# Colloquia

# July 2017

05. July 2017

**RTG Colloquium**

Where: University of Bremen, ZARM

Talk 1: **Mariafelicia de Laurentis** (Goethe University Frankfurt) "Exploring Gravity at Galactic Center" - Abstract

Talk 2: **Annette Eicker** (HafenCity University Hamburg) "The attraction of water - Using GRACE stallite gravity data to improve our understanding of the global water cycle" - Abstract

Talk 3:** Lisa Wörner** (ZARM, University of Bremen) "Exploring the Limits of Quantum Mechanics" - Abstract

A preliminary program can be found here.

# May 2017

31. May 2017

**RTG Colloquium**

Where: Leibniz Universität Hannover

Talk 1: **Nadine Neumayer** (MPI for Astronomy, Heidelberg) "The build-up of galactic centers - how do black holes get there?" - Abstract

Talk 2: **Anupam Mazumdar** (Van Swinderen Institute, University of Groningen) "Constructing Universal Laws of Gravity at Short distances and Small time scales" - Abstract

Journals’ Club: **Paul Jefremov** (ZARM, University of Bremen) "Exact solutions to force-free electrodynamics in black hole backgrounds" by T. D. Brennan, S. E. Gralla and T. Jacobson, CQG, Vol. 30 19 (2013)

Student's Seminar: **Vojtech Witzany **(ZARM, University of Bremen) "Generalized conservation law near spinning black holes" - Abstract

A preliminary program can be found here.

03. May 2017

**RTG Colloquium**

Where: | University of Oldenburg |

When: | 3. May 2017 |

Talk 1: **Troels Harmark** (Niels Bohr Institute, Copenhagen, Denmark) - "Spin Matrix theory limit of the AdS/CFT correspondence" - Abstract

Talk 2: **Ehsan Hatefi** (Vienna University of Technology, Vienna Austria) - “Black hole formation and Critical Collapse in the Axion-Dilaton System in Diverse Dimensions” - Abstract

Student's Seminar: **Dennis Philipp **(University of Bremen)** -** "Timelike congruences, acceleration and redshift potential"

Journals’ Club: **Xiao Yan Chew **(University of Oldenburg) - “Spherical Photon Orbits Around a Kerr Black Hole”

The program can be found here.

# February 2017

07. February 2017

**RTG Colloquium**

When: | 07.02.2017, 10:30 - 17:30 hrs |

Location: | University of Bielefeld, Rooms D6-135 and D6-136 |

Talk 1: | Dr. Stanislav Babak (AEI, MPI for Gravitational Physics, Potsdam) - Universe in gravitational waves with Laser Interferometer Space Antenna |

Talk 2: | Prof. Carsten van de Bruck (University of Sheffield) - Dark energy and new interactions in the dark sector |

Student's Seminar: | Kai Flathmann (University of Oldenburg) - Analytic solutions of the geodesic equation for U(1)^{2} dyonic rotating black holes |

Journals' Club: | Paul Jefremov (ZARM, University of Bremen) - Neutrino-cooled accretion disks around spinning black holes (Chen & Beloborodov, The Astrophysical Journal , 657:383-399, 2007) |

The program can be found here.

# January 2017

10. January 2017

**RTG Colloquium**

Where: | ZARM, University of Bremen |

Speaker: | Carlos A.R. Herdeiro |

Title: | Kerr black holes with bosonic hair: theory and phenomenology |

Abstract: | Over the last two years it has been found that new classes of asymptotically flat black hole solutions, regular on and outside the event horizon, bifurcating from the vacuum Kerr solution, exist in General Relativity, with simple matter contents that obey all energy conditions, namely Kerr black holes with scalar hair and Proca hair. In this talk I will review the general mechanism that allows these solutions to exist, intimately connected to superradiance, how these solutions circumvent well known no-hair theorems and some of their phenomenology (shadows and x-ray spectra) which can be considerably distinct from that of Kerr. |

Key references | C. Herdeiro, E. Radu: Kerr black holes with scalar hair, Phys. Rev. Lett. C. Herdeiro, E. Radu, H. Runarsson: Kerr black holes with Proca hair, Class. Quant. Grav. P. Cunha, C. Herdeiro, E. Radu, H. Runarsson: Shadows of Kerr black holes with scalar hair, Phys. Rev. Lett. |

Speaker: | Gleb Arutyunov, II. Institute for Theoretical Physics, Hamburg University |

Title: | Solution of N=4 super Yang-Mills theory via the Mirror TBA |

Abstract: | How the Mirror Thermodynamic Bethe Ansatz approach solves for the spectrum of scaling dimensions of composite gauge-invariant operators in N=4 supersymmetric Yang-Mills theory. |

The program can be found here.

# November 2016

22. November 2016

**RTG Colloquium**

Where: | Jacobs University Bremen, Conference Room, Campus Center Directions |

When: | 22.11.2016, 10:30 - 17:15 hrs |

Speaker: | Iva Lovrekovic (Vienna) and Dorothea Bahns (Göttingen) |

Program | |

10:30 - 11:00 | Student´s Seminar: Patric Hölscher (Bielefeld):"Intodruction to conformal gravity" |

11:00 - 11:30 | Students´ Meeting |

11:30 - 12:00 | Women´s Assembly |

12:00 - 12:30 | Coffee break |

12:30 - 13:00 | Journals´ Club: Lucas Gardai Collodel (Oldenburg):"Relativistic Archimedeslaw for fast moving bodies and the general-relativistic resolution of the 'submarine paradox' George E. A. Matsas, Phys. Rev. D 68, 027701 (2003) |

13:00 - 14:15 | Lunch break / board meeting |

14:15 - 15:15 | (Vienna) "Conformal Gravity" (Abstract)Dr. Iva Lovrekovic |

15:15 - 15:45 | Coffee break |

15:45 - 16:45 | (Göttingen)Prof. Dr. Dorothea Bahns “On a notion of Volume in a Noncommutative World” |

16:45 - 17:15 | Coffee break and discussions |

Evening | Dinner in downtown Bremen |

# July 2016

28 July 2016

**RTG Students' Colloquium**

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

When: | 14:00 hrs |

14:00-15:00 | Ivan Kolar, Charles University, Prague"Higher dimensional spacetimes with a separable Klein-Gordon equation" Abstract: We study a class of higher dimensional spacetimes that lead to a separable Klein-Gordon equation. Motivated by Carter's work in four dimensions, we introduce an ansatz for the separable metric in higher dimensions and find solutions of the Klein-Gordon equation. For such a metric we solve the Einstein equations and regain the Kerr-NUT-(A)dS spacetime and an Einstein-Kahler metric of a Euclidean signature. We construct a warped geometry of two Klein-Gordon separable spaces with a properly chosen warped factor. We show that the corresponding Klein-Gordon equation can also be solved by separation of variables. By solving the Einstein equations for the warped geometry we find new solutions. We discuss general commutativity conditions for classical observables and analogous scalar field operators. Then we investigate the fulfillment of these conditions in the Kerr-NUT-(A)dS spacetimes (and relate spaces) and find the most general form of the weak electromagnetic field compatible with the complete integrability of the particle motion and the commutativity of the scalar field operators. For such a field we solve the charged Hamilton-Jacobi and Klein-Gordon equations by separation of variables. |

Antonia M. Frassino, Frankfurt Institute for Advanced Studies "Black hole chemistry and other phenomena beyond Einstein's gravity" Abstract: After introducing the phase transitions of asymptotically AdS black holes and the concept of cosmological constant as a thermodynamic pressure, I will talk about the effects of higher curvature Lovelock gravity corrections on the phase diagram structure and in the context of gauge/gravity duality. | |

16:00-16:30 | Coffee break |

16:30-17:30 | Andreas Schreiber, IMPRS for Astronomy and Cosmic Physics, Heidelberg"Why planets should not form - The missing link in planet formation theory?" Abstract: The formation of planets is a beneficial side effect of star formation. When in our milky way a large gas cloud collapses under its own weight to create a star in its center, then some of the material stays in an orbit around the newborn star forming a disk, simply as a consequence of angular momentum conservation. These disks are found around stars that are younger than 10 million years and the disks are slightly larger than our solar system. Not too surprisingly we believe these disks to be the origin of planetary system like ours. But, the exact processes of planet formation remain completely unclear and are under heavy debate. This talk will give in its first part an introduction into the known planet formation processes with its benefits and drawbacks. In the second part we will go deeper into the 'gravotubulent planetesimal formation' scenario, which is most favored from community perspective, but still has a 'missing link' between µm sized dust and km sized planet embryos, called planetesimals. I will show newest results from dust-to-planetesimals simulations and the way we try to predict planetesimal sizes from only a few protoplanetary disk parameters. |

17:30-18:30 | Michael Fennen, ZARM Bremen"Friedmann-like distributions of black holes on the 3-sphere"Abstract: In the very successful standard model of cosmology, matter is described as a homogeneous fluid. But actually, it is not known how to average tensors in a curved space-time so that it is at least reasonable to question this ansatz. We consider an exact vacuum solution of the initial value problem in GR, where we distribute black holes on a 3-sphere to model an inhomogeneous universe. We present conditions under which such universes have a Friedmann-like behaviour. |

ca. 19:00 | Barbecue |

6 July 2016

**RTG Colloquium**

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

When: | 11:00-16:00 hrs |

11:00 - 11:30 | Students' SeminarM.Sc. Vojtech Witzany, ZARM, University of Bremen"Carter-Lichnerowicz form of perfect fluid equations and accretion in the Kerr spacetime" |

11:30 - 11:45 | Coffee Break |

11:45 - 12:15 | Journals' Club , University of Oldenburg M.Sc. Stephan Reimers "An Example of a New Type of Cosmological Solutions of Einstein's Field Equations of Gravitation", K.Gödel |

12:15 - 13:45 | Lunch break for students / Board Meeting with Lunch |

13:45 - 14:45 | Dr. Gesine Grosche, PTB, Braunschweig "Interferometric optical fibre links for long-distance frequency > transfer with 10^(-18) resolution" |

14:45 - 15:00 | Coffee Break |

15:00 - 16:00 | Dr. Monika Moscibrodzka, Department of Astrophysics, IMAPP. Radboud University Nijmegen"General relativistic magneto hydrodynamics simulations of black hole in the Galactic center: confronting theory with astronomical observations" |

17:30 | Early Dinner |

# June 2016

# May 2016

11 May 2016

**RTG Colloquium **

Where: | University of Hannover, Room 027, Appelstr. 4 |

When: | 10:00 - 17:30 hrs |

10:00 - 11:00 | Students´ Seminar Lukas Brunkhorst "Minimal length and Hopfian Relativity" |

11:00 - 11:15 | Coffee break |

11:15 - 12:00 | Women´s Assembly |

12:00 - 12:15 | Coffee break |

12:15 - 13:15 | Journals´ Club Stephan Reimers "An Example of a New Type of Cosmological Solutions of Einstein's Field Equations of Gravitation", K. Gödel |

13:15 - 14:15 | Lunch break |

14:15 - 15:15 | Dr. Jan Pieter van der Schaar, University of Amsterdam and Nikhef"Probing the physics of (eternal) inflation" |

15:15 - 15:45 | Coffee break |

15:45 - 16:45 | NASA USRA / NASA MSFC, Huntsville, AlabamaDr. Adam Michael Goldstein, "Fermi gamma-ray burst monitor observations of GW150914 and near-future orospects for electromagnetic follow-up of gravitational wave signals" |

16:45 - 17:00 | Coffee break |

17:00 - 17:30 | General Assembly |

Prior to the colloquium, 17 of our phd students and postdocs will be visisting GEO600. A report on their experiences and impressions will be available soon afterwards. Please come back for this and click here.

# April 2016

13 April 2016

**RTG Colloquium**

Where: | University of Oldenburg, W2 - 1 146; W2 - 1 143 |

When: | 10:15-17:00 hrs |

10:15 - 11:00 | Women Assembly |

11:00 - 11:15 | Coffe break |

11:15 - 12:15 | Prof. Dr. Joris Verbiest, University of Bielefeld"Pulsar Timing Tests of Gravity" Abstract Millisecond pulsars are highly stable "Einstein" clocks that often inhabit extreme gravitational environments. In many ways, they're ideal probes of extreme gravity. Consequently, they are regularly used for tests of relativistic gravity -- and might even lead to a direct detection of nHz gravitational waves in the near future. In this talk, I will briefly review the various types of radio pulsars that are known to exist and discuss some of the more typical (and most sensitive) gravity tests performed with these pulsars. Furthermore, I will review the current efforts at making a direct detection of gravitational waves through pulsar timing; and will clarify which types of sources could be observed and investigated with such a detection. Finally, I briefly present what can be looked forward to in the context of gravitational-wave science with pulsar timing in the coming decade. |

12:15 - 12:30 | Coffe break |

12:30 - 13:00 | Students' SeminarPaul Jefremov, ZARM"Relativistic disk accretion. "Polish Doughnuts" model". |

13:00 - 14:00 | Lunch break at the Cafeteria |

14:00 - 14:30 hrs | Journal's ClubFech Scen Khoo, Jacobs University Bremen"A master equation for gravitational pertubations of maximally symmetric black holes in higher dimensions" |

14:30 - 15:00 | Coffe break |

15:00 - 16:00 | Dr. Térence Delaste, Université de Mons, Mons, Belgium"Slowly rotating neutron stars in general scalar-tensor theory: The case of non minimal derivative coupling" |

16:00 - 17:00 | Board Meeting |

Apprx. 17:30 | Dinner |

# March 2016

# February 2016

17 February 2016**RTG Colloquium**

Where: | University of Bielefeld, Lecture Hall 10 |

When: | 11:00 - 17:15 hrs |

10:30 - 11:00 | Students' SeminarXiao Yan Chew, University Oldenburg"On the Reproduction of Field Configurations via Retarded Green's function in de Sitter Spacetime" |

11:00 - 12:00 | Students' Meeting |

12:00 - 12:30 | Journal's ClubUniversity BremenKris Schroven, "Gas dynamics of semidetached binaries", S. H. Lubow and F. H. Shu (adsabs.harvard.edu/abs/1975ApJ...198..383L) |

12:30 - 13:00 | General Assembly |

13:00 - 14:15 | Lunch |

14:15 - 15:15 | Jun.-Prof. Annalisa Bonafede, University Hamburg "Magnetic fields and cosmic rays in galaxy clusters" |

Abstract The extreme physical conditions in the intra-cluster medium of galaxy clusters are beyond anything achievable in any laboratory on Earth, and offer us a unique tool to study magnetic fields and cosmic rays on the largest scales in the Universe. A big challenge of modern astrophysics is understanding the origin of radio emission spread over the volume of some galaxy cluster. This emission is a mystery because it requires relativistic electrons moving around magnetic field lines, but both the origin of the magnetic fields and of the electrons are unknown. We are entering into a golden age to address these fundamental problems, thanks to the advent of a new generation of radio telescopes, such as LOFAR, the JVLA, and ASKAP. At the same time, the new eROSITA X-ray satellite is going to provide us with a wealth of new data on the most distant and less massive galaxy clusters and groups. In this talk, I will review our current knowledge about magnetic fields and cosmic ray particle sin galaxy clusters, and I will illustrate the potential of the new generation of radio instruments to answer the many open questions about the origin and evolution of magnetic fields an cosmic rays. | |

15:15 - 15:45 | Coffee and discussions |

15:45 - 16:45 | Dr. rer. nat. Sebastien Clesse, RWTH Aachen"Chameleon and K-mouflage models of modified gravity: cosmological, astrophysical and laboratory constraints" |

Abstract Understanding the current acceleration of the Universe expansion is one major challenge in cosmology. Present and future experiments aim to distinguish between a cosmological constant, a dark energy fluid, and modifications of gravity. Most modified gravity models involve at least one scalar field coupled to matter. An environmental dependance eventually leads to a screening mechanism suppressing long-range fifth force effects in galaxies, in the solar system and in laboratory experiments. Three possible screening mechanisms have been proposed: chameleon (screening when the Newtonian potential is large), Vainshtein (screening when the spatial curvature is large) and K-mouflage (screening when the gravitational acceleration is large). In this seminar, I will focus on chameleon and K-mouflage models. After a description of the scalar field dynamics in different contexts, and of the linear cosmological perturbations, I will give a review of the cosmological, astrophysical, and laboratory constraints that can be set on chameleon and K-mouflage models, including constraints from Lunar Laser Ranging and from atom-interferometry experiments. | |

16:45 - 17:15 | Coffee and discussions |

18:30 | Dinner |

# January 2016

13 January 2016

**RTG Colloquium**

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

10:30 - 13:00 | General Assembly |

13:00 - 14:30 | Board Meeting incl. lunch (Rm 1280), Lunch for students university mensa |

14:30 - 15:30 | Dr. Manuel Hohmann, University of Tartu“Parametrized post? Newtonian formalism of Horndeski´s theory of Gravity” |

15:30 - 16:00 | Coffee break |

16:00 - 16:30 | Journal´s Club |

16:30 - 17:30 | Dr. Gudrun Wolfschmidt, University of Hamburg"The Einstein-Tower in Potsdam - A monument with cultural significance as well as for the history of science" |

18:00 | Dinner |

# December 2015

16 December 2015

**RTG Colloquium**

Where: Leibniz University of Hannover (Anreise)

10:00 - 10:30 | Welcome coffee for early arrivals |

10:30 - 11:30 | Talks from RTG members (20 minutes each): 1. Dr. Saskia Grunau, University of Oldenburg "Geodesic motion in black string and black ring spacetimes" (working title) 2. Dr. Daniela Kunst, University of Bremen, ZARM “Allgemein relativistische Dynamik von Spinteilchen“ (working title) Followed by discussion of the talks. |

11:30 - 12:00 | General Assembly |

12:00 - 13:00 | Lunch break at mensa |

13:00 - 14:00 | First poster presentations incl. coffee break |

14:00 - 15:00 | Dr. Kazuya Koyama (University of Portsmouth) "Cosmological tests of gravity" |

15:00 - 15:30 | Coffee break |

15:30 - 16:30 | Dr. Astrid Eichhorn (Imperial College London) "Quantum gravity and matter in the asymptotic safety scenario" |

17:00 - 18:00 | Questions and answers session |