Field theory

Previous Seminars

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                    Prof. Anupam Mazumdar
        Van Swinderen Institute, University of Groningen

  "Towards singularity free theory of gravity and possible experimental tests"

      Thursday, 02. November 2017, 14:30, ZARM, Room 1280


ABSTRACT:
I will discuss how to resolve the classical singularity problems for gravity by extending Einstein's action by
Ghost free theory of gravity in 4 dimensions. I will briefly discuss how to possibly test these ideas in a laboratory and in
astrophysics.

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                        Dr Oleg Yu. Tsupko

(Space Research Institute of Russian Academy of Sciences)

  "Influence of plasma on the black hole shadow: analytical approach "

(Coauthors: Volker Perlick and Gennady S. Bisnovatyi-Kogan)

      Monday, 16 October 2017, 16:00

   University of Bremen, ZARM, Room 1280


ABSTRACT:
We consider static black hole solutions to Einstein--Euler--Heisenberg
system of equations. By using the Newman--Janis algorithm the
corresponding rotating solutions are obtained. The energy conditions for
the rotating solutions are analyzed. The event horizons and ergoregions
are studied. Also the test particles circular orbits and the shadow of
the  electrically charged rotating black hole are shown.

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                    Prof. Alfredo Macias
        Universidad Autonoma Metropolitana-Iztapalapa Mexico

  "Rotating Black Holes in the Einstein--Euler--Heisenberg theory"

      Thursday, 03. August 2017, 17:00, Room W2 3-349


ABSTRACT:
We consider static black hole solutions to Einstein--Euler--Heisenberg
system of equations. By using the Newman--Janis algorithm the
corresponding rotating solutions are obtained. The energy conditions for
the rotating solutions are analyzed. The event horizons and ergoregions
are studied. Also the test particles circular orbits and the shadow of
the  electrically charged rotating black hole are shown.

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        Tuesday, June 13, University of Bremen, ZARM, Room 1280

1) 14:30 - 15:30
Dr. Norman Guerlebeck (ZARM): The Meissner Effect for Isolated Horizons

2) 15:30 - 16:30
Dr. Audrey Trova (ZARM): Equilibrium configurations of rotating magnetized tori


Abstracts:

1) Black holes are important astrophysical objects describing an end state of
stellar evolution. There are theoretical predictions that Kerr black holes
with high spins expel magnetic fields. Analogously to a similar property of
superconductors, this is called Meissner effect. However, Kerr black holes
are pure vacuum solutions, which do not include, e.g., accretion disks,
which are essential for many observational techniques. Moreover, previous
investigations are mainly limited to weak magnetic fields. By employing the
formalism of weakly isolated horizons, we will be able to study generic
rapidly spinning black holes in equilibrium including those deformed by
accretion disks or other matter. We show for such black holes that they
expel even strong magnetic and electric fields. Thus, the Meissner effect
is not specific to Kerr black holes but rather a property of any axisymmetric
black hole in equilibrium. We assess shortly the impact on creation of jets
by accreting black holes and compare these results with recent observations.


2) Studies of equilibrium of toroidal structures of a perfect fluid are
important to understand the physics of accretion disks in active galactic
nuclei (AGN) and the dense self-gravitating tori around stellar mass
black holes. I will present the equilibrium structures of rotating magnetized
tori in the Newtonian regime and in the General Relativity framework with
various assumptions. My main interest was about to study the impact of the
self-gravity of such objects and the impact of the magnetic field on the
morphology and existence of theses equilibrium configurations.
For instance, the existence of these solutions is possible for certain values
of the model parameters, such as the rotation law, the polytropic index and
the magnetic field intensity.

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                Prof. Panayotis Kevrekidis      
             
         Dept. of Mathematics & Statistics, UMass, Amherst

  "Existence, Stability and Dynamics of  Solitary Waves, Vortices and
   Vortex Rings in Bose-Einstein Condensates: From Theory to Experiments"

        Friday, 9. June 2017, 14:15, Room W2 1-143

ABSTRACT:

In this talk, we will present an overview of some of our recent
theoretical, numerical and experimental efforts concerning the static,
stability, bifurcation and dynamic properties of coherent structures that
can emerge in one- and higher-dimensional settings within Bose-Einstein
condensates.  We will discuss how this ultracold setting can be approximated
at a mean-field level by a deterministic PDE of the nonlinear Schrodinger
type and what the fundamental nonlinear waves of the latter are, such as dark
solitons (in 1d) and vortices (in 2d), as well as vortex lines and rings (in 3d).
Then, we will try to go to a further layer of simplified description via
nonlinear ODEs encompassing the dynamics of the waves within the traps that
confine them, and the interactions between them. Finally, we will attempt to
compare the analytical and numerical implementation of these reduced
descriptions to recent experimental results and speculate towards a number
of interesting possibilities for the future.

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                     Claudio Paganini
             
              Albert-Einstein Institute, Golm

  "The Fingerprints of Black Holes - Shadows and their Degeneracies"

       
    Thursday, 23. May 2017, 14:30, University of Bremen, ZARM, Room 1280

ABSTRACT:

First I will introduce the concept of the shadow of
a black hole and what it means for the shadows of
two observers to be degenerate. I will then present
preliminary results showing that no continuous
degenerations exist between the shadows of observers
at any point in the DOC of any Kerr-Newman-(anti)-
De-Sitter black hole spacetime of unit mass. Therefore
an observer can, by measuring the black holes shadow,
in principle determine the angular momentum, the
charge and the cosmological constant of the black
hole under observation, as well as his radial distance
from the black hole and his angle of elevation above
the equatorial plane.

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                 Roberto Tanzi
            
  "Quantum signatures of non-metric geometries"

      
    Thursday, 23. May 2017, following the talk by Claudio Paganini,
    University of Bremen, ZARM, Room 1280

ABSTRACT:

Ray optics effectively fail to detect an eleven-parameter family of deviations
from a metric spacetime geometry. I show that, however, these ray-optically
invisible deviations are detectable in quantum electrodynamics scattering
amplitudes and I provide few examples.

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                      Ales Flandera
              
               Charles University Prague
            

  "Kerr-Newman black hole in the formalism of isolated horizons"

      
    Friday, 19. May 2017, 14:00, University of Bremen, ZARM, Room 1280

ABSTRACT:

We study the Kerr-Newman black hole in the formalism of weakly isolated
horizons using a near horizon solution of Einstein\u2019s equations in the
Bondi-like coordinates employed by Krishnan in 2012. This solution
establishes the existence of a null tetrad adapted to a non-twisting null
congruence. However, the explicit construction of such a tetrad for the
Kerr-Newman metric is not given. We formulate appropriate initial
data and firstly construct the tetrad in a perturbative way in the
neighbourhood of the horizon. Finally, we find also its exact form
everywhere in the Kerr-Newman space-time.

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                     Claudio Paganini
            
              Albert-Einstein Institute, Golm

  "The Fingerprints of Black Holes - Shadows and their Degeneracies"

      
    Thursday, 23. May 2017, 14:30, University of Bremen, ZARM, Room 1280

ABSTRACT:

First I will introduce the concept of the shadow of
a black hole and what it means for the shadows of
two observers to be degenerate. I will then present
preliminary results showing that no continuous
degenerations exist between the shadows of observers
at any point in the DOC of any Kerr-Newman-(anti)-
De-Sitter black hole spacetime of unit mass. Therefore
an observer can, by measuring the black holes shadow,
in principle determine the angular momentum, the
charge and the cosmological constant of the black
hole under observation, as well as his radial distance
from the black hole and his angle of elevation above
the equatorial plane.


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                     Dr. Kamal Hajian
            
       Institute for Research in Fundamental Sciences
                      Tehran, Iran

     "From black hole information paradox to firewall paradox"

      
          Thursday, 2. March 2017, 16:15, Room W2 3-349

ABSTRACT:

Black hole information paradox has been a concomitant of realizing
these objects/solutions as thermodynamic systems.  The paradox can
be explained as: in the semi-classical regime which gravity is dealt
classically,  black hole evaporation is not a unitary process.
A famous resolution to this paradox was presented in 1993, which is
called ``black hole complementarity".  This resolution can be roughly
explained as: the set of observers standing outside of the black hole
and the set of observers falling freely into it, should be considered
complement to each other. One can not see the world from the point of
view of both sets of observers. Using this complementarity, the
information paradox has been considered to be resolved. But, it has
been shown that black hole complimentarity yields another paradox
which is known as ``firewall paradox". In this talk, I will review
the two paradoxes mentioned above.


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                   Vittorio De Falco
            
         ISSI, Bern, Switzerland


      "Approximate analytical calculations of photon
           geodesics in the Schwarzschild metric"
      
           Tuesday, 28 February 2017, 2:30pm, ZARM, Room 1280

ABSTRACT:

In this talk, we show a method for deriving approximate analytical
formulae to integrate photon geodesics in a Schwarzschild spacetime.
Based on this, we derive the approximate equations for light bending and
propagation delay that have been introduced empirically already
respectively by Beloborodov 2002 and Poutanen & Beloborodov 2006. We
then derive for the first time an approximate analytical equation for
the solid angle. We discuss the accuracy and range of applicability of
the new equations and present a few simple applications of them to known
astrophysical problems, that are: light curve from an emitting clump
orbiting a black hole, emission line profile from an accretion disk
around a black hole and light curve from a hot spot on the surface of a
rotating neutron star.


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                  Dr. Souvik Banerjee 
         
                  Uppsala University 
             

"Lectures on Quantum Mechaniocs and General Relativity"

     Tuesday,   January 24, 2017, ZARM, room 1280 at 14:30
     Wednesday, January 25, 2017, ZARM, room 1280 at 16:00

Abstract:

These are lectures on black holes, the black hole information paradox,
in general on the conflict between GR and QM, locality,
a possible resolution of the information paradox,
and on some further recent developments.

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           Marko Toros
       
     Department of Physics,
     University of Trieste

     "Quantum Mechanics beyond Galileo"

       Tuesday, December 6th 2016, 14:30,
     ZARM, University of Bremen, Room 1280


ABSTRACT:
We will discuss some problems related to
combing relativity and quantum mechanics. We will
first introduce Foldy's framework for quantum
mechanics in Minkowski space-time. We will present
some new results within the theory of relativistic
open quantum systems. In particular, we will discuss
dynamical maps, Lorentz covariance and non-inertial
reference frames. In addition, we will also look
at the implications for fundamental non-unitary
modifications of quantum mechanics.

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                     Anna Hedegaard Lukawska
            
        "Correlation of cosmological observables
            in the effective field theory of inflation"

          Friday, 02. December 2016, 14:00,
       ZARM, University of Bremen, Room 1730

Abstract:
The temperature fluctuations in early universe are encoded in statistical
properties of the CMB anisotropies. If the fluctuations follow a Gaussian
statistics, then the connected three-point correlation function, four-point,
and higher order correlation functions referred to as non-Gaussianities,
are all vanishing, and all statistical information is given by the two-point
correlation function. So far there are neither high statistical significance
detections of a non-zero value for the three-point nor higher-order correlation
functions. However, if non-Gaussianities of the primordial fluctuations are there,
the correlations can provide valuable information on inflation.
I will introduce this topic and present a study I have done on how big
non-Gaussianities can be generated by single field models using the effective
field theory of inflation.
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                      Albert Samoilenko
             
       Department of Theoretical Physics and Astrophysics
              Belarusian State University, Minsk

"Gauged Baby Skyrme Model with and without Chern-Simons Term"

     Thursday, 1. December 2016, 16:15, Room W2 3-349

Abstract:

The properties of the multisoliton solutions of the U(1) gauged
modification of the 2+1 dimensional planar Skyrme model with and
without Chern-Simons term are investigated numerically. Coupling to
the Chern-Simons term allows for existence of the electrically charged
solitons which may also carry magnetic fluxes. Two particular choices
of the potential term are considered: (i) the weakly bounded potential
and (ii) the double vacuum potential. In the absence of the gauge
interaction in the former case the individual constituents of the
multisoliton configuration are well separated, while in the latter
case the rotational invariance of the configuration remains unbroken.
We analyze the dependency of the structure of the solutions, the
energies, angular momenta, electric and magnetic fields of the
configurations on the gauge coupling constant g, and the electric
potential. It is found that, generically, the coupling to the
Chern-Simons term strongly affects the usual pattern of interaction
between the skyrmions, in particular the electric repulsion between
the solitons may break the multisoliton configuration into partons. On
the other hand, in the strong coupling limit the coupling to the gauge
field results in effective recovering of the rotational invariance of
the configuration and both the magnetic flux and the electric charge
of the solutions become quantized, although they are not topological
numbers.

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                       Andrea Fuster
          Eindhoven University of Technology

                "Finsler gravitational waves"

          Tuesday, 28. November 2016, 14:30,
       ZARM, University of Bremen, Room 1730

ABSTRACT:
In this work we present a Finslerian version of the well-known gravitational
pp-waves, which generalizes the very special relativity (VSR) line element.
Our Finsler pp-waves are an exact solution of Finslerian Einstein's equations
in vacuum.

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                       Peter Hogan
               University College Dublin

"Colliding Impulsive Gravitational Waves and a Cosmological Constant"


          Wednesday, 26. October 2016, 15:00,
       ZARM, University of Bremen, Room 1280

ABSTRACT:
A space-time model is derived of the collision of two homogeneous,
plane impulsive gravitational waves propagating in a vacuum before
collision and for which the post collision space-time has constant
curvature. The profiles of the incoming waves are Dirac delta functions
multiplied by constants k and l respectively. The delta functions are
singular on intersecting null hypersurfaces. The cosmological constant
in the post collision region of the space-time is -6kl.

 

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                          Prof. Syed Afsar Abbas
                     Centre for Theoretical Physics
                    JMI University, New Delhi, India

   "Neutrinos in the Quantized Charge Standard Model"

    Tuesday, 18. October 2016, 14:15, Room W2 3-349

In the conventional Standard Model (SM) based on the group structure
${SU(3)_C} X {SU(2)_L} X {U(1)_Y}$, the electric charge is
defined as Q=$T_3$ + Y/2 ( or $T_3 + {Y^\prime}$ ) where $T_3$ is the
diagonal generator of $SU(2)_L$ and Y ( or $Y^\prime$ ) of $U(1)_Y$.
Herein the electric charge is not quantized. We discuss and
consistently derive another expression of the electric charge wherein
the colour degree of freedom pops up in the expression of the electric
charge itself. Moreover the electric charge is properly quantized as
well. We call this model structure the
Quantized Charge Standard Model (QCSM).

This QCSM reproduces all the successes of the conventional SM.
However the QCSM brings in some significant and major differences with
respect to the SM. In particular it shows that the Right-Handed-Neutrino
is a different kind of creature all together. This leads to a demand
that the neutrinos behave differently in the "disappearance" and the
"appearance" experiments. The QCSM predicts that the neutrinos in the
disappearance experiments do travel with subluminal velocities while
those that arise in the appearance experiments shall be observed to
travel with superluminal velocities. As of now, all experiments have
detected neutrino velocities only in disappearnce experiments. The
experitmentalists are urged to confirm this unique prediction of the
QCSM in appearance experiments in the future.


 

                            Sarah Kahlen
                    University of Oldenburg

     "Five-dimensional wormholes at the core of rotating boson stars"
 
         Thursday, 20. October 2016, 12:15, Room W2 3-349

ABSTRACT:

Wormholes are solutions of the Einstein equations which can be
thought of as tunnels between distinct universes. In the presence
of exotic matter wormholes become traversable, allowing travel between
the universes. Although wormholes have not yet been observed in Nature,
they attained an increasing interest in recent years. On the other hand,
boson stars are hypothetical objects made up of self-gravitating
Bose-Einstein condensates with self-interaction. The mixed system
consists of a wormhole at the core of a boson star.

In this talk I will present my studies of wormholes at the core of boson
stars in five dimensions. The solutions have been constructed nummerically
for the non-rotating and rotating cases.
Special focus lies on the domain of existence and the physical properties
like mass, angular momentum and geometrical features.
In particular I will show how rotation might stabilize the notoriously
unstable wormhole solutions.


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                             Rico Berner
                             (TU Berlin)

      "Kinematic constraints in parametrized nonlinear electrodynamics"


      Monday, 12. September 2016, 4:15 p.m.,ZARM, Uni Bremen, Raum 1730

ABSTRACT:

The aim of the talk is to give a concise introduction to the parametrization
approach by Kucha\u0159 and present its application to nonlinear
electrodynamics. We show how the motion of physical fields, in particular
the electromagnetic potential, is connected with the choice of a space and
time decomposition of the background spacetime manifold. The relation of the
field dynamics and its kinematic description is derived. With regard to
generally covariant theories, the arising kinematic constraints are
investigated for the class of nonlinear electrodynamics. We address the
problem of finding the explicit form of the super-Hamiltonian constraint
and reduce it to the problem of solving two nonlinear equations. By this,
a plug-and-play approach to find the whole set of constraints for an
arbitrary theory of nonlinear electrodynamics is provided and applied to
selected examples.

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                         Dr  Marco Maceda
             UAM-I, Mexico City

                 "Noncommutative Landau problem in Podolsky's
                        generalized electrodynamics"

       Tuesday, 16 August 2016, 2:30pm, ZARM, Room 1280

ABSTRACT:

We discuss the Landau problem in Podolsky's
electrodynamics at the classical level and also within the
context of noncommutative quantum mechanics. For the
latter, we use Bopp shifts to obtain corrections to the
standard expressions for the energy eigenvalues; their
consequences are then analyzed.

 

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                              Jiri Ryzner
                    Charles University Prague

      "Majumdar-Papapetrou: special cases"
 
       Tuesday, 26. July 2016, 2:30 p.m., ZARM, Seminar Room 1280

ABSTRACT:

One of important analytical solutions of Einstein-Maxwell equations is the
Majumdar-Papapetrou solution, which consists of extremal sources of
gravitational and electromagnetic field. We investigate the properties
of two particular cases: two extremal Reissner-Nordstroem black holes in
equilibrium and the field of an infinitely long, straight, extremally
charged string.

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                    Anastasia Golubtsov                          Laboratory of Theoretical Physics                      JINR, Dubna      "Holographic Wilson loops in Lifshitz-like backgrounds"        Friday, 24. June 2016, 11:15, Room W2 3-349ABSTRACT:In this talk we discuss the evolution of Wilson loops in Lifshitz-likebackgrounds which can describe anisotropic quark-gluon plasma using theholographic approach. We evaluate the Wilson loops in both static andtime-dependent cases. The anisotropic time-dependent plasma is dual toLifshitz-Vaidya background, while we use a black brane in theLifshitz-like spacetime for a finite temperature plasma in equilibrium.To probe the system we calculate Wilson loops oriented in differentspatial directions. We find that anisotropic effects in the Lifshitz-likebackgrounds are more visible for the Wilson loops lying in thetransversal direction unlike the Wilson loops partially oriented inthe longitudinal one. 

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                       Dr. Dennis Raetzel
                       University Potsdam

          "How light gravitates: a brief exploration"

                Thursday, June 14, 2016, 14:30
              Room 1280, ZARM, University Bremen

ABSTRACT:

As Einstein's equations tell us that all energy is a source of gravity, light must gravitate. However, because changes of the gravitational field propagate with the speed of light, the gravitational effect of light differs significantly from that of massive objects. In particular, the gravitational force induced by a light pulse is due only to its creation and annihilation and decays with the inverse of the distance to the pulse.

We can expect the gravitational field of light to be extremely weak. However, the properties of light are premisses in the foundations of modern physics: they were used to derive special and general relativity and are the basis of the concept of time and causality in many alternative models. Studying the back-reaction of light on the gravitational field could give new fundamental insights to our understanding of space and time as well as classical and quantum gravity.

In this talk, a brief overview is given of the gravitational field of light pulses in the framework of general relativity. A glimpse is caught of the gravitational interaction of two single photons which turns out to depend on the degree of their polarization entanglement.


 

 

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              Michael Kalisch
   ( Friedrich-Schiller-Universitaet, Jena)  



   "Pseudo-spectral construction of highly deformed
    non-uniform black string solutions"
    
    
Friday, 22 April 2016, 11:15, University of Oldenburg, W02-3-349

Abstract:

Black strings are static solutions to Einstein's vacuum field equations in
spacetimes with a compactified dimension. The horizon of such an object
wraps the compact dimension. I will explain in detail how we constructed
black string solutions in a critical regime, in which the horizon is close
to pinching off. The use of an appropriately designed pseudo-spectral scheme
allowed us to obtain very accurate and unprecedented results. In particular,
we observed three clearly pronounced turning points in the curves of
thermodynamic quantities, resulting in a spiral curve in the black string's
phase diagram.

 

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                   Dr Maxim Eingorn

  (North Carolina Central University, Durham, NC, USA)    

"All-scale cosmological perturbations and screening of gravity     in inhomogeneous Universe"

Tue, 19 Apr 2016, 14:30, Bremen, ZARM, Room 1280

Abstract

Without exceeding the limits of the concordance cosmologicalmodel, all-scale scalar and vector perturbations of the homogeneousbackground are derived analytically for arbitrarily distributedinhomogeneities (discrete gravitating masses) as theirnonrelativistic sources. The obtained expressions for the metriccorrections are gauge-invariant, converge everywhere in voids,have zero average values, and conform to Minkowski backgroundlimit and Newtonian cosmological approximation as particularcases. Moreover, the uniform matter distribution limit as onemore crucial test is easily passed as well. It is rigorously proventhat gravitational attraction between inhomogeneities is governedby Yukawa law, covering the whole space and coming up to takeplace of Newtonian gravitation, which is restricted exclusively tosub-horizon distances. The finite time-dependent screeninglength (amounting to 3.7 Gpc at present) is determined by theaverage rest mass density of nonrelativistic matter and providesnatural estimates of the homogeneity scale, the upper limit ofthe cosmic structure dimension, and the bound to a spatial domainof probable structure development. The definition of Yukawainteraction range is generalized to the extended cosmologicalmodel with extra continuous perfect fluids and nonzero spatial

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                        Christian Knoll
                    University of Oldenburg

      "Dirac equation in Myers-Perry black hole geometry
                   with equal angular momenta"



      Friday, 11. March 2016, 12:15, Room W2 3-349


ABSTRACT:
This talk is the defense of my Master Thesis. The main result is the
solution of the angular part of the Dirac equation in the geometry of
the five dimensional Myers-Perry black hole with equal angular momenta.
I will begin to introduce the formalism and fix the notation for the
Dirac equation in curved spacetimes.
After that I will briefly consider the Dirac equation in the
geometry of the Kerr black hole. The decoupled Dirac equation of
Chandrasekhar is reproduced using a method from the literature.
A minimal example is constructed and some physical interpretations are
discussed. One set of special solutions to the angular equations is derived.

Next is the Dirac equation in the geometry of the five dimensional
Myers-Perry black hole with equal angular momenta. Again, the equations are
decoupled using a method from the literature. A minimal example is constructed
with which some physical interpretations are discussed. The angular equations
are solved.

The last part involves possible future work. A possible condition for
physical solutions to the complete equations considered before is discussed.
The Dirac equation in the Myers-Perry black hole geometry with equal angular
momenta in any uneven dimension is formulated.
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                        Philipp Heinrich
                    University of Oldenburg

      "Analysis of UV and EUV emission from impacts on the Sun
               after 2011 June 7 eruptive flare"


      Friday, 18. December 2015, 12 Uhr c.t. Raum W2 3-349


ABSTRACT:
On 2011 June 7 debris from a large filament eruption fell back to the Sun
causing bright ultraviolet (UV) and extreme ultraviolet (EUV) splashes
across the surface. These impacts may give clues on the process of stellar
accretion. I will give an introduction to the subject and subsequently
discuss how the impact emission is influenced by structures in the
falling ejecta and at the solar surface.

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                   Dr. Usha Kulshreshtha
             Department of Physics and Astrophysics
                     University of Delhi

   "Constraint Quantization of D-brane Actions in ADM Formalism
          (-- using Shift Vectors and Lapse Function)"

      Thursday, 22. October 2015, 12 Uhr c.t. Lecture Hall W2 3-349

Abstract:

In this talk, I would consider the constraint quantization
of several D-brane actions.
I would first consider the actions involving the square-roots, like the
relativistic point particle, Nambu-Goto action, Born-Infeld-Nambu-Goto
action and the Dirac-Born-Infeld-Nambu-Goto action.
Then I would consider the actions without the square roots like
the Polyakov-type of actions which do not involve a square.
I would then consider the conformally gauge-fixed Polyakov D1-brane
I would then consider the conformally gauge-fixed Polyakov D1-brane
action  (CGFPD1BA) and subsequently consider its instant-form and
light-front quantization. Then I would consider this action by putting in
more gauge fields in to the action like the 2-form gauge field, the axion
field and the U(1) gauge field. I would also consider the string gauge
symmetries in this context. Then I would consider the Polyakov D1-brane
action in the ADM formalism and discuss its quantization using the
shift vectors and the lapse function.

                             and

                  Prof. Dr. Daya Kulshreshtha
             Department of Physics and Astrophysics
                     University of Delhi


          "The Boson Stars and Boson Shells (- in Gravity Theory)"

         Thursday, 22. October 2015, 16 Uhr c.t. Lecture Hall W2 3-349

Abstract:

In this talk I would consider the compact boson stars and boson shells
in the
gravity theory. I would consider the models in the presence of a
cosmological
constant in the de Sitter as well as in the Anti de Sitter space. I
would also
present the solutions corresponding to the so-called phantom boson stars and
phantom boson-shells. Some comparisions would be made with the work of
the Oldenburg-Bremen-Sao Paulo group.

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                                       Andrey A. Shoom
                                   University of Alberta

            "Local Invariants Vanishing on Stationary Horizons"

            Friday, 25. September 2015, 11:00
            Lecture room W2 3-349, University of Oldenburg


Abstract:
We construct local scalar polynomial curvature invariants that vanish on the horizon
of a stationary black hole:  the squared norms of the wedge products of n linearly
independent gradients of scalar polynomial curvature invariants, where n is the
cohomogeneity of the spacetime.

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Tuesday, 15. September 2015

BREMEN-OLDENBURG RELATIVITY SEMINAR

- Field Theory, Gravity, Relativity -

Where: ZARM, University of Bremen, Room 1730

When: 10:00 am

Speaker 1: Valeri Frolov

Title: Mass gap for mini-black-hole formation in ghost-free gravity

Existence of singularities is an inherent problem of the General Relavitiy. It is
generally believed that in spacetime domains, where the curvature becomes large,
the Einstein-Hilbert action should be modified. There exists a wide class of the
modified theories of gravity proposed to solve fundamental problems of black holes
and cosmology. I consider a special class of such theories called ghost-free gravity,
which was proposed recently. I briefly discuss the ghost-free theory of gravity and
focus on the problem of gravitational collapse of small masses in such a theory.
For this purpose I shall use linearized equations of the ghost-free gravity.
It will be demonstrated how non-local modifications of gravity equations regularize
static and dynamical solutions. First I derive static solutions for a point mass.
Boosting a static solution of the linearized equations for the gravitational potential
I obtain a solution for the field of the ultra-relativistic source. Using the latter
I construct solutions for the collapsing spherical (thin and thick) null shell.
I also discuss head-on collision of ultra-relativistic particles in the ghost-free
gravity. I show that in both cases there exists a mass gap for the mini-black-hole
formation. In conclusion I briefly discuss possible applications of the presented results.

Coffee Break: 11:00 am

Speaker 2: Alexander Zhuk

When: 11:30 am

Title: Problematic aspects of Kaluza-Klein models with Einstein internal spaces

We consider Kaluza-Klein (KK) models where internal spaces are compact Einstein spaces.
These spaces are stabilized by background matter (e.g. monopople form-fields).
We perturb this background by a compact matter source
(e.g. the system of gravitating masses) with zero pressure in the external/our space
and an arbitrary pressure in the internal space. We show that in case of curved
Einstein spaces the Einstein equations are compatible only if the matter source is
smeared over the internal space and the perturbed metric components do not depend on
coordinates of extra dimensions. The latter means the absence of KK modes corresponding
to the metric fluctuations. There are two possibilties to satify the gravitational tests.
First, the gravitating source (e. g. our Sun) should have the black string equation of
state in the internal space. This result does not depend on the size of the internal space.
Second, if the equation of state in the internal space is arbitrary (e. g.dust like),
then the size of the internal space should be small enough and radion is very massive
particle.

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                     Kai Flathmann
                   University of Oldenburg

          Analytic solutions of the geodesic equations
            in U(1)^2 gauged supergravity spacetime

             Monday, 24. August 2015, 10:00 c.t.
        Lecture room W2 3-349, University of Oldenburg

ABSTRACT:

In this talk I consider a family of black hole solutions
found by Chow and Compère in 2014. They are characterized
by 8 constants: mass, angular momentum, NUT parameter,
cosmological constant, 2 electric and 2 magnetic charges.
First I discuss some properties of these solutions.
Then I analyze the geodesic equations in these spacetimes.
I present the whole set of analytical solutions in terms of elliptic
functions for the case without a cosmological constant.
In the presence of a non-vanishing cosmological constant
the analysis must be performed in terms of hyperelliptic functions.

-------------------------------------------------------------------------------

                     Kevin Eickhoff
                   University of Oldenburg

             White Dwarfs in Scalar-Tensor Theory

             Wednesday, 19. August 2015, 10:00 c.t.
        Lecture room W2 3-349, University of Oldenburg

ABSTRACT:

In this talk I present the results of my thesis on white dwarfs.
I begin with a discussion of the equation of state for white dwarfs.
Then I derive the field equations within GR and discuss the solutions
for white dwarfs to set the stage. Next I introduce scalar-tensor theory
and derive the generalized field equations to be solved.
Looking for white dwarf solutions with spontaneous scalarization
I find some interesting features.

-------------------------------------------------------------------------------


                   Dr. Elias Castellanos
                   University of Chiapas

            Scalar field as a Bose-Einstein condensate
              in a Schwarzschild-de Sitter spacetime

                  Tuesday, 28. July 2015, 14:30
              Room 1280, ZARM, University of Bremen


ABSTRACT:

We analyze some properties of a scalar field configuration, as a trapped Bose-Einstein
condensate in a Schwarzschild-de Sitter space-time. The curved space-time endows in a
natural way an effective trapping potential for the scalar field configuration and allows
us, in principle, to explore some thermodynamical properties of the system. Additionally,
the curvature of the space-time also induces a position-dependent self-interaction
parameter, that can be interpreted as a kind of gravitational Feshbach resonance,
that could affect the stability and could be used to obtain information about the
interactions among the components of the system.


-------------------------------------------------------------------------------


                     Christian Hoffmann
                   University of Oldenburg

             Boson Stars with Nontrivial Topology

             Thursday, 26. March 2015, 11:00 c.t.
        Lecture room W2 3-349, University of Oldenburg

ABSTRACT:

In this talk I discuss boson star solutions in the presence of a
phantom field, which allows for a nontrivial topology of the solutions.
The wormholes reside at the core of the configurations and lead to a
number of qualitative changes with respect to ordinary boson stars.
In particular, the typical spiraling dependence of the mass and the
particle number on the frequency of the boson stars is lost. Instead,
the boson stars with nontrivial topology approach a singular
configuration in the limit of vanishing frequency. Depending on the
value of the coupling constant, the wormhole geometry changes from a
single throat configuration to a double throat configuration,
featuring a belly inbetween the two throats.
Depending on the mass of the boson field and its self-interaction,
the mass and the size of these objects cover many orders of magnitude,
making them amenable to various astrophysical observations.
A stability analysis reveals, that the unstable mode of the Ellis
wormhole is retained in the presence of the bosonic matter. However,
the negative eigenvalue can get very close to zero, by tuning the
parameters of the self-interaction potential appropriately.

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                         Prof Gabriel Luchini
     Universidade Federal do Espírito Santo, São Mateus, BRASIL

                  "The integral Yang-Mills equations"

      Friday, 06. February 2015, 14 Uhr c.t. lecture hall W4 1-162


ABSTRACT:
The integral equations of electromagnetism are well known, and in fact,
they consisted of the first formulation of the electromagnetic theory,
before Maxwell's differential equations. On the other hand, Yang-Mills
equations were born as a non-abelian generalisation of these differential
equations and apparently until recently their integral version were not
known. In order to accomplish that we have to understand how Gauss, Faraday,
and Ampére's equations are naturally organised in the Loop Space; then, the
integral Yang-Mills equations can be obtained in this framework. All this
effort is not only to rewrite a set of equations we already know. The
plot-twist in this story is that the integral equations lead to the
definition of gauge invariant conserved quantities, which cannot be
obtained from the differential equations; moreover, these quantities are
not topologically conserved, but dynamically conserved. The most intriguing
part in this is that we recently found that there are infinitely many
conserved charges! What they mean is something yet to be revealed.
For instance, for the Wu-Yang monopole, the charges are powers of a
fundamental one... but for more intriguing configurations, such as the
2-monopole, who knows?! Finally, another surprise in this drama is that
there are new contributions to the usual Yang-Mills equations; In order
to illustrate that we show the non-abelian integral Faraday law.
Following the hints given by these new terms we are able to find new
solutions to Yang-Mills theory.

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                       Charlotte Beelen
                   Universitaet Oldenburg

              "Quark Stars in Einstein-Gauß-Bonnet
                      Dilaton Theory"

      Wednesday, 17. December 2014, 11:15 Room W2 3-349


ABSTRACT:
Quark stars are compact stars that are entirely or in part made of strange
quark matter. This results in interesting properties. In this talk
quark stars are considered in general relativity and in Einstein-Gauß-Bonnet
dilaton theory. The theories are motivated and the structure of quark stars
is explained. Numerical solutions are shown for both theories and the
massradius
diagram as well as the internal structure of the stars is discussed.
Finally,
a comparison to current observational data is made and it is considered
whether some neutron stars that have been observed so far might in fact turn
out to be quark stars.

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 Monday, 15 December 2014, 14:00
ZARM University Bremen, Room 1730

Dr. Matthias Hanauske (J. W. v. Goethe-University Frankfurt)     

"Numerical General Relativity in the Context of the Hadron-Quark
Phase Transition in Compact Stars"   

ABSTRACT:

The properties of compact stars are mainly determined by two fundamental forces:
Quantum chromodynamics (QCD) and general relativity. Relativistic hydrodynamical
simulations of collapsing neutron stars and binary neutron star mergers depend
strongly on the high density properties of the equation of state (EoS) of hadronic
and quark matter. The appearance of the QCD - phase transition (the transition
from confined hadronic to deconfined quark matter) will change the properties
of neutron stars [3]; eg. usually it is assumed that the loss of stability of
a neutron star, exceeding its maximum mass, leads to the collapse into a black hole.
However, realistic calculations within QCD-motivated models show that a neutron
star collapse could be stopped before the black hole forms [1]. Within such a
collapse scenario the neutron star would be transformed into a hybrid star with
a deconfined quark matter phase at its inner core. Several astrophysical observables
of the Quark-Gluon-Plasma (QGP) will be discussed during the talk [2,4].
Whether these observables will be visible with telescopes and gravitational
wave detectors depends strongly on the EoS and on the order and construction
of the phase transition [5,6].

References:
[1] I.N. Mishustin, M. Hanauske, A. Bhattacharyya, L.M. Satarov, H. Stöcker, W. Greiner,
Catastrophic rearrangement of a compact star due to quark coreformation,
 Physics Letters B 552, p.1-8 (2003)
[2] Matthias Hanauske; How to detect the Quark-Gluon-Plasma with telescopes;
 GSI Annual Report, p.96 (2003)
[3] I. Shovkovy, M. Hanauske, M. Huang, Nonstrange hybrid compact stars with color
superconducting matter, Phys. Rev. D 67, 103004 (2003)
[4] A. Bhattacharyya, S.K. Ghosh, M. Hanauske, and W. Raha; Rotating Twin Stars
and Signature of Quark-Hadron Phase Transition; Astron. Astrophys. 418, p.795-799 (2004)
[5] M. Hanauske, Dissertation: Properties o f Compac t Stars within QCD motivated
Models; University Library Publication (2004)
[6] Sarmistha Banik, Matthias Hanauske and Debades Bandyopadhyay; Strange matter
in rotating compact stars; J.Phys. G 31 p. 841-848 (2005)

-------------------------------------------------------------------------------

                            Gleb Zhilin
                Universitaet Minsk, Weissrussland

                         "Gauged Hopfions"

      Montag, 17. November 2014, 14 Uhr c.t. Raum W2 3-349


ABSTRACT:
Hopfions are topological solitons with non-zero Hopf index value.
They appear as stable solutions in the Faddeev-Skyrme model.
I shall give a brief introduction into the Faddeev-Skyrme model
and its U(1) gauged version.
Then I will show the effects of coupling a hopfion to a magnetic field,
and how a magnetic field mimics the topology of the hopfion.

-------------------------------------------------------------------------------

                 Prof. Alexander Gorbatsievich
               Universitaet Minsk, Weissrussland

             "Quantenmechanik in einer gekrümmten Raum-Zeit
   und die  klassischen Mathisson--Papapetrou Bewegungsgleichungen"


     Montag, 10. November 2014, 12 Uhr c.t. Raum W2 3-349


ABSTRACT:
Es wird gezeigt, dass die allgemeinkovariante Dirac-Gleichung immer als
die spezielle Darstellung der traditionellen quantenmechanischen
Bewegungsgleichungen im Hilbertraum interpretiert werden kann.
Auf dieser Basis wird die widerspruchsfreie Quantenmechanik in einer
gekrümmten Raum-Zeit aufgebaut. Weiter wird gezeigt, dass die
Erwartungswerte von Orts- und Spin-Operator eines Dirac-Teilchens
den klassischen Mathisson--Papapetrou Bewegungsgleichungen genügen.
Zum Schluss werden die Bewegungsgleichungen für ein Atom als Ganzes
in einem äußeren Gravitationsfeld abgeleitet.

-------------------------------------------------------------------------------

                      Martin Scholtz
         Charles University, Prague, Czech Republic

      "Bondi mass of Einstein-Maxwell-Klein-Gordon spacetimes"


            Tuesday, 04. November 2014, 3:00 pm
        ZARM, University of Bremen, Room 1280


ABSTRACT:
The concept of quasi-local energy-momentum and angular momentum
belongs to the most challenging problems in general relativity. Many
useful suggestions how to define quasi-local quantities are based on
the so-called Nester-Witten form and we briefly review some of them.
In particular, we start with the motivation behind the Penrose
quasi-local mass which naturally leads to the twistor equation. As a
slight digression, we discuss the notion of the twistor in some detail
and present its geometrical and physical interpretation. After
discussing the drawbacks of the Penrose mass, we present some other
approaches and relate them to the surface integral of the
Nester-Witten form. Finally we apply this formalism to calculate the
Bondi mass of asymptotically flat spacetimes with interacting
electromagnetic and scalar field sources and discuss the positivity of
the Bondi mass and the properties of corresponding massloss formula.

-------------------------------------------------------------------------------

Fr, 26 Sep 2014, 14:30
Bremen, ZARM, Room 1280

Dr Alexei Zayats
Kazan Federal University, Tatarstan, Russia

On the self-force in the Bopp-Podolsky electrodynamics

Abstract: In the framework of Bopp-Podolsky electrodynamics
we obtained the self-force expression for various cases. Explicit
formulas are found and analyzed for two examples: first, when
a charged particle moves on the Minkowski space-time along a
straight line with a uniform acceleration, and, second, for a rest
particle on the background of the global monopole space-time.

-------------------------------------------------------------------------------

                       Hendrik Neumann
                   Universitaet Oldenburg

              "Thermodynamics of a rotating black hole
            in minimal 5-dimensional gauged supergravity"


      Tuesday, 23. September 2014, 10 Uhr c.t. Room W2 3-349


ABSTRACT:
After an introduction to the thermodynamics of black holes in 4
dimensions, where I derive the thermodynamical properties of the
Schwarzschild, the Reissner-Nordstroem and the Kerr black hole,
I present the thermodynamical properties of the interesting black holes
obtained by Chong et al., Phys.Rev.Lett.95 (2005) 161301.
These rotating black holes possess two independent angular momenta
and carry electric charge. They are solutions of the 5-dimensional
Einstein-Maxwell-Chern-Simons theory with a cosmological constant.

-------------------------------------------------------------------------------

Prof. Eckehard W. Mielke
Universidad Autonoma Metropolitana\u2013Iztapalapa
Mexico

Colliding Boson Stars

Monday, 2014-08-18, 16:00
Lecture room W2 3-349, University of Oldenburg

ABSTRACT:

-------------------------------------------------------------------------------

Dr Shohreh Abdolrahimi
University of Oldenburg
Models of Gravity

"Large Randall-Sundrum II Black Holes"

Monday, 14. July 2014, 12:00, Room W04 1-171


ABSTRACT:
Using a novel numerical spectral method, we have constructed an AdS5 -CFT4
solution to the Einstein equation with a negative cosmological constant
that is asymptotically conformal to the Schwarzschild metric. This method is
independent of the Ricci-DeTurck-flow method used by Figueras, Lucietti,
and Wiseman. We have perturbed the solution to get large static black hole
solutions to the Randall-Sundrum II (RSII) braneworld model. Our solution
agrees closely with that of Figueras et al. and also allows us to deduce
the new results that to first order in 1/ (M2 ), the Hawking temperature
and entropy of an RSII static black hole have the same values as the
Schwarzschild metric with the same mass, but the horizon area is increased
by about 4. 7/ (lambda ).

-------------------------------------------------------------------------------

                       Olga Hauser
              Universitaeten Bremen und Oldenburg

     "Wurmloecher mit Phantom- und Einstein-Yang-Mills Feldern"


      Friday, 13. December 2013, 15 Uhr c.t. Room W2 3-349


ABSTRACT:
Die Darstellung der Schwarzschildloesung durch Kruskal-Szekeres-Koordinaten
enthaelt ein sogenanntes Wurmloch. Ein Wurmloch ist eine Verbindung zwischen
zwei asymptotischen Bereichen, welche als zwei Universen angesehen werden.
Das Schwarzschildwurmloch erlaubt jedoch keine Kausalverbindung zwischen
diesen
Gebieten.
In meiner Masterarbeit wurden Wurmloecher betrachtet, die durch eine Metrik
beschrieben werden, welche im Gegensatz zur Schwarzschildmetrik ein Wurmloch
beschreiben, das Kausalverbindungen erlaubt. Diese Wurmlochmetrik wurde mit
Phantom-und Einstein-Yang-Mills-Feldern betrachtet. Die Loesungen wurden
numerisch ueber das Newton-Verfahren ermittelt. Bei Bestimmung der Eichfeld-
Funktion K konnte frei gewaehlt werden, wie oft diese die Abzisse schneidet.
Fuer die Anzahl dieser sogenannten Knoten k wurden die Werte 1 bis 6
gewaehlt.
Fuer unterschiedliche Werte von k ergaben sich unterschiedliche
Wurmlochformen.
Fuer jeden Wert von k wurde auch gleichzeitig der Werte der
Kopplungskonstanten
variiert. Ist der Wert k ungerade, so weisen die Wurmloecher ein anderes
Verhalten auf, als wenn der Wert k gerade ist.  Insbesondere koennen
Wurmloecher mit ungeradem k zwei Haelse haben, zwischen denen sich ein
Aequator befindet.

-------------------------------------------------------------------------------

                       Matthias Kruse
                   Universitaet Oldenburg

     "Bahnkurven um rotierende dyonische schwarze Löcher
                 in der Kaluza-Klein-Theorie"


      Thursday, 05. December 2013, 11 Uhr c.t. Room W2 3-349


ABSTRACT:
Die analytische Lösung der Geodätengleichungen, welche sich
aus einer Extremallösung für ein schwarzes Loch in der
Kaluza-Klein-Theorie ergeben, wird präsentiert.
Außerdem werden die möglichen Bahnkurven für massebehaftete
und masselose Testteilchen in dieser Raumzeit untersucht
und klassifiziert. Die Bewegungsgleichungen können analytisch mit
Hilfe der elliptischen Funktionen gelöst werden, womit eine exakte
Darstellung der Bahnkurven möglich wird.

\
-------------------------------------------------------------------------------

Stephan Reimers
Universitaet Oldenburg

"Dynamics of test particles in the five-dimensional Myers-Perry-spacetime"


Thursday, 07. November 2013, 10 Uhr c.t. Room W2 3-349


ABSTRACT:
We present the complete set of analytical solutions of the geodesic
equations
in the five-dimensional Myers-Perry-spacetime in terms of the Weierstraß
p-, zeta- and sigma-functions. We study the underlying polynomials in the polar and
radial equation, which depend on the parameters of the metric and conserved
quantities of a test particle and characterize the motion. We exemplify the
efficiency of the analytical method on the orbits of test particles by
elaborating some observables of this spacetime.

-------------------------------------------------------------------------------

Hemwati Nandan
Department of Physics, Gurukula Kangri University, Haridwar, India


"Geodesic flows in rotating black hole backgrounds"

Wednesday, 2. October 2013, 11 Uhr c.t. Room W2 3-349



    We study the kinematics of timelike geodesic congruences, in the spacetime
    geometry of rotating black holes. We consider the BTZ black hole in three
    dimensions and the Kerr black hole in four dimensions. The evolution (Raychaudhuri)
    equations for the expansion, shear and rotation along geodesic flows in such
    spacetimes are obtained. For the BTZ case, the equations are solved analytically.
    The effect of the negative cosmological constant on the evolution of the expansion,
    for congruences with and without an initial rotation is noted. Subsequently,
    the evolution equations, in the case of a Kerr black hole in four dimensions
    are written and solved numerically, for some specific geodesics flows.
    It turns out that, for the Kerr black hole, there exists a critical value
    of the initial expansion below (above) which we have focusing (defocusing).
    We delineate the dependencies of the expansion, on the black hole angular
    momentum parameter, as well as on initial rotation. Further, the role of
    angular momentum and initial rotation on the time (affine parameter) of
    approach to a singularity (defocusing/focusing) is studied.

-------------------------------------------------------------------------------

                  Dr. Efstathios G. Charalampidis
               Aristotle University of Thessaloniki

           "Topological Properties of the Skyrme Model"

      Friday, 27. September 2013, 11 Uhr c.t. Room W2 3-349


ABSTRACT:
In this talk we will try to present some topological aspects of the
Skyrme model, where the latter, is a nonlinear field theory admitting
topological soliton solutions, known as skyrmions. In the first part
of this talk, we focus on the so-called "Derrick's theorem", which is
a non-existence theorem in the sense, that one is able to certify for
field theories the non-existence of topological solitons. Having laid
down the ground rules, we apply Derrick's theorem in the Skyrme model
where some novel identities are obtained. The latter can be used in
order to improve known approximate solutions, in particular, solutions
given by the so-called rational map ansatz. Thus, we study particular
cases of skyrmions and present our results.
The second part of this talk is concerned with a lattice version of
the Skyrme model by taking into account its Bogomol'nyi energy bound.
Starting from the latter, a novel discretization scheme for the radial
problem is obtained by using the so-called harmonic map ansatz. This
way, static lattice skyrmion configurations can be constructed
numerically and their stability in question is studied. Finally, the
stability analysis is checked against direct dynamical evolution
forward in time.

-------------------------------------------------------------------------------

                  Prof. Carlos A.R. Herdeiro
                Universidade de Aveiro, PORTUGAL

              "Shock wave collisions in D dimensions"

     Monday, 22. April 2013, 14 Uhr c.t. Room W2 3-349


ABSTRACT:
I will describe work on  "Shock wave collisions" as a semi-analytical
technique to understand the collision of two black holes, head-on,
at very high speeds in D space-time dimensions. I shall describe a
perturbative framework to obtain the radiated energy in gravitational
waves and present a remarkable pattern for the inelasticity in terms
of the space-time dimension, obtained in first order perturbation theory.
Comments will be made about the applicability of perturbation theory,
higher order corrections and comparisons with collisions of black holes
and other compact objects in numerical relativity.

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Wednesday, 12 December 2012, 16:00,
Uni Oldenburg, Room W2 3-349

Cristian Stelea,
Al. I. Cuza University,
Iasi, Romania
 

"On multi-black hole systems in five dimensional spaces with Kaluza-Klein asymptotics"

ABSTRACT:

Exact solutions of Einstein field equations (with or without matter fields)
play a key role in our understanding of gravitational physics in four and higher
dimensions.
In this talk I review a recent solution generating technique based on the symmetries of
the dimensionally reduced Lagrangian of the Einstein-Maxwell-dilaton system in five
dimensions.
I describe applications of this technique to generate the double black hole system,
the black saturn system in five-dimensional backgrounds with Kaluza-Klein asymptotics.
While generically the static multi-black hole solutions are plagued with unavoidable
conical singularities, in the particular case of black objects on the Taub-bolt
instanton such singularities can be eliminated and the multi-black hole configuration
remains in equilibrium. In particular, I show that a static black ring on the Taub-bolt
instanton can be in equilibrium against collapse under its own gravity and the
conical singularities can be completely eliminated.

-------------------------------------------------------------------------------

 Christine Gruber
Freie Universit"at Berlin

Monday, 3. December 2012, 15:00 s.t. lecture room W2 3-349

Title: "Quantum Phenomena in the realm of Cosmology"

 

Abstract:

The talk will discuss two applications of quantum phenomena in the large-scale
framework of cosmology and astrophysics.

The first part will deal with the so-called dark energy problem of cosmology
i.e. the observation that the universe is expanding in an accelerated way.
Among the abundance of models trying to explain this kinematic feature
of the universe, one of them is to consider the vacuum fluctuations of quantum fields,
an energy density constant in space, to cause the expansion. The vacuum energy is a
divergent quantity though, and is thus usually discarded as a possible candidate
for dark energy. However, by balancing contributions of different quantum fields,
a finite value can be achieved, which can correctly account for the expansion of
the universe.

The second part will deal with the occurrence of Bose-Einstein condensates (BECs)
in astrophysical contexts, i.e. in compact objects such as neutron stars and white
dwarfs. As unlikely as it may seem, conditions in such environments allow for the
formation of BECs due to a favourable combination of temperature and density,
and thus it is of interest to investigate the condensation of bosonic particles
under the influence of gravitational interactions in the framework of a Hartree-Fock
theory. Results can be compared to observations through the predicted density profiles
and masses of the objects.


-------------------------------------------------------------------------------

Dr Andrey Shoom
University of Alberta
Edmonton, Canada

Spinoptics in a Stationary Spacetime

Friday, 21. September 2012, 11 Uhr c.t. Raum W2 3-349


ABSTRACT:

We study how polarization of photons affects their motion in a gravitational
field created by a rotating massive compact object. First, we study
propagation of the circularly polarized beams of light in a stationary
gravitational field. We use (3+1)-form of the Maxwell equations to derive a
master equation for the propagation of monochromatic electro-magnetic waves
of the frequency \omega with a given helicity. We first analize its solutions
in the high frequency approximation using the \u2018standard\u2019 geometrical optics
approach. After that we demonstrate how this \u2018standard\u2019 approach can be
modified in order to include the effect of the helicity of photons on their
motion.

Using the modified geometric optics approximation we study scattering of
polarized light by a rotating (Kerr) black hole of the mass M and the angular
momentum J. We demonstrate that a photon moves along a null curve, which in
the limit of the standard geometrical optics becomes a null geodesic. We focus
on the scattering problem for polarized light. Namely, we consider the
following problems: (i) How does the photon bending angle depend on its
polarization; (ii) How does position of the image of a point-like source
depend on its polarization; (iii) How does the arrival time of photons depend
on their polarization. We perform the numerical calculations that illustrate
these effects for an extremely rotating black hole and discuss their possible
applications.

-------------------------------------------------------------------------------



                   Dr. Hideo Iguchi
College of Science and Technology (CST) Nihon University

"Black Di-ring: Solution generation and physical properties"

               Wednesday, 2012-08-22, 11:15
           room W2 3-349, University of Oldenburg

ABSTRACT:
Using solitonic solution generating techniques we constructed an exact
stationary asymptotically flat 5-dimensional vacuum solution describing a "black
di-ring":
concentric black rings rotating on the same plane. There are two different
solution generating techniques.  First a solitonic method based on Neugebauer's
Backlund transformation was applied to construct the black di-ring solution.
After the above attempt Belinsky and Zakharov's inverse scattering method was used
to generate the black di-ring. The representations of these two solutions are very
different. It was shown that these two solution sets of black di-rings are
completely equivalent. One of the most important features of black di-rings is
continuous non-uniqueness.
This fact can be confirmed by a random systematical sampling in the
phase diagram. It was shown that black di-rings can realize states of thermodynamical
equilibrium where both rings have the same temperatures and same angular momenta.

-------------------------------------------------------------------------------


Yulia Bezvershenko
(Bogolyubov Institute for Theoretical Physics, Kyiv, Ukraine)

"Laguerre polynomials and functional Bethe ansatz for the rational Gaudin model"

Thursday, 2012-08-30, 15.30
ZARM, University Bremen
Lecture room 1730


ABSTRACT:


We consider the rational Gaudin model [1] with non-zero magnetic field which
physically corresponds to the central spin problem. The space of states is
described in terms of separated variables [2]. The states of spin system are
given by the rational (up to exponential factor) functions of these variables
on the Lagrangian submanifold. We build the representation of su(2) symmetry
algebra of the model in terms of Laguerre polynomials and extend it to the
representation of corresponding affine algebra. The eigenfunctions of
considered model are presented as solutions of some generalization of
Knizhnik-Zamolodchikov equation to the case of non-zero magnetic field.
This approach is analogous to Feigin-Frenkel-Reshetikhin construction for
Gaudin model with zero magnetic field [3].

References

1. M. Gaudin, J. Physique 37, 1087 (1976).
2. E.K. Sklyanin, J. Math. Sci. 47, 2473 (1989).
3. B. Feigin, E. Frenkel, N. Reshetikhin, Commun. Math. Phys. 166, 27 (1994).

-------------------------------------------------------------------------------

Dr. Hideo Iguchi
(College of Science and Technology (CST) Nihon University)

"Black Di-ring: Solution generation and physical properties"

Wednesday, 2012-08-22, 11.15
University of Oldenburg
Lecture room W2 3-249


ABSTRACT:


Using solitonic solution generating techniques we constructed an exact stationary
asymptotically flat 5-dimensional vacuum solution describing a "black di-ring":
concentric black rings rotating on the same plane. There are two different solution
generating techniques. First a solitonic method based on Neugebauer's Backlund
transformation was applied to construct the black di-ring solution.
After the above attempt Belinsky and Zakharov's inverse scattering method
was used to generate the black di-ring. The representations of these two
solutions are very different. It was shown that these two solution sets of
black di-rings are completely equivalent. One of the most important features
of black di-rings is continuous non-uniqueness. This fact can be confirmed by
a random systematical sampling in the phase diagram. It was shown that black
di-rings can realize states of thermodynamical equilibrium where both rings have
the same temperatures and same angular momenta.

-------------------------------------------------------------------------------

Prof. Eckehard W. Mielke
(Universidad Autonoma Metropolitana-Iztapalapa, Mexico)

"Einstein gravity with cosmological constant from a topological action"

Wednesday, 2012-07-18, 14 c.t.
University of Oldenburg
Lecture room W2 1-143


ABSTRACT:


A topological field theory of gravity in four-dimension is proposed which is finite
after quantization. Since such \u2018minimal\u2019 BF type models for the high energy
limit are physically not quite realistic, a tiny symmetry breaking is needed to recover
standard Einsteinian gravity for the macroscopic metrical background.

-------------------------------------------------------------------------------

Christian Pfeifer
(Universität Hamburg)

"Finsler geometric extension of Einstein gravity "

Wednesday, 2012-05-15, 15:00
ZARM, University of Bremen


ABSTRACT:


In this talk I will show how to use Finsler geometry instead of Lorentzian metric
geometry to formulate an action based theory of gravity which contains general
relativity in the metric limit. Speaking as a physicist, Finsler geometry is based
on a generalized clock postulate which then leads to a spacetime geometry based on
a function on the tangent bundle instead of a metric on the spacetime manifold.
I will briefly review these concepts and present in detail how physical considerations
then lead to our definition of Finsler spacetimes which generalize Lorentzian metric
spacetimes as background for physics. On these generalized backgrounds I will discuss
how to construct well defined action integrals and how to use them to lift physical
matter field theories from metric spacetimes to Finsler spacetimes. I will then
introduce our action for the dynamics of Finsler spacetimes which leads to the
gravitational field equation. It will become clear that General relativity is a
special case of the gravity theory presented here, since the gravity equation is
equivalent to the Einstein equations in the metric limit. Finally I will explain
how to define symmetries of Finsler spacetimes and use these to present a solution
of the gravity equation to first order beyond metric geometry.

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Saskia Grunau
University of Oldenburg

"Geodesic Motion of Test Particles in Black Ring Space-Times"

Master Thesis

Friday, 2012-03-16, 10:00 c.t.
Lecture room W2 3-349, University of Oldenburg


ABSTRACT:


For five dimensional black ring space-times it does not appear possible to separate the
Hamilton- Jacobi-equation in general. Therefore I discuss recent analytical solutions of
the geodesic equations of test particles and light for special cases: the motion of
massless particles within the ergosphere and the motion of particles along the axes.
I characterize and discuss the associated orbits. I compare the motion around singly
spinning black rings, doubly spinning black rings and charged supersymmetric black rings.

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Dr. Frederic P. Schuller
Albert-Einstein-Institute, Golm

"Spacetimes beyond Einstein"

Wednesday, February 1st, 2011, 14:30
Room 1280, ZARM, University Bremen


Summary:


The recent announcement of superluminal neutrino propagation by the OPERA collaboration
is a reminder that the spacetime geometry may after all not be encoded in a Lorentzian
manifold. But which alternative geometries on a smooth manifold can serve as a spacetime
structure instead? And what are their dynamics? In this talk I will show -- through an
interplay of the theory of partial differential equations, real algebraic geometry and
convex analysis -- that there is a rather comprehensive mathematical answer.
The talk will be aimed at a general theoretical physics audience.

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Prof. Dr. Alexander Gorbatsevich
University of Minsk, Belorus

"The equations of motion of a binary star in the field of a supermassive black hole"

Friday, January 27 2012, 14:00 c.t.
W2-3-349, University Oldenburg


Abstract:


By the use of Einstein--Infeld--Hoffmann method, the equations of motion of a binary
star system in the field of a supermassive black hole are derived. In spite of the fact
that the motion of a binary system as a whole can be relativistic or even
ultra-relativistic with respect to the supermassive black hole, it is shown, that under
the assumption of non-relativistic relative motion of the stars inbinary system, the motion
of the binary system as a whole satisfies the Mathisson--Papapetrou equations with
additional terms depending on quadrupole moments. Exemplary case of ultra-relativistic
motion of a binary neutron star in the vicinity of non-rotating black hole is considered.
It it shown that the motion of binary's center of mass may considerably differ from
geodesic

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Mareike Haberichter
Jodrell Bank Centre for Astrophysics, Manchester

"Nuclei as Classically Spinning Skyrmions?"

Monday, August 29th, 2011, 16:15
W2-3-349, University Oldenburg


Abstract:


Skyrmions arise as topological soliton solutions in the Skyrme model which has been
shown to be a low energy effective theory of QCD in the large NC limit. If quantized
suitably, they can be used to describe structure and properties of nucleons and nuclei.
We argue that the semiclassical rigid-body quantization, commonly used in the literature,
has several shortcomings and that removing this simplication may result in a more accurate
description of nucleons and nuclei. We present an extensive numerical study of spinning,
arbitrarily deforming Skyrmions of topological charge B=1-4.

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Dr. Derek Harland
Department of Mathematical Sciences
University of Durham, UK

"Knotted solitons and elastic rods"

Thursday, 9th June 2011, 16:30
Room 1730, ZARM, University Bremen



Summary:

The Skyrme-Faddeev model has been proposed by Faddeev as an effective description both
of 2-component Ginzburg-Landau superconductors, and of low-energy Yang-Mills theory.
A striking feature of this model is the presence of solitons which resemble knotted
strings. I will show that many features of these knotted solitons are captured by a
much simpler model of Kirchhoff elastic rods. This suggests an intriguing connection
between elasticity theory and field theory.

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Prof. Dr. Joerg Frauendiener
Department of Mathematics and Statistics
University of Otago, New Zealand

"The static spherically symmetric body in relativistic elasticity"

Wednesday, 25 May 2011, 14:30
Room 1280, ZARM, University Bremen



Summary:

In this talk, a class of static spherically symmetric solutions of the general
relativistic elasticity equations is discussed . The main point of the discussion
is the comparison of two matter models given in terms of their stored energy functionals,
i.e., the rule which gives the amount of energy stored in the system when it is deformed.
Both functionals mimic (and for small deformations approximate) the classical
Kirchhoff-St Venant materials but differ in the strain variable used. We discuss the
behavior of the systems for large deformations.