Seminars
Winter term 2019/20
Quantum phasefield: from de Broglie–Bohm double solution program to doublon networks
Abstract: We study different variants of linear and nonlinear field equations, socalled ‘phasefield’ equations, in application to the de Broglie–Bohm double solution program. This defines a framework in which elementary particles are described by peaked nonlinear wave solutions moving by the quidance of a linear pilot wave. First, we consider the phasefield order parameter as a phase for the pilot wave, second as the pilot wave, third as a moving soliton which describes the particle. In the last case, we intoduce a superwave which amplitude is responsible for the particle moving in accordance to the de Broglie–Bohm theory. Lax pairs for the coupled problems are found in order to discover the phasefield equations and to draw analogies to the de Broglie–Bohm double solution program. Finally, doublons in 1+1 dimensions are constructed as self similar solutions of a nonlinear phasefield equation. The doublons set the frame for a Schrödinger type linear wave equation determining the energetics of the coupled system. Applying a conservation constraint and using general symmetry considerations the doublons are arranged as a network in 1+1+2 dimensions where nodes are interpreted as elementary particles.
Quantum creation of a universeantiuniverse pair
Abstract: The creation of two universes, one contracting and another expanding, filled with matter can also be interpreted as the creation of two expanding universes, one of them filled with matter and the other filled with antimatter, forming a universeantiuniverse pair. In that case, the total amount of matter and antimatter in the two universes is completely balanced, restoring the (apparent) matterantimatter asymmetry observed in each single universe. Furthermore, the creation of universes in pairs would entail observational consequences, perhaps distinguishable, in the properties of the CMB of a universe like ours, making testable the whole multiverse proposal.
Spatial curvature of cosmic structures
Abstract: Curvature of spatial hypersurfaces is usually considered in the context of globally homogeneous cosmological models, however it can also play a nonnegligible role below the scale of homogeneity. Relativistic Lagrangian perturbations allow us to get insight into mildly nonlinear stages of structure formation, substantially exceeding the standard Eulerian regime. In my talk, I will mainly focus on the spatial curvature estimates utilizing the Relativistic Zel'dovich approximation which is the first order solution to Einstein equations in Lagrangian form. Several theoretical and numerical results will be presented including the value of scalar curvature and averaged scalar curvature at the turnaround epoch for a wide set of initial conditions. Potential observational consequences will be put into perspective.
Quantum black hole structure with the Event Horizon Telescope, an overview of S. Gidding's paper
Abstract: We will review the ideas presented in and motivating the paper “Searching for quantum black hole structure with the Event Horizon Telescope” (Giddings, arXiv:1904.05287) and give a brief overview of possible perturbations for the EHT image presented in “Event Horizon Telescope Observations as Probes for Quantum Structure of Astrophysical Black Holes” (Giddings and Psaltis, arXiv:1606.07814).
On the factor ordering problem in Quantum Cosmology
Abstract:
Choosing the three metric as a configuration variable and applying the Dirac
quantization method to general relativity leads to Quantum Geometrodynamics.
The main equations of the theory are the Wheeler–DeWitt equation and the
quantized version of the diffeomorphism constraints. Apart from technical
issues (the Wheeler–DeWitt equation is illdefined) the canonical
quantization reveals some conceptual problems. These are most prominently the
problem of time, the Hilbert space problem and the factor ordering problem.
All of these problems are in fact intimately connected.
The symmetry reduction to spatially homogeneous cosmological models, the so
called minisuperspace models, provides us with mechanical system analogues of
full general relativity.
Hence the application of Dirac quantization yields quantum mechanical analogues
of Quantum Geometrodynamics. This allows us, among other things, to address the
problems encountered in the full theory in a simplified setup.
In this talk I will introduce a generalized mechanical system endowed with the
main features of a typical minisuperspace model. We study the geometry of this
model and pave the way for the canonical quantization of the system.
Our main focus will be on the factor ordering problem.
On the construction of diffeomorphisminvariant observables
Abstract: We describe a method of construction of gaugeinvariant operators (Dirac observables or “evolving constants of motion”) from the knowledge of the eigenstates of the gauge generator of timereparametrisation invariant mechanical systems. These invariant operators evolve unitarily with respect to an arbitrarily chosen time variable. We emphasise that the dynamics is relational, both in the classical and quantum theories. In this framework, we show how the “emergent WKB time” often employed in quantum cosmology arises from a weakcoupling expansion of invariant transition amplitudes, and we illustrate an example of singularity avoidance in a vacuum Bianchi I (Kasner) model.
Quantum gravitational computations in de Sitter
Abstract:
In Wheeler–DeWitt quantum cosmology, a classical universe can
emerge from a realistic wave function at a suitable scale. It has been
argued that such wave functions are strongly peaked around classical
solutions; in other words, the ridgeline of a physical wave packet
corresponds to a classical solution.
We give quantitative descriptions of the ridgeline of a wave packet.
With explicit examples of wave packets, it is shown that ridgelines can
deviate from classical trajectories at classical scales. Possible
interpretations are discussed.
Quantum gravitational computations in de Sitter
Abstract: Inflation is an extremely efficient particle collider. Particles up to Hubble energies (roughly 10^{14} GeV) can be excited, giving us the chance to explore a vast particle spectrum. A common feature of all the inflationary models is that they predict at least 2 massless particles: The socalled comoving curvature perturbation ζ and the graviton field. The corresponding primordial power spectra as well as the nongaussian correlators of these fields are a central topic of study nowadays in cosmology. In the current talk, some issues related to quantum loop corrections to these tree level results will be discussed. In particular, the focus will be mostly on the interaction (computations performed at 1loop order) between gravitons and scalar fields in an exact de Sitter background.
Perturbations on a quantum cosmological spacetime
Abstract: With the use of canonical General Relativity and coherent state quantisation procedure one obtains a quantum model of spacetime. For cosmological spacetimes such models produce singularity avoidance scenarios, resulting in a smooth evolution of the universe undergoing the Big Bounce. Introducing quantum perturbations to the quantum cosmological spacetime allows to consider interesting observables such as primordial amplitude spectra, and to study how they are influenced by genuinely quantum properties of the cosmological background. Discussed theoretical effects may in principle probe the quantum era of the universe and should be further developed.
Date  Time  Speaker  Topic  Room 

October 8, 2019  12:00 
Leonardo Chataignier (Universität zu Köln) 
Conference report  Konferenzraum 1 (Neubau) 
October 15  12:00 
Ingo Steinbach (RuhrUniversität Bochum) 
Quantum phasefield: from de Broglie–Bohm double solution program to doublon networks  Konferenzraum 1 (Neubau) 
October 22  12:00 
Salvador RoblesPérez (Estación Ecológica de Biocosmología de Medellín (Spain)) 
Quantum creation of a universeantiuniverse pair  Konferenzraum 1 (Neubau) 
November 5  12:00 

Literature seminar: Quantum information in quantum gravity  Konferenzraum 1 (Neubau) 
November 19  12:00 
Jan J. Ostrowski (Narodowe Centrum Badań Jądrowych, Warsaw) 
Spatial curvature of cosmic structures  Konferenzraum 1 (Neubau) 
November 26  12:00 
Alexander Hermanns (Universität zu Köln) 
Quantum black hole structure with the Event Horizon Telescope, an overview of S. Giddings' paper  Konferenzraum 1 (Neubau) 
December 3  12:00 
Nick Kwidzinski (Universität zu Köln) 
On the factor ordering problem in Quantum Cosmology  Konferenzraum 1 (Neubau) 
December 10  12:00 
Leonardo Chataignier (Universität zu Köln) 
On the construction of diffeomorphisminvariant observables  Konferenzraum (Altbau) 
December 17  12:00 
Claus Kiefer (Universität zu Köln) 
Conference Report and Beyond  Konferenzraum 1 (Neubau) 
January 14, 2019  12:00 
YiFan Wang (Universität zu Köln) 
Towards a classical correspondence of wave packets in quantum cosmology  Konferenzraum 1 (Neubau) 
January 21  12:00 
Vasilis Fragkos (Universiteit Utrecht) 
Quantum gravitational computations in de Sitter  Konferenzraum 1 (Neubau) 
January 28  12:00 
Artur Miroszewski (Narodowe Centrum Badań Jądrowych, Warsaw) 
Perturbations on a quantum cosmological spacetime  Konferenzraum 1 (Neubau) 
Past seminars
Summer term 2019
Winter term 2018/19
Summer term 2018
Winter term 2017/18
Summer term 2017
Winter term 2016/17
Summer term 2016
Winter term 2015/16
Summer term 2015
Winter term 2014/15
Summer term 2014
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Winter term 2003/04
Summer term 2003