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  1. Jan 18, 2015 #76


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    First Erlangen Workshop on Cosmology and Quantum Gravity
    Quantum gravity effects are expected to play a prominent role in the early universe as for example in the context of primordial gravitational waves or are expected to manifest in finger prints in the cosmic microwave background. After the expected publications of the new Planck data by the end of 2014 this workshop aims to bring together researchers from quantum gravity as well as from cosmology with links to current experiments, which are interesting also from the quantum gravity perspective.

    The workshop will be held at the FAU-Erlangen-Nürnberg from 9th to 13th of February and participants will be experts equally distributed from both research areas. The workshop will be comprised of introductory talks of quantum gravity and comsomology with a particular focus on topics interesting for both communities as well as specialized talks and ample time for discussion, to stimulate interaction between the participants.

    Invited Speakers
    • Niayesh Afshordi (University of Waterloo
    • Ivan Agullo (Louisiana State University)
    • Abhay Ashtekar (Pennsylvania State University)
    • Tessa Baker (Oxford University)
    • Camille Francois Bonvin (CERN)
    • Latham Boyle (Perimeter Institute)
    • Edmund Copeland (University of Nottingham)
    • Raphael Flauger (Princeton University)
    • Ghazal Geshnizjani (University of Waterloo
    • Steffen Gielen (Imperial College London)
    • Jean-Luc Lehners (Albert Einstein Institute Golm)
    • Lucas Lombriser (University of Edinburgh)
    • Oriol Pojolas (IFAE and Universitat Autonoma de Barcelona)
    • Tehseen Rug (LMU Munich)
    • Angnis Schmidt-May (ETH)
    • Thomas Thiemann (University of Erlangen-Nürnberg)
    • Edward Wilson-Ewing (Albert Einstein Institute Golm)
    Local organizing Committee
    • Kristina Giesel (University of Erlangen-Nürnberg)
    • Stefan Hofmann (LMU Munich)
    • Jochen Weller (LMU Munich)
    The workshop will start on Monday 9th of February at 3 pm and will end on Friday 13th of February at 1 pm. The detailed program will be available on the homepage soon.
    My comment: invited speakers with Loop involvement are Agullo, Ashtekar, Gielen, Thiemann, Wilson-Ewing. Likewise one of the organizers, Giesel.
    It makes sense for some Loop researchers to become increasingly interested in cosmology because that is where QG has the greatest contact with observation.

    AFAICS recovering classical GR was accomplished in the CHRR paper (Chirco, Haggard, Riello, Rovelli) showing that GR can simply be the equation of state of QG degrees of freedom, after Jacobson 1995.
    So one has at least one valid QG theory---the next important order of business is to test theory by confronting it with early universe observations.
    There can be several versions of Loop QG all of which recover GR as EoS. One wants to test them to see which best fits observations of ancient light.

    A representative paper along these lines is the "LambdaCDM bounce" paper by Cai and Wilson-Ewing. [to get it simply google LambdaCDM bounce]. Notice that Wilson-Ewing is one of the speakers at the workshop. The C&W-E paper puts the standard cosmic model LCDM together with the loop cosmology bounce and gets constraints on some observable numbers.
    Last edited: Jan 18, 2015
  2. Jan 18, 2015 #77


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    Spring 2015 ILQGS schedule http://relativity.phys.lsu.edu/ilqgs/
    Code (Text):

    DATE         Seminar Title                         Speaker              Institution
    J27  Rainbows from quantum gravity               Andrea Dapor    University of Warszaw
    F10  No firewalls in quantum gravity              Alejandro Perez    CNRS Marseille
    F24  Cosmology with group field theory condensates Steffen Gielen    Imperial College
    M10    TBA                                   Muxin Han    Florida Atlantic University
    M24  Information loss                            Matteo Smerlak    Perimeter Institute
    A7  Explicit computation of the evaporation of a quantum BH    Jorge Pullin    LSU
    A21 Separability and quantum mechanics               Fernando Barbero    CSIC, Madrid
    My5 Generalized GFT condensates and Cosmology       Lorenzo Sindoni    AEI
    Last edited: Feb 23, 2015
  3. Jun 13, 2015 #78


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    Strings 2015 will be held in Bangalore 22-26 June, so in a week or so.
    These talk titles have been posted:

    Timothy Adamo
    Field theory as a string theory

    Mohsen Alishahiha
    Holographic entanglement entropy for singular surfaces in hyperscaling violating geometry

    Benjamin Basso
    Structure Constants and Integrable Bootstrap in N=4 SYM Theory

    Sayantani Bhattacharyya
    A membrane paradigm at large D

    Jacob Bourjaily
    The On-Shell Analytic S-Matrix

    Alejandra Castro
    Wilson lines in AdS3/CFT2

    Bartek Czech
    Integral geometry: from tensor networks to holography

    Justin David
    Higher spin corrections to entanglement entropy

    Roberto Emparan
    Black holes in the 1/D expansion

    Matthias Gaberdiel
    Strings from a Higher Spin Perspective

    Abhijit Gadde
    Aspects of 2d (0,2) theories

    Simone Giombi
    Generalized F-theorem and the epsilon expansion

    Daniel Harlow
    Bulk Locality and Quantum Error Correction in AdS/CFT

    Yasuyuki Hatsuda
    Large N Non-Perturbative Effects in ABJM Theory

    Jonathan Heckman
    Geometry of 6D SCFTs

    Kentaro Hori
    The Grade Restriction Rule

    Min-Xin Huang
    Topological String on elliptic CY 3-folds and the ring of Jacobi forms

    Ken Intriligator
    Anomalies, RG flows, and the a-theorem in six-dimensional (1,0) theories

    Romuald Janik
    String Field Theory vertex from integrability

    Robert de Mello Koch
    Anomalous Dimensions of Heavy Operators from Magnon Energies

    Charlotte Kristjansen
    One-point Functions in dCFT and Integrability

    Dieter Luest
    Large N Graviton Scattering and Black Hole Production

    Juan Maldacena
    Some speculations on the black hole interior

    Gautam Mandal
    Thermalization in 2D field theories and holography

    Sameer Murthy
    Functional determinants and index theorems for exact quantum black hole entropy

    Vasilis Niarchos
    Exact correlation functions in 4d N=2 superconformal field theories

    João Penedones
    Mellin amplitudes: the scattering amplitudes of AdS/CFT

    Eric Perlmutter
    Holographic Duals of Conformal Blocks

    Ramadevi Pichai
    Knot polynomials, homological invariants and topological strings

    David Poland
    Conformal Bootstrap Review

    Fernando Quevedo
    Low energy SUSY and String Compactifications: The Last Attempts?

    Loganayagam Ramalingam
    A topological gauge theory for the entropy current

    Mukund Rangamani
    Brownian branes, emergent symmetries, and hydrodynamics

    Balt van Rees
    Bootstrapping the six-dimensional (2,0) theories

    Daniel Roberts
    The butterfly effect in spin chains and 2d CFT

    Sudipta Sarkar
    Holographic entanglement entropy and second law for black holes

    John Schwarz
    AdS5 X S5 Superspace Geometry

    Ashoke Sen
    Surviving in a metastable de Sitter space-time

    Masaki Shigemori
    Habemus Superstratum

    Eva M Silverstein
    String spreading and S-matrix data

    Aninda Sinha
    Some analytic results from conformal bootstrap

    Marcus Spradlin
    Cluster Algebras and Scattering Amplitudes

    Douglas Stanford
    A bound on chaos

    Andrew Strominger
    Memory, Soft Theorems and Symmetries

    Sandip Trivedi
    Constraints From Conformal Invariance on Inflationary Correlators

    Angel M. Uranga
    Trans-planckian axion field ranges and string theory

    Cumrun Vafa
    6d (1,0) Supersymmetric Theories and Their Compactifications

    Edward Witten
    An Overview Of Worldsheet and Brane Anomalies

    Alberto Zaffaroni
    A topologically twisted index for three-dimensional supersymmetric theories

    Alexander Zhiboedov
    Conformal Bootstrap With Slightly Broken Higher Spin Symmetry
    Earlier yesterday (12 June) I checked and found 2 talk titles have been posted, by later in the day. there were 11. They were starting to put the titles on line. Now it's 50, not yet the full roster since 67 "confirmed invited speakers" are listed here:
    There are so far around 270 participants registered.
    Last edited: Jun 13, 2015
  4. Jul 1, 2015 #79


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    loops 15 (6-10 July) Parallel Session abstracts

    Parallel Session: Foundations of Covariant LQG (Spin Foams)

    Monday, 14:30 - 16:00, Room: Seminar Room 5
    Chair: Muxin Han
    Foundations of spin foam models: a report from the front
    14:30 - 15:00, Daniele Oriti (Albert Einstein Institute, Germany)
    We outline the main issues in establishing the foundations of spin foam models as quantum gravity models. We identify them in: 1) the complete (formal) definition of the covariant theory; 2) the relation with the canonical formulation; 3) the problem of quantisation (and construction) ambiguities; 4) the definition of the continuum limit; 5) the extraction of effective continuum physics. Then, we report on recent technical results aimed at addressing each of them.
    Duality between the 2D Ising model and 3D Spinfoams
    15:00 - 15:30, Etera Livine (ENS Lyon (CNRS), France)
    We present a duality between the 3D quantum gravity amplitude, given by spin network evaluations on the boundary, and the 2D Ising model. We show how it is realized through a supersymmetry, with the Ising spins representing the fermionic degrees of freedom. Finally we discuss the relation between the Ising critical points and the stationary points of the spin network generating function.
    Coupling Yang-Mills to Spin foams - a toy model
    15:30 - 16:00, Sebastian Steinhaus (University of Hamburg, Germany)
    The universal coupling of matter and gravity is one of the most important features of general relativity, allowing (precise) experimental tests. In quantum gravity, in particular spin foams, matter couplings have been defined in the past, yet the mutual dynamics are hardly explored. This is related to the definition of matter and gravity on the discretisation; on top of the difficulties in pure gravity, as broken diffeomorphism invariance and non-uniqueness, we face similar issues for the matter part and, moreover, its coupling to gravity. To lift these issues, one eventually has to renormalize these theories, e.g. via coarse graining.
    Due to a structure similar to spin foams, pure (lattice) Yang-Mills theory is a natural candidate to study these questions further. We propose a coupling to spin foams by choosing a 'local' coupling constant, which depends on the geometric data of the spin foam. To demonstrate the scope of this idea and the effect of different couplings on both matter and geometry, we coarse grain a simple toy model: We study Z_2 Ising spins coupled to a dynamical 2D (quantum group SU(2)_k) background and present the model's phase diagram(s).

    Parallel Session: Foundations of Canonical LQG
    Monday, 14:30 - 16:00, Room: Lecture Hall
    Chair: Alok Laddha
    Quantization of scalar fields coupled to point masses
    14:30 - 15:00, Fernando Barbero (Instituto de Estructura de la Materia, CSIC, Spain)
    We will discuss the Fock quantization of a compound classical system consisting of point masses coupled to a scalar field in one dimension (a string attached to point masses). This toy model is useful to understand basic features of physical systems for which some relevant degrees of freedom may be associated with boundaries (such as black holes in LQG). A rigorous Hamiltonian description is used to characterize in a precise way the real Hilbert space of classical solutions to the equations of motion and construct the Fock space. The quantization procedure that we follow can be seen as a generalization of standard QFT in curved spacetimes and, hence, may be of interest beyond the simple model that we discuss. The space of states that we find displays some interesting features, the most striking one being the impossibility of factoring it as a tensor product of Hilbert spaces naturally associated with the point masses and the field. Some consequences of this fact will be discussed both within the context of QFT and quantum mechanics.
    Quantization of Plane Gravitational Waves: Kinematic and dynamic considerations
    15:00 - 15:30, Seth Major (Hamilton College, USA)
    Aspects of the quantization of a (1+1)-dimensional midi-superspace model for gravitational plane waves will be discussed. These aspects include a flat space-time condition, the algebra of constraints, regularization of the constraints and the role of geometric quantities, a class of kinematic states, and initial forays into the quantization of the
    Hamiltonian constraint.
    On the Hamiltonian Constraint for plane gravitational waves
    15:30 - 16:00, Franz Hinterleitner (Masaryk University, Brno, Czech Republic)
    The Hamiltonian constraint for an effectively 1+1 dimensional midi-superspace model for plane gravitational waves is analyzed. Different versions of the operator are compared. Among others, the resulting possibilities of spatial inhomogeneity propagation serves as a test. Conditions for a solution modeling flat space are formulated.

    Parallel Session: Asymptotic Safety and Renormalisation Techniques (in Spin Foams)
    Monday, 14:30 - 16:00, Room: Seminar Room 3
    Chair: Roberto Percacci
    Towards coarse-graining of spin foams in three dimensions
    14:30 - 14:55, Sebastian Mizera (University of Cambridge, United Kingdom)
    Understanding of the continuum limit constitutes a major outstanding problem in Loop Quantum Gravity. Encouraged by the early numerical results in the dimensionally reduced models, we introduce a new self-consistent coarse graining scheme in three dimensions. Here the effective building blocks take the form of a tensor network decorated by the original variables of the system, which explicitly preserves the gauge invariance at each step of the RG flow. This technique allows for the efficient computation of observables in the generalized lattice gauge theories, such as spin foam models. Joint work with B. Dittrich and S. Steinhaus (gr-qc/1409.2407).
    Coarse-graining of 3D spin foam models with finite non-abelian structure groups
    14:55 - 15:20, Clement Delcamp (Perimeter Institute, Canada)
    One of the most pressing issues of spin foam models is the construction of the large scale limit. We will discuss in this talk first results of coarse-graining for 3D spin foam models with finite non-abelian structure groups.
    Physical states of the theory can be constructed via a coarse-graining procedure which iteratively improves the amplitude in the path integral. Tensor network renormalization techniques allow a concrete realization of this coarse-graining scheme. The procedure based on a gluing of cubical building blocks successfully deals with the large redundancy of degrees of freedom. Furthermore, our structure allows the implementation of simplicity constraints which converts topological BF models to spin foam models. We should therefore be able to address the question of the fate of the constraints throughout the coarse-graining procedure as well as investigating the existence of additional phases.
    Gauge and parametrization dependence in Quantum Gravity
    15:20 - 15:40, Benjamin Knorr (FSU Jena, Germany)
    We critically examine the gauge- and field-parametrization-scheme-dependence of renormalization group flows in the vicinity of non-Gaussian fixed points in quantum gravity. While physical observables are independent of such calculational specifications, the construction of quantum gravity field theories typically relies on off-shell quantities such as beta-functions and generating functionals and thus faces potential stability issues with regard to such generalized parametrizations. We analyze a two-parameter class of covariant gauge conditions and a one-parameter class of field parametrizations. The principle of minimum sensitivity is used to identify stationary points in this parametrization space.
    Matter renormalization: perfect fluid coupled to asymptotically safe gravity
    15:40 - 16:00, Vadim Belov (University of Hamburg, Germany)
    Asymptotic safety program is based on the existence of the UV fixed point of RG flow. In applications to astrophysical and cosmological problems, one usually tries to incorporate the emerging running of the couplings with appropriately chosen scale. We point towards the fact that renormalization in the matter sector might as well contribute on a par with gravity. Motivated by these applications we perform a first step towards the incorporation of the effective perfect fluid description into the picture of the flows. By coupling it with ADM-decomposed gravity we study how the renormalization of the latter may impact on the running of effective of hydrodynamical parameters. The corresponding flow equation is derived in addition to its gravitational counterparts.

    Parallel Session: Other Related Topics including NCG, CDT, Causal Sets
    Monday, 14:30 - 16:00, Room: Seminar Room 4
    Chair: Astrid Eichhorn
    Aspects of the Bosonic Spectral Action
    14:30 - 15:00, Mairi Sakellariadou (King's College London, UK)
    I will first introduce the bosonic (cutoff based) spectral action and highlight some of its cosmological and high energy physics consequences. I will then propose a novel definition of the bosonic spectral action using zeta function regularization, in order to address the issues of renormalizability and spectral dimensions.
    Spontaneous emergent geometry
    15:00 - 15:30, Pierre Martinetti (Università di Trieste, Italy)
    Recent developments in noncommutative geometry, following the discovery of the Higgs boson, open some ways to physics beyond the Standard Model. The Higgs mass is below the threshold of stability of the electroweak vacuum, which might be the sign of the existence of an extra-scalar field. Noncommutative geometry allows to interprets this extra-field as the relic of a pre-geometric phase which spontaneously breaks to the standard model. We will present a model based on a mixing of the spinorial and gauge degrees of freedom, together with a twisted version of Connes spectral triple.
    Quantum non-commutative geometry
    15:30 - 16:00, John Barrett (University of Nottingham, UK)
    The talk will describe aspects of a project devoted to defining and investigating quantum gravity as a functional integral over non-commutative geometries. This approach also gives the coupling of quantum gravity to matter fields, the particles appearing as defects in a topological state sum model.

    Parallel Session: Foundations of Covariant LQG (Spin Foams)
    Monday, 16:30 - 18:00, Room: Seminar Room 5
    Chair: Muxin Han
    What is renormalization useful for in Spin Foams?
    16:30 - 17:00, Sylvain Carrozza (Aix-Marseille University, France)
    My aim will be to critically review the conceptual and technical arguments which support the use of renormalization theory in spin foams. I will in particular argue that it is necessary to: 1) consistently complete the definition of the theory at the fundamental level; 2) understand its effective low-energy limit. By means of simple toy-models, I will finally illustrate why the GFT formalism is well suited to the task.
    Resumming Spin Foams
    17:00 - 17:30, Jeff Hnybida (Radboud University, Netherlands)
    We show how to resum spin foam amplitudes, and general SU(2) lattice gauge theory amplitudes in terms of spinors. We show how this allows for an explicit reduction to twisted geometry variables.
    Pachner moves in a 4d Riemannian holomorphic Spin Foam model
    17:30 - 18:00, Lin-Qing Chen (Perimeter Institute, Canada)
    In this work we study a Spin Foam model for 4d Riemannian gravity, and propose a new way of imposing the simplicity constraints that uses the recently developed holomorphic representation. Using the power of the holomorphic integration techniques, and with the introduction of two new tools: the homogeneity map and the loop identity, for the first time we give the analytic expressions for the behavior of the Spin Foam amplitudes under 4-dimensional Pachner moves. It turns out that this behavior is controlled by an insertion of nonlocal mixing operators. In the case of the 5-1 move, the expression governing the change of the amplitude can be interpreted as a vertex renormalisation equation. We find a natural truncation scheme that allows us to get an invariance up to an overall factor for the 4-2 and 5-1 moves, but not for the 3-3 move. The study of the divergences shows that there is a range of parameter space for which the 4-2 move is finite while the 5-1 move diverges. This opens up the possibility to recover diffeomorphism invariance in the continuum limit of Spin Foam models for 4D Quantum Gravity.

    Parallel Session: Foundations of Canonical LQG
    Monday, 16:30 - 18:00, Room: Lecture Hall
    Chair: Alok Laddha
    A quantum kinematics for asymptotically flat gravity
    16:30 - 17:00, Miguel Campiglia (Universidad de la República, Uruguay)
    Aside from cosmology, most physically relevant problems in gravity are modeled with asymptotically flat boundary conditions at infinity. It is thus of interest to explore how one may impose such conditions at the quantum level. In this talk I present a proposal for such conditions, in the context of the so-called Koslowski-Sahlmann representation. Based on work with M. Varadarajan.
    Fractal Coherent States
    17:00 - 17:30, Suzanne Lanéry (FAU Erlangen, Germany)
    Motivated by obstructions to the construction of semi-classical states on the holonomy-flux algebra, i will discuss how a discrete subalgebra can be extracted while preserving universality and diffeomorphism invariance. This paves the way for the construction of states whose semi-classicality is enforced step by step, starting from collective, macroscopic degrees of freedom and going down progressively toward smaller and smaller scales.
    Constraint quantization and chaos
    17:30 - 18:00, Philipp Hoehn (Perimeter Institute, Canada)
    There is strong evidence that a generic general relativistic spacetime features chaotic dynamics. This has severe (and often ignored) repercussions for the quantization and interpretation of the dynamics as a chaotic (Hamiltonian) constrained system generally does not give rise to a Poisson algebra of Dirac observables. Nevertheless, in certain cases one can explicitly quantize such systems. By means of toy models, I will discuss general challenges and some surprising consequences for the quantum theory of chaotic constrained systems which presumably will also appear in canonical quantum gravity.

    Parallel Session: Foundations of Covariant LQG (Spin Foams)
    Monday, 16:30 - 18:00, Room: Seminar Room 3
    Chair: Emanuele Alesci
    The geometry of relative locality
    16:30 - 17:00, Laurent Freidel (Perimeter institute, Canada)
    Relative locality is the principle that spacetime is an entity which is probe dependent. By opposition absolute locality is the hypothesis that spacetime is an absolute entity, this hypothesis permeate all of physics. I would review many recent development in which we understood how this new principle forces a formulation of physics in phase space, and how gravity and the quantum naturally equip phase space with several natural geometrical structure. One is a metric that descend from the probability measure of quantum mechanics. The other one is another metric that come from spacetime localization. Absolute locality results in the requirement that the localization metric is flat. Relaxing this condition we promote this localization metric to a new dynamical entity. This allow to put together gravity and the quantum on equal footing. We will also present the first example known, modular spacetime that incorporate these principles. We will also show for the first time the generalization of Einstein equation that naturally follow from demanding compatibility between gravity and the quantum.
    Quantum black hole in the full theory
    17:00 - 17:30, Daniele Pranzetti (FAU Erlangen, Germany)
    I present the construction of a spherically symmetric quantum black hole within the full theory by means of a generalized class of quantum gravity condensate states. The construction relies on the group field theory formalism, which provides a second quantized version of loop quantum gravity. The black hole condensate is defined by an infinite superposition of graph-based states encoding in a precise way the topology of the spatial manifold. I impose the isolated horizon boundary condition and show how the entropy calculation can be performed.
    Loop gravity with non-zero cosmological constant
    17:30 - 18:00, Jonathan Ziprick (University of New Brunswick, Canada)
    We propose a new way to include a cosmological constant in loop gravity. Starting from the continuous Ashtekar variables, we take the curvature of the connection to be piecewise-constant and develop a symplectomorphism between such geometries and a spin network phase space. This data is suitable for the study of (anti) de Sitter cosmology, as well as non-homogenous models.

    Parallel Session: Other Related Topics including NCG, CDT, Causal Sets
    Monday, 16:30 - 18:00, Room: Seminar Room 4
    Chair: Astrid Eichhorn
    Exploring fuzzy space through Monte Carlo Methods
    16:30 - 16:55, Lisa Glaser (Nottingham University, Great Britain)
    A spectral triple is the collection of a fermion space and a Dirac operator.
    The collection of all Dirac operators for a given fermion space defines its space of geometries.
    Fuzzy spaces are a special case of spectral triples with simple matrix algebras.
    They are cases of matrix geometries and have a simple representation.
    These simple repesentations allow us to explore the space of geometries using Markov Chain Monte Carlo methods.
    In attempting to explore the space of geometries we have to face many open questions:
    - Which action should we use?
    - What observables can we measure in it?
    In this talk I will present our answers to these questions and show some of the data generated.
    Towards (3+1) gravity through Drinfel'd doubles with cosmological constant
    16:55 - 17:15, Pedro Naranjo (University of Burgos, Spain)
    We present the generalisation to (3+1) dimensions of a quantum deformation of the (2+1) (Anti)-de Sitter and Poincar\'e Lie algebras that is compatible with the conditions imposed by the Chern-Simons formulation of (2+1) gravity. Since such compatibility is automatically fulfilled by deformations coming from Drinfel'd double structures, we believe said structures are worth being analysed also in the (3+1) scenario as a possible guiding principle towards the description of (3+1) gravity. To this aim, a canonical classical r-matrix arising from a Drinfel'd double structure for the three (3+1) Lorentzian algebras is obtained. This r-matrix turns out to be a twisted version of the one corresponding to the (3+1) κ-deformation, and the main properties of its associated noncommutative spacetime are analysed. In particular, it is shown that this new quantum spacetime is not isomorphic to the κ-Minkowski one, and that the isotropy of the quantum space coordinates can be preserved through a suitable change of basis of the quantum algebra generators. Throughout the paper the cosmological constant appears as an explicit parameter, thus allowing the (flat) Poincar\'e limit to be straightforwardly obtained.
    (Re)constructing Spacetime Geometry from Quantum Dynamics
    17:15 - 17:40, Matti Raasakka (Independent researcher, Finland)
    I will present some encouraging recent results in extracting effective spatiotemporal notions such as locality and duration directly from quantum dynamics. The most concrete results concern for now quantum systems with finite-dimensional observable algebras, but should generalize at least to hyperfinite algebras (such as in QFT). I will also discuss a background-independent algebraic framework for quantum physics, where these methods are directly applicable.
    Semiclassical Field Theory
    17:40 - 18:00, Eugene Kur (University of California, Berkeley, USA)
    Recent developments in multisymplectic geometry have clarified how to apply the Hamiltonian formalism of field theory to arbitrary spacetime slicings. This allows us to use various semiclassical approaches (such as the WKB approximation of the path integral) in a much more general context. I will discuss applications of these semiclassical ideas to the Unruh effect and black hole entropy, as well as describe the modifications to the symplectic structure and Hamilton's principle function that come from looking at general spacetime slicings and general spacetime boundaries.
    Last edited: Jul 1, 2015
  5. Jul 1, 2015 #80


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    Tuesday parallel sessions (7 July) Loops 2015
    Parallel Session: Foundations of Covariant LQG (Spin Foams)
    Tuesday, 14:30 - 16:00, Room: Lecture Hall
    Chair: Muxin Han
    The cosmological constant in the Loop Quantum Gravity framework.
    14:30 - 15:00, Maite Dupuis (University of Waterloo, Canada)
    The Loop Quantum Gravity framework has been mostly studied in the case of a zero vanishing cosmological constant. A Loop Quantum Gravity model with a cosmological constant is not well understood even in the 3d toy model case. For the 3d case, several approaches to define a model of quantum gravity exist and in the case of a non-vanishing cosmological constant, the models such as the Turaev-Viro spin foam model or the Chern-Simons model are written in terms of a quantum group. To reconcile the Loop Quantum Gravity approach with these models, we deform the Loop Quantum Gravity framework using quantum groups or Poisson-Lie groups. Following this line, a topological model with a Hamiltonian constraint for 3d gravity with a cosmological constant has been defined. The Hamiltonian constraint can be solved and the solutions can be related to the Turaev-Viro spin foam amplitude. Moreover, geometric observables for quantum hyperbolic geometries have been defined.
    I am going to present an overview of this program consisting in introducing a cosmological constant in the 3d Loop Quantum gravity framework and will comment the 4d case.
    3d loop gravity and the cosmological constant.
    15:00 - 15:30, florian Girelli (University of Waterloo, Canada)
    Using inspiration from Chern-Simons theory, I will explain how one can use Poisson Lie groups to deform the classical phase space of loop quantum gravity to introduce a cosmological constant. I will focus on the 3d case and present the geometric details behind the construction. This model is quantizable and solvable and I will highlight some of these steps if time permits.
    Encoding Curved Tetrahedra in Face Holonomies
    15:30 - 16:00, Hal Haggard (Bard College, USA)
    I will present a generalization of Minkowski’s classic theorem on the reconstruction of tetrahedra from algebraic data to homogeneously curved spaces. Euclidean notions such as the normal vector to a face are replaced by Levi-Civita holonomies around each of the tetrahedron’s faces. This new approach allows the reconstruction of both spherical and hyperbolic tetrahedra within a unified framework. Several interesting mathematical structures arise in setting up a phase space for these curved tetrahedra such as group-valued moment maps and quasi-Poisson spaces. Curved tetrahedra also provide a natural starting point for thinking about discrete and quantum gravity in spacetimes with a cosmological constant.

    Parallel Session: Foundations of Canonical LQG
    Tuesday, 14:30 - 16:00, Room: Seminar Room 5
    Chair: Wojciech Kaminski
    New applications for canonical LQG
    14:30 - 15:00, Jerzy Lewandowski (Uniwersytet Warszawski, Polen)
    I will outline several new findings in canonical LQG.
    New perspectives for canonical LQG dynamics
    15:00 - 15:30, Mehdi Assanioussi (University of Warsaw, Poland)
    This talk is about the implementation of a new scalar constraint operator for canonical LQG. I will briefly present the regularization procedure and I will discuss the quantum algebra and the possible construction of a symmetric quantum operator. I will conclude with a qualitative description of the physical states and few remarks about the physical Hamiltonian, in case of gravity coupled to a free scalar field, obtained with a similar regularization.
    Symmetry Reduction and Quantum Configuration Spaces
    15:30 - 16:00, Christian Fleischhack (Paderborn University, Germany)
    First, we review the C*-algebraic foundations of loop quantization, focusing on configuration spaces and symmetry implementation. Then, we apply these findings to loop quantum cosmology. In particular, we derive an embeddability criterion for the configuration spaces and review the Hanusch results on non-commutativity of quantization and symmetry reduction.

    Parallel Session: Other Related Topics including NCG, CDT, Causal Sets
    Tuesday, 14:30 - 16:00, Room: Seminar Room 3
    Chair: Karim Noui
    Null canonical gravity, integrability and quantization
    14:30 - 15:00, Michael Reisenberger (Universidad de la Republica, Uruguay)
    The talk will focus on issues connected with the quantization of initial data for vacuum general relativity on null hypersurfaces. After a brief review of a classical canonical formulation of general relativity in terms of unconstrained null data, I will present recent joint work with Andreas Fuchs bearing on the quantization of the main null initial data. We note that the Poisson brackets of these data are almost the same in cylindrically symmetric gravity as in the full theory, and we use a non-linear and non-local change of variables to transform the Poisson brackets in the cylindrically symmetric case into a form that has a known quantization. Note that the talk deals with putting the classical theory in a quantizable form. Very little will be said about the quantum theory itself.
    Shape Dynamics: A Progress Report
    15:00 - 15:30, Sean Gryb (Radboud University, Netherlands)
    There exists a particularly useful foliation for General Relativity where the local gauge-invariant degrees of freedom are conformally invariant. Shape Dynamics is an approach to gravity where this observation is taken seriously to motivate an ontological shift from a spacetime picture to a picture of evolving conformal 3-geometry. Several exciting things arise if one embraces such an ontology: different possibilities exist for what happens behind the horizons of black holes, new insights are gained about the arrow of time, and, perhaps most promising, new scenarios arise for describing a UV completion of General Relativity. In this talk, I will review the basic formalism for Shape Dynamics, discuss some recent results and speculate about the implications for the quantum theory.
    Parity Horizons in Shape Dynamics
    15:30 - 16:00, Gabriel Herczeg (University of California, Davis, United States)
    Recent work has shown that the black hole solutions of shape dynamics are physically different than general relativistic black holes at and within their event horizons. The physical difference arises as a result of an inversion or reflection symmetry about the horizon of a shape dynamic black hole. I define the notion of a parity horizon and show that not only event horizons, but also Cauchy horizons and observer-dependent horizons belong to this class. I also show that this notion of parity endows charged shape dynamic black holes with CPT invariance, and discuss the implications for chronology protection.

    Parallel Session: Classical and/or Quantum non-LQG Cosmology: theory and/or experiment
    Tuesday, 14:30 - 16:00, Room: Seminar Room 4
    Chair: Anna Ijjas
    GFT states for homogeneous cosmologies
    14:30 - 15:00, Lorenzo Sindoni (Albert Einstein Institute, Potsdam, Germany)
    I will present recent work in the construction of kinematical states, belonging to the GFT
    Fock space, that can be used to describe cosmological spacetimes.
    The construction, based on suitably defined refinement moves, follows closely the idea of condensate states, introduced in previous work. The new states encode the geometric and topological information of homogeneous cosmologies, while realizing, at the same
    time, a form of coarse graining in terms of sum over a family of connected graphs.
    Cosmological perturbations in GFT condensates
    15:00 - 15:20, Steffen Gielen (Imperial College London, UK)
    Quantum cosmology can be viewed as an effective hydrodynamic approximation to quantum gravity, in which one describes the universe as a 'condensate' of quanta of geometry. This idea has been made concrete in group field theory (GFT) where these quanta correspond to tetrahedra or open spin network vertices. In the mean-field approximation of the condensate, one obtains a semiclassical description in terms of a statistical distribution of quanta over minisuperspace. I will review the basic ideas behind the formalism and show how one can extract information about the homogeneous mode and about cosmological inhomogeneities from this distribution. I will also discuss some interesting general consequences for cosmology, especially in the connection to CMB observations.
    Time and unitarity around a cosmological bounce
    15:20 - 15:40, Antonin Coutant (Albert Einstein Insitute, Potsdam, Germany)
    I will discuss the notion of time and unitarity in the vicinity of a bounce in quantum cosmology, that is, a turning point for the scale factor. It has been obtained in several approaches that a proper notion of time and evolution emerges form the Hamiltonian constraint of quantum cosmology if at least one degree of freedom is semiclassical enough. In particular, WKB solutions provide a key tool to analyze the possible interpretations of the wave function. Unfortunately, WKB methods fail drastically near a turning point. To address this issue, I developed a semiclassical framework in the momentum representation. I will then discuss the physics of matter degrees of freedom, when that momentum plays the role of time. The regime at hands goes beyond the Born-Oppenheimer approximation, and no classical background exists. I will show this by considering first an adiabatic evolution and then quantum transitions (i.e. particle creation).
    A Noncommutative Extension of Effective Loop Quantum Cosmology
    15:40 - 16:00, Abraham Espinoza García (University of Guanajuato, Mexico)
    A noncommutative modification of the Loop Quantum Cosmology effective scheme of the open FLRW model in the presence of a standard scalar field is proposed. We start from the holonomized Hamiltonian and implement a canonical noncommutativity (theta deformation) among the matter degree of freedom and the holonomy variable (volume representation) by performing a shift in such configuration variables, thus obtaining an effective noncommutative Hamiltonian. We also obtain a noncommutative extension of the modified Friedmann equation for a particular case of the theta deformation. It remains to be tackled the physical interpretation of such extension.

    Parallel Session: Foundations of Covariant LQG (Spin Foams)
    Tuesday, 16:30 - 18:00, Room: Lecture Hall
    Chair: Muxin Han
    From a curved-space reconstruction theorem to a 4d Spinfoam model with a Cosmological Constant
    16:30 - 17:00, Aldo Riello (Perimeter Institute, Canada)
    I will discuss the first steps towards a definition of a spinfoam model for 4d gravity with a cosmological constant, via complex Chern-Simons theory with defects. The proposal hinges on a reconstruction theorem assessing the correspondence between a class of flat connections on a S3 graph complement (related to the 4-simplex 1-skeleton) and the geometries of a constant-curvature Lorentzian 4-simplex. The main result consists in showing that in the semiclassical approximation of the vertex amplitude the Regge action of simplicial general relativity correctly appears. This construction borrows ingredients from the EPRL/FK model and adapts them to the curved case. Time allowing I will also comment on the phase space structure of the boundary states of the model.
    Compactification of LQG phase space
    17:00 - 17:30, Francesca Vidotto (Radboud University Nijmegen, Netehrlands)
    In order to introduce the cosmological constant in a simplicial geometry, simplex faces should be taken of constant curvature. This yields a compactification of the phase space and the finiteness of the Hilbert space for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space.
    Turaev-Viro amplitudes from a Hamiltonian constraint with positive cosmological constant
    17:30 - 18:00, Julian Rennert (University of Waterloo, Canada)
    I will give an update on the current status of the relation between the Turaev-Viro spinfoam model and a canonical quantum theory of q-deformed spin-networks. Building on the recent work of Girelli, Dupuis, Livine and Bonzom on hyperbolic discrete geometries and their quantization with a real deformation parameter I will focus on the quantum theory with q being a root of unity. This case is associated with a positive cosmological constant and indeed we find the expected spherical geometries from generalized length and angle operators. What's more, one finds that q-deformed spin-networks that solve a certain Hamiltonian constraint have q-deformed 6j-symbols as their amplitudes, which are the building blocks of the Turaev-Viro model. Hence, we have a promising model of 3D loop quantum gravity with positive cosmological constant.

    Parallel Session: Foundations of Canonical LQG
    Tuesday, 16:30 - 18:00, Room: Seminar Room 5
    Chair: Wojciech Kaminski
    New Hamiltonian constraint operator for loop quantum gravity
    16:30 - 16:55, Yongge Ma (Beijing Normal University, China)
    A new symmetric Hamiltonian constraint operator is proposed for loop quantum gravity. On one hand, it inherits the advantage of the original regularization method of Thiemann, so that its regulated version in the kinematical Hilbert space is diffeomorphism covariant and creates new vertices to the spin networks. On the other hand, it overcomes the problem in the original treatment, so that there is no ambiguity in its construction and its quantum algebra is anomaly-free in a suitable sense. The regularization procedure for the Hamiltonian constraint operator can also be applied to the symmetric model of loop quantum cosmology, which leads to a new quantum dynamics of the cosmological model.
    Conformal symmetry in LQG
    16:55 - 17:20, Miklos Långvik (Helsinki university & CPT Marseille, Finland)
    Conformal symmetry plays a key role in modern quantum field theory and many authors have suggested that it could be relevant to understand quantum qravity non-perturbatively. As a first step towards exploring this idea within loop quantum gravity, we present some results on the action of the Minkowskian $\mathfrak{su}$(2,2) conformal symmetry on spin networks. Somewhat counterintuitively, the action of the generator of dilatations changes the discrete extrinsic curvature while keeping the areas and 3d volumes intact. We also construct a possible semi-classical picture for conformal spin networks in terms of twistors, which points towards a certain self-dual octahedron in complexified Minkowski space.
    The Conformal Nature of the Barbero-Immirzi Parameter
    17:20 - 17:40, Patrick Wong (University of Cologne, Germany)
    The Barbero-Immirzi parameter of loop quantum gravity is a one parameter ambiguity of the theory whose physical significance is as-of-yet unknown. It is an inherent characteristic of the quantum theory since it appears in the spectra of geometric operators. The parameter's appearance in the area and volume spectra imply that it plays a role in determining the fundamental length scale of space. This appearance as a rescaling of lengths motivates a possible conformal interpretation. Presented here is an analysis of the conformal scaling of the triad formalism and the revelation that the Barbero-Immirzi parameter precisely corresponds to the conformal scale factor. Furthermore, at the kinematical level the conformal scale factor materializes as a scalar field coupled to gravity. The development of this conformal scalar field to the quantum sector of the theory is sketched.
    Real Change in Hamiltonian GR Observables from Equivalence to the Lagrangian
    17:40 - 18:00, J. Brian Pitts (University of Cambridge, United Kingdom)
    Change has seemed missing in Hamiltonian GR. The lack of a worry for Lagrangian GR motivates reexamining where equivalence to the Lagrangian has been lost. Since the 1980s some authors, such as Mukunda, Castellani, Sugano, Pons, Salisbury, Shepley, and Sundermeyer, have aimed to recover Lagrangian equivalent gauge transformations at least on shell using a 3plus1 version of the Rosenfeld Anderson Bergmann gauge generator, a specially tuned sum of first-class constraints, primary and secondary, as opposed to Dirac's view that any first-class primary alone and perhaps any first-class secondary alone generates a gauge transformation. Are the views equivalent for observables? Pons's proof that Dirac should not have stopped at lowest infinitesimal order and hence with primaries is illuminated by direct calculations in electromagnetism and GR showing that each constraint alone falsifies the Lagrangian constraints, viz., Gauss's law, Gauss Codazzi relations, by affecting the initial data, an effect that Dirac's subtractive calculation cancels out. Likewise a first class constraint fails to leave the canonical action int dt p dq/dt - H quasi-invariant, but the gauge generator G does so. The canonical momenta tend not to notice the spoiling of the Lagrangian constraints because individual constraints also spoil the relations dq/dt-dH/dp equals 0 that gives physical meaning to the momenta, which are mere auxiliary fields in the canonical action and hence dispensable. As argued by Pons, Salisbury and Sundermeyer, observables should be defined in terms of a Poisson bracket with the gauge generator G, not separate constraints.
    Given the role of the transport term in Lie differentiation, requiring observables in GR to have 0 Poisson bracket with G is analogous to requiring sameness at 1 am British Summer Time and 1 am GMT an hour later---immediately but implausibly requiring constancy. This argument systematizes Kuchar's critique of observables. A revised definition of observables O is proposed, that the Poisson bracket of O with G is 0 when G generates internal symmetries, but it is the Lie derivative of a geometric object in the classical Nijenhuis sense of components in all coordinates and a transformation law, when G generates 4-dimensional coordinate transformations. Thus Hamiltonian observables, like Lagrangian observables, are internally gauge-invariant geometric objects, including the electromagnetic field strength and the metric.

    Parallel Session: Other Related Topics including NCG, CDT, Causal Sets
    Tuesday, 16:30 - 18:00, Room: Seminar Room 3
    Chair: Karim Noui
    Causal fermion systems as an approach to quantum gravity
    16:30 - 17:00, Felix Finster (Universität Regensburg, Germany)
    The theory of causal fermion systems is an approach to describe fundamental physics. It gives quantum mechanics, general relativity and quantum field theory as limiting cases and is therefore a candidate for a unified physical theory including gravity. Instead of introducing physical objects on a preexisting space-time manifold, the general concept is to derive space-time as well as all the objects therein as secondary objects from the structures of an underlying causal fermion system. The dynamics is described by the causal action principle.
    The aim of the talk is to give a simple introduction, with an emphasis on conceptual issues. In particular, it will be outlined how quantum gravity is to be described in this framework.
    Statistical mechanics of reparametrization invariant systems
    17:00 - 17:20, Thibaut Josset (Aix Marseille Université, France)
    It is notoriously difficult to apply statistical mechanics to generally covariant systems, because the notions of time, energy and equilibrium are seriously modified in this context. However, for a system invariant under reparametrization (without additional gauge), one can generalize the notion of time average. Ergodicity is then used to define statistical states. Finally, if the system splits into subsystems (in some appropriate way), standard results of statistical mechanics and thermodynamics remain valid. This new approach might be a first step in understanding statistical properties of classical or quantum spacetime.
    joint work with Goffredo Chirco and Carlo Rovelli, arXiv:1503.08725
    Operationalization of basic observables for relativistic dynamics
    17:20 - 17:40, Bruno Hartmann (Humboldt University Berlin, Germany)
    We start with thought experiments on measurement practice as Einstein did for the foundation of relativistic Kinematics. Seizing on a programmatic proposal by Heinrich Hertz we arrive at quantification of energy-momentum and then, give a relativistic revision. We define these observables from elemental ordering relations for ''capability to execute work'' and ''impact'' in a collision and apply Helmholtz method for quantification, according to which a basic measurement consists in a reconstruction of the measurement object with a material model of concatenated units (reference process of irrelevant internal structure). From simple physical (light principle, principle of inertia, impossibility of Perpetuum Mobile) and measurement methodical principles we derive all fundamental equations of classical and relativistic dynamics. In this foundation, which explains the mathematical formalism from the operationalization of basic observables, one can address and understand scope and limitations of the formalism, with significance also for other formalisms in physics.
    Intrinsic Time Quantum Geometrodynamics
    17:40 - 18:00, Eyo Ita (US Naval Academy, USA)
    Quantum Geometrodynamics with intrinsic time development and momentric variables is presented. An underlying SU(3) group structure at each spatial point regulates the theory. The intrinsic time behavior of the theory is analyzed, together with its ground state and primordial quantum fluctuations. Cotton-York potential dominates at early times when the universe was small; the ground state naturally resolves Penrose's Weyl Curvature Hypothesis, and thermodynamic and gravitational 'arrows of time' point in the same direction. Ricci scalar potential corresponding to Einstein's General Relativity emerges as a zero-point energy contribution. A new set of fundamental commutation relations without Planck's constant emerges from the unification of Gravitaion and Quantum Mechanics.

    Parallel Session: Homogeneous and Hybrid Loop Quantum Cosmology (LQC)
    Tuesday, 16:30 - 18:00, Room: Seminar Room 4
    Chair: Edward Wilson-Ewing
    LQC, Non-Gaussianity and CMB anomalies
    16:30 - 17:00, Ivan Agullo (LSU, USA)
    This talk will summarize the prediction of LQC for the spectrum of Non-Gaussianity and its role as a potential source for the power asymmetry observed at large angular scales in the CMB
    Suppresion of power at large scales in loop quantum cosmology
    17:00 - 17:30, Brajesh Gupt (IGC Penn State, USA)
    An important feature of singularity resolution in loop quantum cosmology (LQC) is the occurrence of the quantum bounce when the spacetime curvature becomes Planckian leading the pre-inflationary evolution of the universe to be greatly modified. Due to the modified dynamics in the pre-inflationary era the initial conditions for both the background and cosmological perturbations are different from those in the standard inflationary scenario. We find that such modifications can lead to observational signatures on the cosmic microwave background (CMB) anisotropy spectrum. In particular we find that there exist initial states in LQC that could lead to suppression of power at low multipoles in the temperature anisotropy spectrum--a 3$\sigma$ anomaly observed in the recent CMB experiments. In this talk we describe these initial conditions, discuss their consequences on the inflationary power spectrum, and compare our results with data from recent CMB experiments.
    Phenomenology of Starobinsky inflation in LQC
    17:30 - 18:00, Beatrice Bonga (Penn State, USA)
    Recent observations by the Planck collaboration favor Starobinsky inflation over other inflationary models. However, predictions in the setting of LQC have only been made in the context of a quadratic potential. In this work, we close this gap by studying the singularity resolution, probability of inflation and powerspectrum of cosmological perturbations for the Starobinsky model using the framework of QFT on quantum space-time in the effective description of LQC. We investigate the parameter space of initial data at the bounce that lead to slow-roll inflation consistent with observations. We find that, similar to the quadratic potential, there is a limited range in which the LQC predictions differ from the standard inflationary scenario. Phenomenological consequences are also discussed.
    Last edited: Jul 1, 2015
  6. 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
  7. 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
  8. 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.
  9. 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
  10. 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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