#### Field theory

# Previous Seminars

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Dr Jafar Khodagholizadeh

(Farhangian University, Tehran, Iran)

"The effect of cosmic neutrinos on gravitational waves"

Thursday, 22 Feb 2018, 10:00, TAB Room 2.63

Abstract:

We talk about an integro-differential equation for propagation

of cosmological gravitation waves in distinct eras where the traceless

transverse part of the anisotropic stress tensor includes free streaming

neutrinos and antineutrinos which has been traveling essentially without

collisions since the temperature dropped below 10^(10) K. We study

the amplitude of primordial gravitational waves concerning the effect of

neutrinos. Our results show that the gravitational wave amplitude has been

reduced and this reduction is less in the Lambda-dominated era. At the

end, we take a glance on the results of the same investigation in a closed

spacetime.

--------------------------------------------------------------------------------------------------------------- Dr. Kamal Hajian

Institute for Research in Fundamental Sciences

Tehran, Iran

"Cosmological constant is a conserved charge"

Thursday, 22 February 2018, 11:15, Room W2 3-349

Abstract:

The cosmological constant can (always) be considered as the on-shell value

of a top form in gravitational theories. The top form is the field strength

of a gauge field, and the theory enjoys a gauge symmetry. After a historical

review of the cosmological constant, I will show that the cosmological

constant

is the charge of the global part of the gauge symmetry of the top form,

and is

conserved irrespective of the dynamics of the metric and other fields.

In addition, I will introduce its conjugate chemical potential, and prove

the generalized first law of thermodynamics which includes variation of the

cosmological constant as a conserved charge. At the end, I will discuss

how our new term in the first law is related to the volume-pressure term.

This talk is based on the paper arXiv:1710.07904 in collaboration with

Dmitry Chernyavsky.

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Andreas Krut

La Sapienza University, Rome, Italy

"Novel constraints on fermionic dark matter from galactic observables"

Thursday, 11 January 2018, 14:30, 12:00

University of Bremen, ZARM, Room 1280

Abstract:

We have recently introduced a new model for the distribution of dark

matter (DM) in galaxies, the Ruffini-Argüelles-Rueda (RAR) model, based

on a self-gravitating system of massive fermions at finite temperatures.

The RAR model for fermion masses above keV successfully describes the DM

halos in galaxies and predicts the existence of a denser quantum core

towards the centre of each configuration. We demonstrate here that the

introduction of a cutoff in the fermion phase-space distribution,

necessary to account for the finite Galaxy size, defines a new solution

with a compact quantum core which represents an alternative to the

central black hole (BH) scenario for SgrA*. For a fermion mass in the

range mc² = 48 - 345 keV the DM halo distribution fulfils the most

recent data of the Milky Way rotation curves while harbours a dense

quantum core of 4E6 Msun within the S2 star pericentre. In particular,

for a fermion mass of mc² = 48 keV the model is able to explain the DM

halos from typical dwarf spheroidal to normal elliptical galaxies while

harbouring dark and massive compact objects from 1E3 Msun up to 1E8 Msun

at their respective centres. The model is shown to be in good agreement

with different observationally inferred universal relations, such as the

ones connecting DM halos with supermassive dark central objects.

Finally, the model provides a natural mechanism for the formation of

supermassive BHs as heavy as few 1E8 Msun. We argue that larger BH

masses (1E9 - 1E10 Msun) may be achieved by assuming subsequent

accretion processes onto the above heavy seeds, depending on accretion

efficiency and environment.

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Kevin Eickhoff

University of Oldenburg

"Axial quasi-normal modes of Neutron Stars

in shift-symmetric Horndeski Theory"

Friday, 15. December 2017, 12:00 c.t.

Lecture room W2 3-349, University of Oldenburg

Abstract:

In this talk I present the results of my thesis on neutron stars in

Horndeski theory. I begin with a discussion of the formation and

therefore the equations of state for neutron stars. Then I briefly

introduce Horndeski theory and focus on the shift-symmetric part.

I derive the modified TOV-equations and show the static solutions

for a realistic EOS. Afterwards I discuss the perturbation theory

in curved spacetime for a Schwarzschild black hole in General

Relativity to set the stage. I explain the quasi-normal mode (QNM)

formalism for this example and present the results. Next I discuss

the formalism for neutron stars with a time-independent scalar-field,

derive the respective perturbation equations and present my results.

Finally I also find universal relations for the axial QNMs in the

shift-symmetric Horndeski theory, which differ from the ones in

General Relativity.

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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.

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.

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.

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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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.

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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.