first approximation. The amplitude determines the tunnelling time as a function of the mass. This is the key information to evaluate the astrophysical relevance of this process. The calculation offers a template and a concrete example of how a background-free quantum theory of gravity can be used to compute a realistical observable quantity.

http://arxiv.org/abs/1606.04444
Statistical mechanics of covariant systems with multi-fingered time
Goffredo Chirco, Thibaut Josset
(Submitted on 14 Jun 2016)
Recently, in [Class. Quantum Grav. 33 (2016) 045005], the authors proposed a new approach extending the framework of statistical mechanics to reparametrization-invariant systems with no additional gauges. In this work, the approach is generalized to systems defined by more than one Hamiltonian constraints (multi-fingered time). We show how well known features as the Ehrenfest- Tolman effect and the J\"uttner distribution for the relativistic gas can be consistently recovered from a covariant approach in the multi-fingered framework. Eventually, the crucial role played by the interaction in the definition of a global notion of equilibrium is discussed.

http://arxiv.org/abs/1607.00364
Quantum Gravity Effects around Sagittarius A*
Hal M. Haggard, Carlo Rovelli
(Submitted on 1 Jul 2016 (v1), last revised 5 Jul 2016 (this version, v2))
Recent VLBI observations have resolved Sagittarius A* at horizon scales. The Event Horizon Telescope is expected to provide increasingly good images of the region around the Schwarzschild radius rS of Sgr A* soon. A number of authors have recently pointed out the possibility that non-perturbative quantum gravitational phenomena could affect the space surrounding a black hole. Here we point out that the existence of a region around 76rS where these effects should be maximal.

http://arxiv.org/abs/1607.04121
Universe's memory and spontaneous coherence in loop quantum cosmology
Tomasz Pawłowski
(Submitted on 14 Jul 2016)
The quantum bounce a priori connects several (semi)classical epochs of Universe evolution, however determining if and how well the semiclassicality is preserved in this transition is highly nontrivial. We review the present state of knowledge in that regards in the isotropic sector of loop quantum cosmology. This knowledge is next extended by studies of an isotropic universe admitting positive cosmological constant (featuring an infinite chain of large Universe epochs). It is also shown, that such universe always admits a semiclassical epoch thanks to spontaneous spontaneous coherence, provided it is semiclassical in certain constant of motion playing the role of energy.

Thanks to @fuzzyfelt for the first 3 suggestions! To everyone: If I've missed any good stuff, please add it directly here on marcus's bibliography on loop quantum gravity and other non-string QG approaches - of course, if strings and LQG converge that would be great too, at least for Smoliners, not sure about Rovellians ;) As I understand, marcus welcomed everyone to contribute. The main rule to observe is that this thread should a bibliography, so if there is a paper we would like to discuss, a new thread dedicated to it should be started (in addition to adding to this bibliopgraphy).