# Past Events

# October 2015

29 October 2015

**Philosophisches Kolloqium**

Wo: | Universität Bremen, SFG 3070 |

Wann: | 18:15-20:00 Uhr |

Sprecher/in: | Prof. Dr. Holger Lyre, Universität Magdeburg |

Titel: | "Metaphysik im 'Handumdrehen': Spiegelsymmetrie, inkongruente Gegenstücke u nd Paritätsverletzung" |

Abstract: | Der Vortrag behandelt die zum Teil bemerkenswerten Fragen und Probleme, die im Zusammenhang mit der Eigenschaft der Händigkeit in physikalischer, ontologischer und kognitiver Hinsicht auftreten. So hat beispielsweise John Earman argumentiert, dass Kants berühmtes Gedankenexperiment der 'einsamen Hand' als Argument für den absoluten Raum in Gestalt der Paritätsverletzung in der Physik seine Wiederauferstehung erlebt. Im Vortrag werden die jeweiligen Vor- und Nachteile der relationalistischen und er substantialistischen Raumauffassung kritisch bewertet und einer neuartigen Lösung zugeführt. Der Vortrag endet mit einem Ausblick auf Aspekte verkörperlichter Kognition. |

26 October 2015

**Philosophisches Kolloquium**

Wo: | Universität Bremen, SFG 3070 |

Wann: | 16:00-18:00 Uhr |

Sprecher/in: | Prof. Dr. Helmut Pulte, Ruhr Universität Bochum |

Titel: | "Zwischen Newton und Kant: Leonhard Eulers Theorie des Raumes im Kontext seiner wissenschaftlichen Metaphysik" |

22 October 2015

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

**- Field Theory, Gravity Relativity -**

Where: | University of Oldenburg, Lecture Hall W2 3-349 |

When: | 16:00 hrs |

Speaker: | Dr. Daya Kulshrestha, University of Delhi, Department of Physics and Astrophysics |

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

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 comparison would be made with the work of the Oldenburg-Bremen-Sao Paulo group. |

22 October 2015

**Physikalisches Kolloqium**

Wo: | Universität Bremen, NW 1, Hörsaal 3 |

Wann: | 16:00 Uhr |

Sprecher/in: | Dr. Eva Hackmann |

Titel: | "Himmelsmechanik in starken Gravitationsfeldern" |

Abstract: | Stark gravitierende Körper wie schwarze Löcher und Neutronensterne gehören wohl zu den faszinierendsten Objekten im Universum. In den kommenden Jahren werden die experimentellen Verfahren zur hochgenauen Beobachtung solcher Objekte durch Großprojekte wie das Square Kilometre Array und das Event Horizon Telescope erheblich verbessert werden. Damit bietet sich auch die Möglichkeit die Physik in der Umgebung schwarzer Löcher besser zu verstehen und die Allgemeine Relativitätstheorie als solche sowie besondere Aspekte, wie z.B. die Kosmische Zensur oder das Keine-Haare-Theorem, zu überprüfen. Da das Gravitationsfeld nicht direkt beobachtet werden kann, müssen dafür relativistische Effekte auf Materie, z.B. Staubwolken, Sterne, Akkretionsscheiben etc., und auf Licht definiert und vorhergesagt werden. Dazu werden in diesem Vortrag analytische Methoden der relativistischen Himmelsmechanik erläutert und ihre Anwendungen im Hinblick auf hochgenaue Beobachtungen starker Gravitationsfelder vorgestellt. |

# September 2015

16 September 2015

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

** - Field Theory, Gravity, Relativity - **

Where: | University of Bremen, ZARM, Room 1730 |

When: | 14:00 hrs |

Speaker: | Prof. (adj.) Dr. Markus Lazar, Darmstadt University of Technology |

Title: | "Gradient Elasticity Theory and Non-Singular Dislocations" |

Abstract: | In this talk the theory of gradient elasticity is presented. The fundamental problem of non-singular dislocations in the framework of gradient elasticity will be investigated. A general theory of non-singular dislocations is developed for linearly elastic materials. Using the calculus of variations and the framework of incompatible elasticity, we derive the field equations of gradient elasticity, which are inhomogeneous partial differential equations of fourth order. In order to solve "eigendistortion problems" in such a framework, we derive the Green tensor of anisotropic gradient elasticity with up to six independent length scale parameters as a special version of Mindlin's form II anisotropic gradient elasticity theory [1] and as the anisotropic generalization of gradient elasticity of Helmholtz type [2,3,4]. The framework models materials where anisotropy is twofold, namely the bulk material anisotropy and a weak non-local anisotropy relevant at the nano-scale [5,6]. The continuum theory of anisotropic gradient elasticity is an excellent candidate for eigenstrain-problems up to the nano-scale [7]. Using the Green tensor of the theory of gradient elasticity, the non-singular fields which are produced by dislocations are given. All obtained dislocation fields are non-singular due to the regularization of the classical singular fields. The results have a direct application to numerical implementation and computer simulation of non-singular dislocations within the so-called (discrete) dislocation dynamics [8]. Therefore, these non-singular formulas of the dislocation fields are suitable for the numerical implementation in 3D dislocation dynamics without singularities. Such a dislocation dynamics without singularities offers the promise of predicting the dislocation microstructure evolution from fundamental principles and on sound physical grounds. Thus, a dislocation-based plasticity theory can be based on the gradient theory of non-singular dislocations. |

15 September 2015

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

**- Field Theory, Gravity, Relativity - **

Where: | University of Bremen, ZARM, Room 1730 |

When: | 10:00-12:30 hrs |

Speaker 1: | Prof. Dr. Valeri P. Frolov, University of Alberta, Canada |

Title: | "Mass gap für mini-black-hole formation in ghost-free gravity" |

Abstract: | 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. |

Speaker 2: | Prof. Dr. Alexander Zhuk, Odessa National University, Odessa |

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. |

# August 2015

24 August 2015

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

**- Field Theory, Gravity, Relativity -**

Where: | University of Oldenburg, W2 3-349 |

When: | 10:00 hrs |

Speaker: | Dr. Kai Flathmann, University of Oldenburg |

Title: | "Analytic solutions of the geodesic equations in U(1)" |

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. |

19 August 2015

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

**- Field Theory, Gravity, Relativity - **

Where: | University of Oldenburg, W2 3-349 |

When: | 10:00 hrs |

Speaker: | Dr. Kevin Eickhoff, University of Oldenburg |

Title: | "White Dwarfs in Scalar-Tensor Theory" |

Abstract: | In this talk Kevin Eickhoff presents the results of his thesis on white dwarfs beginning with with a discussion of the equation of state for white dwarfs. Then he derives the field equations within GR and discusses the solutions for white dwarfs to set the stage. Next he introduces scalar-tensor theory and derives the generalized field equations to be solved. Looking for white dwarf solutions with spontaneous scalarization he will find some interesting features. |

# July 2015

28 July 2015

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

**- Field Theory, Gravity, Relativity - **

Where: | University of Bremen, ZARM, Room 1280 |

When: | 14:30 hrs |

Speaker: | Dr. Elias Castellanos, University of Chiapas |

Title: | "Scalar Field as a Bose-Einstein Condensate in a Schwarzschild-de Sitter Spacetime" |

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. |

# June 2015

# May 2015

# April 2015

# March 2015

9 March 2015

BREMEN-OLDENBURG RELATIVITY SEMINAR

**- Field Theory, Gravity, Relativity -**

Where: | University of Bremen, ZARM, Room 1280 |

When: | 14:00 hrs |

Speaker: | Dr. Ziri Youns, Goethe University, Frankfurt am Main |

Title: | "Time-dependent general relativistic radiative transfer and episodic outflows from black holes" |

Abstract: | In this talk I will describe how we can use radiative transfer (RT) in full general relativity (GR) to investigate sporadic outflows from near the black hole event horizon in the form of plasmoid ejecta. I will first describe the background to GRRT and ray-tracing before applying the formulation to several test cases of plasmoid ejecta. The electromagnetic emissions from these plasmoids are calculated in the form of lightcurves and I discuss how we can use these light curves to probe and infer key properties of their parent back hole. |

# February 2015

# January 2015

26 January 2015

Physikalsiches Kolloquium

Where: University of Oldenburg

16:15 **Prof. Dr. Anton Zensus** (Max Planck Institute Bonn)

*Peering into the heart of AGN with millimeter VLBI *- W2-1-148

15 January 2015

BREMEN-OLDENBURG RELATIVITY SEMINAR

**- Field Theory, Gravity, Relativity -**

Where: | University of Bremen, ZARM, Room 1280 |

When: | 14:00 hrs |

Speaker: | Dr. Sayantani Lahiri |

Title: | "Radion induced cosmological and spherically symmetric solutions in the warped braneworld" |

Abstract: | We study the effective gravity on the brane: where our Universe is assumed to be located. Set in the generalised warped braneworld scenario, we look for the origin of cosmological constants of the Universe and other possible cosmological solutions that typically arise as an induced effect of higher spatial dimensions specially the radion field. On the brane we also look for spherically symmetric, static solutions sourced by radion field as well as additional on-brane matter which in turn will be shown to take a decisive role in stabilising the radion field. |

# December 2014

19 December 2014**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Where: | ZARM University Bremen; Room 1730 |

When: | 10:00 hrs |

Speaker: | Yuri Obukhow (Russian Academy of Sciences, Moscow) |

Title: | "Spin-torsion coupling: Theory and experimental bounds" |

Abstract: | We give an overview of the basic facts about the gauge-theoretic approach to gravity. The geometrical and physical structures are presented and the viable gravitational models are recalled. The prospects of detecting the spacetime torsion are discussed using the observations for the motion of extended test bodies and of the quantum dynamics of fermions in the electomagnetic and the Poincare gauge gravitational field. The theoretical analysis is applied to obtain the new bounds on the spacetime torsion from the experimental data. |

18 December 2014

**Theoriekolloquium**

Where: | University of Oldenburg, Room W2 1-143 |

When: | 14:15 hrs |

Speaker: | Herr Prof. Dr. M. Avila (Erlangen) |

Title: | "Simple laboratory and computational models of gas flows in accretion disks" |

Abstract: | Turbulent transport of angular momentum is a necessary process to explain accretion in astrophysical disks. However, hydrodynamic stability theory predicts that Keplerian gas flows should be laminar, which raises the question of what physical mechanisms are responsible for the emergence of turbulence. In this talk I will discuss simple models of accretion disks that have been put forward to infer transport properties of Keplerian flows. I will focus on numerical simulations of such models and comparisons to the results from laboratory experiments. Particular attention will be paid to whether current experiments are well suited to model accretion processes. |

15 December 2014**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Where: | ZARM, University of Bremen, Room 1730 |

When: | 14:00 hrs |

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

Title: | "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 durint the talk [2,4]. Whether these observables will be visible with telescopes and gravitational wave detectors depends strongly on the EoS and on the ordner 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-Gloun-Plasma with telescopes; GSI Annual Report, p.96 (2003) [3] I. Shovkovy, M.m Hanauske, M. Huang, Nonstrange hybrig 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 of Compact Stars with 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) |

# November 2014

28 November 2014

Where: | West Hall 4, Jacobs University Bremen |

When: | 14:00 hrs |

Speaker: | Jan Vysoký (Jacobs University) |

Title: | "Introduction to Generalized Geometry and Applications" |

Abstract: | Generalized geometry has recently become a powerful tool in modern physics. It is an intention of this talk to introduce the basics of the theory, with stress on the possible applications in theoretical physics, especially the string and gravity theory. We review the definition of a generalized metric, Courant algebroid and related objects, providing the examples used in physics. |

17 November 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Where: | Room W2 3-349, University of Oldenburg |

When: | 14:00 hrs |

Speaker: | Gleb Zhilin(University of Minsk, Belarus) |

Title: | "Gauged Hopfions" |

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. |

# October 2014

# September 2014

26 September 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR - Field Theory, Gravity, Relativity -**

Where: | ZARM, University of Bremen, Room 1280 |

When: | 14:30 hrs |

Speaker: | Dr Alexei Zayats Kazan Federal University, Tatarstan, Russia |

Title: | "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. |

24 September 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR- Field Theory, Gravity, Relativit, Relativity**

Where: | ZARM, University of Bremen, Room 1730 |

When: | 14:00 hrs |

Speaker: | Belinka González-Fernández (Universidad Nacional Autónoma de México UNAM, México) |

Title: | "Analogue models of gravity and the viscous case" |

Abstract | We make a brief review of the relevant aspects and recent contributions to the field of analogue gravity,particularly concerning the fluid-gravity correspondence. We develop a specific case: a viscous, barotropic, incompressible fluid in which the flow is irrotational, though possibly time dependent. We will show that the equation of motion for the velocity potential describing an acoustic disturbance corresponds to an inhomogeneous d’Alembertian equation of motion for a minimally coupled massless scalar field, propagating in a (3+1)-dimensional Lorentzian geometry. Finally, in order to understand the energetic state of the system, we will calculate the stress-energy tensor and find it is not constant, so that there would exist energy interchange between the scalar field and the modelled space-time: the field would transfer part of its energy to increase the curvature of the space-time. |

# August 2014

# July 2014

14 July 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR - Field Theory, Gravity, Relativity -**

Where: | Room W04 1-171 |

When: | 13:45 hrs |

Speaker: | (University of Alberta, Edmonton)Christos Tzounis |

Title: | "Radiation from an emitter revolving around a magnetized non-rotating black hole" |

Abstract: | One of the methods of study of black holes in astrophysics is based on broadening of the spectrum of radiation of ionized Iron atoms. The line K$\alpha$ associated with Iron emission at 6.4 keV is very narrow. If such an ion is revolving around a black hole, this line is effectively broadened as a result of the Doppler and gravitational redshift effects. The profile of the broaden spectrum contains information about the gravitational field of the black hole. In the presence of a regular magnetic field in the vicinity of a black holes the characteristics of the motion of charged ions are modified. In particular, their innermost stable circular orbits become closer to the horizon. The purpose of this work is to study how this effect modifies the spectrum broadening of lines emitted by such an ion. Our final goal is to analyze whether the change of the spectrum profiles can give us information about the magnetic field in the black hole vicinity. |

14 July 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR - Field Theory, Gravity, Relativity -**

Where: | Room W04 1-171 |

When: | 12:00 hrs |

Speaker: | Dr. Shohreh Abdolrahimi (University of Oldenburg) |

Title: | "Large Randall-Sundrom II Black Holes" |

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 ). |

# June 2014

25 June 2014

Where: | University of Bielefeld, room: D6-135, Institute for Physics |

When: | 13:00 hrs |

Speaker: | Erandy Ramirez (ICN-UNAM Mexico) |

Title: | "Exploring the non-linearity of the collaps approach in the context of inflationary cosmology" |

Abstract: | We calculate perturbed and background quantities for two wave modes using a mechanism that reliesin the collapse of the wave function to explain the quantum-to-classical transition in in ationarycosmology and the emergence of the seeds of cosmic structure. This formalism has already beenapplied consistently for one mode and we develop it at linear order for two modes and consider howthe non-linearity of the theory is manifested. more information |

23 June 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR- Field Theory, Gravity, Relativity - **

Where: | ZARM, University of Bremen, Room 1730ZARM |

When: | 11:00 hrs |

Speaker: | Abhishek Majhi, (Saha Institute of Nuclear Physics; Astroparticle Physics and Cosmology Division) |

Title: | "Energy spectrum of quantum horizons of equilibrium black holes" |

Abstract: | abstract |

19 June 2014

Where: | University of Oldenburg, Room W010-006 |

When: | 14:00 hrs |

Speaker: | Keno Eilers (Oldenburg) |

Title: | "Calculating k:Periods of second kind differentials of (n, s)-curves" |

Abstract: | For elliptic curves expressions for the periods of elliptic integrals of the second kind in terms of theta constants have been known since the middle of the 19th century. In this talk I consider the problem of generalizing these results to curves of higher genera, in particular to a special class of algebraic curves, the so-called (n, s)-curves. It is shown that the representations required can be obtained in two ways: The first uses the comparison of twoequivalent expressions for the projective connection and the second involves the general solution of the Jacobi inversion problem. By that we obtain not only the desired generalization, but also a couple of relations of derivatives of theta constants. |

5 June 2014

Where: | University of Oldenburg Room W2-1-143 |

When: | 14:15 hrs |

Speaker: | Oliver Rinne (Potsdam) |

Title: | "Critical phenomena in gravitational collapse" |

Abstract: | In general relativity, sufficiently small asymptotically flat initial data disperse to flat spacetime, whereas sufficiently strong data collapse to form a black hole. What happens at the threshold between the two outcomes? Critical phenomena in gravitational collapse were discovered through numerical simulations by Choptuik in 1993 and bear a striking analogy to thermodynamic phase transitions. I will give an introduction to this subject, stressing computational aspects and recent developments. Finally I will discuss some open problems. |

5 June 2014

Where: | University of Bremen, building NW1 room H3, Colloquium of the Department for Physics |

When: | 16:15 hrs |

Speaker: | Dr. Sven Herrmann (ZARM, University Bremen) |

Title: | "Cold atoms in the drop tower for fundamental tests of quantum mechanics and gravity" |

Abstract: | Materiewellen-Interferometer mit kalten Atomen haben sich in den letzten Jahren immer mehr zu zuverlässigen und sehr genauen Messinstrumenten für physikalische Präzisionsmessungen verschiedenster Art entwickelt. So wurden mit ihnen bereits sehr genaue Messungen der Feinstrukturkonstante oder der Newtonschen Gravitationskonstante durchgeführt. Ebenso lassen sich mit ihnen empfindliche Inertialsensoren und Gravimeter realisieren. Viele dieser Anwendungen sind dabei begrenzt durch die beschränkte Propagationszeit der frei fallenden Materiewellen im Interferometer. Eine schwerelose Umgebung bietet hier die besondere Möglichkeit diese Propagationszeit und damit die Empfindlichkeit der Messungen deutlich zu steigern, zumal die Empfindlichkeit der Messungen in der Regel quadratisch mit der freien Propagationszeit ansteigt Daher werden im Rahmen des QUANTUS (Quantengase unter Schwerelosigkeit) Verbundprojekts seit einigen Jahren Versuche zu Materiewelleninterferometrie mit Bose-Einstein-Kondensaten im Fallturm in Bremen durchgeführt. Der Vortrag wird die bisherigen Ergebnisse und die laufenden Arbeiten hierzu vorstellen und mögliche Anwendungen in grundlagenphysikalischen Experimenten diskutieren, unter anderem z.B. die Möglichkeit einen Test des Einsteinschen Äquivalenzprinzips durchzuführen. |

4 June 2014

**BREMEN-OLDENBURG RELATIVITY SEMINARField Theory, Gravity, Relativity **

Where: | ZARM, University of Bremen, Room 1730 |

When: | 17:30 hrs |

Speaker: | Dr. David Kofron (Charles University Prague, Czech Republic) |

Title: | "The C-metric: most general form, geodesic motion and Meissner effect" |

Abstract: | Boost rotation symmetric spacetimes form an important family of solutions to Einstein-Maxwell equations. They are highly symmetric so that they can be treated analytically and at the same time they admit a gravitational radiation emitted from accelerated sources. I will briefly revisit the boost rotation symmetric spacetimes in general and then I will discuss the C-metric in detail. C-metrics are a special subclass of BR symmetric spacetimes (of algebraic type D) which describe uniformly accelerated charged and rotating black holes. Except two Killing vectors it also possess a conformal Killing-Yano tensor, so that the null geodesics are completely integrable. Briefly I will mention Newtonian and Minkowskian limit upon which is the physical interpretation based. Then I will summarize the Ernst procedure of removing the nodal singularities and use the Ernst formalism to solve the test Maxwell equations and show how the Meissner effect can be calculated and visualised. |

# May 2014

28 May 2014**BREMEN-OLDENBURG RELATIVITY SEMINAR Field Theory, Gravity, Relativity**

Where: | Room W2 3-349 |

When: | 12:05 hrs |

Speaker: | Dr. Sayantani Lahiri (Relativity and Cosmology Centre Jadavpur University, India) |

Title: | "Radion Dynamics in Randall-Sundrum Two-Brane Model" |

Abstract: | In the backdrop of generalised RS braneworld scenario, we look for origin of an effective 4D cosmological constant on the visible 3-brane due to the effects of bulk curvature and radion field. On the brane we also look for cosmological and spherically symmetric, static solutions sourced by radion field as well as additional on-brane matter which in turn will be shown to take a decisive role in stabilising the radion field. |

20 May 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR Field Theory, Gravity, Relativity**

Where: | Room W2 3-349 |

When: | 10:15 hrs |

Speaker: | Prof. Naresh Dadhich (Jamia Millia Islamia, New Delhi Centre for Astronomy & Astrophysics, Pune) |

Title: | "On pure Lovelock gravity" |

Abstract: | Universality of basic gravitational dynamics would be established in odd, 2N+1, and even, 2N+2, dimensions where N is the degree of Lovelock polynomial. That is, as Einstein gravity for N=1 is kinematic in 3-dimension and so is N=2 Gauss-Bonnet gravity in 5-dimension for the properly defined quadratic GB curvature, and thereby we have analogues of BTZ black hole in all odd, 2N+1 dimensions. On the other hand, bound orbits around static body can exist only in 4-dimension for Einstein gravity while they do for all even, 2N+2, dimensions for pure Lovelock gravity. |

# April 2014

# March 2014

13 March 2014

**BREMEN-OLDENBURG-RELATIVITY SEMINAR- Field Theory, Gravity, Relativity -**

Where: | ZARM, University of Bremen, Room 1730 |

When: | 11:00 hrs |

Speaker: | Randall A. Corell (NASA Ames Research Center, Moffet Field, CA USA) |

Title: | "The Applicability of Emerging Quantum Computing Capabilities to Space Science?" |

Abstract: | In conjunction with the Universities Space Research Association and Google, Inc., NASA Ames has acquired a quantum annealing computing device built by DWAVE Systems with approximately 512 qubits. Quantum computers in principle have performance that scales exponentially with the number of qubits, thus promising a large increase in computational performance via quantum parallelism. Quantum annealing computers are a special type of quantum computer designed specifically for optimization problems. Quantum computing might have significant applicability to space science, exploration, and aviation as well as general computing applications. NASA Ames's Quantum Artifical Intelligence Laboratory (QUAIL) is studying the performance of the DWAVE quantum annealing computer and developing quantum annealing algorithms applicable to NASA missions, such as searching for transiting exoplanets, and mission planning optimization, and fault detection. While experiments to date with initial algorithms run successfully on the DWAVE machine, they are not yet implemented efficiently. Improving the implementation of algorithms on the DWAVE architecture is a current foucs. Longer term applications might be based on algorithms that require new designs for observation and experiments that would collect data that better fits the architecture of quantum computer hardware. Thus we are beginning to explore quanturm annealing algorithms that would support future missions such as mSTAR, a proposed space-based mission to test for Lorentz Invariance Violation. |

3 March 2014

Where: | University of Bielefeld, seminarroom D5-135 |

When: | 11:30 hrs |

Speaker: | Harald Skarke (TU Wien) |

Title: | "The effect of inhomogeneity on the evolution of the universe" |

Abstract: | A recently introduced method for analysing the evolution of the volume of an inhomogeneous irrotational dust universe is presented. In this framework the Buchert formalism is circumvented by working with a mass weighted average instead of the usual volume average. This makes it possible to go beyond pertubation theory in a numerical analysis. If the initial state of the universe is Einstein-de Sitter with small Gaussian pertubations, then the inhomogeneities, albeit strongly affecting the evolution, cannot explain the observed acceleration. Possible loopholes to this conclusion will be discussed. |

# February 2014

27 February 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR - Field Theory, Gravity, Relativity -**

Where: | ZARM, University of Bremen, Room 1730 |

When: | 09:15 hrs |

Speaker: | Dipl.-Phys. Oliver Gabel (Technische Universität Darmstadt) |

Title: | "Tidal and Relativistic Corrections for Free Falling Bose Einstein Condensates" |

Abstract: | The recent developments of matter-wave interferometry hold promise for measuring general relativistic effects to high accuracy. With the demonstration of Bose-Einstein condensates (BECs) and matter wave interferometers in free fall, the QUANTUS collaboration is at the forefront of this endeavour and aims at the verification of Einstein's equivalence principle, the foundation of general relativity, in a future experiment. In this context, it has become relevant to extend the usual Newtonian description of BECs to general relativity and to study the arising corrections in a systematic way. Beginning with a short introduction to BECs, we review Riemann and Fermi normal coordinates as the natural extension of local inertial frames to curved space-time in terms of a covariant Taylor expansion. Employing a generalisation of the Gross-Pittaevskii equation in terms of the non-linear Klein-Gordon equation in Fermi coordinates then allows for a straightforward mean-field escription of free falling BECs. As an application of this formalism, we consider a BEC falling on radial, as well as on circular equatorial geodesics in Schwarzschild space-time, obtaining -- in the non-relativistic limit -- the tidal and general relativistic corrections to its evolution. |

21 February 2014

**BREMEN-OLDENBURG RELATIVITY SEMINAR- Field Theory, Gravity, Relativity -**

Where: | University of Oldenburg, Room W2 3-349 |

When: | 14:15 hrs |

Speaker: | Dr. David Foster (Canterbury University, UK) |

Title: | "Skyrmion scattering" |

Abstract: | The Skyrme model is a model of nuclei which can be understood as a geometric energy functional. I shall introduce the Skyrme model. Then I will discuss how its solutions scatter and show how this can be approximated by treating them as point particles. I will also show some interesting and possibly unique dynamics. |

10 February 2014

**BREMEN-OLDENBURG-RELATIVITY SEMINAR- Field Theory, Gravity, Relativity -**

Where: | ZARM, University of Bremen Room 1730 |

When: | 16:15 hrs |

Speaker: | Dr. Vladimir Karas (Charles University Prague) |

Title: | "Relativistic spectral features from accretion disc spots" |

Abstract: | The inner parts of accretion flows have been extensively studied by means of X-ray spectroscopy. I will summarize several aspects of how the observed spectral features from black-hole accretion discs are influenced by strong gravity. Starting with a brief summary of the equations describing light intensity (and polarization) propagation through strong gravitational fields, relativistic effects are discussed in terms of geometrical optics -- a well-suited approach to interpret various flavours of `hot spots' on the disc surface (and a patchy corona above it). Theoretical approaches have been developed that can tackle more complicated situations, such as the case of dispersive media. This may be able to address, more accurately, simultaneous observations in mutually remote parts of the electromagnetic spectrum. In the second part I mention flares and spots as a model for X-ray variability of active galactic nuclei: multiple spots are created on the surface of an accretion disc following the intense irradiation, and the observed signal is modulated by an interplay of relativistic effects. This scheme captures many properties of present observations. Mean spectra of orbiting spots resemble those of axisymmetric rings and we briefly discuss this degeneracy that is inherent to time integrated observations; this can be resolved by increased sensitivity of future high-throughput technology. More complex geometries of the emitting region should be also explored, such as spiral waves propagating across the disc, but we can safely conclude that strong gravity of the central black hole is very likely the main agent which shapes the overall form of the X-ray spectral features from the inner disc. |

# January 2014

30 January 2014, 2:15 pm

**COLLOQUIUM OF THE DEPARTMENT OF PHYSICS**

Where: | University of Oldenburg, Room W2-1-143 |

When: | 14:15 hrs |

Speaker: | PD Dr. Brigitte Rödiger (University Hamburg) |

Title: | "Dynamic galaxy clusters" |

Abstract: | On scales larger than millions of light years the universe is threaded with the "cosmic web". Matter collapses along the web's filaments due to gravity. The "knots" of the web form clusters of galaxies, which can contain hundreds to thousands of galaxies. However, the bulk of the baryonic matter in galaxy clusters resides in the intra-cluster medium (ICM), the hot tenuous plasma that fills the clusters. Galaxies as well as ICM are bound by the clusters' dark matter halos. Galaxy clusters are the largest bound structures in the universe, but they are still growing via infall of galaxies or groups of galaxies, and via spectacular mergers between clusters. The ICM records traces of the dynamic and thermal history of the clusters, and is an interesting astrophysical plasma in itself. I will give an overview of theoretical and observational studies of galaxy cluster dynamics and ICM plasma properties. |

# December 2013

18 December 2013

**BREMEN-OLDENBURG-RELATIVITY SEMINAR - Field Theory, Gravity, Relativity -**

Where: | ZARM, University of Bremen, Room 1730 |

When: | 14:00 hrs |

Speaker: | Dr. Yuri N. Obukhov (Russian Academy of Sciences) |

Title: | "Dynamics of spin in a gravitational field" |

Abstract: | We study the quantum mechanics of a Dirac fermion on a curved spacetime manifold. The metric of the spacetime is completely arbitrary, allowing for the discussion of all possible inertial and gravitational field configurations. In this framework, the Hermitian Dirac Hamiltonian for an arbitrary classical external field is derived. In order to discuss the physical content of the quantum-mechanical model, we apply the Foldy-Wouthuysen transformation, and find the quantum equations of motion for the spin and position operators. The semiclassical limit of these equations is obtained. These results are compared with the dynamics of a classical particle with spin in the framework of the standard Mathisson-Papapetrou theory and in the classical canonical theory. The comparison of the quantum mechanical and classical equations of motion of a spinning particle in an arbitrary gravitational field shows their complete agreement. |

12 December 2013

**Theoriekolloquium**

Wo: | University of Oldenburg, Room W2 1-143 |

Wann: | 14:15 hrs |

Sprecher/in: | Herr Dr. Thomas Müller (University Stuttgart) |

Titel: | "Interaktive relativistische Visualisierung" |

Abstract: | Die relativistische Visualisierung gliedert sich im Wesentlichen in zwei Bereiche: Die Darstellung aus der Ich-Perspektive und die visuelle Exploration von zum Beispiel feld-basierter Daten oder Daten entlang von Geodäten.In diesem Vortrag konzentriere ich mich auf die Darstellung aus der Ich-Perspektive (first-person view), die sich sowohl an die breite Öffentlichkeit richtet, als auch in der Lehre die Relativitätstheorie und deren Effekte veranschaulicht.Hierfür wird die physikalische Ausbreitung von Licht umgekehrt und vom Beobachter rückwärts in die Szene verfolgt. In der Relativitätstheorie muss dieses Raytracing- Verfahren auf vier Dimensionen und auf beliebige Raumzeiten erweitert werden. Allerdings ist diese Methode sehr zeitaufwendig und für eine interaktive Visualisierung nur bedingt geeignet. Basierend auf moderner Grafikhardware stelle ich verschiedene Techniken vor, die eine interaktive Visualisierung in der Speziellen (SRT) und der Allgemeinen Relativitätstheorie (ART) ermöglichen. In der ART sind hierfür vor allem analytische Lösungen der Geodaetengleichung notwendig. Als Beispiele dienen das lock-key paradox inder SRT, das Morris-Thorne Wurmloch, und eine dünne Akkretionsscheibe um ein statisches Schwarzes Loch. |

# November 2013

29 November 2013

**BREMEN-OLDENBURG-RELATIVITY SEMINAR- Field Theory, Gravity, Relativity - **

Where: | ZARM, University of Bremen, Room 1280 |

When: | 16:00 hrs |

Speaker: | Dr. Oliver Rinne (Albert-Einstein-Institute, Golm) |

Title: | "Formation and decay of coloured black holes and solitons in the large" |

Abstract: | With V. Moncrief I developed a constrained ADM-like formulation of the Einstein equations on hyperboloidal spacetime slices reaching out to future null infinity. Here I focus on the Einstein-Yang-Mills system in spherical symmetry. Type I critical phenomena in the gravitational collapse of one-parameter families of regular initial data are studied numerically. The critical solutions, which serve as intermediate attractors, are the n=1 Bartnik-McKinnon soliton and coloured black hole. |

# October 2013

30 October 2013

**HEP Seminar**

Where: | University of Bielefeld, Room D6-136 |

When: | 14:15 hrs |

Speaker: | Dr. Marc Walker (Manly Astrophysics, Manly, Australia) |

Title: | "The direct detection of G2" |

Abstract: | Right now, at the centre of our Galaxy, a tiny gas cloud called ’G2’ is speeding to a destructive encounter with Sgr A*. This event will be closely watched by many telescopes, because everyone wants to see how a black hole eats. But what is G2? and where did it come from? The nature and origin of G2 are controversial topics, but taken at face value the data suggest that planetary-mass gas clouds are commonplace in the central regions of our Galaxy. I will describe how G2 might have appeared - as a cold, dense and highly durable molecular cloud - before it fell under the in?uence of Sgr A*. In isolation such clouds can be practically invisible, and G2 may be our ?rst direct detection of a dark matter ’particle’. |

21 October 2013

**COLLOQUIUM OF THE DEPARTEMENT OF PHYSICS**

Where: | C.-v.-O. University of Oldenburg, Room W2-1-148 |

When: | 16:15 hrs |

Speaker: | PD Dr. Silke Britzen (Max-Planck-Institute for Radioastronomy, Bonn) |

Title: | "Black hole observations - towards the Event Horizon" |

Abstract: | Black Holes are the most attractive and most compact objects in our Universe. They are expected to be at the very centers of the brightest galaxies. Their masses range from Millions to Billions of solar masses. Black Holes can not be observed directly but astronomers have gathered a lot of indirect evidence for their existence. In the talk I will present highest resolution observations of the most important effects of Black Holes on their environment. I will also describe the current status of the endeavor to image the Event Horizon of the Galactic Center Black Hole. The physical nature of a Black Hole can not be fully described within Einstein's theory of General Relativity. Trying to understand these objects better is a major challenge but promises great potential. |

21 October 2013

**BREMEN-OLDENBURG-RELATIVITY SEMINAR**

Where: | ZARM, University of Bremen, Room 1730 |

When: | 11:00 hrs |

Speaker: | Dr. Carlos Schat (CONICET - University of Buenos Aires) |

Title: | "Effective field theories for nucleons and neutron stars" |

Abstract: | Modern effective theories are an important theoretical tool to study the non-perturbative physics of the strong interactions. Their fundamental theory is quantum chromodynamics (QCD), which describes the interactions at the level of quarks and gluons, but at larger scales all the richness of nuclear physics emerges. I will show how effective theories can provide a simple description for the complex phenomenology by discussing baryons, the nuclear force and very recent work relevant for nuclear matter in the neutron star crust and core. |

15 October 2013

**HEP Seminar**

Where: | University of Bielefeld, Room D6-135 |

When: | 14:15 hrs |

Speaker: | Dr. Christiano Germani (LMU, Munich) |

Title: | "Cosmological consequences of non-minimal gravitational interactions" |

Abstract: | The only fields so far discovered in Nature are the one of the Standard Model of particle physics and gravity. In this talk, I will show that these may suffice to explain the early time cosmology in compatibility with the latest observations. In order to do so, new gravity-matter interactions (that do not introduce new degrees of freedom) must be introduced. Finally, I will also show that the missing Dark Matter might be just related to the QCD axion, if non-minimally coupled to gravity. |

14 October 2013

**COLLOQUIUM OF THE DEPARTEMENT OF PHYSICS**

Where: | University of Oldenburg, Room W2-1-148 |

When: | 16:15 hrs |

Speaker: | Chair: Prof. Dr. Claus Lämmerzahl (ZARM, University of Bremen)Prof. Dr. Matthias Steinmetz (Leibniz-Institute of Astrophysics, Potsdam) |

Title: | "The Milky Way as a Galaxy Formation Laboratory" |

Abstract: | The Milky Way is the galaxy we can study in most detail. Large spectroscopic campaigns such as the Radial Velocity Experiment RAVE combined with photometric and astrometric surveys provide us an integrated view on the chemical and kinematical history of the Galaxy and its various stellar populations. Combined with the results of cosmological gas dynamical simulations we thus can disentangle the various physical processes that have shaped the Galaxy's structure and get a detailed view of the complex structure of the Milky Way and its convoluted formation history. |

10 October 2013

**BREMEN-OLDENBURG RELATIVITY SEMINAR- Field Theory, Gravity, Relativity -**

Where: | University of Oldenburg, Room W2 3-349 |

When: | 12:00 hrs |

Speaker: | Hemwati Nandan (Department of Physics, Gurukula Kangri University, Haridwar, India) |

Title: | "Geodesic flows in rotating black hole backgrounds" |

Abstract: | 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. |

# September 2013

# August 2013

30 August 2013

**BREMEN-OLDENBURG RELATIVITY SEMINAR **

- Field Theory, Gravity, Relativity -

Where: | ZARM, University of Bremen, Room 1280 |

When: | 14:00 hrs |

Speaker: | Dr. Dinesh Singh (University of Regina, Saskatchewan, Canada) |

Title: | "Assessing the Conceptual Challenges and Future of Quantum Gravity Research" |

Abstract: | For over 70 years, the search for a viable theory of quantum gravity has taken on many creative approaches to date. Beyond the lack of any physical data to justify the approaches taken thus far, these respective theories also fundamentally disagree with each other in terms of the set of conceptual, mathematical, and philosophical assumptions that define them. This underlying issue becomes most evident when assessing the relative merits of the two leading contenders for quantum gravity, namely string theory (ST) and loop quantum gravity (LQG). Furthermore, the mathematical complexities of present and future quantum gravity theories render it very difficult to properly gauge the level of progress gained beyond purely theoretical considerations, especially in the hope of generating signatures that can be unambiguously assessed within an experimental setting. The overall purpose of this presentation is to identify and offer a critical assessment of the conceptual challenges that come from studying the overlap region between gravitational and quantum mechanical phenomena in the ongoing search for quantum gravity. In particular, a set of well-defined research findings are presented to illustrate the appearance of non-trivial physical predictions several orders of magnitude larger than any effects predicted by ST, LQG, and other approaches to quantum gravity. If one or more of these predictions are subsequently confirmed by observations, they have the potential to rule out any number of existing quantum gravity theories in existence, with important implications for future research endeavors. |

# July 2013

19 July 2013

**BREMEN-OLDENBURG RELATIVITY SEMINAR **

Where: | University of Oldenburg, Room W2 3-349 |

When: | 14:00 hrs |

Speaker: | Prof. Alfredo Macias (Universidad Autonoma Metropolitana-Iztapalapa Mexico) |

Title: | "No-hair conjecture for Einstein-Plebanski nonlinear electrodynamics static Black Holes" |

Abstract: | The no-hair conjecture statement is enhanced to include static regular and singular black-hole solutions, based on the class of nonlinear lectrodynamics proposed by Pleban´ski coupled to Einstein theory. In particular, we focus, as example, on regular black-hole solutions, i.e., black holes where the space-time metric everywhere is nonsingular, in the framework of General Relativity theory. Timereversal invariance implies the existence of two separate non-overlapping cases: a purely gravito-electric case, and a purely gravito-magnetic one. We prove the enhanced no-hair conjecture for both cases. Moreover, the generalization of the conjecture including also the couplings to non-Abelian matter is also discussed. |

# June 2013

# May 2013

29 May 2013

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | ZARM, University of Bremen, Room 1280 |

When: | 14:30 hrs |

Speaker: | Dr. Matias Dahl, Institute of Mathematics, Helsinki University of Technology (Finland) |

Title: | "Classification of electromagnetic media using phase velocity" |

Abstract: | If we are given an electromagnetic medium tensor we can compute the speed of a propagating signal. For example, in a homogeneous medium we can compute the phase velocity using plane waves. A less well understood question is the converse: If we know the behaviour of phase velocity in all possible directions for an unknown medium, how much can we say about the (bi)anisotropic structure of that medium? In this talk we describe a number of results on this question in the setting of linear, local, and non-dissipative (skewon-free) media. In particular, we discuss the classification of medium tensors where wave propagation is determined by one or two Lorentz null cones. |

27 May 2013

**Physikalisches Kolloquium**

Where: | University of Bielefeld, Hörsaal 6 |

When: | 16:15 hrs |

Speaker: | Prof. Dr. Hartmut Abele (Atominstitut, TU Wien) |

Title: | "The Quantum Bouncing Ball and Gravity Tests with a Quantum Interference" |

Abstract: | Newton’s Law of Gravity is considered valid from sub-millimetre distances up to inter-galactic space, but fails to describe important features of cosmology like the accelerating expansion component of our universe. While the most straightforward candidate for such a component is Einstein’s cosmological constant, a plausible alternative is dynamical vacuum energy, or “quintessence”, changing over time. Although it is traditional to neglect (or set to zero) the couplings of this light scalar to the standard model, it is natural for a scalar quintessence field to evolve on cosmological time scales today while having couplings to matter, as expected from string theory. Hence the presence of such a field would provide energy changes to Newton’s gravity potential of the earth at short distances invisible to electromagnetic interactions. We present a novel direct search strategy with neutrons based on Rabispectroscopy of quantum transitions in the gravity potential of the earth. The sensitivity for deviations on Newton’s gravity law is right now E = 10?14eV , providing a severe restriction on quintessence fields and on any possible new interactions on that level of accuracy. If some undiscovered dark matter or dark energy particles interact with a neutron, this should result in a measurable energy shift of the observed quantum states. In the case of some dark energy scenarios with a coupling to matter, the experiment has the potential to find or exclude these hypothetical particles in full parameter space. |

# April 2013

22 April 2013

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | University of Oldenburg, Room W2 3-349 |

When: | 14:00 hrs |

Speaker: | Prof. Carlos A.R. Herdeiro, Universidade de Aveiro (Portugal) |

Title: | "Shock wave collisions in D dimensions" |

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. |

References:

http://www.springerlink.com/content/t5812l42161255p2/?MUD=MP

http://prl.aps.org/abstract/PRL/v108/i18/e181102

http://arXiv.org/abs/arXiv:1206.5839

http://arXiv.org/abs/arXiv:1301.1073

11 April 2013 14:00, Room 1280, ZARM, University of Bremen

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | ZARM, University of Bremen, Room 1280 |

When: | 14:00 hrs |

Speaker: | Prof. Dr. Chunfang Li, Shanghai University (China) |

Title: | "Photon’s inner reference frame and angular momentum" |

Abstract: | Due to the constraint of transversality condition on the vector wavefunction, the photon’s spin and orbital angular momentum (OAM) about the origin of the laboratory reference frame are not constants of motion. Such a phenomenon is usually interpreted as the non-separability of the spin from the OAM. This talk addresses the problem of the photon’s angular momentum from a different point of view. From the transversality condition we introduce a two-component representation, called the Jones representation. The wavefunction in the Jones representation is free of any constraints. Two constants of motion are identified in the Jones representation, the spin and the OAM about the origin of the so-called inner reference frame. Because the position with respect to the inner reference frame is canonically conjugate to the momentum, we are allowed to canonically quantize the radiation field at the inner reference frame. Besides, a new degree of freedom that appears as a unit vector is also identified to specify the inner reference frame. At last, it is shown in the Jones representation that the operator of the OAM about the origin of the laboratory reference frame depends on the helicity and satisfies the commutation relations that were obtained by van Enk and Nienhuis [J. Mod. Opt. 41, 963 (1994)] from a consideration of the second quantization. |

5 April 2013

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | Jacobs University Bremen, Seminar Room (Room 50), Research 3 |

When: | 12:30 hrs |

Speaker: | Dr. Gabriel Luchini, Department of Mathematical Sciences, Durham University (United Kingdom) |

Title: | "Hidden symmetries and gauge theories" |

Abstract: | The description of electromagnetism in terms of equations of flux, known as the integral formulation, had a major role in the understanding of electromagnetic phenomena. The Yang-Mills theory, on the other hand, was first formulated in terms of differential equations. In this seminar we present an integral formulation for Yang-Mills theory and discuss how it leads (in a very natural way) to conserved charges that are invariant under general gauge transformations and reparametristion. |

5 April 2013

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | ZARM, University of Bremen, Room 1280 |

When: | 10:00 hrs |

Speaker: | Prof. Dr. Jose Pons, University of Alicante (Spain) |

Title: | "Strongly magnetized neutron stars as laboratories for relativistic astrophysics" |

Abstract: | For decades, neutron stars have been used as laboratories for matter under extreme conditions for nuclear physics, gravitation, plasma physics, particle physics or almost any other subfield. In particular, much interest has been paid in the last years to the "magnetar" subclass, which are young neutron stars characterized by high X-ray quiescent luminosities, outbursts, and, in some cases, sporadic giant flares. They are believed to be powered by ultra-strong magnetic fields but the diversity of their observed behaviors is not well understood. Following the coupled magnetic, thermal, and rotational evolution, we try to establish evolutionary links between the (apparently different) observed phenomenology. From our results, we can give a qualitative description of the main stages in the evolution of a neutron star, from its youth to its old age, and connect them with observations. |

# March 2013

# February 2013

27 February 2013

Where: | University of Oldenburg |

When: | 11:00 hrs |

Speaker: | Chandrachur Chakraborty |

Title: | "Lense-Thirring precession in strong gravitational fields" |

Abstract: | An exact expression for the rate of dragging of inertial frames (Lense-Thirring (LT) precession) in a general stationary spacetime, is derived without invoking the weak field approximation. This expression, when used for the Kerr metric, leads to the LT precession frequency in the strong gravity regime appropriate to compact gravitating objects like rotating neutron stars and black holes. Numerical values of the precession rate are computed for a few known cases of pulsars and compared to the precession rates in the weaker gravity regimes of the earth and the sun. We also derive the exact LT precession rate for Plebanski-Demianski spacetimes. From this result, we show that in the case of spherically symmetric, zero angular momentum NUT spacetimes, the frame-dragging effect does not vanish. |

# January 2013

# December 2012

19 December 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | ZARM, University of Bremen, Room 1730n |

When: | 14:00 hrs |

Speaker: | Alberto Favaro (University of Cologne) |

Title: | "Fresnel versus Kummer surfaces: geometrical optics in dispersionless linear (meta)materials and vacuum" |

Abstract: | Geometrical optics describes, with good accuracy, the propagation of high-frequency plane waves through an electromagnetic medium. Under such approximation, the behaviour of the electromagnetic fields is characterised by just three quantities: the temporal frequency ?, the spatial wave (co)vector k, and the polarisation (co)vector a. Numerous key properties of a given optical medium are determined by the Fresnel surface, which is the visual counterpart of the equation relating ? and k. For instance, the propagation of electromagnetic waves in a uniaxial crystal, such as calcite, is represented by two light-cones. Kummer, whilst analysing quadratic line complexes as models for light rays in an optical apparatus, discovered in the framework of projective geometry a quartic surface that is linked to the Fresnel one. Given an arbitrary dispersionless linear (meta)material or vacuum, we aim to establish whether the resulting Fresnel surface is equivalent to, or is more general than, a Kummer surface. To read an extended abstract, see: http://www.thp.uni-koeln.de/gravitation/mitarbeiter/favaro.html |

12 December 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | University of Oldenburg, Room W2 3-349 |

When: | 16:00 hrs |

Speaker: | Cristian Stelea (Al. I. Cuza University, Iasi, Romania) |

Title: | "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. |

4 December 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity -

Where: | ZARM, University of Bremen, Room 1280 |

When: | 16:00 hrs |

Speaker: | Bobomurat Ahmedov (Institute of Nuclear Physics & Ulugh Beg Astronomical Institute Uzbekistan Academy of Sciences, Tashkent; Uzbekistan National University, Tashkent) |

Title: | "Plasma Magnetosphere and Electromagnetic Fields of Oscillating NSs and Magnetars in General Relativity" |

4 December 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity -

Where: | ZARM, University of Bremen, Room 1280 |

When: | 11:30 hrs |

Speaker: | Prof. Dr. Christoph Lienau (Institute of Physics, University of Oldenburg, Oldenburg) |

Title: | "Small semi-black holes for light: New light and electron sources" |

3 December 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | University of Oldenburg, Room W2 3-349 |

When: | 15:00 hrs |

Speaker: | Christine Gruber (Freie Universität Berlin) |

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. |

# November 2012

# October 2012

19 October 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | University of Oldenburg, Room W4 1-171 |

When: | 14:00 hrs |

Speaker: | Thomas Albin (University of Oldenburg and ESTEC) |

Title: | "Maximizing the ground based detection of near-Earth objects" |

Abstract: | Near-Earth Objects are a special case of comets and asteroids in the solar system. Over 100,000 objects belong to the main belt, a region of objects between Mars' and Jupiter's orbits whose orbit is within both planets. In contrary to that the Near-earth objects specifies objects, which intersect Earth's orbit or which come close to it. Consequently, these objects might be a potential hazard for life on Earth depending on their size.Currently, approximately 9000 objects have been discovered so far and several 100,000 Tunguska-sized objects are still unknown. It is the task and responsibility of ESA's SSA programme to discover further objects and compute their probability of hitting the Earth.Future NEO surveys, which are observing and scanning the sky systematically, need effective search strategies. Therefore it is important to know, especially for surveys with limited observation time or a limited field of view, whether there are certain areas on the sky with high detectability probabilities. This report will give an answer to this scientific objective for certain existing or future telescopes. It will be shown, that optical properties of the telescope and CCD properties affect the detectability. For example, reducing the resolution of a CCD can double the detection probability for certain sky areas. |

# September 2012

21 September 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | University of Oldenburg, Room W2 3-349 |

When: | 11:00 hrs |

Speaker: | Dr. Andrey Shoom (University of Alberta, Edmonton, Canada) |

Title: | "Spinoptics in a Stationary Spacetime" |

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 ‘standard’ geometrical optics approach. After that we demonstrate how this ‘standard’ 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. |

20 September 2012

Where: | Jacobs University Bremen, Research III Lecture Hall, Jacobs University Bremen |

When: | 13:00 hrs |

Speaker: | Dr. Floris van der Tak, SRON (Netherlands Institute for Space Research) |

Title: | "The first results of the Herschel-HIFI mission" |

Abstract: | This talk reviews the results from the first years of observations with the HIFI instrument onboard ESA's Herschel space observatory. The talk starts by outlining the goals and possibilities of far-infrared and submillimeter astronomy, the limitations of the Earth's atmosphere, and the scientific scope of the Herschel-HIFI mission. The presentation of science results from the mission follows the life cycle of gas in galaxies as grouped into five themes: Structure of the interstellar medium, First steps in interstellar chemistry, Formation of stars and planets, Solar system results and Evolved stellar envelopes. The HIFI observations paint a picture where the interstellar medium in galaxies has a mixed, rather than a layered structure; the same conclusion may hold for protoplanetary disks. In addition, the HIFI data show that exchange of matter between the various bodies within planetary systems is a common process. |

18 September 2012

Where: | University of Oldenburg, Room W2 3-349 |

When: | 16:00 hrs |

Speaker: | Keno Eilers (University Oldenburg) |

Title: | "Computational aspects and outstanding problems with multivariate sigma-functions" |

# August 2012

30 August 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | ZARM, University of Bremen, Room 1730 |

When: | 15:30 hrs |

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

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

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). |

22 August 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | University of Oldenburg, Room W2 3-349 |

When: | 11:15 hrs |

Speaker: | Dr. Hideo Iguchi (College of Science and Technology (CST), Nihon University, Japan) |

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

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. |

6 August 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | ZARM, University of Bremen, Room 1280 |

When: | 15:00 hrs |

Speaker: | Dr. Jan Steinhoff (CENTRA, Instituto Superior Tecnico, Lisboa, Portugal) |

Title: | "The PN Approximation beyond Point-Masses" |

Abstract: | The inspiral of compact objects like black holes or neutron stars can be approximated using point masses very well. However, very interesting astronomical information is contained in effects to gravitational waves arising from the object's higher multipoles (or their finite size). Some of these effects can be modeled by an extension of the point mass action. Based on such an action, contributions of dipole (i.e., spin) and quadrupole to the post-Newtonian (PN) approximation can be obtained. The quadrupole effects are the first which encode information of the internal structure of the compact objects, e.g., they allow an distinction between black holes and neutron stars and also different equations of state. |

# July 2012

31 July 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | ZARM, University of Bremen, Room 1280 |

When: | 14:30 hrs |

Speaker: | Ricardo Gallego Torrome (Universidade de Sao Paulo) |

Title: | "On Fermat's principle in Finsler spacetimes" |

Abstract: | In this talk Fermat's principle for causal curves on time orientable Finsler spacetimes is presented. We show how the second variation of the arrival time functional along a geodesic can be calculated in terms of the index form associated with the Lagrangian $L$. Then the character of the critical point of the arrival time functional is investigated. |

18 July 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | University of Oldenburg, Lecture room W2 1-143 |

When: | 14:00 hrs |

Speaker: | Prof. Eckehard W. Mielke (Universidad Autonoma Metropolitana-Iztapalapa Mexico) |

Title: | "Einstein gravity with cosmological constant from a topological action" |

Abstract: | A topological field theory of gravity in four-dimension is proposed which is finite after quantization. Since such 'minimal' 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. |

# June 2012

7 June 2012

Where: | University of Bremen, Room H3, Building NW1 |

When: | 16:00 hrs |

Speaker: | Prof. R. Meinel (University of Jena) |

Title: | "Gleichgewichtsfiguren rotierender Flüssigkeiten in der Einsteinschen Gravitationstheorie" |

24 May 2012

Where: | University of Oldenburg, Room: W2 1-142 |

When: | 14:15 hrs |

Speaker: | Dr. Georgios Loukes-Gerakopoulos (University of Jena) |

Title: | "Chaos in bumpy black hole spacetime backgrounds" |

Abstract | Bumpy black holes are stationary and axisymmetric perturbations of black holes spacetimes. The absence of a Carter-like constant in these perturbed spacetimes allows the appearance of chaos. Although, chaos in frequency analysis corresponds to noise, certain non-linear effects of these non-integrable systems can be observed. I will present some of these effects and their corresponding imprints on the frequency spectra. |

# May 2012

15 May 2012

**Bremen-Oldenburg Relativity Seminar**

-Field Theory, Gravity, Relativity-

Where: | ZARM, University of Bremen, Room 1280 |

When: | 15:00 hrs |

Speaker: | Christian Pfeifer (Hamburg University) |

Title: | "Finsler geometric extension of Einstein gravity" |

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. |

# April 2012

# March 2012

# February 2012

1 February 2012

Where: | ZARM, University of Bremen, Room 1280 |

When: | 14:30 hrs |

Speaker: | Dr. Frederic P. Schuller (Albert-Einstein-Institute, Golm) |

Title: | "Spacetimes beyond Einstein" |

Abstract: | 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. |