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  1. Jul 1, 2015 #81


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    Thursday parallel sessions (9 July) Loops 2015

    Parallel Session: Foundations of Covariant LQG (Spin Foams)
    Thursday, 14:30 - 16:00, Room: Seminar Room 5
    Chair: Benjamin Bahr
    The area-law sector of loop quantum gravity
    14:30 - 15:00, Eugenio Bianchi (Penn State, USA)
    In this talk I present a concrete realization of the conjecture that semiclassical states in quantum gravity satisfy an area law. In loop quantum gravity the entanglement entropy of a random spin-network state scales linearly with the volume of a region of space. I describe a class of spin-network states that are fully characterized by the expectation value and the 2-point correlation function of geometric observables. Such states are semiclassical, have non-vanishing graviton-graviton correlations, and satisfy the area law. The framework extends previous results about the black hole horizon entropy to all regions of space.
    Area-law and its consequences for LQG
    15:00 - 15:30, Antonino Marciano (Fudan University, China)
    We will first comment on the importance of quantum entanglement entropy in characterizing semiclassical properties of quantum theories. We will then introduce techniques mutuated from quantum many-body theory in order to calculate entanglement in LQG, taking into account gauge invariance. In preparation for Mingyi Zhang seminar, we will present a one-link state that satisfies the area-law, and is a viable building block for constructing new semi-classical states in LQG. We will finally comment on the eigenvalues distribution for these newly found states.
    Derivation of the area law in LQG
    15:30 - 16:00, Mingyi ZHANG (Fudan University, China)
    I will present a preliminary result on computing the entanglement entropy in the context of loop quantum gravity. The gauge invariance is concerned, and the area law can be obtained in the semi-classical limit and low energy limit of spin foam model. This talk follows Prof. Antonino Marciano's talk.

    Parallel Session: Foundations of Canonical LQG
    Thursday, 14:30 - 16:00, Room: Lecture Hall
    Chair: Norbert Bodendorfer
    Coherent States and Intertwiners
    14:30 - 15:00, John Schliemann (University of Regensburg, Germany)
    Coherent states provide a natural connection of quantum systems to their classical limit and are employed in various fields of physics. Here we derive general systematic expansions, with respect to quantum parameters, of expectation values of products of arbitrary operators within both oscillator coherent states and SU(2) coherent states. In particular, we generally prove that the energy fluctuations of an arbitrary Hamiltonian are in leading order entirely due to the time dependence of the classical variables. We apply our general results to the calculation of expectation values within coherent intertwiners in spin networks. For expectation values of rotationally invariant operators (and these are the only ones contributing) one finds here a subleading correction to the classical limit given in terms of universal (i.e. operator-independent) expansion coefficients which contain only geometric information about the network node and show an interesting formal analogy to classical inertia tensors of rigid bodies.
    A stability criterion for coherent states
    15:00 - 15:30, Antonia Zipfel (University of Warsaw, Poland)
    Coherent states play an important role in the semiclassical analysis of a given quantum theory. In order to also study the semiclassical features of the dynamics it is necessary to construct a system of states that does not loose its semiclassical properties during the evolution. I will introduce and discuss a stability criterion for complexifier coherent states, which are used in LQG. It turns out that already for quantum mechanical system the introduced criterion is quite restrictive. However, there is a way to circumvent these restrictions by introducing action-angle coordinates. These mechanisms are potentially also applicable in symmetry reduced models such as LQC.
    Typicality and local thermalisation in spin networks
    15:30 - 16:00, Goffredo Chirco (CPT - AMU, France)
    We investigate the notion of quantum typicality in spin networks, by applying the general approach proposed by Popescu, Short and Winter in 2006, in the context of LQG. In particular, we focus on a basic spin network building block consisting in a N-valent SU(2) intertwiner with fixed total spin, the equivalent of a space of convex polyhedra with N face and fixed total boundary area at the classical level. On the fixed-area subspace of the intertwiner, we study the reduced state associated to a small region of the boundary surface.
    By exploiting the "concentration of measure phenomenon", we show how the distribution for such a state is highly peaked around the "thermal state" for almost all pure states of the global intertwiner. We obtain a Gibbs state written in terms of the area preserving generator of the U(N) group, the area having the role played by the energy in the standard canonical picture. Local thermalisation arises as the result of the degree of correlations between local state and environment.
    We study the temperature of the local surface patch state and we confront the specific structure of correlations of our result with the previous derivations of a single link thermal state present in the literature.

    Parallel Session: Isolated Horizons and Quantum Black Holes
    Thursday, 14:30 - 16:00, Room: Seminar Room 4
    Chair: Jonathan Engle
    BF Theory Explanation of Isolated Horizon Entropy in Loop Quantum Gravity
    14:30 - 15:00, Yongge Ma (Beijing Normal University, China)
    We consider an isolated horizon as the inner boundary of a four-dimensional asymptotically flat spacetime region. Due to the isolated horizon condition, it turns out that in certain gauge the boundary degrees of freedom can be described by a SO(1,1) BF theory with sources. This
    provides a new alternative approach to the usual one using Chern-Simons theory to study the black hole entropy. To count the microscopical degrees of freedom with the boundary BF theory, the entropy of the isolated horizon can also be calculated in the framework of loop quantum gravity. This approach is applicable in arbitrary dimensions of spacetime.
    Neutron Star Masses : A Black Hole Entropic Perspective
    15:00 - 15:30, Parthasarathi Majumdar (Ramakrishna Mission Vivekananda Univ, India)
    This talk is motivated by the need to understand the observed absence of neutron stars heavier than 2 solar masses. Extant approaches to this involve an adaptation of Chandrasekhar’s ideas of a limiting mass for white dwarf stars, based on hydrostatic equilibrium, to situations where general relativity can no longer be ignored. We survey the complications that this might introduce to the already difficult problem of determining the equation of state of this superdense system as an essential part of hydrostatic equilibrium. We then describe an alternative perspective, still under construction, to this problem, based on certain results from the analysis of (quantum) black hole entropy and thermal stability. These results, derived from a non perturbative, background-independent formulation of canonical quantum gravity (Loop Quantum Gravity), will be discussed in some detail. Some attempts to incorporate entanglement entropy ideas into the construction are also to be discussed. The aim here is a formulation of the issue of neutron star instability in terms of the stability and growth of a nascent quantum horizon deep inside a collapsing neutron star. If this perspective works, this might be a first indirect evidence of quantum gravity playing an important role in the gravitational collapse of neutron stars.
    Dilatonic black holes in LQG
    15:30 - 16:00, Saeed Rastgoo (Universidad Autonoma Metropolitana, Mexico)
    We review some of the more recent results about the dilatonic black holes in LQG. These include the singularity resolution in the CGHS model, derivation of a true local Hamiltonian in this generic class, to be used in reduced phase space methods, and the problem of access to the saddle point approximation in these systems, and how and if, one can overcome this issue using polymerization.
    We will also briefly comment on some of the works in progress.

    Parallel Session: Group Field Theory and Tensor Models
    Thursday, 14:30 - 16:00, Room: Seminar Room 3
    Chair: John Barrett
    FRG approach for tensorial group field theory
    14:30 - 15:00, Joseph Bengeloun (Max-Planck Institute, AEI, Germany)
    The functional renormalization group approach for tensorial group field theory models will be introduced. Focusing on concrete examples, the peculiarities of the FRG method when applied to tensorial models will be then emphasized.
    FRG methods for a tensorial group field theory on R
    15:00 - 15:30, Riccardo Martini (Alma Mater Studiorum, University of Bologna, Italy)
    I will present an application of Functional Renormalization Group methods to a rank 3 Tensorial Group Field Theory model endowed with phi^4 interaction and defined over three copies of R. This is the first renormalization analysis for TGFT's on non-compact groups. The model has non-Gaussian fixed points which suggest the existence of phase transitions in the IR regime.
    Conservation laws in Group Field Theory
    15:30 - 16:00, Alexander Kegeles (Albert Einstein Institute, Germany)
    Group Field Theories are quantum field theories which perturbative description is strongly connected to spin foam models in Loop Quantum Gravity. On the other hand it was recently shown that already classical equations of motion of Group Field Theory can encode cosmological data. From this perspective a natural question arise: what are the fundamental implications to spin foams and/or cosmology of the general frame work of Group Field Theory. In this talk I am addressing the consequences of symmetries in Group Field Theory and show that for such theories the usual conservation equations have to be modified, which can lead to interesting results in applications to spin foams and cosmology.

    Parallel Session: Foundations of Covariant LQG (Spin Foams)
    Thursday, 16:30 - 18:00, Room: Seminar Room 5
    Chair: Benjamin Bahr
    On the observable algebra of local covariant effective field theories
    16:30 - 17:00, Jose A. Zapata (Centro de Ciencias Matematicas UNAM, Mexico)
    In previous work we developed a geometrical formalism for local covariant effective field theories: we developed a discrete version of the first jet bundle and a multisymplectic geometry in it. Here we describe aspects of the algebraic counterpart of the formalism. The approach follows the spirit of general boundary field theory.
    Regge Quantum Gravity
    17:00 - 17:30, Aleksandar Mikovic (Lusofona University, Portugal)
    If we assume that the short-distance structure of the spacetime is a picewise linear manifold corresponding to a triangulation of a smooth manifold, then we can obtain a finite state-sum model which is a generalization of the Regge path integral for GR. By using the effective action approach we show how to compute the quantum corrections and how the diffeomorphism invariance appears. We also show how the model can solve the problem of the cosmological constant.
    Spincube model of QG and connection to CDT
    17:30 - 18:00, Marko Vojinovic (GFM Lisbon, Portugal)
    Spincube model is a categorical generalization of spinfoam models, based on the Poincare 2-group. Its main advantage is the automatic and explicit presence of the tetrads as basic degrees of freedom, which simplifies the model and allows for seamless coupling of all matter fields. An additional unexpected property is that, if the simplicity constraint is enforced strongly, the model contains the Causal Dynamical Triangulations state-sum as its special case. This relationship between SC model and CDT is a novel result, and will be explained in detail, along with other interesting properties of the SC approach to QG.

    Parallel Session: Foundations of Canonical LQG
    Thursday, 16:30 - 18:00, Room: Lecture Hall
    Chair: Norbert Bodendorfer
    Correlations and entanglement in loop quantum gravity
    16:30 - 16:55, Alexandre Feller (ENS LYON, France)
    In a background independent context, correlations are essential to understand the emergence of geometry, locality and a metric to describe gravity. Typical kinematical states have trivial correlations but physical states are expected to have non trival ones, reproducing for instance the graviton propagator.
    Another landmark of quantum gravity is black hole entropy and one of its interpretation as entanglement entropy.
    What I wish to present here is a first investigation of those topics in loop quantum gravity with a quantum information and condensed matter perspective by constructing Ising and toric code types states and analysing their properties and structure, in particular which constraints they solve for the first and the area law for entanglement entropy for the second. Such a point of view has the potential to shed some lights on the coarse graining of spin networks and the dynamic of loop gravity.
    Entangled spin network states
    16:55 - 17:20, Lucas Hackl (IGC, Pennsylvania State University, USA)
    We present a class of entangled spin network states that are labeled by symplectic matrices and are generated via unitary transformations of the Ashtekar-Lewandowski vacuum. We show that the entanglement entropy between a subgraph and its complement can be calculated analytically as a function of the symplectic matrix labeling the state. In particular, we identify states whose entanglement entropy satisfies an area law. Projecting these states onto the kinematical Hilbert space provides a new candidate for highly entangled semi-classical states.
    [based on work with Eugenio Bianchi & Nelson Yokomizo]
    Tagged and Loopy Spin Networks
    17:20 - 17:40, Christoph Charles (ENS Lyon, France)
    In the context of the coarse-graining of loop quantum gravity, we introduce tagged and loopy spin networks, which generalize the standard spin network states to account explicitly for non-trivial curvature and torsion. We illustrate their use in some simple dynamical toy-models.
    Statistical geometry: classical and quantum picture
    17:40 - 18:00, Seramika Ariwahjoedi (CPT Marseille, France)
    We study the statistical aspect of a system of quanta of space in both classical discrete and canonical loop quantum gravity picture, by proposing a coarse-graining and averaging procedure. This procedure could provide a link between theory described by finite degrees of freedom with a theory described by infinite degrees of freedom (its 'continuum limit'), both in the classical discrete and quantum picture.

    Parallel Session: Quantum Gravity Phenomenology
    Thursday, 16:30 - 18:00, Room: Seminar Room 3
    Chair: Aurelien Barrau
    Time, vacuum energy, and the cosmological constant
    16:30 - 17:00, Viqar Husain (University of New Brunswick, Canada)
    I review the assumptions (and their limitations) underlying the standard formulation of the cosmological constant problem. I then describe a link between the cosmological constant problem and the problem of time in quantum gravity. This arises by examining the relationship between the cosmological constant and vacuum energy in non-perturbative formulations of quantum gravity.
    Dimensional reduction and scale-invariant primordial fluctuations
    17:00 - 17:30, Giulia Gubitosi (Imperial College, UK)
    I discuss the properties of vacuum fluctuations in general theories with modified dispersion relations which represent dimensional running at high energies, showing that in all theories which run to 2 dimensions in the ultraviolet the vacuum fluctuations, in this regime, are scale-invariant. This is true in flat space but also for “inside the horizon” modes in an expanding universe. I discuss the conditions upon the gravity theory for this scale-invariance to be preserved as the modes are frozen-in outside the horizon.
    Planck scale modified dispersion relations in FRW spacetime
    17:30 - 18:00, Giacomo Rosati (University of Wroclaw, Poland)
    In recent years Planck scale modifications of the dispersion relation have been attracting increasing interest also from the viewpoint of possible applications in astrophysics and cosmology, where spacetime curvature cannot be neglected. Nonetheless the interplay between Planck scale effects and spacetime curvature is still poorly understood, particularly in cases where curvature is not constant. These challanges have been so far postponed by relying on an ansatz, first introduced by Jacob and Piran. We here propose a general strategy of analysis of the effects of modifications of dispersion relation in FRW spacetimes, applicable both to classical cases where the relativistic equivalence of frames is spoiled (preferred frame scenarios) and to the alternative possibility of theories that are fully relativistic but with relativistic laws deformed so that the modified dispersion relation is observer independent.

    Parallel Session: Homogeneous and Hybrid Loop Quantum Cosmology (LQC)
    Thursday, 16:30 - 18:00, Room: Seminar Room 4
    Chair: Mercedes Martin-Benito
    Loop quantization of the linearly polarized Gowdy T3 model with local rotational symmetry
    16:30 - 17:00, Daniel Martín de Blas (Universidad Nacional Andrés Bello, Chile)
    In this talk we will present a full quantization of the vacuum linearly polarized Gowdy three torus model with local rotational symmetry using the techniques of loop quantum gravity. First, we consider a redefinition of the constraints such that the new scalar constraint Poisson-commutes with itself. Then we apply of the canonical quantization program of loop quantum gravity. We identify the exact solutions of the constraints and the physical observables, and we construct the physical Hilbert space. It is remarkable that quantum spacetimes are free of singularities. Besides, new quantum observables (not present at the classical level) appear codifying partially the discretization of the geometry.
    Approximate solutions of the hybrid quantum Gowdy model with FRW dynamics
    17:00 - 17:30, Beatriz Elizaga de Navascués (Instituto de Estructura de la Materia - CSIC, Spain)
    We construct approximate solutions of the hybrid quantum Gowdy cosmology with three-torus topology, linear polarization, and local rotational symmetry, in the presence of a massless scalar field. More specifically, we determine some families of states for which the complicated inhomogeneous and anisotropic Hamiltonian constraint operator of the Gowdy model is approximated by a much simpler one. Our quantum states follow the dynamics governed by this simpler constraint, while being at the same time also approximate solutions of the full Gowdy model. This is so thanks to the quantum correlations that the considered states present between the isotropic and anisotropic sectors of the model. Remarkably, this simpler constraint can be regarded as that of a flat Friedmann-Robertson-Walker universe filled with different kinds of perfect fluids and geometrically corrected by homogeneous and isotropic curvature-like terms. Therefore, our quantum states, which are intrinsically inhomogeneous, admit approximate homogeneous and isotropic effective descriptions similar to those considered in modified theories of gravity.
    Extension of LQC to arbitrary spacetime dimensions
    17:30 - 18:00, Xiangdong Zhang (FAU Erlangen, Germany)
    Loop quantum cosmology(LQC) is the symmetric model of loop quantum gravity. In this talk, we generalize the structure of loop quantum cosmology to the theories with arbitrary spacetime dimensions. The isotropic and homogenous cosmological model in n+1 dimensions is quantized by the loop quantization method. Interestingly, we find that the underlying quantum theories are divided into two qualitatively different sectors according to spacetime dimensions. The effective Hamiltonian and modified dynamical equations of n+1 dimensional LQC are obtained. Moreover, our results indicate that the classical big bang singularity is resolved in arbitrary spacetime dimensions by a quantum bounce. We also briefly discuss the similarities and differences between the n+1 dimensional model and the 3+1 dimensional one.
    Last edited: Jul 1, 2015
  2. Jul 1, 2015 #82


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    Friday parallel sessions (10 July) Loops 2015
    Parallel Session: Foundations of Covariant LQG (Spin Foams)
    Friday, 14:30 - 16:00, Room: Lecture Hall
    Chair: Wolfgang Wieland
    Spin foam sums for polyhedral atoms
    14:30 - 15:00, Johannes Thürigen (AEI Potsdam, Germany)
    While spin foam models are mostly restricted to (locally) simplicial complexes, compatibility with canonical LQG demands a generalization to polyhedral building blocks.
    I will present proposals for polyhedral spin foam sums and discuss the implementation of simplicity constraints on polyhedral atoms in these models.
    Symmetric spin-foam model with the Lorentzian EPRL vertex
    15:00 - 15:30, Marcin Kisielowski (FAU Erlangen, Germany)
    In the Euclidean signature we introduced a version of the EPRL model that is symmetric with respect to certain moves including an edge splitting move and a face splitting move. In this talk we present a symmetric version of the Lorentzian EPRL model. The face splitting move is particularly problematic. In this move there appear vertex graphs that are not 3-edge connected and in the Lorentzian theory the standard regularization is not sufficient to make them finite. We propose a regularization of the vertex amplitudes that solves this problem.
    15:30 - 16:00, Marco Finocchiaro (Albert Einstein Institute, Potsdam-Golm, Germany)
    Spin foam models are constructed by imposing, directly at the quantum level, the required geometricity constraints turning topological quantum simplicial BF theory into quantum simplicial gravity. Therefore they depend on the specific prescription adopted for implementing the constraints, on additional choices in the construction, e.g. the prescription for gluing simplicial cells and on the choice of the quantization map, that is the choice of the operator ordering ambiguities. In my talk I will discuss these aspects and their implications by comparing the resulting spin foam amplitudes (more in detail the corresponding fusion coeffcients) for different models proposed in the literature (and new ones obtained as variations of the known constructions). The analysis will be performed by rewriting the spin foam amplitudes in terms of different sets of variables (spins, group elements, flux variables). Moreover I will introduce a general definition of embedding map applicable to all these models and discuss its properties. Other relevant issues as the choice of the boundary Hilbert space and the requirement of proper behaviour of the amplitudes under composition will be also considered.

    Parallel Session: Foundations of Canonical LQG
    Friday, 14:30 - 16:00, Room: Seminar Room 5
    Chair: Christian Fleischhack
    14:30 - 15:00, Jędrzej Świeżewski (Faculty of Physics, University of Warsaw, Poland)
    During the talk I will present some of the results concerning the radial gauge and employing it to define spherically symmetric loop quantum gravity.
    Spinor approach to 3D Lorentzian loop quantum gravity
    15:00 - 15:30, Giuseppe Sellaroli (University of Waterloo, Canada)
    We consider the generalization of the “spinor approach” to the Lorentzian case, in the context of 3d loop quantum gravity with cosmological constant Λ = 0. The key technical tool that allows this generalization is the recoupling theory between unitary infinite-dimensional representations and non-unitary finite-dimensional ones, obtained in the process of generalizing the Wigner–Eckart theorem to SU(1,1). We use SU(1,1) tensor operators to build observables and a solvable quantum Hamiltonian constraint, analogue of the one introduced by V. Bonzom and his collaborators in the Euclidean case (with both Λ = 0 and Λ = 0). We show that the Lorentzian Ponzano–Regge amplitude is solution of the quantum Hamiltonian constraint by recovering the Biedenharn–Elliott relation (generalized to the case where unitary and non-unitary SU(1, 1) representations are coupled to each other).
    3D gravity with cosmological constant from Yang-Mills theory with dynamical dual
    15:30 - 16:00, Mariano Celada (Cinvestav, Mexico)
    Since 3D general relativity with cosmological constant can be identified with a topological sector of Yang-Mills theory where the Yang-Mills field is the spin connection and the spacetime metric is constructed from the Yang-Mills strength, we start from Yang-Mills equations where the Hodge dual operator is replaced by a dynamical entity, and show that 3D gravity with cosmological constant arises.

    Parallel Session: Isolated Horizons and Quantum Black Holes
    Friday, 14:30 - 16:00, Room: Seminar Room 3
    Chair: Eugenio Bianchi
    Anyonic statistics and large horizon diffeomorphisms for LQG black holes
    14:30 - 15:00, Andreas Pithis (King's College London, UK)
    We investigate the role played by large diffeomorphisms of quantum isolated horizons for the statistics of LQG black holes by means of their relation to the braid group. To this aim the symmetries of Chern-Simons theory are recapitulated with particular regard to the aforementioned type of diffeomorphisms. For the punctured spherical horizon, these are elements of the mapping class group of S^2, which is almost isomorphic to a corresponding braid group on this particular manifold. The mutual exchange of quantum entities in two dimensions is achieved by the braid group, rendering the statistics anyonic. With this we argue that the quantum isolated horizon model of LQG based on SU(2)_k-Chern-
    Simons theory exhibits non-Abelian anyonic statistics. In this way a connection to the theory behind the fractional quantum Hall effect and that of topological quantum computation is established, where non-Abelian anyons play a significant role. (The talk is based on Phys. Rev. D 91, 064053.)
    How big is a black hole?
    15:00 - 15:30, Marios Christodoulou (CPT, Aix-Marseille University, France)
    It is possible to give a coordinate independent definition of the 3D volume of the interior of a black hole. The definition extends that of the volume inside a 2-sphere in flat-spacetime by making use of the existence of well defined geometrical features of such spacetimes; maximal surfaces. Using this result, we can answer the physical question of the amount of spatial volume inside the horizon of a collapsed star. The result is surprising: the volume is very large and increases monotonically with the time since the collapse. The internal volume is still large even if the black hole horizon is shrank to Planckian size by the back reaction of the Hawking radiation. These results have implications for the non-singular black holes and remnant scenarios, for the information paradox, the notion of gravitational entropy and the plausibility of holography.
    A heuristic energy quantization of equilibrium black hole horizons
    15:30 - 16:00, Abhishek Majhi (Institute of Mathematical Sciences, India)
    We propose a heuristic model Hamiltonian operator for equilibrium black hole horizons, modeled as quantum isolated horizons in loop quantum gravity framework, followed by a heuristic top-down analysis. We explain how the motivations behind adopting the heuristic top-down approach to the concerned problem germinate from the bulk-boundary interplay manifested by the underlying quantum theory. The reasons behind the {\it necessity} of the requirement of the commutation of the Hamiltonian operator and the area operator of a quantum isolated horizon are explained. The analysis is carried out for generic quantum isolated horizons admitting {\it arbitrary} number of topological defects (a.k.a. punctures), explaining the observations of both local and asymptotic observers. In addition to these, we highlight a crucial quantum mechanical aspect of our findings -- even though we require that the Hamiltonian operator$(\hat H_S)$ commutes with the area operator$(\hat A_S)$ and the classical energy of the horizon is a function of its classical area, we {\it do not} require $\hat H_S\equiv \hat H_S(\hat A_S)$; indeed we can have a very generic form of the Hamiltonian operator such that $\hat H_S\not\equiv \hat H_S(\hat A_S)$, which still respects the commutativity and yields the classical energy as a function of its classical area. We find it logical enough to speculate that this particular aspect of our work might prove to be of inherent merit by finding its applicability for larger class of quantum mechanical systems having Hilbert space structures similar to that of a quantum isolated horizon and admitting smeared operators.

    Parallel Session: Homogeneous and Hybrid Loop Quantum Cosmology (LQC)
    Friday, 14:30 - 16:00, Room: Seminar Room 4
    Chair: Parampreet Singh
    Quantum Reduced Loop Gravity: theory and perspectives
    14:30 - 15:00, Francesco Cianfrani (University of Wroclaw, Poland)
    I will give an overview of Quantum Reduced Loop Gravity and of its implications in Quantum Cosmology.
    Quantum scalar field coupled to reduced loop gravity
    15:00 - 15:30, Jakub Bilski (Fudan University, China)
    Working within the framework of Quantum Reduced Loop Gravity (QRLG), I will show the quantization of the Hamiltonian constraint for the Einsteinian theory of gravity minimally coupled to a scalar field. This procedure relies on the method proposed by T. Thiemann (QSD V) and developed in the collaboration with E. Alesci and C. Cianfrani. The scalar field is described in terms of point-holonomies, located at the nodes of the graph. I will show how to regularize the action of the scalar field Hamiltonian in the diffeomorphism invariant Hilbert space of QRLG, and present the resulting action of the Hamiltonian constraints operator, which contains only analytic coefficients. I will then explain how in the large j limit, the Hamiltonian eigenvalues approach the classical Hamiltonian at the leading order. The next-to-the-leading order corrections are purely quantum, and can be discussed in conjunction with their possible phenomenological implications.
    A Dynamical Cosmological Sector in Loop Quantum Gravity
    15:30 - 16:00, Phillip Mendonca (Florida Atlantic University, USA)
    We introduce diffeomorphism and gauge-covariant functions on phase space, show they are first-class, and then quantize them, whereupon they become a set of symmetry constraints for LQG. We then investigate using them as the foundation for an embedding scheme. We show how the embedding works in cosmological models, and how we can then gain information about one model from the other.

    Parallel Session: Foundations of Covariant LQG (Spin Foams)
    Friday, 16:30 - 18:00, Room: Lecture Hall
    Chair: Wolfgang Wieland
    Graviton propagator of the "proper" vertex
    16:30 - 17:00, Atousa Chaharsough Shirazi (Florida Atlantic University, USA)
    The “proper” spin-foam vertex amplitude was obtained from the EPRL vertex by projecting out all but a single gravitational sector, in order to enable correct semi-classical behavior. We calculated the gravitational two-point function predicted by the proper spin-foam vertex to lowest order in the vertex expansion. We find the same answer as in the EPRL case, so that the theory is consistent with the predictions of linearized gravity in the regime of small curvature.
    Proper Vertex asymptotics and Graviton Propagator
    17:00 - 17:30, Ilya Vilensky (Florida Atlantic University, US)
    The EPRL vertex amplitude provides a consistent formulation of dynamics of loop quantum gravity states. However, its semi-classical limit does not exactly match classical Regge calculus. We present a modification of the EPRL amplitude - the proper vertex amplitude - that has the correct semi-classical limit. We use the proper vertex amplitude to calculate graviton propagator and find that in semi-classical limit it agrees with the result from Lorentzian Regge calculus.
    A new way to impose simplicity constraints
    17:30 - 18:00, Andrzej Banburski (Perimeter Institute, Canada)
    I will present a new way to impose simplicity constraints in a Riemannian holomorphic spin foam model. Rather than imposing the constraints on the boundary spin network, one can impose the constraints on the spin foam propagator. Surprisingly, to first order in the asymptotic expansion (the one giving Regge calculus) the two ways of imposing constraints are equivalent, with differences appearing at higher order. This means that one can get the same asymptotic behavior with wildly different full quantum theory.

    Parallel Session: Foundations of Canonical LQG
    Friday, 16:30 - 18:00, Room: Seminar Room 5
    Chair: Christian Fleischhack
    Quantum Cauchy Surfaces in Canonical Quantum Gravity
    16:30 - 16:55, Chun-Yen Lin (Physics Dep., National Central University, Taiwan)
    For certain Dirac theories of canonical quantum gravity, I propose an exact and quantum notion of Cauchy surfaces. Similar to its classical counterpart, a quantum Cauchy surface is defined in conjunction with the Dirac observables representing the complete quantum fluctuations localized at the surface. Each of the Dirac theories (most importantly canonical loop quantum gravity) has a timeless physical Hilbert space $\mathbb H$, which is the image of a rigging map $\hat P$ acting on a kinematic Hilbert space $\mathbb K$ to extract the kernel of the quantum scalar constraints. The proposed quantum Cauchy surfaces would be given by the inverse maps of $\hat P$, each of which describes $\mathbb H$ with a specific subspace of $\mathbb K$ that has only partial quantum fluctuations in $\mathbb K$, and with the rest of the fluctuations being frozen. Each quantum Cauchy surface leads to the Dirac observables representing the specified fluctuations, measured at the moment labeled by the frozen fields' values. Among these observables, certain exact and complete conjugate pairs may define a Schr\"odinger theory. The goal of the proposal is thus to re-introduce the Schr\"odinger pictures in a timeless Dirac theory, as viewed from the different observer frames. The explicit application to a simple model will be demonstrated.
    A gauge connection formulation for general relativity
    16:55 - 17:20, Diego Gonzalez (Cinvestav, Mexico)
    We propose an action principle for GR with either a vanishing or a nonvanishing cosmological constant, which depends functionally on a gauge connection and a complex-valued 4-form. In particular, with a vanishing cosmological constant, it leads to a new action principle that turns out to be a modification of the CDJ action principle. While, in the case of a nonvanishing cosmological constant, it leads to the Krasnov's action principle.
    Discrete WKBJ method for the volume operator
    17:20 - 17:40, Dimitri Marinelli (University of Waterloo, Canada)
    In this talk, I will present Braun's WKBJ method for discrete three terms recurrence relations. This method can be applied to LQG geometric operators with discrete spectrum to disclose their classical dynamics. In particular, I will focus on the volume operator. Also, I will present a set of orthogonal polynomials associated with it.
    Entropic Motion and Quantum Gravity
    17:40 - 18:00, Manuel Garcia-Islas (National Autonomous University of Mexico, Mexico)
    Entropic forces result from an increase of the entropy of a thermodynamical physical system. It has been proposed that gravity is such a phenomenon and many articles have appeared on the literature concerning this problem. We propose an idea which may reproduce an entropic force in loop quantum gravity.

    Parallel Session: Group Field Theory and Tensor Models
    Friday, 16:30 - 18:00, Room: Seminar Room 4
    Chair: Aristide Baratin
    A new representation for loop quantum gravity
    16:30 - 17:00, Marc Geiller (ICG Penn State, USA)
    One of the key results of loop quantum gravity is the existence of a diffeomorphism-invariant representation of the holonomy-flux algebra of observables, and the construction of a continuum inductive limit Hilbert space. After briefly recalling the properties and the role played by the so-called Ashtekar-Lewandowski vacuum state in this construction, I will describe how a dual formulation can be obtained by trading the roles of the holonomies and the fluxes. This dual representation is built upon a vacuum based on states of topological BF theory, and therefore cast canonical loop quantum gravity in a formulation closer to the spirit of spin foam models. Furthermore, this new vacuum allows for the construction of a continuum limit Hilbert space carrying a (unitarily inequivalent) representation of the holonomy-flux algebra, and gives a new perspective on the derivation of quantum geometry and on the extraction of physics from the theory.
    Continuum limit of canonical tensor model and general relativity
    17:00 - 17:30, Naoki Sasakura (Yukawa Institute for Theoretical Physics, Kyoto University, Japan)
    Canonical tensor model is a rank-three tensor model formulated as a totally constrained system in Hamilton formalism. The constraints satisfy a non-linear first-class constraint algebra, which has a similar structure as the Dirac algebra in the ADM formalism of GR. We discuss a formal limit, in which the constraint algebra of the canonical tensor model exactly reproduces the Dirac algebra of ADM. The limit is described by an almost diagonal form of a rank-three tensor, and the lowest of a moment expansion of off-diagonal components can be identified with the metric tensor field of GR. We also give a preliminary discussion on a possible dynamical origin of the formal limit.
    Random Tensor Networks and Canonical Tensor Model
    17:30 - 18:00, Yuki Sato (University of the Witwatersrand, South Africa)
    Canonical tensor model (CTM) is a model of dynamical fuzzy space based on the canonical formalism aiming for a model of quantum gravity; a canonically conjugate pair of rank-three tensors with cardinality N are dynamical variables characterising a fuzzy space.
    We have proposed a different and relatively simpler model which is supposed to be related to CTM, i.e.,
    a statistical system on random networks or random tensor networks (RTN), in which rank-three tensors appear as vertices of random networks.
    In this talk I will show that the boundary of the Hamiltonian vector flow of CTM coincides with the phase transition line of RTN at least for N=2. This coincidence happens because the renormalisation-group flow of RTN is given by the Hamiltonian vector flow of CTM, which is true for arbitrary N. My talk is based on the works with Naoki Sasakura (YITP, Japan).

    Parallel Session: Quantum Gravity Phenomenology
    Friday, 16:30 - 18:00, Room: Seminar Room 3
    Chair: Hal Haggard
    Spinning Particles in the Worldline Formalism
    16:30 - 17:00, Trevor Rempel (Perimeter Institute, Canada)
    In this talk I present a wordline formulation of the classical spinning particle. After establishing a suitable parameterization of the phase space action I consider the three point interaction vertex and derive a necessary and sufficient condition for consistency. A novel reinterpretation of the model as a classical system of two coupled particles is then presented. I conclude with some preliminary results arising when the phase space is reparameterized in terms of spinors.
    Comparison of primordial tensor power spectra from the deformed algebra and dressed metric approaches in loop quantum cosmology
    17:00 - 17:30, Boris Bolliet (LPSC Grenoble, France)
    Loop quantum cosmology tries to capture the main ideas of loop quantum gravity and to apply them to the Universe as a whole. Two main approaches within this framework have been considered to date for the study of cosmological perturbations: the dressed metric approach and the deformed algebra approach. They both have advantages and drawbacks. In this talk, we will compare their predictions. In particular, we present the computation of the associated primordial tensor power spectra. We will see – numerically and analytically – that the large scale behavior is similar for both approaches and compatible with the usual prediction of general relativity. The small scale behavior is, the other way round, drastically different. Most importantly, in a range of wavenumbers explicitly calculated, both approaches do agree on predictions that, in addition, differ from standard general relativity and do not depend on unknown parameters. These features of the power spectrum at intermediate scales might constitute a universal loop quantum cosmology prediction that can hopefully lead to observational tests and constraints.
    The Volume Operator in Loop Quantum Gravity: The Spectrum in the High Valence Limit
    17:30 - 18:00, Felix Winterhalter (FAU Erlangen, Germany)
    Last edited: Jul 1, 2015
  3. Aug 17, 2015 #83


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    The International Society for General Relativity and Gravitation (ISGRG) holds triennial conferences. GR20 was in 2013, in Warsaw. There were over 800 participants.

    The next one, GR21, will be in New York. July 10-15 2016, at Columbia University (upper West side of Manhattan, nice location)
    I suspect we won't know very much until after January, when the organizers start taking proposals/abstracts for talks.
  4. Aug 25, 2015 #84


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    A small conference on the black hole puzzles is taking place this week at Stockholm KTH --24-29 August. Participants include a number of prominent physicists. I've highlighted those giving talks (16) and the conclusions wrap-up at the end.
    • Stephen Hawking, University of Cambridge
    • Jim Bardeen, University of Washington, Seattle
    • Philip Candelas, University of Oxford
    • Steve Christensen, UNIX Packages LLC
    • Ulf Danielsson, Uppsala University
    • Paul Davies, Arizona State University
    • Fay Dowker, Imperial College London
    • Michael Duff, Imperial College London
    • Larry Ford, Tufts University
    • Katie Freese, Nordita
    • Steve Fulling, Texas A&M University
    • Jim Hartle, University of California, Santa Barbara
    • Gerard t’Hooft, Utrecht University
    • Gary Horowitz, University of California, Santa Barbara
    • Werner Israel, University of Victoria
    • Claus Kiefer, University of Cologne
    • Jorma Louko, University of Nottingham
    • Laura Mersini-Houghton, University of North Carolina
    • Charles Misner, University of Maryland
    • Emil Mottola, Florida Atlantic University
    • Jack Ng, University of North Carolina at Chapel Hill
    • Jerome Novak, French National Centre for Science
    • Don Page, University of Alberta
    • Leonard Parker, University of Wisconsin, Milwaukee
    • Malcolm Perry, Cambridge University
    • Joe Polchinski, University of California, Santa Barbara
    • Carlo Rovelli, Aix-Marseille University
    • Philippe Spindel, University of Mons
    • Kelly Stelle, Imperial College London
    • Andy Strominger, Harvard University
    • Bo Sundborg, Stockholm University
    • Gerard 't Hooft, University of Utrecht
    • Paulo Vargas Moniz, Universidade da Beira Interior
    • Francesca Vidotto, Radboud University Nijmegen
    • Bob Wald, University of Chicago
    - See more at: http://global.unc.edu/events/hawkingradiation/#sthash.ZTJ0XAXx.dpuf
    Last edited: Aug 25, 2015
  5. Aug 25, 2015 #85


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    The program:
    MONDAY, 24 August
    10.45 – 11.00 Welcome Remarks P. Gudmundsson, KTH Chancellor;
    A. Söderbergh Widding, Stockholm University Chancellor;
    C. L. Folt, UNC Chancellor;
    K. Freese, Nordita Director;
    L. Mersini-Houghton, UNC, Organizer of the Conference
    11.00 – 11.45 "Backreaction and Conformal Symmetry" G. 't Hooft

    14.15 – 15.00 "Backreaction of Hawking Radiation and Singularities" L. Mersini-Houghton

    16.00 – 16.45 "Physical interpretation of the semi-classical energy-momentum tensor in a Schwarzschild background" J. Bardeen

    19.00 http://www.nordita.org/hawkingradiation/program/lecture/index.php [Broken], at Stockholm Waterfront Congress Centre

    TUESDAY, 25 August
    11.00 – 11.45 "The Information Paradox" S. Hawking

    14.15 - 15.00 "Black Hole Memory" M. J. Perry

    16.00 – 16.40 "Black to White Hole Tunnelling: Before or After Hawking Radiation?" C. Rovelli

    16.40 – 17.10 "A new Quantum Black Hole Phenomenology" F. Vidotto

    WEDNESDAY, 26 August
    11.00 – 11.45 "Black Holes as Open Quantum Systems" C. Kiefer

    14.15 - 15.00 "Particle Creation from vacuum in gravitational expansion and collapse" L. Parker

    16.00 - 16.45 "Gravitational Condensate Stars or What's the (Quantum) Matter with Black Holes?" E. Mottola

    19.30 Conference Banquet and Concert at the Gold Foyer of the Royal Opera

    THURSDAY, 27 August
    11.00 – 11.45 "Did the chicken survive the firewall" J. Louko

    14.15 – 15.00 "Gravity = (Yang-Mills)^2" M. Duff

    16.00 – 16.45 "Black holes and other solutions in higher derivative gravity" K. Stelle

    FRIDAY, 28 August
    11.00 - 11.45 "Quantum Damping or Decoherence: Lessons from Molecules, Neutrinos, and Quantum Logic Devices" L. Stodolsky

    14.15 – 15.00 "Puzzle Pieces: Do any fit?" Ch. Misner

    16.00 – 16.45 "The Generalised Second Law and the unity of physics" F. Dowker

    SATURDAY, 29 August
    11.00 – 12.00 Group Discussion: Summary of Results and Open Questions
    12.00 – 12.30 Status Report S. Hawking [TBC]
    12.30 – 13.00 Conclusions P. Davies
    Last edited by a moderator: May 7, 2017
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