Winter term 2017/18


Singularity Avoidance in Bianchi Type I Models in Quantum Cosmology

Abstract: In this thesis the possibility of avoiding cosmological singularities in Bianchi type I models in the framework of quantum geometrodynamics is investigated. Therefore three different models are considered: one with a positive cosmological constant, one with matter that fulfills the barotropic equation of state p=ωρ, and one with a generalized Chaplygin gas. Those exhibit Big Bang, Big Rip and Big Brake singularities, respectively. In all cases the classical behaviour as well as the behaviour of the solution of the corresponding Wheeler-DeWitt equation is analyzed and compared to the isotropic flat FLRW case. It is found that the there are solutions fulfilling the DeWitt criterion for singularity avoidance, that in some cases the classical behaviour is modified by the anisotropy (i.e. introducing new or changing present singularities) and that the additional degrees of freedom can lead to dispersive behaviour of the wave packets.


Geometry transition in covariant LQG: black to white

Abstract: Black holes may end their lifes by becoming white holes, as the result of a non-perturbative quantum-gravitational process. To the best of our knowledge, the transition is not forbidden by any physical principle and quantum theory should be in a position to describe relevant physics. In particular, estimates of characteristic timescales are within reach with currently available tools. We use the EPRL model to define a transition amplitude. The EPRL model represents the current state-of-the-art of the spinfoam quantization program, which provides a tentative definition for the regularized path-integral of gravity in the context of Loop Quantum Gravity. We will review the conceptual setup of this ongoing project, present results for the characteristic timescales and discuss implications for the information paradox.


Quantum gravity: the effective way

Abstract: In this talk I will review recent progress in using effective field theoretical (EFT) methods to deal with quantum gravity. I will show that EFT techniques enable us to make model independent calculations and to identify features of the underlying, unknown, theory of quantum gravity. Equipped with these tools, we reconsider problems such as the calculations of quantum gravitational corrections to black hole metrics, stars or Newton's potential and comment on applications to gravitational waves and other astrophysical probes of quantum gravity.


Radiative Symmetry Breaking in Classically Conformal Extensions of the Standard Model

Abstract: We will examine the dynamical breakdown of the electroweak symmetry in classically conformal extensions of the Standard Model known as Higgs Portal Models. In such extensions, the validity of perturbation theory for arbitrary field values can be guaranteed (or improved) with the renormalisation group (RG). To achieve this, we will discuss a new method to RG improve the effective potential with an arbitrary number of scalar fields. The method amounts to solving the renormalisation group equation for the effective potential with the boundary conditions chosen on the hypersurface where quantum corrections vanish. We will show how this procedure opens the possibility of studying the effective potential of different Higgs Portal Models across a large range of energy scales and, in particular, how it clarifies the issue of stability of the improved potential.


Integrable Cosmological Models with Liouville Scalar Fields

Abstract: An alternative method is proposed for homogeneous cosmological models with a Liouville field, which is a scalar field with an exponential potential. The key of the method is to use an integral of motion to eliminate the lapse function as the redundant degree of freedom. Applying this method in isotropic (Friedmann-Lemaître) as well as anisotropic (Bianchi-I) universes, explicit equations of time-independent classical trajectories in minisuperspace are derived, which makes their correspondence with the Wheeler--DeWitt quantum cosmological theory more directly.
The completeness and orthogonality of cosmological wave functions in quantum context are carefully reconsidered, so is the Hermiticity of the phantom model, which as a result admits a discrete spectrum unusually. The orthonormal eigenfunctions of the Wheeler--DeWitt equation pushes one step further to the calculation of observables of physical quantities.
The physical wave packets are established based on two definitions of norm, one is non-relativistic, the other is introduced from the techniques of pseudo-Hermitian quantum mechanics. Numerical results of both cases are given and discussed.


3-forms and the recent speed up of the Universe

Abstract: In this talk, we consider 3-form dark energy (DE) models with interactions in the dark sector. We aim to distinguish the phenomenological interactions that are defined through the dark matter (DM) and the DE energy densities. We identify the non-interacting 3-form DE model which generically leads to an abrupt late-time cosmological event which is known as the little sibling of the Big Rip (LSBR). We classify the interactions which can possibly avoid this late-time abrupt event. we make also some preliminary analysis constraint of our model, for example, in light of the SDSS III data on the measurement of the linear growth rate of structure.


Inducing the Einstein action in QCD-like theories

Abstract: We evaluate the induced value of Newton’s constant which would arise in QCD. The ingredients are modern lattice results, perturbation theory and the operator product expansion. The resulting shift in the Planck mass is positive. A scaled-up version of such a theory may be part of a quantum field theory treatment of gravity.


Abstract: Cosmic strings are interesting objects both from an experimental and a theoretical point of view. In this talk, we discuss two scenarios:

(i) The stress-energy tensor of a string without internal structure satisfies a simple equation of state (?stringy matter?). We demonstrate that spherical deformations of the Reissner-Nordström-(A)dS metric give rise to such stringy matter, which in turn can be interpreted as a continuous distribution of cosmic strings.

(ii) For stationary spacetimes admitting a non-degenerate closed conformal Killing-Yano 2-form (e.g. the Kerr-NUT-(A)dS geometry), we construct a special stationary string configuration from the principal null congruence and the timelike Killing vector. In the special case of the Kerr metric (and higher-dimensional generalizations thereof) these strings extend from the black hole horizon to spatial infinity and extract angular-momentum from the black hole. We interpret this as the action of an asymptotic torque.


[1] J.B. and V. P. Frolov, "Stationary black holes with stringy hair," arXiv:1711.06357 [gr-qc], to appear in Phys. Rev. D (2018)
[2] J.B. and V. P. Frolov, "Principal Killing strings in higher-dimensional Kerr-NUT-(A)dS spacetimes," arXiv:1801.00122 [gr-qc].


Dark Energy: Theoretical Approaches, Observational Constraints, and Cosmic Future Singularities

Abstract: In this thesis we will try to get a fundamental understanding of dark energy, starting by fully deriving the cosmological background model from a FLRW spacetime. In order to understand the observations that we want to discuss later on, we need to discuss both the matter species in our universe aswell as the concept of cosmic distances. The derivation of the equation of state of dark energy concludes the first section of this thesis. We follow up by discussing multiple examples of observations to find out, that they require dark energy to exist. This leads to constraints on the cosmic parameters and we will try to understand how these constraints are obtained. In the last section of this thesis we are going to discuss the fate of our universe under the existence of (phantom) dark energy before we fit a scalar field to our previous calculations to create a model for phantom dark energy.


Beyond General Relativity: exponential gravity as a particular case

Abstract: In this talk I will review the basis of some types of modified gravities, their pros and cons, the possibility of reproducing late-time acceleration and their viability. A particular case will be analysed, based on exponential terms in the gravitational action, such a model may be capable of reproducing both inflation as the late-time acceleration. I will discuss the constraints coming from different sources of data, both at the inflationary epoch as at late times by using Sne Ia, BAO and Hubble data. As will be shown, exponential gravity may be kept as a promising candidate for dark energy and inflation.


Date Time Speaker Topic Room
October 10 12:00 Organizational meeting
(Konferenzraum alte Theorie)
October 17 12:00 Friedrich Hehl A short note on the Wheeler-DeWitt metric Konferenzraum 1 (Neubau)
November 14 10:00 Dennis Piontek
(Master Colloquium)
Singularity Avoidance in Bianchi Type I Models in Quantum Cosmology HS 118, Room 1.019, AVZ I in Bonn
November 21 12:00 Marios Christodoulou (Marseille)
Geometry transition in covariant LQG: black to white Konferenzraum 1 (Neubau)
November 28 12:00 Xavier Calmet (Brighton)
Quantum gravity: the effective way Konferenzraum 1 (Neubau)
December 5 12:00 Leonardo Chataignier
Radiative Symmetry Breaking in Classically Conformal Extensions of the Standard Model Konferenzraum 1 (Neubau)
December 12 12:00 YiFan Wang
Integrable Cosmological Models with Liouville Scalar Fields Konferenzraum 1 (Neubau)
December 15 14:00 Mariam Bouhmad-López (EHU-Ikerbasque, Bilbao)
3-forms and the recent speed up of the Universe TBA
December 19 12:00 Gabriel Menezes (Rio de Janeiro)
Inducing the Einstein action in QCD-like theories Konferenzraum 1 (Neubau)
January 9 12:00 Jens Boos (University of Alberta)
Cosmic strings in stationary black hole geometries: stringy matter and principal Killing strings Konferenzraum 1 (Neubau)
January 12 11:00 Lucas Kiefer
Dark Energy: Theoretical Approaches, Observational Constraints, and Cosmic Future Singularities (Bachelor colloquium) Seminar room of the Astrophysics Institute
January 30 12:00 Diego Sáez-Chillon Gómez (Barcelona, IEEC)
Beyond General Relativity: exponential gravity as a particular case Konferenzraum 1 (Neubau)
February 5 12:00 Anirudh Gundhi
Scale Invariant Inflation Konferenzraum 1 (Neubau)


Past seminars

Summer term 2017
Winter term 2016/17
Summer term 2016
Winter term 2015/16
Summer term 2015
Winter term 2014/15
Summer term 2014
Winter term 2013/14
Summer term 2013
Winter term 2012/13
Summer term 2012
Winter term 2011/12
Summer term 2011
Winter term 2010/11
Summer term 2010
Winter term 2009/10
Summer term 2009
Winter term 2008/09
Summer term 2008
Winter term 2007/08
Summer term 2007
Winter term 2006/07
Summer term 2006
Summer term 2005
Winter term 2004/05
Summer term 2004
Winter term 2003/04
Summer term 2003