| Date |
Time |
Speaker |
Topic |
Room |
| October 18 |
12:00 |
various speakers
|
Conference reports
|
Konferenzraum 1 (Neubau) |
| October 18 |
|
Matthias Dahlmanns (Duisburg)
|
fo
Particle creation in an expanding universe with a Big-Brake singularity
|
Konferenzraum 1 (Neubau) |
Particle creation in an expanding universe with a Big-Brake singularity
Abstract: As Leonard Parker has shown in his Ph.D. thesis, particles are created in an
expanding universe if one applies Quantum Field Theory in Curved Spacetimes. In my Bachelor
thesis, I investigated this mechanism for flat FLRW universes with a certain kind of future
singularity which occur in the Big-Brake model. They are caused by a vanishing energy density
at a finite time. Therefore it might be questioned if Quantum Field Theory in Curved Spacetimes
offers a possibility to avoid this singularity by changing the energy density in the universe
due to the created particles.
After a short introduction of the mentioned mechanism, I will talk about the results of my numerical
computations and there results on the evolution of the particle number while approaching the singularity.
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| October 25 |
12:00 |
Mariusz Klimek (Darmstadt)
|
Discretization of Maxwell's Equations for Non-inertial Observers Using
Space-Time Algebra
|
Konferenzraum 1 (Neubau) |
Discretization of Maxwell's Equations for Non-inertial Observers Using
Space-Time Algebra
Abstract: We employ Maxwell's equations formulated in Space-Time Algebra to
perform discretization of moving geometries
directly in space-time. All the derivations are carried out without any
non-relativistic
assumptions, thus the application area of the scheme is not restricted
to low velocities.
The 4D mesh construction is based on a 3D mesh stemming from a
conventional 3D mesh generator.
The movement of the system is encoded in the 4D mesh geometry,
enabling an easy extension of well-known 3D approaches to the space-time
setting.
As a research example, we study a manifestation of Sagnac's effect in a
rotating ring resonator.
In case of constant rotation, the space-time approach enhances the
efficiency of the scheme,
as the material matrices are constant for every time step, without
abandoning the relativistic framework.
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| November 3 |
14:00 |
Bethan Cropp (IISER, Trivandrum)
|
Hints of quantum gravity from the horizon fluid
|
Konferenzraum 1 (Neubau) |
Hints of quantum gravity from the horizon fluid
Abstract: For many years researchers have tried to glean hints about quantum
gravity from black hole thermodynamics. However, black hole thermodynamics
suffers from the problem of Universality --- at leading order, several
approaches with different microscopic degrees of freedom lead to
Bekenstein-Hawking entropy. We attempt to bypass this issue by using a
minimal statistical mechanical model for the horizon fluid based on
Damour-Navier-Stokes (DNS) equation. For asymptotically flat black hole
spacetimes in General Relativity, we show explicitly that at equilibrium the
entropy of the horizon fluid is the Bekenstein-Hawking entropy. Further we
show that, for the bulk viscosity of the fluctuations of the horizon fluid
to be identical to Damour, a confinement scale exists for these
fluctuations, implying quantization of the horizon area. The implications
and possible mechanisms from the fluid point of view are discussed.
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| November 8 |
12:00 |
Matthias Hanauske (Frankfurt am Main)
|
Gravitational waves and the interior structure of hypermassive
neutron stars from binary mergers
|
Konferenzraum 1 (Neubau) |
Gravitational waves and the interior structure of hypermassive
neutron stars from binary mergers
Abstract: One hundred years after Albert Einstein developed the field equations of
general relativity and predicted the existence of gravitational waves
(GWs), these curious spacetime-ripples have been observed from a pair of
merging black holes by the LIGO detectors. As GWs emitted from merging
neutron star binaries are on the verge of their first detection, it is
important to understand the main characteristics of the underlying merging
system in order to predict the expected GW signal. Based on a large number
of numerical-relativity simulations of merging neutron star binaries, the
emitted gravitational wave and the interior structure of the generated
hypermassive neutron stars (HMNS) have been analyzed in detail. This talk
will focus on the internal HMNS properties (e.g. differential rotation
profiles, structure of the space-time metric, particle composition) and
their connection with the emitted GW signal.
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| November 22 |
12:00 |
Dirk Puetzfeld (Bremen)
|
Measurement of the gravitational field in General Relativity
|
Konferenzraum 1 (Neubau) |
Measurement of the gravitational field in General Relativity
Abstract: In General Relativity, the comparison of test bodies moving along adjacent
world lines is of direct operational significance. The observation of a suitably prepared
set of test bodies allows for the determination of the components of the curvature.
We present some recent results on generalized deviation equations for test bodies and
the measurement of the gravitational field by means of these equations.
Close
| November 29 |
12:00 |
Jamir Marino (Cologne)
|
Thermal and non-thermal signatures of the Unruh effect in Casimir-Polder forces
|
Konferenzraum 1 (Neubau) |
Measurement of the gravitational field in General Relativity
Abstract: I will discuss how Casimir-Polder forces between two relativistic uniformly
accelerated atoms exhibit a transition from the short distance thermal-like behavior predicted by the
Unruh effect to a long distance nonthermal behavior. This phenomenology extends the Unruh thermal response
detected by a single accelerated observer to an accelerated spatially extended system of two particles, and we identify the
characteristic length scale for this crossover with the inverse of the proper acceleration of the two
atoms. Furthermore, at the end of the talkI will discuss some preliminary results
on analogue effects for the Casimir force of two atoms close to a black
hole.
Close
| December 6 |
12:00 |
Alexander Hermanns
|
The time warp: Changing the progression of time by using Alcubierre drive-like spacetime structures
|
Konferenzraum 1 (Neubau) |
The time warp: Changing the progression of time by using Alcubierre drive-like spacetime structures
Abstract: Since the beginning of space exploration, humanity has encountered a critical problem:
the vast extend of space and the immense distances between objects seem to make
probing a very time consuming activity. This fact is due to the limitation discovered by
Einstein that no particle with mass can travel with the speed of light and nothing can
exceed this speed. Therefore a journey to our closest neighbour galaxy, the Andromeda
Galaxy, would take about 2.5 million years, even if we could travel at the speed of light.
This has led to many searches for a way to exceed this speed limitation, proven by
general relativity, without breaking the theory. Inspired by the science fiction way to
solve this problem, Miguel Alcubierre developed a spacetime which acts like a "warp
drive" and provides a theoretical solution to travel at superluminal velocities.
In the first part of this work, we will give an introduction of the Alcubierre metric,
followed by a short summary of discoveries which have been made while studying it (we
don’t take quantum physics into account) and point out the problems that arise in the
theory. This is followed by the analysis of the Alcubierre metric in Cartan calculus, which
will give us its Einstein equations. In the second part we will use central structures of
the Alcubierre metric to derive a "time warp". This being an region of space in which
we can modify the progression of time compared to its outside. In the third part we will
try to combine the aspects of both metricies i.e. a moving time warp structure which
uses Alcubierres drive to move through space at arbitrary speeds.
Close
| December 16 |
10:00 |
Mariam Bouhmadi-Lopez (Bilbao/Covilha)
|
Possible doomsdays - what are observations and theory telling us
at this regard?
|
0.03 (Neubau) |
Possible doomsdays - what are observations and theory telling us
at this regard?
Abstract: In this talk, we will present a brief review on dark energy
singularities and abrupt events. We will start with a classical
description of several models, including their behavior at the
perturbative level. Then, we will contrast our results with
observational data. On the second part of the talk, we will focus on
the quantum analysis of these singularities/abrupt events. The
analysis will encompass mainly modified theories of gravity. If time
allows, we will as well show how phantom dark energy models might not
at all imply dark energy singularities in Einstein-Cartan theory.
Some references (on which the talk is based): arXiv:1611.00392,
arXiv:1608.01679, arXiv:1611.03100, arXiv:1604.08365,
arXiv:1609.00700
Close
| December 16 |
10:00 |
Prado Martin-Moruno (Madrid)
|
Horndeski accelerating solutions as a result of
an adjustment mechanism
|
0.03 (Neubau) |
Horndeski accelerating solutions as a result of
an adjustment mechanism
Abstract: In the context of Horndeski cosmologies, we consider a dynamical
adjustment mechanism able to screen any value of the vacuum energy of the
matter fields leading to a fixed de Sitter geometry. Thus, we present the
most general scalar-tensor cosmological models without higher than second
order derivatives in the field equation that have a fixed spatially flat
de Sitter critical point for any kind of material content or vacuum energy.
These models allow us to understand the current accelerated expansion of the
universe as the result of the evolution towards the critical point when it
is an attractor.
Close
| December 16 |
10:00 |
Nick Kwidzinski
|
Classical and Quantum Cosmology of Kantowski-Sachs Models
|
0.03 (Neubau) |
Classical and Quantum Cosmology of Kantowski-Sachs Models
Abstract: It is well known that by crossing the event horizon of an eternal Schwarzschild black hole the
timelike Killing vector field tips over and becomes spacelike. Consequently staticity is traded for homogeneity.
The interior Schwarzschild solution can therefore be regarded as belonging to a certain class of homogeneous but anisotropic cosmological models.
These are the so called Kantowski-Sachs models which will be at the center of this talk.
Special attention shall be given to the singularities occurring in these models and their resolution after quantization, which will be performed within the framework of quantum geometrodynamics.
Close
| December 20 |
12:00 |
Branislav Nikolic
|
What can we learn from treating first order Lagrangians as higher derivative theories:
The case of Einstein-Hilbert and non-minimally coupled scalar field theories.
|
Konferenzraum 1 (Neubau) |
What can we learn from treating first order Lagrangians as higher derivative theories:
The case of Einstein-Hilbert and non-minimally coupled scalar field theories.
Abstract: If a Lagrangian possesses a symmetry, then in its Hamiltonian formulation one ends up with first class constraints which reflect this symmetry.
If this symmetry is not present, the corresponding constraints still present, but are now 'spoiled' and become second class.
On an example of conformally coupled scalar field it is shown in the talk that in the standard Hamiltonian formulation of the theory there is no any
first class constraints related to the conformal symmetry of this Lagrangian, even though the theory is conformally invariant. How can that be?
Furthermore, standard Hamiltonian formulation of the Einstein-Hilbert theory (General Relativity) gives only 4 first class constriants
reflecting the reparametrization invariance symmetry, but no second class constraints are found, in spite of the
fact that conformal symmetry of GR is broken. It seems as if standard Hamiltonian formulation of GR and the scalar field does not give
information about the conformal properties of these theories. Where is that information?
The answer lies in treating the extrinsic curvature as an independent variable, and thus treating the mentoned theories as if they were higher derivative theories,
ignoring their first order nature. The result is that the Hamiltonian theory now looks like it has additional degrees of freedom, but the missing information about
conformal properties naturally reveals itself precisely in the additional first or second class constraints, and in this seminar we will try to understand why.
Close
| January 17 |
12:00 |
YiFan Wang
|
Hawking radiation: Comparison of pure-state and thermal description
|
Konferenzraum 1 (Neubau) |
Hawking radiation: Comparison of pure-state and thermal description
Abstract: We present the power spectral densities for a quantum field in the background of a
Schwarzschild black hole and compare the cases of the field being in a pure state and in the usual
thermal state. In the low-energy regime the densities strongly differ, while at high energies they
practically coincide. We define a distance measure between the resulting two density operators and
evaluate it between the pure and thermal states. We find that the distance is exponentially small with
respect to the Hawking temperature. We finally discuss the operational meaning of the vanishing distance.
Close
| January 20 |
16:30 |
William G. Unruh (University of British Columbia, Vancouver)
|
Measuring Black Hole radiation in the lab
|
TP seminar room 0.03 (Neubau) |
Measuring Black Hole radiation in the lab
Abstract: Hawking's prediction that black holes should radiate was one of the biggest surprizes
in physics of the late 20th century. It has dominated much of the study in the interface between
gravity and Quantum Mechanics ever since. The calculation Hawking presented is problematic, and thus
experimental confirmation would be good. Unfortunately, finding small black holes is hard.
However, there turn out to be analog systems, systems with horizons which one can and is studying in the lab.
I will present the arguments and the current experimental effort to see the radiation, and the arguments
that some experiments have already seen it.
Close
| January 23 |
10:15 |
Anton Krieger
|
Consequences of a Black Hole Firewall: Dynamics of a Dust Shell
|
Bonn, Wegelerstr. 10, Konferenzraum 1 (W 0.027) |
Consequences of a Black Hole Firewall: Dynamics of a Dust Shell
Abstract:
Close
| January 24 |
12:00 |
Kurt Sundermeyer (Berlin)
|
On Some Astounding Consequences of Klein-Noether Identities
|
Konferenzraum 1 (Neubau) |
On Some Astounding Consequences of Klein-Noether Identities
Abstract: It is well-known that local symmetries of an action lead by Noether’s second theorem to identically
fulfilled relations involving the Euler derivatives of the Lagrangian. Less well-known is the fact that
these identities are a consequence of a larger set of identities discovered by Felix Klein in 1918.
These Klein-Noether identities do have astounding consequences. (1) Assuming a specific local
symmetry for the fields involved, this largely fixes the action of a theory. (2) The Klein-Noether identities -
if expressed through phase-space variables - directly entail primary and secondary constraints.
At this stage no Hamiltonian is needed. (3) A ”Noether charge” can be defined which turns out to be a
specific linear combination of the first-class constraints. On the other hand, the first-class constraints
can be read off from the charge - without referring to a Hamiltonian. (4) For Legendre projectable
symmetry transformations this Noether charge is the generator of the transformations.
After defining the Klein-Noether identities and discussing its generic consequences, the considerations are
particularized for examples of Yang-Mills theories and generally covariant theories. The
history-of-science story of the Klein-Noether identities and its relation to the field of constrained
dynamics is presented as story of early discovery and later rediscovery.
Close
| February 1 |
11:00 |
YiFan Wang
|
Hawking Radiation: Comparison of Pure-state and Thermal Description
|
Bonn, Wegelerstr. 10, Konferenzraum 1 (W 0.027) |
Hawking Radiation: Comparison of Pure-state and Thermal Description
Abstract: The cases of the Hawking radiation field being described by a pure
state and by the usual thermal state are compared within the CGHS dilaton gravity model.
The field-strength fluctuations of the Fourier modes are computed, showing a discrepancy
in the low-energy regime, while coinciding at high energies. Then by defining a distance
for density operators and evaluating it for the cases concerned, the difference between
the pure and thermal descriptions is quantified and found to be exponentially small
with respect to the Hawking temperature. Possible physical interpretations are discussed.
Close
| February 9 |
14:00 |
Christian Steinwachs (Freiburg)
|
Quantum properties of theories with anisotropic scaling
|
Konferenzraum 1 (Neubau) |
Quantum properties of theories with anisotropic scaling
Abstract: We discuss the quantum properties of theories with an anisotropic scaling of space and
time. Such theories break fundamental Lorentz invariance. New techniques have to be
applied to analyze general renormalization properties and to perform explicit calculations
– among them a non-local gauge fixing procedure. We present a method how calculations
of quantum divergences in a non-covariant theory can be related to the well known heat
kernel technique for covariant theories and discuss the application of this technique to the
model of Hoˇrava-Lifshitz gravity.
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