home UU | home department

Abstracts Grafiti
 

Grafiti 230
Igor Khavkine (UU) - December 5 2011

Time delay observable in classical and quantum geometries

IGiven a certain clock synchronization thought experiment, the inherent, operationally defined, gravitationally induced time delay can be seen as a diffeomorphism invariant observable. It is sensitive to the deviation of the ambient geometry from flat space-time and satisfies interesting inequalities related to the causal structure. In the linearized gravity approximation, the time delay defines a gauge invariant observable, which can be explicitly evaluated in both classical and quantum states. Partial results are available for the quantum Fock vacuum state. (reference: arXiv 1111.7127)

Grafiti 229
Andreas Kreienbuehl (ITF, Utrecht) - November 14 2011

Quantum cosmology and polymer matter

In this talk I will first summarize a project from 2009 in which I performed a reduced phase space quantization of an isotropic, spacially flat FRW Universe sourced by a non-negative cosmological constant and a massless scalar field. I will show how I chose the scale factor as clock, made use of a Schroedinger representation for the scalar field, and concluded that no semi-classical state avoids the big bang singularity. This result is in contrast to the outcome of earlier work by Ashtekar, Pawlowski, and Singh, who found that the big bang singularity can be avoided if the scalar field is chosen as clock and the remaining geometric degrees of freedom are polymer quantized. In the second part of the talk I will therefore introduce the polymer quantization method by applying it to a free particle and explain how it can be used for the quantization of the scalar field in a FRW model. If time permits, I will mention results form 2010 by Hossain, Husain, and Seahra, who found that a polymer quantized scalar field can naturally lead to a nonsingular Universe with inflation and a graceful exit therefrom.

Grafiti 228
Timothy Budd (UU) - November 7 2011

Probing moduli space using dynamical triangulations

The model of Dynamical Triangulations can be viewed as a regularization of non-critical string theory or 2d Euclidean gravity coupled to conformal matter. In this talk I will demonstrate how to assign a moduli parameter to a random triangulation of the torus, which describes its conformally invariant shape. This method is then used to obtain numerically the distribution in moduli space corresponding to the ensemble of genus-1 dynamical triangulations of pure gravity (c=0) as well as gravity coupled to matter with conformal charge c=-2. I will show that the numerical data converges to the continuum expressions known from Liouville theory.

Grafiti 227
Chris van den Broeck (NIKHEF) - October 24 2011

Binary neutron stars and black holes as laboratories for testing general relativity

Coalescences of binary neutron stars and/or black holes are amongst the most promising sources of directly detectable gravitational waves with the upcoming Advanced Virgo/LIGO interferometer network. They will also provide us with our very first empirical access to the genuinely strong-field dynamics of general relativity (GR). I will first give an overview of how we aim to detect and analyze these signals. Next I outline a parameter estimation method which uses them to search for generic violations of GR, irrespective of the detailed nature of the deviation. The technique tests the consistency of a signal's phase with the prediction of GR. It will easily lend itself to combining information from multiple sources to increase confidence in GR being correct or otherwise. I will end with showing some preliminary results obtained by analyzing simulated signals containing small deviations from GR.

Grafiti 226
Sean Gryb (ITF, Utrecht) - October 10 2011

Shape dynamics: the conformal backbone of general relativity

Shape dynamics is an equivalent description of general relativity that is, itself, free of the local problem of time. Its discovery was motivated by Julian Barbour's relational program based on a precise interpretation of Mach's writings. The key features of shape dynamics that distinguish it from general relativity are: 1) that all local symmetries, which include local Weyl invariance of the spatial metric, are linear in the momenta and can be formally solved in any dimension, and 2) that the dynamics is generated by a single global Hamiltonian constraint. We will give a summary of the key features of shape dynamics and discuss some interesting future research directions including the possibility of quantizing and a promising link to gauge/gravity dualities.

Grafiti 225
Samo Jordan (ITF, Utrecht) - October 3 2011

A second-order phase transition in CDT

Covariant approaches to Quantum Gravity, such as Causal Dynamical Triangulations (CDT), generically need to implement a discretization of the system to make the path integrals well-defined. This either leads to a discrete theory or to a lattice regularization of a continuum theory. We are interested in the question whether CDT in 3+1 dimensions possesses a continuum limit and thus fits into the latter category. One typically expects such a continuum limit to be found at a critical point in the phase diagram. Past searches for critical points in dynamically triangulated models have been mostly fruitless. In this talk I present results based on Monte-Carlo measurements, which strongly support the existence of a second order phase transition line in CDT in 3+1 dimensions.

Grafiti 224
Gianluca Calcagni (AEI Golm) - July 5 2011

Multi-fractal spacetimes and quantum gravity

We introduce multi-fractional spacetime, a continuum model with multi-fractal geometry. Its dimension, symmetry and geometry change with the scale. At ultra-small scales, symmetries are discrete and conventional geometry is broken down by oscillations in the action measure. At intermediate scales, spacetime is continuous and two-dimensional, and flows to an infrared limit where the dimension is 4 and Lorentz invariance is restored. The UV properties of field theories living therein are discussed.

Grafiti 223
Yi Wang (McGill) - June 27 2011

Inflation in a landscape

What if inflation is more complicated than single field slow roll? In this talk I will discuss three possibilities: quasi-single field inflation, multi-stream inflation and single field chain inflation. Observational signatures such as power spectrum and non-Gaussianities are addressed for these scenarios.

Grafiti 222
Rainer Verch (U. Leipzig)- June 20 2011

Quantum Dirac field on Moyal-Minkowski spacetime - illustrating quantum field theory over Lorentzian spectral geometry

A sketch of an approach towards Lorentzian spectral geometry (based on joint work with Mario Paschke) is described, together with a general way to define abstractly the quantized Dirac field on such Lorentzian spectral geometry. Moyal-Minkowski spacetimes serve as an example. The scattering of the quantized Dirac by a non-commutative (Moyal-deformed) action of an external potential is investigated. It is shown that differentiating the S-matrix with respect to the strength of the scattering potential gives rise to quantum field operators depending on elements of the non-commutative algebra entering the spectral geometry description of Moyal-Minkowski spacetime, in the spirit of "Bogoliubov's formula", in analogy to the situation found in external potential scattering by a usual scalar potential.

Grafiti 221
Sundance Bilson-Thompson - June 6 2011

Is matter made of spacetime?

Approaches to quantum gravity, like CDT and loop quantum gravity, can manifest a wide range of topological degrees of freedom. It is appealing to think that the ways different quanta of spacetime connect to each other may give rise to fermions and bosons. I will describe work which attempts to explore this possibility, and pose some questions about what conditions must be met to yield an interacting theory, and how these ideas could be modelled within the framework of CDT, with the goal of fostering discussion and collaboration between Utrecht and Adelaide.

Grafiti 220
Gemma de las Cuevas (Innsbruck U.) - May 30 2011

A quantum information approach to discrete quantum gravity

We apply quantum information concepts and tools to certain models of discrete quantum gravity, namely to Causal Dynamical Triangulation (CDT). We have previously tackled classical spin models from a quantum information perspective, and this has allowed us to prove new results for these models. CDT provides a natural route to generalize our previous work, inasmuch as the central quantity has the form of a partition function, and classical spin models are used as toy models of matter. We therefore expect to gain new insight into CDT by studying it from the angle of quantum information. We will present the first steps in this direction.

Grafiti 219
Yuko Urakawa (Barcelona) - May 23 2011

Implications of genuine gauge-invariance on primordial perturbations

The conventional cosmological perturbation theory has been performed under the assumption that we know the whole spatial region of the universe with infinite volume. This is, however, not the case with the actual observations because the observable portion of the universe is limited. To give a theoretical prediction to the observable fluctuations, gauge-invariant observables should be composed of the information in our local observable universe with finite volume. From this point of view, we reexamine the primordial non-Gaussianity in single-field and multi-field models of inflation, requesting the gauge-invariance in the local observable universe. Our result requests the significant modification of the conventional predictions.

Grafiti 218
Astrid Eichhorn (Jena U.) - April 11 2011

The functional Renormalisation Group: Physics on different scales

Quantum field theories are defined by the microscopic action, which describes interactions in the ultraviolet. Integrating out quantum fluctuations in the path integral we get the effective action in the infrared, which can be vastly different from the microscopic action. We are interested in describing physics at different scales, and thus connecting the microscopic description in the ultraviolet to the macroscopic description in the infrared. As a tool we introduce the functional Renormalisation Group, which can be applied to many different theories. As an example, we consider QCD, where we know the microscopic interactions in the ultraviolet, but the resulting macroscopic physics at low energies is strongly-interacting and exhibits properties such as confinement. We show how the functional Renormalisation Group can be used to investigate confinement at zero as well as finite temperature. On the other hand we consider examples, where we understand the effective theory at low energies, but search for a UV-completion, such as in quantum gravity. Here the functional Renormalisation Group can be applied to search for a UV-completion of gravity within the framework of quantum field theory, which is known under the name of asymptotic safety. We show results suggesting the realisation of asymptotic safety in gravity, and in particular focus on the question if this scenario is compatible with the observed fermionic matter content of our universe.

Grafiti 217
Tomi Koivisto (ITF, Utrecht) - April 4 2011

Unifying Einstein and Palatini gravities

New approaches to nonmetricity in gravity are presented. There the relation of the metric and the connection can have a nontrivial dependence upon the curvature, providing a unified framework for the metric and the metric-affine (or Einstein and Palatini) gravity theories. Furthermore, using the variational principle where the connection is assumed to be generated by an independent metric, even an action linear in the curvature can feature propagating nonmetricity and torsion. Cosmological implications in particular to chaotic inflation are discussed.

Grafiti 216
Felix Finster (U. Regensburg) - March 28 2011

Causal fermion systems: A quantum space-time emerging from an action principle

We begin by introducing an action principle defined on a finite set of points. This action principle is causal in the sense that it generates a relation on pairs or points which distinguishes between spacelike and timelike separation. In this way, minimizing the action gives rise to a "discrete causal structure". By generalizing the action principle to include continuum space-times we get to the setting of "causal fermion systems". We outline how for a given minimizer, one can introduce notions of connection and curvature, which generalize the classical notions and give rise to a proposal for "quantum geometry". In the second part of the talk, it is explained how Minkowski space is formulated in this framework to obtain a formulation of quantum field theory. The differences to standard quantum field theory are explained in a simplified model where Dirac particles interact via an axial field.

Grafiti 215
Jan-Willem van Holten (NIKHEF/Leiden U.) - March 7 2011

The gravity of light

Like matter, light is a source of gravity. In this talk I review the gravitational properties of a light wave and its interaction with matter. The similarities and differences with gravitational waves will be discussed.

Grafiti 214
Claudio Dappiaggi (DESY & Pavia)- February 21 2011

On the relation between free quantum fields and Lambda-CDM

We investigate the backreaction of free quantum fields on a flat Robertson-Walker spacetime. Apart from renormalization freedom, the vacuum energy receives contributions from both the trace anomaly and the thermal nature of the underlying quantum state. The former represents a dynamical realisation of dark energy, while the latter encodes the relic density of thermally produced dark matter. The semiclassical dynamics yield two classes of asymptotically stable solutions. The first reproduces the Lambda-CDM model with additional quantum corrections. The second lacks a classical counterpart, but is in excellent agreement with recent observations.

Grafiti 213
Albert Roura (AEI, Golm)- February 14 2011

One-loop Riemann correlators and de Sitter invariance

After briefly introducing the effective field theory approach to study quantum gravitational phenomena with characteristic length-scales much larger than the Planck length, I will focus on the study of one-loop corrections to the two-point function of the Riemann tensor for metric perturbations around de Sitter (dS) spacetime. This object can provide valuable insight on the possibility of a secular screening of the cosmological constant due to higher-loop effects as well as on the radiative corrections to the spectrum of cosmological perturbations in inflationary models. We have obtained a manifestly dS-invariant result for the correlations of the Ricci tensor, which exhibits arbitrarily long-range correlations for loops of fields with sufficiently small (but non-vanishing) masses, and are currently investigating the correlations of the Weyl tensor. I will conclude with a discussion of the implications for the power spectrum of primordial gravitational waves.

Grafiti 212
David Sloan (ITF, Utrecht) - January 17 2011

The a priori probability of inflation in Loop Quantum Cosmology

In loop quantum cosmology (LQC) the big bang is replaced by a quantum bounce which is followed by a robust phase of super-inflation. Rather than growing unboundedly in the past, the Hubble parameter vanishes at the bounce and attains a finite universal maximum at the end of super-inflation. These novel features allow one to unambiguously define a probability based on the Liouville measure, which leads to an unforeseen implication: in presence of suitable potentials all LQC dynamical trajectories are funneled to conditions which virtually guarantee slow roll inflation with more than 68 e-foldings.

Grafiti 211
Sergey Solodukhin (LMPT, Univ. Tours) - December 6 2010

Entanglement entropy of black holes

In this talk I will review the notion of entanglement entropy and its applications to the problem of black hole entropy.

Grafiti 210
Xavier Calmet (Sussex) - November 29 2010

Unitarity constraints on models of particle physics coupled to gravity

In this talk I discuss the unitarity of the S-matrix for linearized General Relativity coupled to particle physics models. Consequences for different models (grand unified theories, models of low scale quantum gravity, Higgs inflation models) are discussed.

Grafiti 209
Silke Weinfurtner (SISSA, Trieste) - November 22 2010

Measurement of stimulated Hawking emission in an analogue system

There is a mathematical analogy between the propagation of fields in a general relativistic space-time and long (shallow water) surface waves on moving water. Hawking argued that black holes emit thermal radiation via a quantum spontaneous emission. Similar arguments predict the same effect near wave horizons in fluid flow. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include wave horizons. Long waves propagating upstream towards this region are blocked and converted into short (deep water) waves. This is the analogue of the stimulated emission by a white hole (the time inverse of a black hole), and our measurements of the amplitudes of the converted waves demonstrate the thermal nature of the conversion process for this system. Given the close relationship between stimulated and spontaneous emission, our findings attest to the generality of the Hawking process.

Grafiti 208
Y. Jack Ng (UNC Chapel Hill) - November 15 2010

Spacetime Foam, Holographic Cosmology and MoNDian Dark Matter

Probed at small distances, spacetime appears to be very complicated -- something akin in complexity to a turbulent froth which John Wheeler dubbed spacetime foam (aka quantum foam). I will discuss spacetime foam physics involving length measurements and how spacetime foam can be detected. Then I will touch on the cosmology inspired by spacetime foam physics, and a scheme that reconciles the cold dark matter and the modified Newtonian dynamics approaches.

Grafiti 207
Fay Dowker (Imperial College) - November 8 2010

Discrete Spacetime, Lorentz Invariance, Locality and All That

Causal sets form the basis for an approach to the problem of quantum gravity that is fundamentally discrete and yet also respects Lorentz symmetry. This makes the approach radically nonlocal and threatens to de-rail any attempt to recover the local physics - in particular General Relativity - which works so well. I will describe recent progress in causal set theory that indicates that the non-locality can be tamed, in particular a proposal for an action for a causal set that there is reason to believe may be approximately local. I will try to use the causal set story to illuminate issues that will arise in *any* approach to quantum gravity with a discrete flavour.

Grafiti 206
Jonathan Hackett (Perimeter I.) - November 1 2010

The Octonions and Topology in Loop Quantum Gravity

I will present the state of research into understanding the meaning of topological objects in Loop Quantum Gravity. I will also present a related model for octonionic multiplication.

Grafiti 205
Glenn Barnich (Université Libre de Bruxelles) - October 25 2010

Aspects of the BMS/CFT correspondence

After a brief review of purely gravitational aspects of the AdS3/CFT2 correspondence, a similar analysis is performed for asymptotically flat spacetimes at null infinity in 3 and 4 dimensions. In the spirit of two dimensional conformal field theory, it is shown that the symmetry algebra of asymptotically flat spacetimes at null infinity in 4 dimensions can be taken to be the semi-direct sum of supertranslations with infinitesimal local conformal transformations and not, as usually done, with the Lorentz algebra. As a first application, we derive how the symmetry algebra is realized on solution space. In particular, we work out the behavior of Bondi's news tensor, mass and angular momentum aspects under local conformal transformations. We comment on possible implications for the problem of angular momentum in general relativity, for the concept of particles and for quantum gravity.

Grafiti 204
Pietro Falgari (UU) - October 18 2010

Threshold resummation for pair-production of coloured heavy particles at hadron colliders

Pair-production of known (top quarks) or yet undiscovered (squarks, gluinos,...) heavy coloured particles will play an important role in the physics program of the LHC in the years to come. Unfortunately, a precise theoretical description of this class of processes is hampered by the fact that, near the pair-production threshold, large corrections arise from exchange of soft and Coulomb gluons. These terms can in principle lead to a breakdown of perturbation theory, and must thus be resummed to all orders in the strong coupling constant alpha_s. In this talk I will discuss an approach to the problem based on an effective-theory description of the pair-production process near threshold, show how resummation is achieved via renormalisation-group equations in momentum space and present explicit results for squark-antisquark production at the LHC.

Grafiti 203
Juliane Behrend (UU) - October 11 2010

Covariant Averaging of a Poincaré Gauge Theory of Gravity

So far the averaging problem of general relativity is unresolved and thus, for example, the role of backreaction effects in cosmology remains unclear. We argue that the Riemannian framework of general relativity is too restrictive to formulate an averaged theory of gravity and suggest the Riemann-Cartan framework instead. Additionally, a covariant averaging process can be formulated easily when a teleparallel structure exists on a manifold. In our novel approach to the problem we make use of an equivalence between general relativity and a class of teleparallel theories of gravity. Open issues with this approach will be addressed along with an outlook on future research.

Grafiti 202
David Sloan (UU) - October 4 2010

A Hamiltonian Formulation of the BKL Conjecture

The BKL Conjecture is cast in terms of Ashtekar variables and the resulting dynamics explored. Through a truncation of the constraints of General Relativity, a subspace of the full phase space is found which is invariant under the full dynamics and contains the Mixmaster dynamics. The relationship with the strong coupling limit of gravity is discussed, as in the inclusion of a simple form of matter. The Kasner 'u-map' is derived, with related "spike" points. If time permits, a possible link to dimensional reduction on approach to singularities a la Carlip will be presented.

Grafiti 201
Antonia Micol Frassino (Parma U.) - September 13 2010

Uncertainty relations and field quantization in non-commutative space-times

After an introduction to some non-commutative spacetime hypotheses and Doubly special relativity theory, I will discuss the k-deformed phase space in two different bases (defined by commutation relations between boosts and momenta) with the aim of studying the different uncertainty relations. Finally I will derive the k-Minkowski Green functions for a possible quantum field theory of Doubly Special Relativity theory.

Grafiti 200
Martin Bojowald (Penn State U.) - June 21 2010

Space and time in effective loop quantum gravity

Loop quantum gravity has shown several characteristic effects in the quantum geometry it implies via operators. Resulting implications for the dynamics of space-time remain incompletely understood, but they can be probed by means of effective equations. We will review the canonical way of formulating effective equations, as well as recent results about the deformation of space-time symmetries and the nature of time.

Grafiti 199
Fotini Markopoulou (Albert Einstein I. and Perimeter I.) - June 14 2010

A quantum Bose-Hubbard model on an evolving graph as a toy model for emergent spacetime

We present a toy model for interacting matter and geometry that explores quantum dynamics in a spin system as a precursor to a quantum theory of gravity. The model has no a priori geometric properties, instead, locality is inferred from the more fundamental notion of interaction between the matter degrees of freedom. The interaction terms are themselves quantum degrees of freedom so that the structure of interactions and hence the resulting local and causal structures are dynamical. The system is a Hubbard model where the graph of the interactions is a set of evolving quantum variables. We show entanglement between spatial and matter degrees of freedom and study numerically the quantum system and its entanglement dynamics. Finally, we discuss analogues of trapped surfaces and gravitational attraction in this simple model.

Grafiti 198
Jerzy Kowalski-Glikman (Wroclaw U.) - June 7 2010

Fun with BF

In my talk I will recall the construction of gravity in 4D as a constrained BF theory and I will explain how the coupling of gravity to particles can be constructed in this framework. Then I will briefly discuss the perturbation theory around topological vacuum provided by unconstrained BF theory, and its relevance in the context of Doubly Special Relativity.

Grafiti 197
Samo Jordan (ITF, Utrecht) - May 10 2010

Phase transitions in Causal Dynamical Triangulations

The theory of Causal Dynamical Triangulations (CDT) attempts to define a non-perturbative theory of quantum gravity as a sum over space-time geometries. Remarkably CDT has been shown to possess a large scale limit which can be modeled with high accuracy by a four-dimensional de Sitter universe. More precisely this limit is achieved in one of the three phases of geometry, which have been identified in earlier works on CDT. In this talk, after giving a review of the CDT basics, I discuss recent efforts to study the phase transitions, and in particular, the fixed point structure in the CDT phase diagram. Possible connections to the Asymptotic Safety program and also to anisotropic theories of gravity are outlined towards the end of the talk.

Grafiti 196
John Kelley (Radboud University) - May 3 2010

Searching for Quantum Gravity with High-Energy Cosmic Rays and Neutrinos

The high energies and long baselines probed by recent particle astrophysics experiments allow searches for possible effects of quantum gravity. We discuss two of these experiments, involving the detection of high-energy cosmic rays and neutrinos. The Pierre Auger Observatory measures cosmic rays to energies beyond 100 EeV, and the spectrum at such extreme energies can be used to search for violations of Lorentz invariance. High-energy neutrinos can also be used to search for physics beyond the Standard Model, as violation of Lorentz invariance or quantum decoherence can lead to unexpected flavor changes in the neutrino flux. We present limits on these phenomena obtained with the AMANDA and IceCube neutrino telescopes at the South Pole. astrophysical bounds on the system.

Grafiti 195
Marieke Postma (NIKHEF) - March 22 2010

Hidden photons as (luke)warm dark matter?

Dark matter is usually assumed to be cold, although a case can be made for warm dark matter. I will discuss the possiblity of a hidden photon --- the gauge field of an extra U(1) symmetry --- as warm dark matter. At low energies the hidden photon coupling to the standard model degrees of freedom is uniquely defined, and we can calculate its relic density. The results have to be compared with the various cosmological and astrophysical bounds on the system.

Grafiti 194
Gianluca Calcagni ( Albert Einstein Inst. Golm ) - March 8 2010

Fractal universe

I outline a recent proposal for a quantum field theory defined on fractal spacetime. The physical spacetime is two-dimensional in the UV and unfolds to four dimensions at sufficiently large scales. Consequences for renormalization, quantum gravity, and its motivation from other approaches (Causal Dynamical Triangulation, Quantum Einstein Gravity, Horava-Lifshitz gravity) are discussed. Based on arXiv:0912.3142, arXiv:1001.0571.

Grafiti 193
Marius de Leeuw ( UU ) - March 1 2010

The S-matrix of the AdS5 x S5 superstring

In this talk we will discuss the scattering theory of bound states of the AdS5 x S5 superstring. The key feature in this discussion is a so-called Yangian symmetry. We will explain how this symmetry allows one to determine the exact world-sheet S-matrix. After this we will discuss the Bethe ansatz, which can be used to derive the large-volume energy spectrum of these bound states

Grafiti 192
Frank Saueressig ( Univ. Mainz ) - Jan 11 2010

The Asymptotic Safety Program of Quantum Gravity - Status and
Perspectives

Weinberg's asymptotic safety conjecture proposes that gravity constitutes a consistent and predictive Quantum Field Theory within Wilson's generalized framework of renormalization. In this scenario, the
high-energy behavior of the theory is controlled by a non-trivial fixed point of the gravitational renormalization group flow, which ensures the absence of unphysical UV divergences. We give a pedagogical introduction to one of the main tools available for investigating this conjecture,
the functional renormalization group equation for gravity, and summarize the evidence for the existence of the non-trivial UV fixed point underlying the asymptotic safety program.
 

Grafiti 191
Pedro Machado - Dec 14 2009

On the Renormalization Group Flow of Gravity

Understanding the quantum behavior of gravity poses a problem. While general relativity is an extremely successful theory at the classical level, attempts to reconcile it with quantum mechanics by means of perturbative quantization indicate a breakdown of the quantized theory at the Planck scale. A possible solution to this problem is Weinberg's asymptotic safety scenario, which conjectures the existence of a non-trivial fixed point of the gravitational RG flow providing quantized gravity with a predictive and well-defined high-energy limit.

In recent years, this scenario has been actively investigated by means of the functional renormalization group equation, a continuous algorithm that serves as a powerful framework for probing the RG behavior of field theories. In this talk, after giving a brief overview of this framework, I will present recent and not-so-recent results on the RG flow of gravity that provide strong evidence for asymptotic safety.

This talk is based on my PhD thesis.
 

Grafiti 190
Sabine Hossenfelder - Dec 7 2009

Phenomenological Quantum Gravity

The search for a satisfying theory that unifies general relativity with quantum field theory is one of the major tasks for physicists in the 21st century. During the last decade, the phenomenology of quantum gravity has been examined from various points of view, opening new perspectives and testable predictions. I will give a short introduction into these effective models which allow to extend
the standard model and include some of the expected effects of the underlying fundamental theory. I will talk about models with extra dimensions, models with a minimal length scale and those with a
deformation of Lorentz invariance. The focus is on observable consequences, such as black hole and graviton production, modifications of standard-model cross-sections, and an energy dependent speed of light.

Grafiti 189
Miles Blencowe - Nov 30 2009

Analogue Hawking Radiation in a Superconducting Circuit

We propose the use of a nonlinear transmission line formed from an array of superconducting quantum interference devices (SQUIDs) for investigating analogue Hawking radiation. Biasing the array with a space-time varying flux modifies the propagation velocity of the transmission line, leading to an effective metric with a horizon. Being a fundamentally quantum mechanical device, the SQUID array also allows for the investigation of quantum effects such as analogue space-time fluctuations on the Hawking process.
 

Grafiti 188
Tomi Koivisto (UU) - Nov 23 2009

Bouncing universes in modified gravity

The initial singularity can be avoided in bouncing universes. In this talk motivations to consider these scenarios in certain gravity models are discussed. The background expansion and the evolution of the perturbation spectra are then described with some examples.
 

Grafiti 187
Massimiliano Rinaldi (Geneva U.) - Nov 16 2009

Living in rough (space-)times

Minimal lengths in fundamental physics might be interpreted as the frontier beyond which certain laws break down.  Independently of the underlying model, one can try to draw phenomenological consequences based on general arguments. In this spirit, we briefly review some old ideas on modified dispersion relations, and then introduce more recent results about a minimal length arising from non-commutativity. In the light of a new approach to non-commutativity, we review the Unruh effect and some questions about cosmology and black hole physics.  

Grafiti 186
Claudio Perini (CPT, Marseilles) - Nov 10 2009

Rethinking spin foams

The new spin foam models are revised with special attention to contact with loop gravity. We show the matching between LQG and spin foam kinematical states and how transition amplitudes (dynamics) are defined. The generality of the spin foam formalism is such that we have to abandon a picture based on triangulations, and come back to the physical picture suggested by loop gravity: space-time is a relation between its own quanta.

Grafiti 185
Rachel Maitra (UU) - Oct 26 2009

Pain or Gain: The Factor-Ordering Problem and How CDT Helps

The causal dynamical triangulations (CDT) program has for the first time allowed for path-integral computation of correlation functions in full general relativity without symmetry reductions and taking into account Lorentzian signature. One of the most exciting recent results in CDT is the strong agreement of these computations with (minisuperspace) path integral calculations in quantum cosmology. In this talk I will describe my current project to compute minisuperspace (Friedman-Robertson-Walker) path integrals with a range of different measures corresponding to various factor orderings of the Friedman-Robertson-Walker Hamiltonian. The aim is to compare with CDT results and ask whether CDT can shed light on factor-ordering ambiguities in quantum cosmology models.

Grafiti 184
Michele Arzano (UU) - Oct 19 2009

Fun from none: deformed Fock space and hidden entanglement

Attempts to go beyond the framework of local quantum field theory include scenarios in which the action of external symmetries on the quantum fields Hilbert space is deformed.  A common feature of these models is that the quantum group symmetry of their Hilbert spaces induces additional structure in the multiparticle states which in turns reflects a non-trivial momentum-dependent statistics. The richer structure of the deformed Fock space allows for the possibility of entanglement between the field modes and, in particular, when the deformation scale is proportional to the Planck energy, it leads to a "planckian" mode-entanglement invisible to an observer that cannot probe the Planck scale.

Grafiti 183
Gerard 't Hooft (UU) - Oct 12 2009

Quantum gravity without space-time singularities or horizons

In an attempt to re-establish space-time as an essential frame for formulating quantum gravity - rather than an "emergent" one -, we find that exact invariance under scale transformations is an essential new ingredient for such a theory. Use is made of the principle of "black hole complementarity", the notion that observers entering a black hole describe its dynamics in a way that appears to be fundamentally different from the description by an outside observer. These differences can be boiled down to conformal transformations. If we add these to our set of symmetry transformations, black holes, space-time singularities, and horizons disappear, while causality and locality may survive as important principles for quantum gravity.

Grafiti 182
Silke Weinfurtner (UBC Vancouver) - Sept 28 2009

Anisotropic scaling: From condensed matter systems to Horava-Lifshitz gravity

Horava-Lifshitz gravity is based on the assumption of anisotropic scaling of space and time. Such an unequal splitting of spacetime into space and time is expected to regulate the ultraviolet behaviour to the extent of generating a renomalisable quantum field theory of gravity. Although we are lacking any rigorous calculations for Horava-Lifshitz gravity first power-counting arguments seem promising; for a certain degree of anisotropy it is indeed possible to set the gravitational coupling constant to zero.

Given the situation at hand the theory is on shaky grounds. However, we will demonstrate that the very idea of anisotropic scaling is a nature inspired concept arising in all quantum field theories involving an emergent (or induced) spacetime geometry. We will show that for a certain degree of anisotropy it is indeed possible to use anisotropic scaling as a quantum field theory regulator for an arbitrarily interacting (massive or massless) scalar field. Inspired from this well understood quantum field theory we will show how to map the concept of  anisotropic scaling onto gravity; and in this sense onto any field theory.

Joseph Samuel (RRI Bangalore) - June 9 2009

The universe in a soap film

The value of the cosmological constant is one of the major puzzles of modern cosmology: it is tiny but nonzero. Sorkin predicted, from the Causet approach to quantum gravity, that the cosmological constant has quantum fluctuations. The predicted order of magnitude of the fluctuations agrees with the subsequently observed value of the cosmological constant. We had earlier developed an analogy between the cosmological constant of the universe and the surface tension of fluid membranes. Here we demonstrate by computer simulations that the surface tension of a fluid membrane has statistical fluctuations stemming from its discrete molecular structure. Our analogy enables us to view these numerical experiments as probing a small and fluctuating cosmological constant. Deriving insights from our analogy, we show that a fluctuating cosmological constant is a generic feature of quantum gravity models and is far more general than the specific context in which it was originally proposed. We pursue and refine the idea of a fluctuating cosmological constant and work towards making further testable predictions.

G. Cornelissen ( UU, Math ) - May 11 2009

Spectral Invariants from Noncommutative Geometry

I will show various way to construct spectral data (observables) on Riemannian manifolds using a viewpoint advocated in noncommutative geometry, and show some reconstruction theorems of the manifold from the spectral data (so you *can* hear their shape). In case of Riemann surfaces, there is a similar construction on the "boundary of uniformization", reminiscent of a kind of holographic principle.
 

R. Schützhold ( U. Düsseldorf ) - March 23 2009

Emergent horizons in the laboratory

The concept of a horizon known from general relativity describes the loss of causal connection and can be applied to non-gravitational scenarios such as out-of-equilibrium condensed-matter systems in the laboratory. This analogy facilitates the identification and theoretical study (e.g., regarding the trans-Planckian problem) and possibly the experimental verification of 'exotic' effects known from gravity and cosmology, such as Hawking radiation. Furthermore, it yields a unified description and better understanding of non-equilibrium phenomena in condensed-matter systems and their universal features. By means of several examples including general fluid flows, expanding Bose–Einstein condensates and dynamical quantum phase transitions, the concepts of event, particle and apparent horizons will be discussed together with the resulting quantum effects.
 

J. Vink ( SIU, UU ) - February 23 2009

Cosmic ray production by supernova remnants:
evidence for cosmic ray dominated shock acceleration

ABSTRACT: Cosmic rays observed on earth span a large energy range; from 10^9 eV up to ~10^20 eV. It is generally assumed that the cosmic rays up to ~10^18 eV are produced in our Galaxy,
whereas the highest energy cosmic rays must come from extra-galactic sources (active galaxies,
gamma-ray bursts, or large scale shocks).
In this talk I will focus on the origin of cosmic rays below 10^18 eV, which are thought to be
powered by supernova explosions. In particular, I will review the recent observational evidence for efficient cosmic ray acceleration by the shocks of young supernova remnants (those with ages less than 2000 yr). Thisevidence is based on the observation of X-ray synchrotron radiation, TeV gamma-ray radiation, but also on a careful analysis of optical spectroscopic data, with which the energetics of shocks can be measured. From these observations it is clear that young supernova remnants are able to convert a large part of theenergy flux into the shock into cosmic ray energy. I will place the evidence in the framework of so-called diffusive shock acceleration theory.

C. Kiefer ( Universität  zu Köln ) 9 February 2009

Quantum avoidance of classical singularities

ABSTRACT: Any reasonable quantum theory of gravity should avoid the singularities predicted by general relativity. In my talk I shall discuss various possible criteria for such an avoidance and exemplify them in two concrete models: the quantum cosmological counterpart of a classical model with a big-brake singularity and a model for the quantum gravitational collapse of a dust shell. I shall also comment on the situation in loop quantum cosmology.
 

U. Ebert (CWI, Amsterdam) - 12 January 2009

The multiscale dynamics of sparks and lightning

Sparks and lightning in nature and technology evolve on multiple time and length scales. The first stage of the discharge consists of streamers which are cold growing plasma channels very far from equilibrium; they pave the way of hot sparks and lightning leaders with velocities of 10^5 to 10^7 m/s in ambient air. The talk will largely focus on the streamer aspects of natural and technologically used discharges.

The list of physical questions is long: How does lightning start, and how does it reach the ground when in both cases the background electric field is much too low? How quantitatively have we characterized and understood the various lightning processes below, within, between and above thunderclouds? How far do we understand lightning phenomena in the atmospheres of other planets? or ignition processes in argon discharge lamps? Why are streamer discharges in air so efficient in ozone generation (a green house gas, that is very important in waste gas treatment and disinfection technology), when a new generation of pulsed power voltage supplies is used? Do streamers also produce the X-rays observed from approaching lightning leaders? And which lightning process generates the terrestrial gamma-ray flashes that are observed from satellites?

We have developed a range of methods: We now can photograph the evolution of streamers with nanosecond resolution, and we can perform experiments in a wide parameter range and in very
clean gases. We can reconstruct their 3D branched tree structure. We can simulate single streamers with their internal structure and study their (electro-)dynamic behavior. We also begin to model the acceleration of single electrons in the high field region at the streamer tip. 3D simulations with adaptive grid refinement allow us to study streamer interactions. Negative streamers in simple gases can be treated in part analytically based on moving boundary approximations for ionization fronts;
and a relation to a classical problem in hydrodynamics can be established. But much remains to be done.