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 |
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.
Close
November 21 |
12:00 |
Marios Christodoulou (Marseille)
|
Geometry transition in covariant LQG: black to white
|
Konferenzraum 1 (Neubau) |
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.
Close
November 28 |
12:00 |
Xavier Calmet (Brighton)
|
Quantum gravity: the effective way
|
Konferenzraum 1 (Neubau) |
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.
Close
December 5 |
12:00 |
Leonardo Chataignier
|
Radiative Symmetry Breaking in Classically Conformal Extensions of the Standard Model
|
Konferenzraum 1 (Neubau) |
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.
Close
December 12 |
12:00 |
YiFan Wang
|
Integrable Cosmological Models with Liouville Scalar Fields
|
Konferenzraum 1 (Neubau) |
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.
Close
December 15 |
14:00 |
Mariam Bouhmad-López (EHU-Ikerbasque, Bilbao)
|
3-forms and the recent speed up of the Universe
|
TBA |
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.
Close
December 19 |
12:00 |
Gabriel Menezes (Rio de Janeiro)
|
Inducing the Einstein action in QCD-like theories
|
Konferenzraum 1 (Neubau) |
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.
Close
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) |
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.
References
[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].
Close
January 12 |
11:00 |
Lucas Kiefer
|
Dark Energy: Theoretical Approaches, Observational Constraints, and Cosmic Future Singularities
(Bachelor colloquium)
|
Seminar room of the Astrophysics Institute |
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.
Close
January 30 |
12:00 |
Diego Sáez-Chillon Gómez (Barcelona, IEEC)
|
Beyond General Relativity: exponential gravity as a particular case
|
Konferenzraum 1 (Neubau) |
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.
Close
February 5 |
12:00 |
Anirudh Gundhi
|
Scale Invariant Inflation
|
Konferenzraum 1 (Neubau) |
February 16 |
12:00 |
Wolfgang Wieland (Perimeter Institute)
|
New boundary variables for classical and quantum gravity on a null surface
|
Konferenzraum 1 (Neubau) |
New boundary variables for classical and quantum gravity on a null surface
Abstract:
In my talk, I present new canonical boundary variables for classical and
quantum gravity on a null surface. These variables are found by considering
general relativity as a Hamiltonian system in domains with inner null
boundaries. The presence of such lightlike boundaries requires then
additional boundary terms in the action. Using Ashtekar?s original SL(2,C)
selfdual variables, I will explain that the natural such boundary term is
nothing but a kinetic term for a spinor (defining the null flag of the
boundary) and a spinor-valued two-form, which are both intrinsic to the
boundary. The relevance of this new boundary term for the definition of
quasi-local observables and for quantum gravity in particular will be
explained. I will show, in particular, that in quantum gravity the oriented
area of a two-dimensional cross section of the null boundary turns into the
difference of two number operators. The resulting area spectrum is discrete
and agrees with the results from loop quantum gravity. The entire derivation
happens at the level of the continuum theory, and no spin-networks or SU(2)
gauge variables are ever required for deriving this result.
Close
February 20 |
12:00 |
Lisa Glaser (Nijmegen)
|
A partially ordered introduction to Causal Set theory
|
Konferenzraum 1 (Neubau) |
A partially ordered introduction to Causal Set theory
Abstract:
In causal set theory space-time is described as a partially ordered
set. While this makes identifying geometric properties a little more
cumbersome, it does provide a Lorentz invariant discretisation.
One interesting side effect of this discretisation are non-local
effects in field theories that could be searched for in experiments.
Another feature of this discrete description is that we can use
computer simulations to explore the path integral over geometries.
In this lecture I will introduce the basic set up of causal set
theory, tell you a little bit about non-local signatures and then show
some results of computer simulations.
Close
April 24 |
12:00 |
Andrea Zanzi (University of Ferrara)
|
The chronon in M-theory
|
Konferenzraum 1 (Neubau) |
The chronon in M-theory
Abstract:
In this seminar we discuss some aspects of chronon physics in an M-theory
model. We start considering the possibility that quantum fluctuations are
stretched on very large distances allowing a quantum mechanical treatment of
physics on certain macroscopic scales. A crucial element of our analysis is
the relativity of time. Indeed, the presence of a 5D black hole (with its
gravitational field) leads us to a scenario where small quantum fluctuations
produced near the black hole become very large for an asymptotic observer in
harmony with the relativity of time. In the deep IR region, gravity shows
new phenomena (related to an orbifold of time) which cannot be described
through a field theory on the brane and, in this sense, these phenomena
resemble the action-at-a-distance of Newtonian gravity when interpreted from
the standpoint of S-brane physics. If an observer on the brane tries to
analyze quantum gravity in his ground state, the gauge fixing procedure
shows that all the matter fields and all the interactions of the Standard
Model become redundant: the only physical degree of freedom for quantum
gravity in this case is the time (namely the chronon) with its two
dimensions (parametrized by the dilaton and the radion). Remarkably, the
dynamics of the chronon is governed by a modified Schroedinger equation and,
in this sense, the Schroedinger equation is the most fundamental equation of
physics. A dilatonic signal traveling in the bulk can bring information from
the future (of a different dilatonic time dimension) to our present
dilatonic time. The selection of the dilatonic time dimension is related to
the value of the chameleonic radion which is stabilized by some UV dynamics.
Therefore, a modification of the environment produces a small shift in the
radionic coordinate and this selects a totally different dilatonic time
dimension (this mechanism is reminiscent of a butterfly effect). A number of
consequences of this approach will be discussed.
Close