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Abstracts Quist
 

Tim Koslowski (Perimeter I.) 9 December 2011

Shape Dynamics

IShape Dynamics is a reformulation of GR that trades refoliation invariance for local spatial conformal invariance (symmetry trading). The proof of equivalence with GR is based on Barbour's Machian ideas. I will discuss the construction of Shape Dynamics. Then I will show how it can be coupled to standard matter. Depending on individual interest I will discuss Shape Dynamics in 2+1 dimensions, the classical correspondence between large CMC-volume and CFTs, ansaetze for quantization and/or current work on symmetry doubling from symmetry trading.

Norbert Bodendorfer (U. Erlangen) 8 December 2011

Towards loop quantum supergravity and its applications

This talk will deal with a new connection formulation for higher-dimensional (Super)gravity theories and its applications. We will start by reviewing the basic ideas of loop quantum gravity. Next, the derivation of the new connection formulation will be discussed and it will be shown that the quantisation methods developed in the context of loop quantum gravity apply. We comment on applications of the framework, focusing on making contact with String theory.

Bruce Allen (AEI, Hannover) 24 November 2011

The gravitational stochastic background: how will we detect it, and when?

I review the possible origins and properties of a gravitational wave stochastic background, and possible means of detection with both current and future instruments. Different mechanisms to detect a stochastic background are described, along with the prospects for their detection.

Philipp Höhn (ITF, Utrecht) 27 October 2011

Canonical simplicial gravity

In this talk I will introduce a general canonical formalism for discrete theories which can handle varying phase space dimensions and constraints and, furthermore, is fully equivalent to the covariant formulation. In particular, I will apply this canonical formalism to (Euclidean) Regge Calculus.

Juliane Behrend (ITF, Utrecht) 20 October 2011

On the Possibility of a Poincaré Gauge Theory as an Averaged Theory of Gravity

Despite the effort that has been devoted to the averaging problem of general relativity, it still remains unresolved. The reason might be that the framework of general relativity is too restrictive to take account of the averaging effects. In this talk I want to discuss a novel approach to the problem which starts from the teleparallel equivalent of general relativity (TEGR) and in which the averaged theory is described as a Poincré gauge theory. This will be illustrated with an example of cosmological relevance.

Karim Thébault (U of Sydney & Tilburg) 13 October 2011

The role of time in relational quantum theories

We propose a solution to the global problem of time. Our solution stems from the observation that, for theories where the time evolution is generated by a global Hamiltonian constraint, conventional gauge theory methods fail to capture the classical dynamics of the full system. We propose a new strategy for consistently quantizing systems with a relational notion of time that captures the full classical dynamics of the system and allows for evolution parametrized by an equitable internal clock. This proposal contains the minimal temporal structure necessary to retain the ordering of events required to describe classical evolution. In the context of shape dynamics, an equivalent formulation of general relativity that is locally scale invariant and free of the local problem of time, our proposal constitutes a natural methodology for describing dynamical evolution in quantum gravity.

Igor Khavkine (ITF, Utrecht) 8 September 2011

Characteristic geometry and causality in locally covariant field theory

Geroch (gr-qc/9602055) has shown that all equations of relativistic field theory can be cast into symmetric hyperbolic, first order, quaslinear form. Characteristic geometry, a field dependent notion intrinsic to such systems, then appears in theorems on local well-posedness, domains of influence and domains of dependence, replacing Lorentzian geometry as the notion of causal structure. I will discuss the relationship between these theorems of classical field theory and the Brunetti-Fredenhagen-Verch axioms of locally covariant quantum field theory. This line of reasoning also motivates a proposal for the structure of the field commutator in a quantum field theory of gravity.

Artur Tsobanjan (Penn State U.) 18 August 2011

Effective equations for quantum mechanics and semiclassical observables in quantum cosmology

The work is motivated by the canonical approach to quantization of the general theory of relativity. A key issue in this approach is the quantum implementation of the constraint algebra that arises in the Hamiltonian formulation of general relativity. Some aspects of this problem can be studied in a greatly simplified context of homogeneous cosmological models, which, due to their symmetry, possess only a finite number of classical degrees of freedom and the constraint conditions can be set up using standard quantum mechanics. In the first part of the talk I will describe a relatively recent technique for constructing approximate equations of motion for quantum systems in the semiclassical regime. In the second part of the talk, the technique together with additional input will be used to treat the Hamiltonian constraint of a class of homogeneous cosmological models.

Marco Scalisi (AEI Golm) 7 July 2011

Fractal and Noncommutative spacetimes: a connection

We examine, in parallel, fractal and noncommmutative spacetimes. Focusing on the action functional and on its nontrivial measure, we find a mapping between them. Depending on the scale at which the relation is estabilished, two possibilities arise. Near a fractional fundamental scale, identified with the Planck scale, the effective measure coincides with a cyclicity-inducing measure of k-Minkowski. For larger scale, the averaged fractional measure can be obtained as the ciclicity-inducing measure from a certain nonlinear algebra. This fractional algebra is given a physical interpretation as an interpolating spacetime structure between k-Minkowski and canonical noncommutativity.

Andrew Randono (Perimeter I.) 14 April 2011

Torsional Skyrmions: fermions from pure geometry

It is generally assumed that the gravitational field is bosonic. Here we show that a simple propagating torsional theory can give rise to localized geometric structures that can consistently be quantized as fermions under exchange. To demonstrate this, we show that the model can be formally mapped onto the Skyrme model of baryons, and we use well-known results from Skyrme theory. In addition to providing a a new matter-from-geometry approach, it begs the question: Is geometry bosonic or fermionic (or both)?

Tomasz Trześniewski (Utrecht & Krakow) 7 April 2011

The Discrete Semiclassical Action of CDT

CDT model is a nonperturbative approach to quantum gravity, demonstrating a spontaneous generation of the semiclassical solution. Gravitational path integral is approximated by the weighted sum over causality-preserving simplicial manifolds i.e. causal triangulations. Analysis of the data from numerical Monte-Carlo simulations of CDT in 3+1 dimensions shows that behaviour of (4, 1) simplicial building blocks of spacetime can be very well described by an expansion of the classical minisuperspace model. The precise form of the discrete analogue of the minisuperspace action will be discussed. Furthermore, it will be presented that the remaining (3, 2) simplices may also be successfully included in the semiclassical description, leading to the extended discrete action and that the consistency between the two frameworks exists.

Andreas Kreienbuehl (U. New Brunswick) 17 March 2011

Effective quantum gravitational collapse

We present a class of Hamiltonian deformations of the massless Einstein-Klein-Gordon system in spherical symmetry for which the Dirac constraint algebra closes. The system may be regarded as providing effective equations for quantum gravitational collapse. We analyze these equations numerically and find interesting deviations from Choptuik's classical findings.

Federico Piazza (APC Paris VII) 3 March 2011

Modifying gravity in the Infra Red by imposing an "ultra-strong" equivalence principle

The standard framework "gravity + matter fields" deals with some difficulties which already appear at low energy and therefore are likely to be detached from its UV completion. I will give account of a recent attempt to modify General Relativity (GR) in the infra-red (IR) and address those difficulties. The proposed modification does not contain any adjustable parameter as it is effective at length scales comparable with the inverse curvature. The guiding line for this modification is a recently proposed "ultra-strong" version of the equivalence principle, according to which the vacuum expectation value of the (bare) energy momentum tensor is exactly the same as in flat space: constant everywhere and quartically divergent with the cut-off. It is possible to work out explicitly the first IR-corrections to GR in the case of a Friedman Robertson Walker Universe. I will show that, for a matter dominated universe, the proposed modification goes in the direction of an effective positive acceleration, but it is too mild to explain, on its own, supernovae data.

Willem Westra 24 February 2011

The Klein Gordon field as a wave function: causality and localization

Usually in quantum field theory one considers two different interpretations:
1: The field is an infinite number of quantum oscillators, giving rise to a wave functional \Psi(\phi).
2: The positive frequency component of a field, \phi_+(x), is a wave function analogous to standard quantum mechanics.
While interpretation 2 is often only mentioned implicitly it is crucial to standard computations of measurable scattering probabilities.

We extend the interpretation of QFT as relativistic quantum mechanics (option 2) and show how the total Klein Gordon field which consists of positive and negative frequency contributions, \phi = \phi_+ + \phi_- , can be interpreted as a wave function. Our construction manifestly shows that signal propagation in QFT cannot exceed the speed of light. This follows from the replacement of the Feynman propagator by the Wheeler propagator, which is just the time ordered commutator. Our second observation is that interpretation 2 is problematic if one uses the Newton Wigner position operator, therefore we introduce a more natural bilinear position operator. We show by an explicit example that the bilinear operator, contrary to the Newton Wigner operator, allows relativistic particles to be perfectly localized, precisely as in non relativistic quantum mechanics.

Paul Reska (ITF, Utrecht) 17 February 2011

Black holes in CDT quantum gravity and inflationary cosmology

In this talk I will give an overview of my research on black holes in quantum gravity from causal dynamical triangulations (CDT) and in inflationary cosmology. First, the coupling of a point-like mass to CDT and the signature of the Schwarzschild-de Sitter background geometry in this framework will be briefly summarized. Then, I will discuss the implication of the presence of black holes in inflationary cosmology. The correction to the power spectrum of scalar cosmological perturbations on quasi-de Sitter space is then presented for different masses and positions of the black hole. Some comments about observational consequences and the expected prevalence of such objects will also be made.

Maïté Dupuis (ENS, Lyon) 16 November 2010

U(N) framework and simplicity constraints in spin foam models for quantum gravity

In the context of loop quantum gravity and spin foam models, the simplicity constraints are essential in that they allow to write general relativity as a constrained topological theory. I will first recall the spin foam quantization procedure and focus more particularly on the step consisting in implementing the simplicity constraints. Then, I will present the U(N) framework using harmonic oscillators initially developed for SU(2) intertwiners. Finally, I will show how we can apply this new framework to impose the simplicity constraints in the context of 4d Euclidean gravity and present new solutions defined in term of U(N) coherent states.

Igor Khavkine (UU) 19 October 2010

Comment on `Hawking radiation from fluctuating black holes'

In a recent paper (arXiv:1005.0286), Takahashi & Soda have attempted to assess the effect of interaction with quantum, dynamical gravitons on the spectrum of scalar Hawking radiation from a Schwarzschild black hole. Unfortunately, an improper treatment of intermediate divergences makes their results unreliable. I will summarize the logic of their calculations, comment on the divergences encountered in its course and on how the calculations could be completed. A more detailed discussion can be found in (arXiv:1008.5059).

Paul Reska (UU) 23 September 2010

Causal dynamical triangulations with a point-like source

In this talk I will discuss the coupling of a point-like mass to quantum gravity in the non-perturbative setting of causal dynamical triangulations (CDT). To provide a point of comparison for the classical limit of the matter-coupled CDT model, the spatial volume profile of Euclidean Schwarzschild-de Sitter space glued to an interior matter solution will be discussed. For this a proper-time foliation of the Euclidean manifold is introduced that matches the global time slicing present in CDT. Some of the implementation details for numerically measuring the expectation value of the volume profiles will also be presented.

Maarten van de Meent (UU) 8 April 2010

Collisions in 3+1D piecewise flat gravity

In 2008 `t Hooft introduced a locally finite gravity model based on string defects propagating in a locally flat spacetime. In this talk we will explore the issue of colliding string defects. In particular, we will comment on the tension between resolving generic collisions and the imposed locality and causality conditions in the model.

Mark Jackson ( Univ. Leiden ) 25 maart 2010

Observing Quantum Gravity in the sky

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Philipp Hohn ( UU ) 18 Feb. 2010

Joe Henson ( Perimeter Institute ) 7 jan 2010

Spectral geometry as a probe of quantum spacetime

Employing standard results from spectral geometry, we provide strong evidence that in the classical limit the ground state of three-dimensional causal dynamical triangulations is de Sitter spacetime. This result is obtained by measuring the expectation value of the spectral dimension on the ensemble of geometries defined by these models, and comparing its large scale behaviour to that of a sphere (Euclidean de Sitter). From the same measurement we are also able to confirm the phenomenon of dynamical dimensional reduction observed in this and other approaches to quantum gravity -- the first time this has been done for three-dimensional causal dynamical triangulations. In this case, the value for the short-scale limit of the spectral dimension that we find is approximately 2. We comment on the relevance of these results for the comparison to asymptotic safety and Horava-Lifshitz gravity, among other approaches to quantum gravity.
 

HongSheng Zhao ( St. Andrews ) 17 Dec 2009

Fifth force in galaxies?

This talk will be based on the recent article "Universality of galactic surface densities within one dark halo scale-length" (Nature 461, 627-628), http://www.nature.com/nature/journal/v461/n7264/suppinfo/nature08437.html=

Elena Magliaro (CPT, Marseilles) 12 Nov 2009

The new loop quantum gravity propagator

We compute metric correlations (graviton propagator) in loop quantum gravity with the dynamics defined by the new spin foam models. The analysis is done at the lowest order in a vertex expansion and at the leading order in a large spin expansion. The result is compared to the standard graviton propagator of perturbative quantum gravity. In particular we recover the expected scaling with distance for all components of the propagator. Similar techniques can be applied to study the fractal properties of quantum space time.

Tomas Sotiriou (DAMTP Cambridge) 22 Oct 2009

Horava-Lifshitz gravity and projectability

Models of 3+1 dimensional quantum gravity based on anisotropic scaling at a z=3 Lifshitz point, variants of the model initially proposed by Horava, have been attracting remarkable attention lately. The differences between these models will be explained and their shortcomings will be discussed. Emphasis will be given to the model with projectability and without detailed balance, which seems to be the least problematic at this stage, and its theoretical development and phenomenology will be presented.

T. Janssen ( UU ) 2 July 2009

The infrared sector of quantum fields on cosmological space-times

J. Smit ( Amsterdam ) Mei 28 2009

Observables in quantum gravity

G. Stavenga ( UU ) Mei 7 2009

P. Machado (UU) April 23 2009

Taming perturbative divergences in asymtotically safe gravity

I. Pushkina (UU) April 2 2009

Cosmological constant dilemma revisited

A. Franzen ( Köln ) March 19 2009

Classical and quantum gravitational collapse of the Lemaitre-Tolman-Bondi model with positive cosmological constant

I. Prèmont-Schwarz ( Perimeter ) March 5 2009

Emergent Light-Cone Structure in Quantum Gravity

J. McDonald (Florida Atlantic University )-February 25 2009

Regge Calculus: Its Structure and the Incorporation of Non-gravitational Sources

One of the elegant features of classical General Relativity is embodied in the interplay between source and field. This is best exemplified in the Geometrodynamic Steering Principle--"Inertia here (local conformal scale factor) is determined by mass there." While quantum gravity is often represented by discrete geometries--as first described by Tullio Regge--there is insufficient understanding of the coupling between non-gravitational fields and the lattice spacetime to fully understand a discrete or quantum Steering Principle. In this seminar we explore the structural properties of simplicial spacetimes so as to illuminate a natural coupling of source to the lattice. We describe an approximate symmetry relation that ensures automatic conservation of source. We show how this can be put in the form of a Kirchhoff-like conservation principle. This is shown to have clear ramifications for the topological structure of matter coupled to the Regge lattice. We further define geometric observables in terms that are compatible with the suggested structure of matter in Regge Calculus. Moreover, we suggest a general principle for defining non-gravtiational sources in the lattice. We discuss how these findings may be an important contribution for research into discrete quantum gravity.
 

J. Tambornino ( AEI-MPG) - February 19 2009

Do we need matter to make gravity observable?

Due to the complicated structure of General Relativity's constraint algebra it is a highly nontrivial task to construct physical observables, i.e. quantities which are invariant under the action of the diffeomorphism group. In recent years there has been a lot of progress concerning this issue using the framework of ``Relational observables'' -- but most of the results obtained so far were rather formal, and besides that there still remain many questions regarding the interpretation of these quantities. In this talk I want to propose an extension of General Relativity (namely a coupling to a certain kind of dust matter fields first investigated by Brown and Kuchar) which makes it possible to compute diffeomorphism invariant observables, will present some recent tests concerning the validity of this framework and will comment on possible implications for a quantum theory of gravity.

B. Sandhoefer  ( Universität zu Köln ) - February 12 2009

Quantum Cosmologies with large-scale singularities

ABSTRACT: The general expectation is that quantum gravity resolves the singularities of classical general relativity. Much effort has been spent on the quantum cosmological study of the big-bang singularity. But other, more exotic types of singularities also exist. I will discuss the quantum cosmological behaviour of models with two new types of singularity: the so-called big rip occuring in generic phantom dark energy models and the big-brake singularity. Both occur at large scale factor, i.e. in the macroscopic universe. The question then arises whether these singularities are resolved through quantum gravity, or more pointedly, do quantum gravitational effects exist in a macroscopic universe?  

I. Khavkine (UU) - January 29 2009

Time delay in fluctuating geometries

I will describe an operationally defined observable of gravity coupled to a test particle, the clock synchronization time delay. The definition stems from a thought experiment where an observer synchronizes his clock with that of a remote, moving probe using light signals. This observable is thus sensitive to deviations of the geometry from flat space-time. By virtue of being operationally defined, it is gauge invariant and hence its expectation value may be computed in any classical or quantum statistical state. The time delay can be evaluated explicitly in linearized gravity. This is a report on work in progress.