Poll: First quarter 2016 MIP (most important QG papers)

[marcus]

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

http://arxiv.org/abs/1603.08658
The Atoms Of Space, Gravity and the Cosmological Constant
T. Padmanabhan

http://arxiv.org/abs/1603.07931
Short-scale Emergence of Classical Geometry, in Euclidean Loop Quantum Gravity
Vincent Bayle, François Collet, Carlo Rovelli

http://arxiv.org/abs/1603.04170
Effective cosmological constant induced by stochastic fluctuations of Newton's constant
Marco de Cesare, Fedele Lizzi, Mairi Sakellariadou

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

http://arxiv.org/abs/1603.01561
Volume Entropy
Valerio Astuti, Marios Christodoulou, Carlo Rovelli

http://arxiv.org/abs/1603.01128
Diffeomorphism invariant cosmological symmetry in full quantum gravity
Christopher Beetle, Jonathan S. Engle, Matthew E. Hogan, Phillip Mendonca

http://arxiv.org/abs/1603.01117
The Fock Space of Loopy Spin Networks for Quantum Gravity
Christoph Charles, Etera R. Livine

http://arxiv.org/abs/1602.08271
Bouncing cosmologies from quantum gravity condensates
Daniele Oriti, Lorenzo Sindoni, Edward Wilson-Ewing

http://arxiv.org/abs/1602.08104
Quantum cosmology from group field theory condensates: a review
Steffen Gielen, Lorenzo Sindoni

http://arxiv.org/abs/1602.05881
Emergent Friedmann dynamics with a quantum bounce from quantum gravity condensates
Daniele Oriti, Lorenzo Sindoni, Edward Wilson-Ewing

http://arxiv.org/abs/1602.04452
Conceptual issues in loop quantum cosmology
Aurélien Barrau, Boris Bolliet

http://arxiv.org/abs/1602.03237
Can chaos be observed in quantum gravity?
Bianca Dittrich, Philipp A. Hoehn, Tim A. Koslowski, Mike I. Nelson

http://arxiv.org/abs/1602.02295
On Unification of Gravity and Gauge Interactions
Ali H. Chamseddine, Viatcheslav Mukhanov

http://arxiv.org/abs/1602.01861
Twisted geometries, twistors and conformal transformations
Miklos Långvik, Simone Speziale

http://arxiv.org/abs/1601.05707
Projective quantum states for Loop Quantum Gravity coupled to tensor fields
Andrzej Okolow

http://arxiv.org/abs/1601.05688
Quantum self-gravitating collapsing matter in a quantum geometry
Miguel Campiglia, Rodolfo Gambini, Javier Olmedo, Jorge Pullin

http://arxiv.org/abs/1601.01716
Primordial power spectra for scalar perturbations in loop quantum cosmology
Daniel Martín de Blas, Javier Olmedo

 

[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 (I_c) accessible to an eternal observer finite and hence is directly related to it. This relation allows us to determine the numerical value of Λ from the quantum structure of spacetime.
Invited Review; 32 pages; 3 figures

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

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

http://arxiv.org/abs/1603.01764
Accelerated expansion of the Universe without an inflaton and resolution of the initial singularity from GFT condensates
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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

[marcus]

reminder for the next poll (just missed including it in this one):
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