Leading lines of development in LQG, main issues, Loops 15

[Berlin]

Hi Marcus,

I am not from Berlin! Look at my picture! Don't you recognize the bold man with the moustache, from the first Solvay conference? No, not the other bold one from Berlin, who worked so hard in October 1900, chasing the quantum. On the buste the text says 'Door meten tot weten', and it stands in the city
where he found out that liquids are more fun,
where Einstein met his father in physics,
where the cosmos got its saddle point,
where the electron got its spin and
where we had the best physics teacher before Feynman.

berlin :)

 

[marcus]

"Through measurement to knowledge"
Heike Kamerlingh Onnes - Leyden

 

[marcus]

http://www.gravity.physik.fau.de/events/loops15/program.shtml
All the plenary session abstracts have now been posted, most recently at 22:35 hours central on 24 June.
For the latest, scroll down to the abstract of Muxin Han's Friday 10AM talk.

Plenary Session
Monday, 08:45 - 10:45
Opening
08:45 - 09:00, Joachim Hornegger (President FAU Erlangen, Germany)

Spacetime and matter in asymptotic safety
09:00 - 09:45, Astrid Eichhorn (Imperial College, United Kingdom)
I will explain the main conceptual ideas underlying the asymptotic safety scenario in quantum gravity. I will then summarize the status of this approach, and highlight some recent developments. In particular, I will discuss why matter matters in this model of quantum gravity and how this could potentially open a window into experimental tests of quantum gravity.

Aspects of Hamiltonian constraint in Loop Quantum Gravity
10:00 - 10:45, Alok Laddha (Chennai Mathematical Institute, India)
We review some of the recent progress that has been made in constructing the dynamical equations of canonical Loop Quantum Gravity in the Euclidean sector. We start be recalling some of the well known problems which plague the dynamical aspects of canonical theory and set out a strategy aimed at resolving them. After reviewing the lessons which have been obtained in various toy models, we define a continuum quantum Hamiltonian constraint of the theory and show some intriguing evidence that this constraint generates an anomaly free Quantum constraint algebra off-shell.

Plenary Session
Monday, 11:15 - 13:00
Covariant Loop Quantum Gravity: Models and Developments
11:15 - 12:00, Wolfgang Wieland (IGC Penn State, USA)
I start with a pedagogical introduction to covariant loop quantum gravity. I select a number of recent results and discuss the open problems in the field. I present, in particular, a program to derive the spinfoam amplitudes from a classical action for discretized gravity. This action, which is a one-dimensional integral over a system of auxiliary worldlines, has the same field content as loop quantum gravity, which explains its relevance for the theory.

Continuum limit and renormalization in spin foam models
12:15 - 13:00, Benjamin Bahr (University of Hamburg, Deutschland)
Spin foam models provide a tentative proposal for the path integral of quantum gravity. In recent years, there have been very successful candidates for such models, and while their properties on a microscopic level are very promising, little is known about the contuum limit of many building blocks.
In this talk I will give an overview of background-independent renormalization techniques, which can not only provide a framework for investigating the continuum limit, but also as a method to construct the analogue of low-energy effective actions for spin foam models. I will also discuss the connection with diffeomorphism-invariance.

Plenary Session
Tuesday, 09:00 - 10:45
Early-Universe Cosmology: Issues and Opportunities
09:00 - 09:45, Anna Ijjas (Princeton University, USA)
Inflationary cosmology is commonly considered as the “standard model" of the early-universe, though, it has several open issues. Some of the problems have been known since the introduction of inflationary theory in the early 80s but some of the problems have been realized first later as we learned to better understand the theory and gained more experimental data. I will present the main problems with inflation, discuss alternative approaches and point to the opportunities the current situation gives us.

Numerical loop quantum cosmology: overview and recent results
10:00 - 10:45, Parampreet Singh (Louisiana State Universitty, USA)
In this talk, we will discuss some of the recent developments in the investigations on singularity resolution using numerical simulations in loop quantum cosmology. Thanks to the introduction of new techniques, the bounce can now be established for a wide variety of states and the reliability of the effective dynamics can be checked rigorously. These results will be discussed in isotropic and anisotropic models.

Plenary Session
Tuesday, 11:15 - 13:00
Hybrid Loop Quantum Cosmology
11:15 - 12:00, Mercedes Martin-Benito (Radboud University Nijmegen, Netherlands)
I will present an overview of the hybrid approach to quantize inhomogeneous cosmologies in the framework of loop quantum cosmology. Though I will keep the discussion as general as possible, for definiteness I will focus on two particular examples largely studied so far: linearly polarized Gowdy cosmologies with spatial three-torus topology, which is the simplest inhomogeneous cosmological model, and the flat homogeneous and isotropic model minimally coupled to a scalar field and with cosmological perturbations. After explaining the hybrid quantization of these models, I will discuss some aspects of the resulting quantum dynamics, and also comment on approximations that one can introduce to extract physical results.

Loop quantum cosmology and alternatives to inflation
12:15 - 13:00, Edward Wilson-Ewing (Albert Einstein Institute, Germany)
High precision observations of the cosmic microwave background provide strong constraints on the dynamics of the early universe and raise the hope that it may be possible to detect quantum gravity effects. In this talk, I will focus on realizations of the ekpyrotic and matter bounce scenarios in loop quantum cosmology. These are alternatives to inflation where scale-invariant perturbations are generated in a contracting background Friedmann space-time which later bounces due to loop quantum cosmology effects. I will show how it is possible to explicitly calculate the evolution of the perturbations through the non-singular bounce and explain under what conditions scale-invariance is preserved. I will also discuss how loop quantum cosmology can affect observational quantities; one such effect is a damping of the amplitude of tensor modes.

Plenary Session
Wednesday, 09:00 - 10:45
Pathways in Quantum Gravity Phenomenology
09:00 - 09:45, Stefano Liberati (SISSA, Trieste, Italy)
In this talk I will offer a panoramic view on the lessons and the achievements gathered so far in the quest for probing the fabric of spacetime. In particular, I will discuss possible scenarios for the mesoscopic physics between our classical world and full quantum gravity regimes. In doing so I shall focus on the perspectives to test them so to gain some guidance in the development of quantum gravity.

Higher dimensional connection dynamics and applications
10:00 - 10:45, Norbert Bodendorfer (University of Warsaw, Poland)
A reformulation of general relativity in terms of connection variables constitutes the classical basis of loop quantum gravity. In this talk, we will review the construction of a set of connection variables which generalise those of Ashtekar and Barbero, and allow to apply the quantisation techniques of loop quantum gravity also to higher-dimensional gravitational theories. Recent developments such as the computation of black hole entropy in higher dimensions and the treatment of generalised gravity theories will be discussed. Future prospects and promising lines of research will be outlined.

Plenary Session
Wednesday, 11:15 - 13:00
Getting rid of the Barbero-Immirzi parameter?
12:15 - 13:00, Karim Noui (LMPT Tours, France)
The Barbero-Immirzi parameter plays a rather intriguing role in loop quantum gravity. At the classical level, it has no physical relevance whereas it plays a crucial role in the quantum theory (quantum geometry, black hole entropy, spin foams, etc). In a recent series of articles, we have shown that the right value of this parameter could be the complex number +/- i. We will review how this result appears in the contexts of black holes and three dimensional gravity. We will also discuss some consequences of this observation in the full theory.

Isolated and Dynamical Horizons: Overview and recent results
12:15 - 13:00, Jonathan Engle (Florida Atlantic University, USA)
The notion of isolated horizon encodes just that `minimum' of the properties of a stationary black hole necessary to ensure that the laws of black hole mechanics hold. It is quasi-local, and makes possible a well-defined canonical framework in which energy and angular momentum of a black hole can be defined. Dynamical horizons generalize this notion to the non-stationary case. I review the basic ideas of this framework, and discuss its applications, from the statistical mechanics of quantum black holes to numerical relativity. Both classic and more recent results and developments will be reviewed.

Plenary Session
Thursday, 09:00 - 10:45
A New Decay Mode for Black Holes
09:00 - 09:45, Hal Haggard (Bard College, USA)
The recent discovery of a metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole has ignited interest in an alternative mode for black hole decay: through quantum tunneling of its geometry a black hole might turn into a white hole. I will discuss the current status, weaknesses and strengths, of this proposal and review potential phenomenological possibilities for the observation of decaying primordial black holes.

Group field theory for (loop) quantum gravity
10:00 - 10:45, Aristide Baratin (University of Waterloo, Canada)
Group field theory is a higher dimensional analogue of matrix models, which gives a prescription for summing over triangulations in spin foam models. I will give an overview of the framework and discuss some of the (numerous) recent developments in the field.

Plenary Session
Thursday, 11:15 - 13:00
On black hole design
11:15 - 12:00, Matteo Smerlak (Perimeter Institute, Canada)
Black hole design is the art of constructing black hole spacetimes from scratch, i.e. without solving the Einstein equations. This endeavor is motivated by Penrose's realization that classical black hole generically have singularities, and by the belief that these singularities must be resolved by quantum effects. I will share my perspective on the black hole design process, its upshots, and its pitfalls.

Loop quantization of vacuum spacetimes with spherical symmetry
12:15 - 13:00, Javier Olmedo (Louisiana State University, USA)
In this talk I will briefly review the quantization of spherically symmetric vacuum spacetimes. Concretely, I will discuss in more detail the quantization of this midisuperspace setting within loop quantum gravity. I will adopt a redefinition of the classical constraints characterized by an algebra free of structure functions. I will then adopt the Dirac quantization approach together with a representation mimicking the one of loop quantum gravity. I will provide the solutions to the constraints, a suitable inner product, and the quantum observables of the model (some of them without classical counterpart). I will eventually discuss about the applications and extensions to other midisuperspace models.

Plenary Session
Friday, 09:00 - 10:45
Gravity in the radial gauge
09:00 - 09:45, Wojciech Kaminski (University of Warsaw, Poland)
A prominent feature of General Relativity is the presence of diffeomorphism group as gauge transformations. As a result, evolution of the metric tensor in a given space point is not a well defined notion unless coordinate system is uniquely specified. The physically motivated choices are certain versions of normal or Fermi coordinates, which are determined by a single observer moving in space-time. The particularly simple form of the metric in these radial coordinate systems allows to eliminate spurious degrees of freedom. This geometrical meaning attracted a lot of attention. I will give a sketch of recent works (of N. Bodendorfer, P. Duch, J. Lewandowski, J. Swiezewski) on the hamiltonian formulation in terms of the radial gauge. The aim of the construction is to define relational observables.

4-dimensional Spinfoam Amplitude with Cosmological Constant, 3-Manifold, and Supersymmetric Gauge Theory
10:00 - 10:45, Muxin Han (FAU Erlangen, Germany)
In this talk, I give an overview of the recent progress of covariant LQG in 4-dimensions with cosmological constant, with emphasis on the interesting relations with other areas of physics and mathematics. The 4d spinfoam amplitude is written as a finite dimensional integral, which has nice relation with Chern-Simons theory on a (dual) 3-manifold. Moreover the 4d spinfoam amplitude can be formulated as the holomorphic block in 3d, which arises from the holomorphic factorization of a certain 3-dimensional N=2 supersymmetric gauge theory. This formulation relates covariant LQG to M5-brane dynamics and 6d (2,0) theory in String/M-theory.

Plenary Session
Friday, 11:15 - 13:00
Quantum Reduced Loop Gravity
11:15 - 12:00, Emanuele Alesci (Warsaw University, Poland)
We present Quantum Reduced Loop Gravity a gauge fixed version of LQG. We show how QRLG provides a promising framework for a consistent characterization of the early Universe.
The effective semiclassical dynamics, the differences with Loop Quantum Cosmology and the inclusion of a scalar field will be discussed.

Summary
12:00 - 13:00, Abhay Ashtekar, Carlo Rovelli, Jerzy Lewandowski

 

[marcus]

I mentioned several people earlier who have taken part in very interesting research recently but who were not on the list of plenary speakers. Now that the titles and abstracts for the PARALLEL SESSION talks are posted online we can see what those folks are going to be talking about too.

BTW I noticed that Suzanne Lanéry, who did that interesting series of papers with Thomas Thiemann, is one those who have already registered--so one can count on presentation of some of the recent Thiemann et al work in the parallel sessions.
... Gofreddo Chirco (of the CHRR) is another of those listed, who will most likely be presenting results in parallel session. Francesca Vidotto as well. I'm interested in that "compact phase space" result which comes out of incorporating the cosmological constant in simplicial QG.

I have time to mention only a few of the parallel session talks. I have highlighted ones by Haggard, Riello, Vidotto, and Geiller, about recent research discussed here at PF.
Parallel Session: Foundations of Canonical LQG
Monday, 16:30 - 18:00, Room: Lecture Hall
Chair: Alok Laddha
...
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.
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Parallel Session: Foundations of Covariant LQG (Spin Foams)
Tuesday, 14:30 - 16:00, Room: Lecture Hall
Chair: Muxin Han
...
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 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.
=================
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
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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.
=================
Parallel Session: Foundations of Canonical LQG
Thursday, 14:30 - 16:00, Room: Lecture Hall
Chair: Norbert Bodendorfer
...
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: 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.
====================
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.
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