# Talks

# 2019

# December 2019

5. December 2019

**SEMINAR TALK**

Location: University of Bielefeld, seminar room D6-135

When: 05.12.2019, 14 h

Speaker: **Dr. Nan Li **(Shenyang)

Title: **Black hole thermodynamics inthe extended phase space**

The announcement and the abstract can be found here.

3. December 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 03.12.2019 13:00 - 14:00 h

Speaker: **Marco Falconi **(Roma Tre University)

Title: **tba**

Further information can be found here.

# November 2019

28. November 2019

**THEORY COLLOQUIUM**

Location: University of Oldenburg, Room W2 1-143

When: 28.11.2019, 14:15 h

Speaker: **Prof. Dr. Betti Hartmann** (São Paulo, Brazil / Bilbao, Spain)

Title:** The AdS/CFT correspondence -String Theory in the lab?**

**Abstract**: The Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence is one of the most important results of String Theory. It states that a gravity theory in Anti-de Sitter space-time (AdS) is dual to a Conformal Quantum Field Theory on the boundary of AdS. The duality connects strongly coupled Quantum Field Theories to weakly coupled gravity theories. In recent years it has been suggested that this fact can be used to gain easier access to strongly coupled Quantum Field Theories. Examples are real-time, high density processes in Quantum Chromodynamics as well as high temperature superconductivity which is presumably driven by strong interaction processes. In both cases experimental data exists which can be used to test the correspondence. In this talk I will motivate the AdS/CFT correspondence, explain its potential and point out future directions.

28. November 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 28.11.2019 13:00 - 14:00 h

Speaker: **Claus Lämmerzahl **(ZARM)

Title: **General Relativistic Geodesy**

**Abstract**: With new optical clocks and laser interferometers in space new instruments are at work to measure the gravitational field of the Earth with highest precision. The measurement procedures as well as the highest precision make it necessary to employ General Relativity. Accordingly, in the talk a fully general relativistic approach to geodesy is presented. Starting from a stationary situation the fully relativistic geoid is defined. Since relativistic gravity has more degrees of freedom than Newtonian gravity, a second geoid has been found which is related to the rotational degrees of freedom of gravity. The methods to practically measure these geoids are described and the deviation from Newtonian gravity is discussed. In the outlook remaining tasks are outlined.

Further information can be found here.

25. November 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 25.11.2019 13:00 - 14:00 h

Speaker: **Alan Huckleberry **(Jacobs University)

Title: **Aspects and Applications of the Atiyah-Singer Index Theorem II**

**Abstract**: The Index Theorem is a general result which has its roots in classical geometry. In the first of the two talks we will explain its statement and relevant ingredients in the special case of the Hirzebruch-Riemann-Roch Theorem which was extended from algebraic geometry to an arbitrary analytic setting by the Index Theorem.

In the second lecture we will introduce the essential tools for stating the Index Theorem, e.g., the necessary PDE background, and give a very rough idea of its proof.

Further information can be found here.

22. November 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 22.11.2019 13:30 - 14:00 h

Speaker: **Georgios Karagiannis **(Rudjer Boskovic Institute)

Title: **Electric/Magnetic Duality in Linearized Gravity**

Further information can be found here.

22. November 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 22.11.2019 13:00 - 13:30 h

Speaker: **Athanasios Chatzistavrakidis **(Rudjer Boskovic Institute)

Title: **On Lie Algebroids and Sigma Models**

Further information can be found here.

19. November 2019

**SEMINAR TALK**

Location: University of Bielefeld, seminar room D6-135

When: 19.11.2019, 14 h

Speaker: **Dr. Aritra Basu** (University of Bielefeld)

Title: **Pobing ALP dark matter throughpolarisation measurements towads a gravitational lensing galaxy**

The announcement and the abstract can be found here.

18. November 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 18.11.2019 13:00 - 14:00 h

Speaker: **Alan Huckleberry **(Jacobs University)

Title: **Aspects and Applications of the Atiyah-Singer Index Theorem I**

**Abstract**: The Index Theorem is a general result which has its roots in classical geometry. In the first of the two talks we will explain its statement and relevant ingredients in the special case of the Hirzebruch-Riemann-Roch Theorem which was extended from algebraic geometry to an arbitrary analytic setting by the Index Theorem.

In the second lecture we will introduce the essential tools for stating the Index Theorem, e.g., the necessary PDE background, and give a very rough idea of its proof.

Further information can be found here.

# October 2019

30. October 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 30.10.2019 13:00 - 14:00 h

Speaker: **Federico Carta** (DESY Hamburg)

Title: **Supersymmetry Enhancement**

**Abstract**: In the last couple of years it was discovered that some 4d *N=1* quantum field theories flow in the IR to 4d *N=2* superconformal field theories (often of generalized Argyres-Douglas type), therefore showing a phenomenon of Supersymmetry Enhancement at the IR fixed point. The *N=2* IR theory is often non-lagrangian while the *N=1* UV theory is lagrangian, therefore such flows are extremely useful to learn features of the IR non-lagrangian theory, by using the UV formulation to compute RG-flow protected quantities as for example the superconformal index. However, up to date it is not completely clear why such flows exist, and how the SUSY enhancement happens. Limiting ourself to the case of rank one theories, we show how it is possible to understand the enhancement phenomenon in a geometric way, by condering a D3 brane probing a local singularity in F-theory corresponding to a T-brane of seven-branes. It is also possible to understand the enhancement as an hyperkahler restoration on the moduli space of solutions of the (generalized) Hitchin system associated to such theories.

Further information can be found here.

17. October 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 17.10.2019 13:00 - 14:00 h

Speaker: **Eugenia Boffo **(Jacobs University)

Title: **Graded Poisson Algebra and its Applications to Gravity**

**Abstract**: Symplectic structures with an internal integer grading on a manifold *M* can accommodate a metric on TM. This opens up the possibility to construct gravitational theories of very general kind starting with a graded Poisson algebra. In this talk we will review some aspects of graded geometry, hint at relations with Courant algebroids and mention how it can be applied to yield a Hilbert-Einstein-like action functional.

Further information can be found here.

15. October 2019

**INTRODUCTION TO QUANTUM ENTANGLEMENT**

Location: ZARM, University of Bremen, Room 1280

When: 15.10.2019, 14:00 h

Speaker: **Roy Barzel** (ZARM. University of Bremen)

Title:** Introduction to Quantum Entanglement**

The talk is for beginners in the topic.

1. October 2019

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Location: ZARM, University of Bremen, Room 1280

When: 01.10.2019, 15:30 h

Speaker: **Dr. Dirk Puetzfeld** (ZARM. University of Bremen)

Title:** Simplifying nonlocal gravity**

**Abstract**: The structure of a recent nonlocal generalization of Einstein's theory of gravitation by Mashhoon et al. is briefly analyzed. By means of a covariant technique, an expanded version of the nonlocality tensor which constitutes the theory is derived. This leads to a simplification which sheds light on the fundamental structure of the theory and may prove useful in the search for exact solutions of nonlocal gravity.

1. October 2019

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Location: ZARM, University of Bremen, Room 1280

When: 01.10.2019, 14:30 h

Speaker: **Prof. Mahmood Roshan **(Department of Physics, Ferdowsi University Of Mashhad, Iran)

Title:** Evolution of spiral galaxies: distinction between modified gravity and particle dark matter**

**Abstract**: The long-term evolution of the spiral galaxies via high-resolution simulations combined with the relevant observations may help to distinguish between particle dark matter and modified gravity theories. It turns out that there are features associated with particle dark matter which cannot be reconstructed by modified gravity. We study the evolution of spiral galaxies in two well-studied modified gravity theories. These theories are scalar-tensor-vector gravity known as MOG (Moffat 2006), and a model of nonlocal gravity (NLG) (Hehl & Mashhoon 2009). We show that the bar growth and the bar pattern speed evolves differently in modified gravity and standard dark matter halo paradigm. More specifically, we show that the density wave dynamics on the surface of galaxies in these different viewpoints reveals significant differences. For example, disks in modified gravity host fast bars compatible with observations. While disks in the standard viewpoint host slow bars in contrast to observation.

# September 2019

30. September 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 30.09.2019 13:00 - 14:00 h

Speaker: **Alan Huckleberry **(Jacobs University)

Title: **An introduction to Spin Geometry and Dirac Operators**

**Abstract**: The notions of Spin and Dirac Operator are well-known in a physics framework. In this lecture we introduce certain mathematical tools which are involved in spin geometry and indicate some applications.

Further information can be found here.

20. September 2019

**SEMINAR TALK**

Location: University of Bielefeld, seminar room D6-135

When: 20.09.2019, 14:15 h

Speaker: **Dr. Prabharkar Tiwari** (NAOC, Beijing)

Title: **The Large-scale Structure of our Universe with Radio galaxies**

The announcement and the abstract can be found here.

18. September 2019

**MATHEMATICAL AND THEORETICAL PHYSICS SEMINAR**

Location: Jacobs University, Bremen, Seminar Room Research III

When: 18.09.2019 13:00 - 14:00 h

Speaker: **Lea Boßmann **(University of Tübingen)

Title: **Derivation of 1D/2D Gross-Pitaevskii equations for strongly confined 3D bosons**

**Abstract**: We study the dynamics of a system of *N* interacting bosons in a cigar- or disc-shaped trap, which initially exhibit Bose-Einstein condensation and interact via a non-negative interaction potential in the Gross-Pitaevskii scaling regime. The trap is realized by an external potential, which confines the bosons in two/one spatial dimensions to a region of order ε. We study the simultaneous limit (N,ε) → (∞,0) and show that the N-body dynamics preserve condensation. The time-evolved condensate wave function is the solution of a 1D/2D Gross-Pitaevskii equation. Joint work with Stefan Teufel.

Further information can be found here.

# August 2019

21. August 2019

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Location: University of Oldenburg, Lecture Hall W2 3-349

When: 21.08.2019, 11:00 h

Speaker: **Prof. Dr. Usha Kulshreshtha** (Department of Physics, University of Delhi)

Title: **A Class of O(N) Nonlinear Sigma Models**

**Abstract**:In this talk, I would consider a class of 2D nonlinear sigma modelsas well as a class of gauged nonlinear sigma models in differentregularizations.

# July 2019

25. July 2019

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Location: ZARM, University of Bremen, Room 1280

When: 25.07.2019, 14:00 h

Speaker: **Prof. Dr. Alfredo Macias** (Departamento de Física, UAM-I, Ciudad de Mexico)

Title: **Geodesic structure of the Euler-Heisenberg Static Black Hole**

**Abstract**:We derive the electrically charged static black hole spacetime of the Einstein–Euler–Heisenberg theory, in terms of the Plebanski dual variables, and study the shape of its shadow and all possible equatorial trajectories of test particles. This solution is a non–linear electromagnetic generalization of the Reissner–Nordstroem solution and it is characterized by three parameters: mass* M*, electric charge *Qe* and Euler–Heisenberg non–linearity parameter *A*. Moreover, we study the trajectories of photons by means of introducing the effective Plebanski metric related to the geometrical metric and to the electromagnetic energy–momentum tensor. The shape of the shadow of the black hole is also presented and discussed.

16. July 2019

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Location: ZARM, University of Bremen, Room 1730

When: 16.07.2019, 11:00 c.t.

Speaker: **Prof. Dr. Andrey Surzhikov** (PTB, Braunschweig)

Title: **Scattering of relativistic vortex electrons**

**Abstract**:Studies on the scattering of relativistic electrons by atoms and ions have a very long tradition both in theory and experiment. During the recent years, special emphasis in these studies has been placed on the so-called twisted (or vortex) electrons. The beams of twisted electrons, that carry a nonzero projection of the orbital angular momentum (OAM) upon their propagation direction, serve today as a valuable tool for probing the magnetic properties of materials at the nano- and even atomic scale. In this contribution, therefore, we re-visit two fundamental processes involving vortex beams: electron-atom and electron-electron scattering. We derive the differential cross sections for both processes and discuss how these cross sections differ from those obtained for the usual plane-wave electrons. Based on this analysis we will show how vortex electrons may shed more light on the relativistic and magnetic effects in Mott and Møller scattering, and can provide access to the Coulomb phase; a quantity which plays an important role in various collision processes but which cannot be observed in usual, plane-wave-experiments.

# June 2019

24. June 2019

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Location: University of Oldenburg, Lecture hall W2 3-349

When: 24.06.2019, 12:00 c.t.

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

Title: **Compactness of a static charged object**

**Abstract**: A very general and novel prescription for compactness of a static object/star would be proposed. The compactness limit is defined when gravitational field energy exterior to object is less than or equal to half of its non-gravitational energy for a charged object described by the unique Reissner - Nordström electrovac solution of the Einstein equation. This definition makes no reference to the interior at all, may what that be.

(arxiv:1903.03436)

# May 2019

09. May 2019

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

Location: University of Oldenburg, Lecture hall W04 1-171

When: 09.05.2019, 12:00 c.t.

Speaker: **Thien-Sa Tram **(University of Oldenburg)

Title: **Implementation of an Orbit Propagator in FiPS**

**Abstract**: For ArianeGroup’s Simulation tool called FiPS a Simulink based numerical orbit propagator is to be implemented which takes into account the main orbital perturbations. This orbit propagator was validated using precision ephemeris data of the Envisat satellite and the reproduction of well known effects of orbit perturbation. Using the statistics of roughly 1000 test runs, the error growth was evaluated using ephemeris data of Envisat. Here, a non-linear error growth is observed that reaches around 20m after a simulation time of one day for the Envisat orbit.

# January 2019

30. January 2019

**BREMEN-OLDENBURG SYMPOSIUM**

When: 30.01.2019

Where: Uni Oldenburg, Campus Wechloy

**Program:**

11:15, Room W2 1-146

Iuliia Kazarina (Irkutsk) - "Three messenger studies of the high energy Universe in Siberia" - Abstract

12:30, Room W2 1-146

Daria Chernykh (Irkutsk) - "The latest results of cosmic ray experiments" - Abstract

14:15, Room W32-008

Igor Ivanov (Lisbon) - "What many Higgses can do for you" - Abstract

15:30, Room W32-008

Jakov Shnir (Dubna, Minsk) - "Spinning black holes with Skyrme hair" - Abstract

# 2018

# December 2018

14. December 2018

**BREMEN-OLDENBURG RELATIVITY SEMINARDouble Feature**

Location: |
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When: | 14. December 2018 14:00 c.t. | |

Speaker: | Hovhannes Demirchyan (Byurakan Astrophysical Observatory, Armenia) | |

Title: | Astrophysical shock waves as impulsive signals (Abstract below) |

AND | |

When: | 14. December 2018 15:00 c.t. |

Speaker: | Omar Jesus Franca (Santiago National Autonomous University Mexico) |

Title: | Theories with θ-couplings and some concrete application (Abstract below) |

**First Abstract:** Impulsive signals exist in lightlike hypersurfaces where Riemann curvature tensor contains a Dirac delta function. Such hypersurfaces describe astrophysical shock waves arising from supernovae, collision of neutron stars and other cataclysmic events. These shock waves are a burst of matter traveling with the speed of light and gravitational radiation which together form the impulsive signal. We will discuss approaches for constructing singular hypersurfaces in general space-time. As a particularly interesting example we will discuss the construction of singular hypersurfaces in Minkowski space.

**Second Abstract: **Through the last years the coupling of topological terms to certain field theories has obtained a lot of interest in several branches of physics from both experimental and theoretical point of view. In this talk we focus on the Pontryagin term coupled by a scalar function theta to the Maxwell action, known as theta-Electrodynamics, which models the effective behavior of Topological Insulators (TIs). We also study how to add this theta-term to Einstein-Hilbert action, resulting in theta-Gravity. Here we analyze two applications. First, we apply theta-Electrodynamics to the radiation of a point-like particle with constant velocity, whose radiation gives forward and reverse Čerenkov radiation (RČR), which is very surprising because RČR has been observed only in metamaterials. And finally, in the context of theta-Gravity we will expose a black hole-like solution, which provides a possibility to study the gravitational analog of TIs.

# November 2018

30. November 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | 30. November 2018 10:00 c.t. | |

Speaker: | Prof. Anton Galajinsky (Tomsk Polytechnic University, Russia) | |

Title: | Eisenhart lift and its applications |

**Abstract:** The Eisenhart lift is a specific geometrization of the Newtonian mechanics in which a dynamical system with N degrees of freedom is embedded into the null geodesics of a Lorentzian metric in (N+2)–dimensional spacetime. In this talk, we review Eisenhart’s construction and discuss its applications to spacetimes with hidden symmetries, higher derivative theories, and cosmology.

# October 2018

18. October 2018 (**was 17. Oct.**)

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | 18. October 2018 10:15 | |

Speaker: | Sarah Kahlen (University of Oldenburg) | |

Title: | Constraints on neutrino cosmology from the cosmic microwave background |

**Abstract:** The cosmic microwave background (CMB) radiation and its angular power anisotropy spectrum are well explored in the theoretical and experimental sense, e.g. by measurements of the Planck satellite. But apart from the cosmic photon background, the cosmic neutrino background (CNB), which is much more difficult to explore because of the weakly interacting behavior of neutrinos, fills our Universe. In general, it is difficult to obtain information about neutrino physics from measurement. However, experiments among other things proved that neutrinos are massive, although they are massless according to the standard model of particle physics. Furthermore, the CNB impacts cosmology and e.g. influences the expansion rate of the Universe, the Big Bang Nucleosynthesis and finally also the photon background. In my talk I am going to present the basic ideas about cosmological perturbation theory, which is capable of explaining how the structures observed in our Universe nowadays have formed and evolved as well as it is able to predict the angular power spectrum of the microwave background with high accuracy. After the presentation of the formalism of linear perturbation theory, I am applying the theory to neutrinos and derive their so-called Boltzmann hierarchy, which exist for all the different particle species of the Universe. In combination with Einstein's field equations these hierarchies yield the temporal evolution of perturbations in the corresponding particles. These temporal evolutions for all the components of the Universe will briefly be dealt with in the talk. The Boltzmann hierarchies of massive and massless neutrinos differ. They thus influence the angular power spectrum of the CMB in different ways. Non-instantaneous decoupling of neutrinos causes extra energy in the neutrino sector as compared to the standard scenario. This extra energy is degenerate with neutrino temperature and their phase-space distribution function, wherefore the so-called number of relativistic neutrino species N_{eff} has been introduced to parametrize the extra neutrino energy. The degeneracy between neutrino temperature and N_{eff} has been analyzed in my work regarding their influence on the CMB angular power spectrum for massless and massive neutrinos. The influence of the neutrino phase-space distribution function was another important aspect of the analysis. Whereas the Fermi-Dirac distribution function is usually assumed to describe how neutrinos are distributed in phase-space, we used a Gaussian distribution function and showed that, with an appropriate normalization, its influence on the CMB angular power spectrum is rather small, also for massive neutrinos.

08. October 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | 08. October 2018 11:00 c.t. | |

Speaker: | Noa Wassermann (University of Oldenburg) | |

Title: | Analytic solution of the geodesic equation for the black spindle spacetime |

**Abstract:** Since Albert Einstein first predicted the existence of black holes in his theory of general relativity in 1915, many more black hole solutions have been found. However, the spacetime presented in this talk is special because of its extraordinary horizon geometry. It was found by Dietmar Klemm in 2014 as a special case of the Carter-Plebanski solution of Einstein-Maxwell-Λ theory [1]. The black spindle is a four-dimensional, non-rotating black hole whose horizons have two cusp ends, defining its characteristic spindle shape and giving the spacetime its name. Interestingly, the horizons of the black spindle have a finite area, although their poles extend towards infinity. These properties make this spacetime particularly interesting for further investigations, promising deeper insights into black hole physics. Since the existence of black holes is yet to be confirmed, having an effective tool to study them theoretically is important, especially for exotic spacetimes like the black spindle. The orbits of test particles provide information about a black hole, therefore geodesics are an excellent tool to examine black holes. In this talk I will present the derivation and analytic solution of the geodesic equations in terms of the elliptic Weierstraß ℘-, σ- and ζ-functions. To characterize the spacetime more accurately, we determine the location of the singularity and the event horizons. Furthermore, we use parametric diagrams and effective potentials to find all possible orbit types. This will reveal some very interesting looking orbits. With the analytic solution of the geodesic equations we are able to display the photon motion graphically. At the end of my talk I will present examples of some of these orbits plotted in both spherical and cylindrical coordinates.

[1] D. Klemm, “Four-dimensional black holes with unusual horizons,” Phys. Rev. D, vol. 89, no. 8, p. 084007, 2014.

# July 2018

30. July 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | 30 July 2018 14:00 c.t. | |

Speaker: | M.Sc. Omar Jesus Franca Santiago (Institute of Nuclear Sciences National Autonomous University of Mexico) | |

Title: | Green’s function for radiation in planar θ-Electrodynamics |

**Abstract:** We investigate how electromagnetic radiation is modified by the presence of a planar topological insulator (TI). To this end we employ the Green’s function method to find the electromagnetic fields resulting from the interaction between the TI and an external source. This method gives us the Green’s function (GF) in terms of the Fourier’s transverse and frequency transforms. Having obtained an analytical expression for the GF as function of the coordinates, we apply it to a charged particle moving with constant and perpendicular velocity to the TI and study its electromagnetic fields.

We further analyze plane wave behavior of the GF using approximation methods in the far zone regime. As an application of this approximation, we study the radiation of an infinitesimal electric dipole perpendicular to the TI. We compute analytical expressions for the electric field, the angular distribution of radiated power and the total radiated power, and obtain new effects such as a phase shift due to the TI.

12. July 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | 12 July 2018 14:00 c.t. | |

Speaker: | Dr. Andrew Coates (Department of Theoretical Astrophysics, Eberhard Karls Universität Tübingen) | |

Title: | The Weak Equivalence Principle Beyond Weak Gravity |

**Abstract:** Alternative theories of gravity are well motivated due to various shortcomings of GR from a theoretical standpoint, despite its empirical success. Using Lovelock’s theorem, which demonstrates the uniqueness of GR, one can classify alternative theories according to which of the assumptions in the proof of Lovelock’s theorem they break.

One of the assumptions we know to be false already. That is the assumption that there is only the gravitational field, and it is described by a metric. There is matter, and not just a metric, in the Universe. A simple appeal to quantum corrections, and the fact that the Equivalence Principles do not seem to have any basis in symmetry, leads one to expect that all the Equivalence Principles should be broken at some level.

Due to the strength of solar system constraints on such violations, if any appreciable violation occurs in nature then there must be some mechanism which suppresses them in the solar system. An example of such a mechanism for violations of the Strong Equivalence principle has been known since the early 90s. Perhaps surprisingly this same mechanism (and other, more recent, mechanisms) can be adapted to work for violations of the Weak Equivalence Principle.

Unfortunately, without the Weak Equivalence Principle, the space of possible theories grows enormously. There is, however, a redeeming feature. Violations of the Weak Equivalence Principle seem likely to generically have smoking gun phenomena. The motivation for this can be seen by comparing a toy model to a parametric oscillator.

A final interesting takeaway from studying Weak Equivalence Principle violations is that it demonstrates the ambiguity of the term “Alternative theory of gravity”. For example, in Scalar- Tensor theories without the Weak Equivalence Principle there is no such thing as a Jordan frame and there appears to be no general way to distinguish between the scalar field being of a “gravitational” nature or being a matter field.

12. July 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | 12 July 2018 12:15 | |

Speaker: | Prof. Seyed Hossein Hendi (Physics Department and Biruni Observatory College of Sciences, Shiraz University, Iran) | |

Title: | Exact solutions of LoveLock gravity with nonlinear electrodynamics |

**Abstract:** Lovelock gravity and an exponential form of nonlinear electrodynamics are presented. Asymptotically anti-de Sitter black hole solutions are obtained. Geometric and thermodynamic properties are presented in brief. Finally, some suggestions for the future works are given.

# June 2018

18. June 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | Monday, 18. March 2018 14:00 c.t. | |

Speaker: | Prof. Srijit Bhattacharjee (Department of Applied Sciences Indian Institute of Information Technology, Allahabad | |

Title: | Inner horizon instability of black holes |

**Abstract:** It is generally believed the inner horizon of a charged or rotating black hole in asymptotically flat spacetime is a singular surface. The singularity arises due to the backreaction of blue-shifted perturbation near the inner horizon. Poisson-Israel first studied this back-reaction for a charged black hole and found there is an unbounded increase of the black hole's internal mass parameter - known as 'mass inflation' instability. Ori constructed a simple model of this phenomenon and determined an exact solution in the mass inflation region.

In this talk we discuss the Ori model for charged AdS black holes. We also discuss the nature and strength of the singularity.

6. June 2018

**TALKFellow lecture**

Location: | Hanse-Wissenschaftskolleg, Lehmkuhlenbusch 4, 27753 Delmenhorst |

When: | Wednesday, 6. June 2018, 18:00 s.t. |

Speaker: | Dr. Christine Gruber (University of Oldenburg) |

Title: | Black holes, or there and never back again |

Guests are invited to join the talk and a following dinner at the HWK.

Please register with let the reception team (empfang(at)h-w-k.de) until Friday, June 1, 2018.

The full invitation can be downloaded here.

# March 2018

28. March 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | Wednesday, 28. March 2018 11:00 c.t. | |

Speaker: | Merik Juljan Niemeyer (CvO University of Oldenburg) | |

Title: | Black holes in 5D Einstein-Maxwell-Chern-Simons theory in the small-coupling regime |

**Abstract:** In this talk I will present the results of my bachelor thesis on 5D black holes in Einstein-Maxwell-Chern-Simons theory. In five dimensions the familiar Einstein-Hilbert action can be complemented by a Chern-Simons term, which leads to a modification of the Maxwell equations. I will focus on extremal, asymptotically flat black hole solutions with two angular momenta of equal magnitude. Two special cases of this have been discussed before: The pure Einstein-Maxwell case and the case of 5D supergravity. The transition between these two will be the main point of this talk. After a historical overview, I will first discuss near-horizon solutions which will be obtained using the entropy function formalism. The transition from the SUGRA case to the pure Einstein-Maxwell case will be shown in the branch structure. After that I will present the branch structure of the global solutions. In particular the correspondence between global and near-horizon solutions will be considered.

15. March 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
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When: | Thursday, 15. March 2018 11:00 c.t. | |

Speaker: | Kai Flathmann (CvO University of Oldenburg) | |

Title: | Gravitational waves from inspiralling compact binaries in Dilatonic-Einstein-Gauss-Bonnet theory |

**Abstract:** For the first direct detection of gravitational waves by the LIGO collaboration, it was mandatory to compare the data to highly accurate waveforms. Therefore several analytical and numerical techniques are needed to calculate the inspiral, merger and ringdown phase. One analytic approach to describe waveforms of inspiralling black hole binaries, is the Effective-One-Body formalism developed by Buonanno and Damour in 1999. The basic idea is to relate the relativistic dynamics of two bodies to a test particle with reduced mass propagating in an effective one-body metric. In lowest order in General Relativity this metric coincides with a Schwarzschild metric deformed by the symmetric mass ratio. The basic ingredient for this formalism is a high order Post-Newtonian Hamiltonian for the relativistic two-body problem expressed in terms of the radial action variable. In this talk we present this formalism using the example of Dilatonic-Einstein-Gauss-Bonnet theory. For the description of the conservative dynamics we derive the two-body Hamiltonian in the second Post-Newtonian approximation and find a relation to the effective one-body Hamiltonian. In addition we introduce a radiation reaction force to describe the non-conservative dynamics and use the canonical equations to describe the effective motion and the gravitational waveforms of the inspiral phase. Finally we match the inspiral phase to the ringdown at the merger time and present full gravitational waveforms, which in principle can be compared to observations.

# February 2018

22. February 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**University of Oldenburg

Location: |
| |

When: | Thursday, 22 February 2018 11:15 | |

Speaker: | Dr. Kamal Hajian (Institute for Research in Fundamental Sciences Tehran, Iran) | |

Title: | Cosmological constant is a conserved charge |

**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.

22. February 2018

**BREMEN-OLDENBURG RELATIVITY SEMINAR**ZARM, University of Bremen

Location: |
| |

When: | Thursday, 22 February 2018 10:00 | |

Speaker: | Dr. Jafar Khodagholizadeh (Farhangian University, Tehran, Iran) | |

Title: | The effect of cosmic neutrinos on gravitational waves |

**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.

21. February 2018

**TALKFellow at work**

Location: | Hanse-Wissenschaftskolleg, Lehmkuhlenbusch 4, 27753 Delmenhorst |

When: | Wednesday, 21. February 2018, 19:00 s.t. |

Speaker: | Dr. Christine Gruber (National Autonomous University of Mexico, Institute of Nuclear Sciences, Mexico-city) |

Title: | Visit to Bremen Drop Tower |

# January

17. January 2018

**TALKFellow lecture**

Location: | Hanse-Wissenschaftskolleg, Lehmkuhlenbusch 4, 27753 Delmenhorst |

When: | Wednesday, 17. January 2018, 18:00 s.t. |

Speaker: | Dr. Christine Gruber (University of Oldenburg) |

Title: | Black hole thermodynamics: Cosmic refrigerators? |

Guests are invited to join the talk and a following dinner at the HWK.

Please register with let the reception team (empfang@h-w-k.de) until Friday, January 12, 2018.

11. January 2018

**RELATIVITY SEMINAR**

Bremen-Oldenburg

Location: |
| |

When: | Thursday, 11. January 2018 14:30 | |

Speaker: | Andreas Krut (La Sapienza University, Rome, Italy) | |

Title: | Novel constraints on fermionic dark matter from galactic observables |

**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.

10. January 2018**PHD DEFENSE**

Location: | ZARM, room 1730, University of Bremen |

When: | Wednesday, 10 January, at 3 pm |

Speaker: | Gerold Schellstede (ZARM) |

Topic: | Nonlinear electrodynamics |

Everybody is welcome. Please note that the colloquium will be in German.

9. January 2018

**SEMINARTALK**

Location: | Bielefeld University, seminar room E6-135 |

When: | Tuesday, 9. January 2018, 14:15 s.t. |

Speaker: | Carolin Hoefer (Univ. of British Columbia, Vancouver, CA) |

Title: | The Canadian Hydrogen Intensity Mapping Experiment |

**Abstract:** 21 cm Intensity Mapping is a new technique to map the large-scale structure in the Universe in three dimensions using the red-shifted emission from cosmic neutral hydrogen. Without needing to resolve individual galaxies, it can survey unprecedented volumes, which makes it an ideal method to measure the Baryon Acoustic Oscillations as a function of redshift and thus place constraints on the cosmic expansion history *H(z)* and the dark energy equation of state *w(z)*. However, fore-ground removal is quite challenging, since the cosmic signal is several orders of magnitude below the bright galactic signal. The Canadian Hydrogen Intensity Experiment (CHIME) is a new digital radio telescope with no moving parts. Situated in Canada, it will observe the northern sky every day over the wide radio frequency band of 400 - 800 MHz, which maps to a redshift range of 0:8 < *z* < 2:5 for 21 cm emission. This cosmic epoch has been poorly surveyed to date, but it includes the era in when dark energy came to dominate the energy density of the Universe. In this talk I will introduce CHIME, explain its design, its science goals, and its associated data analysis challenges. Analyzing CHIME data is a complex task, so we employ end-to-end simulations of the experiment, with known inputs, to develop and validate our data reduction and foreground filtering techniques.

# 2017

# December

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

Location: | Lecture room W2 3-349, University of Oldenburg |

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

Speaker: | Kevin Eickhoff (University of Oldenburg) |

Title: | Axial quasi-normal modes of Neutron Stars in shift-symmetric Horndeski Theory |

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. |

**Physical Colloquium**

Location: | ZARM, University of Bremen, Hörsaal H3, Geb. NW, Otto-Hahn-Allee |

When: | 14.12.2017, 16:00 p.m. |

Speaker: | PD Dr. Volker Perlick (ZARM) |

Title: | Jetzt beginnt eine neue Art von Astronomie: Gravitationswellen werden direkt beobachtetThe beginning of a new kind of astrononmie: gravitational waves can now be directly detected |

Abstract: | Die Existenz von Gravitationswellen wurde 1916 von Albert Einstein auf der Grundlage seiner Allgemeinen Relativitätstheorie vorausgesagt. Während es schon seit längerer Zeit sehr gute indirekte Evidenz für die Existenz dieser Wellen gibt, gelang ihre direkte Beobachtung erst hundert Jahre nach Einsteins Voraussage mithilfe der LIGO-Detektoren in den USA. Hierfür erhielten Rainer Weiss, Kip Thorne und Barry Barish den Physiknobelpreis 2017. In diesem Vortrag werde ich nach einer kurzen Einführung in die theoretischen Grundlagen die Funktionsweise der interferometrischen Gravitationswellendetektoren erläutern und dann die mittlerweile fünf beobachteten Gravitationswellenereignisse diskutieren. Dabei handelte es sich mit großer Wahrscheinlichkeit viermal um das Verschmelzen von zwei Schwarzen Löchern und einmal um das Verschmelzen von zwei Neutronensternen. Mit diesen Ereignissen hat die Ära der Gravitationswellenastronomie begonnen. |

Höhepunkt und zugleich Abschluss des Kolloquiums im Kalenderjahr ist ja seit einigen Jahren unser Vortrag zum aktuellen Nobelpreis. Wie Ihnen bekannt ist, wurde in diesem Jahr der Nobelpreis für den Nachweis von Gravitationswellen vergeben. Hierfür haben wir einen perfekten Experten im Haus, nämlich Herrn PD Dr. Volker Perlick aus dem ZARM.

Traditionell wird es zum Nobelkolloquium auch weihnachtliches Gebäck und Glühwein geben.

**Physical Colloquium**

Location: | Lecture room W2-1-148, University of Oldenburg |

When: | Monday, 11.12.2017, 4.15 p.m. |

Speaker: | Dr. Eva Hackmann (ZARM, University of Bremen) |

Title: | The influence of charge on the accretion process onto black holes |

Abstract: | Accretion onto black holes is a fundamental astrophysical process as it gives rise to a large range of observational phenomena. To understand the general physical processes analytical models of accretion play a very important role. Here we consider the influence of charge in the accretion process. On the one hand, we consider a charged perfect fluid around a rotating black hole embedded in a large scale uniform magnetic field and analyse the resulting interesting equilibrium configurations. On the other hand, due to the no?hair theorem, black holes are characterised by mass, rotation, and charge. Although a net electric charge of astrophysical black holes is usually vanishingly small, we show that it may nevertheless have a noticeable influence on the accretion process. |

# November

20. November 2017

**TALKShe talks - Karrierewege in Wissenschaft, Wirtschaft und PolitikShe talks - Career paths in science, business and politics**

When: 20.11.2017, 18:30 h

Where: Main-building Leibniz University Hannover

Women in prominent leading positions talk about their career path and how they overcame challenges. They will share their views on leadership, power and reconcilability of career and family.

The speakers are:

- Yasmin Fahimi - Staatssekretärin im Bundesarbeitsministerium
- Prof. Dr.-Ing. Katharina Klemt-Albert - Professorin am Institut für Baumanagement und digitales Bauen, Leibniz Universität Hannover
- Dr. Carla Seidel - Vice President - Business Management Acids & Specialties Europe, BASF SE
- Verena Weiss Vice President - Segment Industrial Applications Air Spring Systems, ContiTech

Admission is from 18:00 h and the talk stars at 18:30 h. They are followed by a get-together.

Registration deadline is 12. November 2017. Please contact info@chancenvielfalt.uni-hannover.de

02. November 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 14:30 - 15:30 |

Speaker: | Dr. Anupam Mazumdar (Van Swinderen Institute, University of Groningen) |

Title: | Towards singularity free theory of gravity and possible experimental tests |

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. |

# October

16. October 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 16:00 - 17:00 |

Speaker: | Dr Oleg Yu. Tsupko (Space Research Institute of Russian Academy of Sciences) |

Title: | Influence of plasma on the black hole shadow: analytical approach (Co-Authors: Volker Perlick and Gennady S. Bisnovatyi-Kogan) |

Abstract: | The influence of a plasma around a black hole on the black hole shadow is discussed. Whereas modelling is made by many groups, we perform the first attempt to investigate the shadow in matter based on analytical calculations. We work in the frame of geometrical optics and take into account effects of general relativity and plasma presence. First of all, we consider general spherically symmetric space-time, and spherically symmetric distribution of plasma. Our main result for this case is an analytical formula for the angular size of the shadow of the black hole surrounded by plasma. As a plasma is a dispersive medium, the radius of the shadow depends on the photon frequency. The effect of the plasma presence is significant only in the radio regime. We find that for an observer far away from the black hole, the non-homogeneous plasma has a decreasing effect on the size of the shadow. Then, we consider the shadow of a Kerr black hole surrounded by a plasma. We have demonstrated that the Hamilton-Jacobi equation is separable, i.e., that a generalized Carter constant exists, only for special distributions of the plasma electron density. The necessary and sufficient condition for separability is found. We have derived analytical formulas for the boundary curve of the shadow on the observer’s sky in terms of two angular celestial coordinates. Examples of the shadow for different density distributions are constructed.Additionally, in this talk we would like to present our recent results about the fully analytical calculation of spin via deformation of the shadow, for the Kerr black hole in vacuum. We succeed to find compact analytical expressions which allow one to easily extract the black hole spin from observations of its shadow, without need to construct or model the entire curve of the shadow. References: 1. V. Perlick, O.Yu. Tsupko and G.S. Bisnovatyi-Kogan, Physical Review D 92, 104031 (2015) 2. V. Perlick and O.Yu. Tsupko, Physical Review D 95, 104003 (2017) 3. O.Yu. Tsupko, Physical Review D 95, 104058 (2017) |

# June

13. June 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

*First talk:*

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

When: | 14:30 - 15:30 |

Speaker: | Dr. Norman Guerlebeck (ZARM, University of Bremen) |

Title: | The Meissner Effect for Isolated Horizons |

Abstract: | 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. |

*Second talk:*

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

When: | 15:30 - 16:30 |

Speaker: | Dr. Audrey Trova (ZARM, University of Bremen) |

Title: | Equilibrium configurations of rotating magnetized tori |

Abstract: | 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. |

09. June 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 14:15 |

Speaker: | Prof. Panayotis Kevrekidis (Dept. of Mathematics & Statistics, UMass, Amherst) |

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

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. |

# May

23. May 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 16:15 |

Speaker: | Roberto Tanzi |

Title: | Quantum signatures of non-metric geometries |

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. |

23. May 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 14:30 |

Speaker: | Claudio Paganini (Albert-Einstein Institute, Golm) |

Title: | The Fingerprints of Black Holes - Shadows and their Degeneracies |

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. |

19. May 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 14:00 |

Speaker: | Aleš Flandera (Charles University Prague) |

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

Abstract: | We study the Kerr-Newman black hole in the formalism of weakly isolated horizons using a near horizon solution of Einstein’s 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. |

5. May 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

Location: | University of Oldenburg, Campus Wechloy, Room W2 3-349 |

When: | 15:15 |

Speaker: | Dr. Ehsan Hatefi (TU Wien) |

Title: | On DBI, Wess-Zumino effective actions of Superstring theories and their all order alpha. Corrections in IIB, IIA |

Abstract: | In this seminar, We would like to talk about all kinds of effective actions in Type IIA, IIB super string theories,involving DBI,Chern-Simons and more importantly new Wess-Zumino actions. Indeed we try to provide a comprehensive explanation not only for BPS branes but also for D-brane-anti D-brane systems. Along those actions, we also introduce various new techniques for mathematicians/string theorists for which getting the exact and final form of the world sheet integrals up to five point mixed closed-open amplitudes to all orders will be presented. Eventually we make various remarks on how to derive without any ambiguity all order ?’ corrections to all type II effective actions. If time allows, we then mention several applications related to those effective actions as well. |

4. May 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

Location: | University of Oldenburg, Campus Wechloy, Room W2 3-349 |

When: | 10:00 c.t. |

Speaker: | Hendrik Neumann (University of Oldenburg) |

Title: | Geodesics in the Myers-Perry-AdS spacetime |

Abstract: | In this talk I consider the Myers Perry AdS black hole, which is characterized by its mass, two independent rotation parameters and a negative cosmological constant. First I discuss some properties of this solution, then I analyze the geodesic equations in this spacetime. I present the whole set of analytical solutions in terms of elliptic functions. With the help of parametric diagrams and effective potentials I discuss the possible orbits and show some example plots. |

# March

9. March 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

Location: | University of Oldenburg, Campus Wechloy, Room W2 1-143 |

When: | 11:15 |

Speaker: | Dr. Kamal Hajian (Institute for Research in Fundamental Sciences Tehran, Iran) |

Title: | A review on covariant phase space formulation |

Abstract: | In this talk, after reviewing the phase space and its canonical construction, I construct step by step the covariant phase space formulation of gravitational theories. Covariant phase space formalism (also known as Wald formalism) yields a convenient method for calculating black hole conserved charges. |

3. March 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

*First talk:*

Location: | University of Bremen, TAB, Room 2.63 E (The building is next to the Drop Tower, on the other side of the street Am Fallturm. Enter from the side facing the Drop Tower (Entrance E), the room is on the second floor.) |

When: | 11:00 |

Speaker: | Alessandro Spallicci (University of Orleans, France) |

Title: | Self-force and motion |

Topic: | An overview of the concept of self-force in General Relativity is given, with some emphasis on the motion of compact stars captured by supermassive black holes. Later, we examine the concept of self-force in Newtonian gravity, the relation with the Equivalence Principle and what the outcome would be when dropping masses in an ideal ZARM-Pisa tower. |

*Second talk:*

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

When: | 14:00 |

Speaker: | Luca Bonetti (University of Orleans, France) |

Title: | Non-Mawellian electromagnetism in astrophysics and in space applications at low radio frequencies |

Topic: | The universe is largely observed with electromagnetic signals – supposedly Maxwellian – though the gravitational wave window was just unbarred. Meanwhile, cosmology is since long confronted with an universe for 96% dark and unknown. Thus, it is pertinent for fundamental physics to start a theoretical and experimental verification of electromagnetism, and its photon messenger. This seminar discusses non-Maxwellian theories, focusing on testable effects. We deal with a range of applications and assess how alternative electromagnetic theories are helpful. We revise the most notable theories, either non-linear or massive. In the former class, for analysis of the self-force, we consider the radiation of an uniformly accelerating Born-Infeld charge; in the Heisenberg-Euler theory, we find a frequency shift induced by an overcritical magnetic dipole, associated to a magnetar. Being the photon the only massless particle in the Standard Model (SM), we explore SM extensions. Indeed for the massive class, in a supersymmetric context, we find non-Maxwellian behaviors arising in even and odd cases of Charge- Parity-Time Reversal (CPT) symmetry: either subluminal propagation or massive dispersive behavior proportional to the Lorentz symmetry breaking vector. Further on, we determine an upper mass limit of 3.2 x 10^{-50} kg by analyzing Fast Radio Bursts. Massive (à la de Broglie-Proca) photon arrival time delays go with the inverse of frequency squared, as photons passing through plasma. Thereby, our interest for low frequency radio-astronomy on ground or in space. An unexplored domain hiding pulsars lies below 15 MHz, and it will be accessible by a swarm of nano-satellites. |

2. March 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

Location: | University of Oldenburg, Campus Wechloy, Room W2 3-349 |

When: | 16:15 |

Speaker: | Dr. Kamal Hajian (Institute for Research in Fundamental Sciences Tehran, Iran) |

Title: | From black hole information paradox to firewall paradox |

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. |

# February

28. February 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 14:30 |

Speaker: | Vittorio De Falco (ISSI, Bern, Switzerland) |

Title: | Approximate analytical calculations of photon geodesics in the Schwarzschild metric |

Topic: | 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. |

16 - 18 February 2017

**SEMINAR**

**7th Central European Relativity Seminar 2017**

Where: | Bremen "Haus der Wissenschaften" - Sandstraße 4/5, 28195 Bremen, Olbers Saal, first floor |

What: | This is the seventh seminar of a series initiated at the ESI (Vienna) January 27 - 29, 2011. This series of seminars is designed to provide a forum for younger researchers to present their work, and to expand their research horizons. The main geographical basin of attraction is Austria, the Czech Republic, Hungary, Poland and Germany. |

Topics: | The scientific program will include keynote lectures by D. Giulini and C. Lämmerzahl, as well as seminars by young researchers selected on the basis of submitted abstracts. The speakers will be chosen mainly amongst graduate students and post-docs. Anticipated are about 24 talks of 20 minutes each. A few longer talks might replace some of the shorter ones if scientifically or pedagogically desirable. The seminar will be accompanied by a public lecture (in english) by Jochen Liske, Hamburger Sternwarte, Universität Hamburg, on 18.02.2017, 15.30, entitled: "The World's Largest Telescope and the Acceleration of the Universe" |

Abstracts: | Abstracts submitted by January 15, 2017, will receive full consideration. Later submissions might be considered if the schedule allows. Abstract submission |

# January

25. January 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 16:00 |

Speaker: | Dr. Souvik Banerjee (Uppsala University) |

Title: | 2. Lecture on Quantum Mechanics and General Relativity |

Topic: | The second of two 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. |

24. January 2017

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 14:30 |

Speaker: | Dr. Souvik Banerjee (Uppsala University) |

Title: | 1. Lecture on Quantum Mechanics and General Relativity |

Topic: | The first of two 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. |

# 2016

# December

6. December 2016

BREMEN - OLDENBURG RELATIVITY SEMINAR

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

When: | 14:00 |

Speaker: | Marko Toroš, Department of Physics University of Trieste |

Title: | Quantum Mechanics beyond Galileo" Time and Place |

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. |

2. December 2016

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 14:00 |

Speaker: | Anna Hedegaard Lukawska |

Title: | Correlation of cosmological observables in the effective field theory of inflation |

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. |

1. December 2016

BREMEN - OLDENBURG RELATIVITY SEMINAR

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

When: | 16:15 |

Speaker: | Albert Samoilenko, Department of Theoretical Physics and Astrophysics, Belarusian State University, Minsk |

Title: | Gauged Baby Skyrme Model with and without Chern-Simons Term |

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. |

# November

**Change of room**

**Now:** TAB Building, Entrance E, Second floor, Room: 2.63

28 November 2016

Replacement Seminar

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

Where: | ZARM, University of Bremen, TAB Building, Entrance E, Second floor, Room: 2.63 |

When: | 14:30 |

Speaker: | Andrea Fuster, Eindhoven University of Technology |

Title: | Finsler gravitational waves |

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. |

21. November 2016

**PHYSICAL COLLOQUIUM**

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

When: | 21.11.16, 4.15 p.m. |

Speaker: | Dr. Erik Curiel (Munich Center for Mathematical Philosophy, Germany) |

Topic: | Can Black Holes be described by Thermodynamics? |

Abstract: | Serious problems have been raised about the idea that black holes "really are thermodynamical objects", for instance: that the Zeroth Law for black holes, constancy of surface gravity, is not equivalent to the deepest formulation of the Zeroth Law for ordinary thermodynamics, transitivity of equilibrium; that black?hole area is neither an extensive nor an additive quantity, as ordinary thermodynamical entropy is; that a lack of a general, localized expression for gravitational energy makes it difficult to construct a thermodynamical internal energy for black holes; and several more along the same lines. The fundamental concern is that the mere fact that the relevant black?hole quantities satisfy formal analogues of the Laws of ordinary thermodynamics does not by itself guarantee that enough of the structure of ordinary thermodynamics is thereby recovered. There is much more to thermodynamics, after all, than just the four Laws. Transitivity of equilibrium, for instance, normally grounds the construction of the state space of an ordinary thermodynamical system and the arguments that isolated systems spontaneously approach equilibrium. I argue in this talk that all these problems can be answered, and that black?hole can meaningfully be treated as thermodynamical objects. |

Please download the invitation here

16. November 2016

**Physics Colloquium**

Where: | Jacobs University, Lecture Hall in Research III Campusring 3 |

When: | 17:15 – 18.15h |

Speaker: | Prof. Dr. Johann Kroha (Professor of Theoretical Physics, Physikalisches Institut, Universität Bonn) Additional Information |

Topic: | Particle Creation in Coupled Bose-Einstein Condensates and in the Early Universe |

Abstract: | Common knowledge tells that many-body systems come to thermodynamic equilibrium by coupling to a heat reservoir. In classical physics, even ideally isolated, macroscopic systems thermalize due to the equivalence of the microcanonical and the canonical ensembles. However, in quantum dynamics it is a fundamental problem how an isolated quantum many-body system can eventually come to rest from an initial nonequilibrium state, and whether the final state is a thermal one. The problem arises because the time evolution of a quantum system is unitary, that is, a single (pure) quantum state will remain pure for all times and can never reach a thermal state. Contrary to this theorem, one typically observes quantum many-body systems behave in a thermal way. This paradox has recently become more and more pressing, as isolated quantum systems can be realized in ultracold atomic gases with unprecedented control. This problem is also central for the evolution of the universe as an ideally isolated system. It is closely related to the creation of elementary particles. In this talk we will review some attempts at resolving the thermalization paradox. As a prototype of an isolated quantum system we will then consider an atomic Bose-Einstein condensate performing Josephson oscillations in a double-well potential trap. We will show that this system thermalizes by a complex dynamics, covered by three different time regimes: an initial period of undamped Josephson oscillations, an intermediate period of avalanche-like creation of bosonic quasiparticles out of the condensate and a slow thermalization regime induced by quasiparticle collisions [1]. We will draw a detailed analogy to the dynamics of elementary particle creation and thermalization during and after the inflationary phase of the early universe. |

For further information please contact:

Prof. Dr. Stefan Kettemann, Professor of Physics, Physics & Earth Sciences

Email: s.kettemann(at)jacobs-university.de

Tel: +49 421 200-3521

Homepage: http://skettemann.user.jacobs-university.de

All are welcome!

**Change of room**

**Now:** 28. November, 14:30, TAB Building, Entrance E, Second floor, Room: 2.63

08 November 2016

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

Where: | ZARM, University of Bremen, TAB Building, Entrance E, Second floor, Room: 2.63 |

When: | 14:30 |

Speaker: | Andrea Fuster, Eindhoven University of Technology |

Title: | Finsler gravitational waves |

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. |

# October 2016

31. October 2016

**Physical Colloquium**

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

When: | 31.10.16, 4.15 p.m. |

Speaker: | Dr. Felipe Guzman (ZARM, Zentrum für angewandte Raumfahrttechnologie und Mikrogravitation, Universität Bremen, Germany) |

Topic: | Laser measurement science and an overview of its groundbreaking applications |

Abstract: | Coherent light enables length measurements of exquisite sensitivity that lie at the core of fascinating observations in fundamental and quantum physics, astrophysics, geodesy and measurement science. In particular, observations from the Laser Interferometer Gravitational-Wave Observatory (LIGO) over the past year not only confirmed crucial gravitational physics effects, but have now also officially launched the era of Gravitational Wave Astronomy and Multi-Messenger observations. Similar laser-interferometric measurements have been demonstrated and are now flying on LISA Pathfinder, exceeding expectations and paving the way for a spaceborne Gravitational Wave Observatory that will allow us to survey the gravitational universe otherwise inaccessible to us from ground. Moreover, GRACE follow-on will continue to provide valuable information about fluctuations of the Earth’s gravitational field to the geophysical and climatology science community starting early 2018, whose observations will be greatly enhanced by interspacecraft laser gradiometric measurements. In the area of cavity optomechanics and novel compact and integrated photonics, the combination of low-loss devices and optomechanically coupled coherent light field is enabling us to reach unprecedented measurement accuracies at the quantum sensing limit, which is revolutionizing the state-of-the-art in several areas of measurement science. I will discuss the advances in coherent light measurements and some of its recent results and upcoming groundbreaking applications to the science community. |

26 October 2016

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

When: | 15:00 hrs |

Speaker: | Peter Hogan, University of Oldenburg |

Title: | Colliding Impulsive Gravitational Waves and a Cosmological Constant |

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. |

20 October 2016

**BREMEN - OLDENBURG RELATIVITY SEMINAR **

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

When: | 12:15 hrs |

Speaker: | Sarah Kahlen, University of Oldenburg |

Title: | Five-dimensional wormholes at the core of rotating boson stars |

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 numerically 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. |

18. October 2016

**BREMEN - OLDENBURG RELATIVITY SEMINAR **

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

When: | 14:15 hrs |

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

Title: | Neutrinos in the Quantized Charge Standard Model |

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 the conventional Standard Model (SM) based on the group structure SU(3) _{C}XSU(2)_{L}XU(1)_{Y}, the electric charge is defined as Q=T_{3} + Y/2 ( or T_{3}+Y' ) where T_{3} is the diagonal generator of SU(2)_{L} and Y ( or Y' ) 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. |

# September 2016

12 September 2016

**BREMEN - OLDENBURG RELATIVITY SEMINAR **

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

When: | 16:15 hrs |

Speaker: | Rico Berner, TU Berlin |

Title: | Kinematic constraints in parametrized nonlinear electrodynamics |

Abstract: | The aim of the talk is to give a concise introduction to the parametrization approach by Kucha 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. |

# August 2016

16 August 2016

**BREMEN-OLDENBURG-RELATIVITY SEMINAR **

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

When: | 14:30 hrs |

Speaker: | Dr. Marco Maceda, UAM-I, Mexico City |

Title: | Noncommutative Landau problem in Podolsky's generalized electrodynamics |

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. |

# July 2016

29 July 2016

**BREMEN-OLDENBURG RELATIVITY SEMINAR **

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

When: | 11:00 hrs |

Speaker: | Prof. Dr. Alfredo Macias, UAM-I, Mexico |

Title: | Do Black Holes exist? |

26 July 2016

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

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

When: | 2:30 hrs |

Speaker: | Jiri Ryzner, Charles University Prague |

Title: | Majumdar-Papapetrou: special cases |

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-Nordström black holes in equilibrium and the field of an infinitely long, straight, extremally charged string. |

21 July 2016

**BREMEN-OLDENBURG-RELATIVITY SEMINAR**

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

When: | 04:00 hrs |

Speaker: | Dr. Elias Castellanos |

Title: | Ouasi-Bound scalar field configurations in curved geometries |

Abstract: | One of the most interesting ideas suggested in recent years is that scalar fields can be relevant candidates to describe dark matter in the Universe. Moreover, the possible existence of super-massive black holes located at the center of galaxies, imposes that these scalar field configurations must be stable, at least for enough time, in the presence of these super massive objects, if these scalar clouds want to be interpreted as a dark matter halo. Furthermore, if the dark matter particle is assumed as a 0-spin boson, this opens the door to the existence of scalar field dark matter as a some kind of Bose-Einstein condensate. In this talk we will discuss some issues related with scalar fields configurations in two specific curved geometries. Also, we wil explore the properties of the scalar cloud when the system can be viewed as some type of trapped Bose-Einstein condensate. |

# June 2016

24 June 2016

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

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

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

When: | 11:15 hrs |

Speaker: | Anastasia Golubtsova, Laboratory of Theoretical Physics JINR, Dubna |

Title: | "Holographic Wilson loops in Lifshitz-like backgrounds" |

Abstract: | 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. |

16 June 2016

**Physikalisches Kolloquium **

Wo: | Universität Bremen, Hörsaal H3, Gebäude NW 1, Otto-Hahn-Allee |

Wann: | 16 hrs |

Sprecher/in: | Prof. Dr. Manfred Stöckler |

Titel: | "EPR, Schrödingers Katze, Bellsche Ungleichung: das Rätsel der Nicht-Lokalität der Quantenmechanik" |

Abstract: | Eigentlich wollten Einstein, Podolsky und Rosen in ihrem Gedankenexperiment aus dem Jahre 1935 die Unvollständigkeit der Quantentheorie zeigen. Heute ist diese Arbeit aber interessant, weil darin die Nicht-Lokalität der Quantentheorie eine zentrale Rolle spielt. Im EPR-Paradoxon wird ein Zustandstyp verwendet, den wir heute verschränkt nennen. Er wurde dann von Schrödinger analysiert und zur Formulierung des Gedankenexperiments mit der Katze benutzt. Seit den sechziger Jahren konzentriert sich die Diskussion um die Nicht-Lokalität im Anschluss an einen Beweis von J.S. Bell auf die Besonderheiten der Korrelationen von Messungen an verschränkten Systemen mit weit voneinander entfernten Messgeräten. Diese belegen, dass (entgegen einer zentralen Prämisse von Einstein) die Quantentheorie nicht lokal ist, und darüber hinaus, dass jede beliebige Theorie, die die gleichen Korrelationen wie die Quantenmechanik vorhersagt, nicht lokal sein kann. Mittlerweile sind die von der Quantentheorie vorhergesagten Korrelationen empirisch hoch bestätigt. Damit ist das von EPR aufgeworfene Problem aber nicht gelöst, weil es keine dynamische Erklärung für den Messprozess an verschränkten Systemen zu geben und die Nicht-Lokalität der Quantentheorie der Relativitätstheorie zu widersprechen scheint. Die genaue Bedeutung der Nicht-Lokalität, insbesondere die Frage, ob sich daraus ein Konflikt mit der Relativitätstheorie ergibt, haben in der Philosophie der Physik eine bis heute offene Kontroverse ausgelöst. Der Sprecher berichtet über einschlägige Einsichten, Missverständnisse und mehr oder weniger verzweifelte Lösungsvorschläge. |

14 June 2016

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

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

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

When: | 14:30 hrs |

Speaker: | Dr. Dennis Raetzel, University Potsdam |

Title: | "How light gravitates: a brief exploration" |

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. |

02. June 2016

**Theory colloquium **

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

When: | 14:15 hrs |

Speaker: | Prof. Dr. Wolfgang Duschl (Kiel) |

Title: | The Evolution of Active Galactic Nuclei - Individually and as an Ensemble |

Abstract: | Quasars, and Active Galactic Nuclei (AGN, for short) in general, are among the most efficient energy sources in the Universe. They balance their energy budget by liberating potential energy through accretion of matter into super-massive black holes. Due to their high luminosity, they are ideal test bodies which can be observed almost throughout the entire Universe's history. As ever so often in astrophysics, we are confronted with the problem of exceedingly long evolutionary timescales of individual objects which are not accessible to direct observation. In the talk, the speaker will adress several problems related to the evolution of individual objects as well as that of entire ensembles of AGN, an example being, for instance, the processes limiting the mass growth of black holes. |

# Mai 2016

19 May 2016

**Theory Colloquium**

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

When: | 14:15 hrs |

Speaker: | Dr. Christian Pfeifer, University of Bremen, University of Hannover |

Title: | "The Potential in General Linear Electrodynamics: Causal Structure, Propagators and Quantization" |

Abstract: | From an axiomatic point of view, the fundamental input for a theory of electrodynamics are Maxwell's equations dF=0 (or F=dA) and dH=J, and a constitutive law H=# F, which relates the field strength 2-form F and the excitation 2-form H. In this talk we consider general linear Electrodynamics, the theory of Electrodynamics which is defined through a liner constitutive law. The best known application of this theory is the effective description of Electro-dynamics inside (linear) media including for example birefringence. We will analyze the classical theory of the electromagnetic potential A thoroughly before we use methods familiar from mathematical quantum field theory in curved spacetimes to quantize it. Our analysis of the classical theory contains the inversion of the principal symbol of the field equations, the construction of the causal structure on the basis of the constitutive law, which replaces the metric one uses in Maxwell Electrodynamics, the construction of the inverse of the field equations and the Puli-Jordan propagator and the classification of the classical symplectic phase space of the theory. This classical analysis then sets the stage for the construction of the quantum algebra of observables and quantum states. |

# April 2016

22 April 2016

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

Where: | University of Oldenburg, W02-3-349 |

When: | 11:15 hrs |

Speaker: | Michael Kalisch, Friedrich-Schiller-Universität, Jena |

Title: | "Pseudo-spectral construction of highly deformed non-uniform black string solutions " |

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 horizion is close to pinching off. The use of an appropiately 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. |

19 April 2016

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

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

When: | 2:30 hrs |

Speaker: | Dr. Maxim Eingorn, North Carolina Central University, Durham, NC, USA |

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

Abstract: | Without exceeding the limits of the concordance cosmological model, all-scale scalar and vector perturbations of the homogeneous background are derived analytically for arbitrarily distributed inhomogeneities (discrete gravitating masses) as their nonrelativistic sources. The obtained expressions for the metric corrections are gauge-invariant, converge everywhere in voids, have zero average values, and conform to Minkowski background limit and Newtonian cosmological approximation as particular cases. Moreover, the uniform matter distribution limit as one more crucial test is easily passed as well. It is rigorously proven that gravitational attraction between inhomogeneities is governed by Yukawa law, covering the whole space and coming up to take place of Newtonian gravitation, which is restricted exclusively to sub-horizon distances. The finite time-dependent screening length (amounting to 3.7 Gpc at present) is determined by the average rest mass density of nonrelativistic matter and provides natural estimates of the homogeneity scale, the upper limit of the cosmic structure dimension, and the bound to a spatial domain of probable structure development. The definition of Yukawa interaction range is generalized to the extended cosmological model with extra continuous perfect fluids and nonzero spatial curvature. |

14 April 2016

**Physics Colloqium **

Where: | University of Bremen, Hörsaal H3, Geb. NW1, Otto-Hahn-Allee |

Speaker: | Prof. Dr. Wiesendanger, Universität Hamburg |

Title: | "Magnetische Knoten auf der Nanometerskala: von der Grundlagenforschung zu neuen Datenspeicherkonzepten" |

Abstract: | Nanoskalige Knoten in der Magnetisierung, sogenannte Skyrmionen, sind zum Gegenstand intensiver weltweiter Forschungsanstrengungen geworden, da man mit ihnen neuartige Konzepte der hochdichten magnetischen Datenspeicherung realisieren kann. Neben ihrer geringen Größe zeichnen sich Skyrmionen durch nicht-triviale topologische Eigenschaften verbunden mit einem ausgezeichneten Drehsinn der Magnetisierung aus, verursacht durch eine zugrundeliegende chirale spinabhängige Wechselwirkung, welche diese nanoskaligen magnetischen Objekte äußerst stabil werden lässt. Die Entdeckung von Skyrmionengittern und insbesondere einzelner nanoskaliger Skyrmionen in ultradünnen metallischen Schichten liefert die Basis für Anwendungen in Form von neuartigen magnetischen Datenspeichern, Logikelementen und Sensoren unter Nutzung der Technologie, welche in den vergangenen Jahrzehnten auf dem Gebiet der Riesen- und Tunnelmagnetowiderstands-Sensoren etabliert wurde. Dieser Vortrag widmet sich sowohl den grundlegenden physikalischen Aspekten dieser neuartigen nanoskaligen Magnetisierungsstrukturen in ultradünnen Schichten und Multilagensystemen als auch deren Anwendungsperspektiven. |

7 April 2016

**Theory Colloquium**

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

When: | 14:15 hrs |

Speaker: | Dr. Roman Konopolya |

Title: | "Detection of gravitational waves from black holes: Is there a window for alternative theories?" |

Abstract: | Recently the LIGO and VIGRO collaborations reported the observation of gravitational-wave signal corresponding to the inspiral and merger of two black holes, resulting into formation of the final black hole. It was shown that the observations are consistent with the Einstein theory of gravity with high accuracy, limited mainly by the statistical error. Angular momentum and mass of the final black hole were determined with rather large allowance of tens of percents. Here we shall show that this indeterminacy in the range of the black-home parameters allows for some non-negligible deformations of the Kerr spacetime leading to the same frequencies of the black-hole ringing. This means that at the current precision of the experiment there remains some possibility for alternative theories of gravity. |

# March 2016

11 March 2016

**BREMEN - OLDENBURG RELATIVITY SEMINAR**

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

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

When: | 12:15 hrs |

Speaker: | Christian Knoll, University of Oldenburg |

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

Abstract: | This talk is the defense of the Speakers' 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. The Speaker will begin to introduce the formalism and fix the notation for the Dirac equation in curved spacetimes. After that the Speaker 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. |

# February 2016

12 February 2016

**Colloquium talk **

Where: | Max Planck Institute for Astrophysics, Garching |

When: | 16 Uhr c.t. |

Speaker: | Eiichiro Komatsu |

# January 2016

14 January 2016

**Physikalisches Kolloquium **

Wo: | Universität Bremen, Hörsaal H3, Geb. NW1, Otto-Hahn-Allee |

Wann: | 16 Uhr c.t. |

Sprecher/in: | Dr. Jan Ingo Flege |

Titel: | "Four (or Five) Quarks for Muster Mark?" - Zur Existenz von Tetra- und Pentaquarks |

Abstract: | Etwa 50 Jahre nach der Aufstellung wesentlicher Grundlagen des Standardmodells zum Aufbau massiver Teilchen aus Elementarbausteinen, sogenannten Quarks, herrscht in der Teilchenphysik eine neue Aufbruchsstimmung: Neueste experimentelle Ergebnisse vom Large Hadron Collider (LHC) in der Schweiz zeigen ungeahnte Einblicke in den subatomaren Mikrokosmos und erhärten die Hypothese, dass es neben den etablierten hadronischen Teilchen wie Mesonen und Baryonen, die aus je zwei bzw. drei Quarks bestehen, auch Teilchen aus einer größeren Anzahl dieser Elementarbausteine gibt. Obzwar nicht die ersten ihrer Art - erste Hinweise auf ein Pentaquark aus fünf Teilchen wurden bereits 2003 veröffentlicht - so sind die aktuellen Funde im Gegensatz zu früheren (teils voreiligen) Meldungen von dramatisch gesteigerter statistischer Signifikanz. Im Vortrag wird zunächst der akzeptierte Wissenstand zum strukturellen Aufbau der Hadronen und ihrer Wechselwirkungen kompakt dargestellt. Aufbauend auf den sich daraus ergebenden Möglichkeiten ihrer experimentellen Untersuchung werden anschließend die jüngsten Befunde der LHCb-Experimente zum Nachweis von Tetra- [1] und Pentaquarks [2] erläutert und ein Überblick über den gegenwärtigen Stand der (sicherlich noch nicht abgeschlossenen) Diskussion um ihre Bedeutung für die Teilchenphysik gegeben. [1] R. Aaij et al. (LHCb Collaboration), Phys. Rev. Lett./ *112*, 222002 (2014). [2] R. Aaij et. al. (LHCb Collaboration), Phys. Rev. Lett. / *115*, 072001 (2015). |

7 January 2016

**Talk: Ralf Lehnert**

Where: | University of Oldenburg |

When: | 14:15 hrs |

Speaker: | Dr. Ralf Lehnert (University of Hannover) |

Title: | "Does Relativity Theory Hold Exactly?" |

Abstract: | Relativity Theory is a cornerstone of physics. Since its conception over a century ago, it has beem scrutinized experimentally with ever increasing precision, but no credible evidence for departures from its underlying symmetry - Lorentz invariance - has been found to date. However, recent theoretical research has established that minute violations of Lorentz symmetry can be accommodated in various approaches to physics beyond the Sandard Model. This seminar provides a brief overview of our group's efforts in this research field. A variety of mechanisms for Lorentz breakdown are reviewed, the effective-field-theory test framework for the emerging low-energy effects is introduced, and a number of present-day and near-future experimental measurements in this context are surveyed. |

# December 2015

18 December 2015

**BREMEN-OLDENBURG RELATIVITY SEMINAR**

**- Field Theory, Gravity Relativity -**

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

When: | 16:00 hrs |

Speaker: | Philipp Heinrich - University of Oldenburg |

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

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. |

17 December 2015

**Talk: Harry Braden**

Where: | University of Oldenburg, W01 0-012 (Wechloy) |

When: | 16:00 hrs |

Speaker: | Harry Braden (University of Edinburgh) |

Title: | "Spectral Curves are Transcendental" |

Abstract: | The modern approach to integrable systems typically proceeds via a curve, the parameters of the curve encoding the actions and its Jacobian (or possibly some related Prym) encoding the angles. Physically relevant families of curves are often described by fixed relations amongst differentials on the curve. We shall look at number theoretic properties of these curves. For many integrable systems the curves are transcendental. |

17 December 2015

**Physics K****olloquium**

Wo: | Universität Bremen, Hörsaal H3, Geb. NW 1, Otto-Hahn-Allee |

Wann: | 16:15 Uhr c.t. |

Sprecher/in: | Prof. Dr. Matthias Günther, MEVIS, Bremen |

Titel: | "Physik-Nobelpreis 2015: Die Entdeckung der Neutrinomasse" |

Abstract: | Der Physik-Nobelpreis geht dieses Jahr an zwei Teilchen-Physiker, den Japaner Takaaki Kajita und den Kanadier Arthur B. McDonald. Sie haben mit ihren Experimenten den Beweis liefern können, dass Neutrinos eine Masse besitzen. Neutrinos sind ungeladene Elemntarteilchen, die nach dem Standardmodell der Teilchenphysik in drei verschiedenen Arten (Flavors) vorkommen undeigentlich masselos sein sollten. Der Nachweis der Neutrino-Oszillationen, bei denen Neutrino ihren Flavor ändern, (und der nun zur Verleihung des Physiknobelpreises an Kajita und McDonald geführt hat) hat nun zweifelsfrei bewiesen, dass Neutrinos nicht masselos sind (zumindest zwei der drei Flavor nicht). In dem Vortrag sollen Neutrinos "greifbar" werden. Die Beschreibung der Neutrino-Eigenschaften wird deutlich machen, vor welchen Herausforderungen die Neutrino-Forscher standen (und stehen). Die faszinierenden Möglichkeiten der Neutrinoforschung werden kurz dargestellt. *Hinweis*: Wie schon ein wenig traditionell werden wir die Nobelvorlesung mit hochadventlichem Glühwein und Weihnachtsgebäck begleiten. Sie werden *ab ca. 16 Uhr *vor dem Hörsaal H3 einen entsprechenden Ausschank vorfinden. Selbstverständlich haben wir auch einige Kannen Tee vorrätig für diejenigen, denen Glühwein nicht zusagt. |

10 December 2015

**Talk: ****Chris Eilbeck**

Where: | University of Oldenburg, W01 0-012 (Wechloy) |

When: | 16:00 hrs |

Speaker: | Chris Eilbeck (Heriot-Watt University) |

Title: | "Generalizations of the Weierstrass sigma function, discriminants, and heat equations" |

Abstract: | The solutions of many interesting nonlinear wave equations can be written down in terms of Weierstrass $\wp$ functions and their generalizations to higher genus. In turn the $\wp$ functions are just the 2nd logarithmic derivatives of a entire function, the Weierstrass $\sigma$ function generalized to genus $g$. We discuss the properties of the $\sigma$ function associated with a plane curve of genus $g$. These functions satisfy sets of interesting linear parabolic PDEs. These PDEs can be used to form linear but complicated recurrence relations for the coefficients of the sigma expansion. A connection with the discriminant of the associated curve is also highlighted. |

# November 2015

26 November 2015

**Talk: ****Victor Enolski **

Where: | University of Oldenburg, W01 0-012 (Wechloy) |

When: | 16:00 hrs |

Speaker: | Victor Enolski (Kiew) |

Title: | "Lesser known Nahm Ansatz in algebro-geometric description of monopole fields" |

Abstract: | In the ADHMN-construction (Atiyah-Drinfeld-Hitchin-Manin-Nahm) calculation of the monopole fields- Higgs field and gauges - reduced to solving linear ODE - Weyl equations with "potentials'" are given by the so called Nahm data. Even in the case of charge two, when the Nahm data represent elliptic functions, the analytic expressions for the monopole fields are unknown until now. We overcome the problem using a lesser known Nahm Ansatz (compared to the ADHMN construction), whose implementation leads to a complete description of monopole fields in terms of algebraic functions and incomplete elliptic integrals. |

26 November 2015

**Physikalisches K****olloquium**

Wo: | Universität Bremen, Hörsaal H3, Geb. NW 1, Otto-Hahn-Allee |

Wann: | 16:00 Uhr |

Sprecher/in: | Prof. Dr. Christopher Kyba |

Titel: | "Artificial light at a crossroads during the International Year of Light" |

Abstract: | For centuries, increases in the luminous efficiency of lamps have resulted in corresponding increases in light use, rather than energy savings. This has resulted in an exponential growth of light pollution, to the point where true night no longer occurs in most of Europe. Will LEDs be the final nail in the coffin for the night, or will they allow us to save energy and bring the stars back over our cities? This talk will touch on concepts and results from remote sensing, lighting design, physiology of vision, biology, ecology, energy economics, and lighting policy, in order to discuss the role of LEDs in the Energiewende, and in the worldwide transition to sustainable and healthy lighting. |