# Colloquia

# 2022

# February 2022

**2 February 2022**

**RTG Colloquium** - Online (orig. ZARM, University of Bremen)

14:00- 16:30 CET

Schedule:

**14:00 - 15:00 ****Dr. Tessa Baker (Queen Mary University of London):** Tests of Gravity with Gravitational Waves

*Abstract*: The past few years of discoveries in gravitational wave astronomy have had a profound impact on cosmology. In particular, they have triggered a host of new ideas on how to probe the fundamental nature of gravity on large scales. As such, gravitational waves are rapidly becoming a crucial pillar in the long-standing challenge to understand dark energy.

In this talk, I’ll explain the essential phenomenology of gravitational wave propagation outside of General Relativity. We’ll see that gravitational wave sirens — both with and without electromagnetic counterparts — enable us to test very general deviations from GR, via modifications of the propagation speed and luminosity distances of gravitational wave signals. I’ll review some of the key results to date, and also discuss the potential of future gravitational wave detectors to further probe the nature of gravity.

15:00 - 15:30 discussion

15:30 - 16:00 Coffee in the (own) kitchen

** 16:00 - 17:00 Dr. Melanie Graf (University of Tübingen):** Singularity Theorems at Low Regularity

*Abstract*: The singularity theorems of R. Penrose and S. Hawking from the 1960s show that a spacetime satisfying certain physically reasonable curvature and causality conditions cannot be causal geodesically complete. Despite their great success these classical theorems still have some drawbacks, one of them being that they require smoothness of the metric while in many physical models the metric is less regular. I will first present a summary of the classical theorems and after which I'll give a general overview of the new challenges arising in the statements and proofs of singularity theorems for metrics of lower regularity and review some of the

more recent results and techniques available. For the last part of the talk we'll focus on a version of Hawking's theorem based on a distributional strong energy condition for metrics that are merely continuously differentiable - a regularity where one still has existence but not uniqueness for solutions of the geodesic equation and which represents the current state of the art for an analytic approach.

17:00 - 17:30 discussion

# January 2022

**12 January 2022**

**RTG Colloquium** - Online (orig. University Bielefeld)

**10:30- 15:00 CET**

Schedule:

**10:30 - 11:30 ****Christos Tsagas (****Aristotle University of Thessaloniki):** Cosmic Acceleration in 'Tilted' Universes

*Abstract*: Tilted cosmological models are spacetimes that allow for two families of observers, moving relative to each other with finite peculiar velocities. By construction, tilted cosmologies provide a more realistic description of the actual universe, where bulk peculiar flows appear to be the norm rather than the exception. Given that relative-motion effects are known to interfere with the way the associated observers interpret their data and understand the world they live in, it is worth investigating the theoretical implications of such large-scale peculiar motions for cosmology. In this talk we consider the implications of bulk peculiar flows for cosmic acceleration and more specifically for the deceleration parameter of the universe. Applying relativistic cosmological perturbation theory to a tilted almost-Friedmann universe, we consider the mean kinematics of typical galaxies, like our Milky Way, which move with respect to the smooth Hubble flow. We find that observers residing in these galaxies can assign very different values to their deceleration parameters, entirely because of their relative motion. In fact, some observers may even experience apparent accelerated expansion, while the host universe is actually decelerating. Although the accelerating effect is a local artefact of relative motion, the affected scales can be large enough to create the false impression of a recent global event.

11:30 - 14:00 Break

**14:00 - 15:00 ****Alberto Sesana (****University of Milan-Bicicca): **The Importance of GW Background Detection for SMBHB Population Studies

*Abstract*: The recent detection of gravitational waves (GWs) marked the opening of a completely new window on the Universe. At nHz frequencies, pulsar timing arrays (PTAs) promise to detect the signal coming from the cosmological population of supermassive black hole binaries (SMBHBs) within the next few years. PTAs will help probing the high mass end of the SMBHB mass function, contributing to our understanding of the cosmic evolution of SMBHs and the physics governing their pairing in binaries and subsequent dynamics. After reviewing the astrophysics of SMBHBs, I will describe the recent detection of a common red signal in PTA dataand discuss its possible implications for SMBHB astrophysics.

More details will follows soon.

# 2021

# December 2021

**8 December 2021**

**RTG Colloquium** - Online (orig. University Oldenburg)

**14:00- 16:30 CET**

Schedule:

**14:00 - 15:00 ****Irene Tamborra:** Messengers from the Universe

*Abstract*: Neutrinos are fascinating elementary particles heralding the dawn of the multi-messenger astronomy era. Neutrinos affect the stellar dynamics, drive the formation of new elements, and carry signatures of the yet mysterious physics ruling the most powerful cosmic fireworks. Recent developments on the role of neutrinos in cosmic sources will be reviewed together with the most exciting detection prospect.

15:00 - 15:30 Coffee break

**15:30 - 16:30 ****Fethi Ramazanoglu: **Unwelcome instabilities in theories of gravity

*Abstract*: Weak field tests already constrain any deviation from general relativity to be small, hence, scenarios in alternative theories where major effects become relevant in the strong gravity regime are particularly interesting. A perfect example of this is the spontaneous scalarization phenomena in scalar-tensor theories where the scalar can naturally grow near compact objects due to a tachyonic instability. The tachyon is welcome, because it is eventually shut off due to nonlinear effects, and leads to stable objects of potential astrophysical relevance. There does not seem to be anything specific to scalars in the instability mechanism at a first sight, hence there have been various efforts to find analogous theories for vectors and other fields. I will summarize these efforts, and then concentrate on our recent finding that such theories are fundamentally different from scalarization. Wheneven an analog of the tachyon in scalarization is introduced in a vector-tensor theory, a ghost (or gradient) instability also appears. Instability time scale of the ghost is faster than that of tachyons, hence it dominates the dynamics. Even more crucially, these ghosts also manifest divergent terms in the field equations, which poses serious questions about the mathematical meaning and physical validity of the theories they appear in. We will discuss these problems, some ideas about potential solutions and factors that hinder the solutions. Reference: arxiv.org/abs/2110.04594

More details can be found here.

# Juli 2021

**14 July 2021**

**RTG Colloquium** - Online (orig. University Bielefeld)

**09:00 - 11:30 CET**

Schedule:

**09:00 – 10:00 Sebastian von Hausegger (University of Oxford): **Testing the Cosmological Principle with Distant Galaxies - presentation slides

*Abstract*: In the first part of this talk I will discuss the Cosmological Principle — the assumption that our universe is homogeneous and isotropic on large scales — and its central role in standard analyses in cosmology. Various tests can be devised to examine the validity of the Cosmological Principle in probes of Large Scale Structure. I will then focus on one such test, that we conducted on a large, full-sky, flux-limited sample of high-redshift galaxies, 1.36 million quasars observed by the WISE satellite, which reveals inconsistency with the Cosmological Principle at 4.9σ — the highest significance of any such finding to-date. I will discuss consequences of this result and comment on possible avenues for future research.

10:00 – 10:30 Break

**10:30 – 11:30 Siyuan Chen (CNRS, Orléans):** From Pulsars to Supermassive Black Holes and Gravitational Waves inbetween

*Abstract*: Pulsar Timing Arrays (PTAs) aim to detect nHz gravitational waves (GWs) by looking for correlated variations of the Times of Arrivals (TOAs) across an array of ultra-stable millisecond pulsars. Comparing the predicted TOAs from our timing model against the measured TOAs gives us the residuals. These contain the imprint of GWs, but also other effects and sources of noise processes. A gravitational wave background (GWB) manifests as a common spatially correlated process across all pulsars, with the characteristic signature being the Hellings-Downs correlation - the smoking gun of the detection of a GWB. One possible source for such a GWB could come a population of merging supermassive black hole binaries (SMBHBs). Three established PTA collaborations: the North American Nanohertz Gravitational Wave Observatory (NANOGrav), the Australian Parkes PTA (PPTA) and the European PTA (EPTA), as well as emerging PTA collaborations from India, China and South Africa all work together in the International PTA (IPTA) consortium towards the common goal of detecting low frequency GWs.

Recently, PTAs have reported the detection of an uncorrelated but spectrally similar signal found amongst a number of pulsars in the array. However, due to the lack of significant evidence for the Hellings-Downs correlation, the signal cannot be confirmed to be a GWB yet. I will present results from the analyses of the most recent datasets of the three PTAs as well as the IPTA data release 2. We find consistent amplitudes across all 4 datasets for this signal and hope to confirm its origin in a future data combination with the IPTA. If this signal is indeed a GWB, it can be interpreted as coming from numerous different astrophysical sources. I will focus on the astrophysical interpretation of such a speculative detection using SMBHBs from galaxy mergers.

More details can be found here.

# May 2021

**12 May 2021**

**RTG Colloquium** - Online (orig. University Oldenburg)

**14:00- 16:30 CET**

Schedule:

** 14:00 - 15:00 Dr. Kamal Hajian (HWK Delmenhorst):** Black hole temperature in Horndeski gravity

*Abstract*: In Horndeski gravities, which are the most generic scalar-tensor theories without ghosts, the speed of graviton can be different w.r.t other massless particles/waves such as photons. We will show that this leads to a black hole temperature which is different from the standard Hawking temperature by an overall factor. The factor depends on black hole properties as well as the Lagrangian. Using this modified temperature, the first law of thermodynamics for black holes in Horndeski gravities is recovered.

15:00 - 15:30 Coffee break

** 15:30 - 16:30 Dr. Ivonne Zavala (Swansea University, Wales): **Dark energy in string theory and supergravity

*Abstract*: I will review progress in understanding present day cosmological acceleration in string theory and supergravity. I will first discuss recent progress and challenges on realising dynamical dark energy in these theories. Then, I will briefly discuss a recent new approach to de Sitter solutions in supergravity and its possible realisation in string theory.

More details can be found here.

# April 2021

**14 April 2021**

**RTG Colloquium Online **

Where: online Zoom-Conference (orig: Jacobs University Bremen)

When: **14.04.2021, 14:00 - 16:30 CET**

Schedule:

**14:00 - 15:00 Andreas Finke** (University of Geneva) - *Dark Siren constraint on modified GW propagation*

15:00 - 15:30 Coffee break

**15:30 - 16:30 Mairi Sakellariadou** (King's College London) - *Hunting for the stochastic gravitational-wave background: implications for astrophysical and high energy physics models*

More details can be found here.

# 2020

# December 2020

**16 December 2020**

RTG Colloquium Online

Where: online Zoom-Conference (orig: Copenhagen)

When: **16.12.202, 14:00 - 16:30 CET**

**14:00 - 15:00**

**Speaker:** Michèle Levi (NBIA, Niels Bohr Institute)

**Title:** QFT for gravity at all scales

Abstract: We will review the state of the art in PN gravity, and in particular its significant advancement via the effective field theory (EFT) of spinning gravitating objects. First, we will introduce the concept of a tower of EFTs for the binary inspiral problem. We will then show the intricate formulation of the EFT of spinning objects. Finally, we will present some advanced recent results accomplished within this framework.

**15:00 - 15:30 Coffee break**

**15:30 - 16:30 **

**Speaker**: Jelle Hartong (University of Edinburgh)

**Title: **On the non-relativistic expansion of General Relativity and Quantum mechanics”

Abstract: Recently progress has been made on how to systematically expand general relativity in powers of 1/c where c is the speed of light. This expansion is similar in spirit to the post-Newtonian expansion, but it has the benefit that it can be applied to any matter system that is coupled to GR. Furthermore, the expansion is covariant and can be applied off shell so that we can use it to define action principles for non-relativistic gravity plus relativistic corrections. I will review these developments. The second part of the talk will be about the question: how to describe the interactions between gravity and matter at the quantum level in the non-relativistic domain? For example, if we treat gravity as a classical background that is 1/c expanded, how do we couple it to a quantum system? Even in this non-relativistic domain a theory of quantum mechanics backreacting on non-relativistic gravity is non-trivial. Time permitting, we will discuss approaches, such as non-relativistic string theory that can potentially address such and other issues.

More details can be found here.

# November 2020

**25 November 2020**

RTG Colloquium Online

Where: **online Zoom-Conference (orig: University of Bremen)**

When: **25.11.2020, 9:30 - 12:00 CET and 14:00-15:00 CET**

**09:30 - 10:30 CET **

**Sergey V. Ketov **(Tokyo Metropolitan University and Kavli IPMU, Japan)

*Models of supergravity for inflation, primordial black holes and gravitational waves *

**11:00 - 12:00 CET**

**Christophe LePoncin-Lafitte** (SYRTE, Paris)

*Testing General Relativity and searching Dark Matter with clocks*

**14:00 - 15:00 CET**

**Nicoleta Voicu **(Transilvania University Brasov)

*Variational completion of differential equations and modified theories of gravity*

Details about the talks and the ZOOM link can be found here.

# October 2020

28. October 2020

**RTG Colloquium****Online **

Where: online Zoom-Conference (orig: University of Hannover)

When: 28.10.2020

**Confirmed speakers**

- 14:00 - 15:00
**Vitor Cardoso**(Universidade Técnica de Lisboa, Portugal) - 15:30 - 16:30
**Jerzy Lewandowski**(University of Warsaw, Poland)

# JULY 2020

9. July 2020

**Student's Seminar and Journal Club**

Where: Online

When: 09.07.2020, 14:00 - 16:00 h

**Students' Seminar**Speaker:

**Roberto Tanzi**

Title:

*Asymptotic symmetries of Yang-Mills fields in Hamiltonianformulation*

Abstract: Asymptotic symmetries are an important feature of theories with long-ranging fields, such as gravity, electromagnetism, and Yang-Mills. I explain how to derive the asymptotic symmetry group of the free SU(N)-Yang-Mills theory from clear-cut first principles using the Hamiltonian formalism. These principles include the minimal assumptions that are necessary to ensure the existence of Hamiltonian structures (phase space, symplectic form, differentiable Hamiltonian) and, in case of Poincaré invariant theories, a canonical action of the Poincaré group. I show that, differently from the electromagnetic case (first studied by Henneaux and Troessaert), the above principles lead to trivial asymptotic symmetries and charges in the Yang-Mills case. This seems to hint at a kind of colour-confinement built into the classical Hamiltonian formulation of non-abelian gauge theories. This talk is based on the paper e-Print: 2006.07268 [hep-th], which my supervisor and I published recently.

**Journal Club**Speaker: tba

1. July 2020

**RTG Colloquium****Online **

Where: online Zoom-Conference (orig: University of Oldenburg)

When: 01.07.2020

**Confirmed speakers:**

**Vanessa Graber**(Institute of Space Sciences (ICE-CSIC), Barcelona, Spain) - “Neutron Stars - Cosmic Superfluids”**Nicolás Sanchis Gual**(Center for Astrophysics and Gravitation (CENTRA), Lisbon, Portugal) - “Light in the dark: numerical relativity, compact objects, and gravitational waves”

Please find the program here.

# JUNE 2020

11. June 2020

**Student's Seminar and Journal Club**

Where: Online

When: 11.06.2020, 14:00 - 16:00 h

**Students' Seminar**Speaker: Orville Damaschke

Title: Lovelock theorem - an important result for GR and its alternatives

**Journal Club:**Speaker: Felix Willenborg

Title: Construction and application of variations on the cylindrical gravitational waves of Weber, Wheeler, and Bonnor

03. June2020

**RTG ColloquiumOnline **

Where: online Zoom-Conference (orig: University of Bielefeld)

When: 03.06.2020, 14:00 h - 16:30 h

**Confirmed speakers:**

**Robert Brandenberger**(McGill University) - “Is Inflationary Cosmology Consistent with Fundamental Physics?”**Stephen Taylor**(Vanderbilt University) - “Charting the next frontier of gravitational-wave astronomy with pulsar-timing arrays”

Please find the program here.

# April 2020

22. April 2020

**RTG ColloquiumJacobs University Bremen - Online**

Where: Jacobs University Bremen - Online

When: 22.04.2020, 14:00 - 16:30 h

The program can be found here.

The slides of the presentation given by **Alessia Platania** with the title *"Quantum gravity in the asymptotic-safety approach"* can be found here and the slides of the presentation given by **Evgeny Skvortsov** with the title *"(Quantum) Higher Spin Gravity and Physics"* here.

# February 2020

5. February 2020

**RTG ColloquiumZARM, University of Bremen**

Where: ZARM, University of Bremen

When: 05.02.2020

**Confirmed speakers: **

- Kai Flathmann (Bremen, Germany) - "Post-Newtonian limit of general scalar-torsion theories of gravity"
- Mourad Halla (Bremen, Germany) - "Applications of the Gauss-Bonnet theorem to gravitational lensing"
- Anna Ijjas (Hannover, Germany) - "The virtues of slow contraction, and other perks of bouncing"
- Yakov Shnir (Dubna, Russia) - "Spinning black holes with synchronized hairs and soliton stars"

The program and the abstracts can be found here.

# 2019

# December 2019

4. December 2019

**RTG ColloquiumUniversity of Hannover**

Where: University of Hannover, "Königlicher Pferdestall”, Appelstraße 7, 30167 Hannover

When: 04.12.2019, 11:00 h ‐ 16:30 h

Confirmed Speakers:

**Bruce Allen**(Hannover, Germany) - "Gravitational wave stochastic background from cosmologicalparticle decay"**Nathalia Aprile**(Sao Paulo, Brazil) - "Holographic Superconductors"**Jafar Khodagholizadeh**(Tehran, Iran) - “Aschenbach effect for spinning particles in Kerr Spacetime”

Please find the preliminary program and abstracts here.

# November 2019

6. November 2019

**RTG ColloquiumUniversity of Oldenburg**

Where: University of Oldenburg, Room A14-1-111 (Senatssitzungssaal)

When: 06.11.2019

New Speakers:

**Christian Hoffmann**(Oldenburg, Germany) - Solitons in gravity and non-linear Systems**Sarah Kahlen**(Oldenburg, Germany) - Einstein-Maxwell-scalar black holes: classes of solutions, dyonsand extremality**Sravan Kumar**(Groningen, Netherlands)**Ahmad Sheykhi**(Shiraz, Iran / Oldenburg, Germany) - Origin of MOND Theory

Please find the new program and abstracts here.

# October 2019

28. - 29. October 2019

**Colloquium: From Classical To Quantum Physics**

on occasion of Domenico Giulini's 60th birthday

Where: ZARM, University of Bremen

When: 28. - 29.10.2019

The program and further information can be found here.

# June 2019

24. June 2019**COLLOQUIUMPhysical Colloquium**

Location: | University of Oldenburg, Campus Wechloy, W2 1-148 |

When: | 24.06.2019, 16:15 |

Speaker: | Prof. Markus Ahlers (Niels Bohr Institute, Copenhagen) |

Titel: | Neutrino Sources in Light of Recent IceCube Results |

Abstract: | The field of high-energy neutrino astronomy is undergoing a rapid evolution. In 2013, the IceCube Observatory at the South Pole reported first evidence of a diffuse flux of astrophysical neutrinos in the TeV-PeV energy range. While the flux is by now observed with high significance, its astrophysical origin is still unknown. Only recently, IceCube was able to report first compelling evidence of neutrino emission from a gamma-ray blazar. The current lack of firmly detected neutrino point sources indicates that the observed neutrino flux is dominated by relatively weak sources. Most likely, the neutrino sky is complex and several source classes contribute. I will summarize the status of these neutrino observations and highlight the strong role of multi-messenger astronomy for their interpretation. |

The announcement can be found here.

13. June 2019

**Habilitation Colloquium - Dr. Eva Hackmann (ZARM)University of Bremen**

Topic:** "Special relativistic elasticity and the Ehrenfest paradox"**

Where: Hörsaal H3, Geb. NW1, Otto-Hahn-Allee 1, 28359 Bremen

When: 13.06.2019, 16:00

5. June 2019

**RTG ColloquiumUniversity of Bielefeld**

Where: University of Bielefeld

When: 05.06.2019

Confirmed speakers:

**Bilel Ben Salem**(Bielefeld) - "A brief introduction to pulsar timing" - presentation slides**Nitesh Bhardwaj**(Bielefeld) - "Cosmology with LoTSS: The angular two-point correlation function"**Michael Janssen**(Nijmegen) - "The first image of a black hole"**Laura Tolos**(Frankfurt) - "A new family of compact objects: Dark Compact Stars"

The preliminary program and the abstracts can be found here.

# May 2019

8. May 2019

**RTG ColloquiumZARM, University of Bremen**

Where: ZARM, University of Bremen

When: 08.05.2019, 10:00 h - 18:00 h

Confirmed Speakers:

**Hadi Godazgar**(Albert Einstein Institute, Potsdam) - “Dual and extended asymptotic charges”**Oldrich Semerak**(Prague) - “Black holes under external influence”**Nezihe Uzun**(Lyon) - “Symplectic ray bundle transfer in general relativity”

The program and the abstracts can be found here.

# April 2019

17. April 2019

**RTG ColloquiumUniversity of Oldenburg**

Where: University of Oldenburg, Campus Wechloy

When: 17.04.2019

**Confirmed speakers:**

**Fech Scen Khoo**(Rudjer Boskovic Institute, Zagreb, Croatia) - The double field theory algebroid from the relaxation of Courant algebroid axioms - Abstract**Anupam Mazumdar**(University of Groningen, The Netherlands) - Nonlocal Star as a Blackhole Mimicker - Abstract**Eugen Radu**(University of Aveiro, Portugal) - No hair conjecture: a review of recent results - Abstract**Dennis Stock**(ZARM, University of Bremen) - The Averaging Problem in Inhomogeneous Cosmology - Abstract**Roberto Tanzi**(ZARM, University of Bremen) - BMS supertranslations and memory in four and higher dimensions - Abstract

The preliminary program can be found here.

# January 2019

23. January 2019

**RTG ColloquiumZARM, University of Bremen**

Where: ZARM, University of Bremen

When: 23.01.2019

Speakers:

- Marvin Pinkwart (Jacobs University, Bremen) - "On Quantum Spacetime and the horizon problem" - Abstract
- Björn-Malte Schäfer (University of Heidelberg) - "Weak lensing and intrinsic alignments of galaxies" - Abstract
- Andrey Shoom (Max Planck Institute, Hannover) - "Metamorphoses of a photon sphere" - Abstract
- Matheus C. Teodoro (University of Oldenburg) - “Simulations of accretion processes onto boson stars” - Abstract
- Oleg Tsupko (Russian Academy of Science, Moscow) - “Shadow of black holes at cosmological distances“ - Abstract

A preliminary program can be found here.

# 2018

# December 2018

05. December 2018

**RTG ColloquiumUniversity of Hannover**

Where: University of Hannover, ITP, Appelstrasse 2

Seminar room 268 and 267

When: 05.12.2018

Confirmed speakers:

**Eugenia Boffo**(Jacobs University Bremen) - "Einstein–Cartan theory" - Abstract**Laura Covi**(Universitaet Goettingen) - "Quantum corrections in curved space-times and the inflationary power spectrum" - Abstract**Jan Plefka**(Humboldt Universitaet Berlin) - "The Classical effective action of Dilaton-gravity from the double copy"**Dennis Philipp**(ZARM, University of Bremen) - “Theoretical Aspects of Relativistic Geodesy” - Abstract

The preliminary program can be found here.

# November 2018

15. November 2018

**COLLOQUIUMTheory Colloquium**

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

When: | 15.11.2018, 14:15 - 16:00 |

Speaker: | Prof. Dr. Ralf Hofmann (Heidelberg) |

Title: | SU(2) Yang-Mills thermodynamics and its implications for the cosmological model |

Abstract: | Based on a non-perturbative and largely analytical approach to Yang-Mills thermodynamics and considering the low-frequency excess of radiance in the present CMB spectrum we put forward the postulate that thermal photon gases are subject to an SU(2) rather than a U(1) gauge principle. Implications for the CMB temperatureredshift relation and the dark sector of the cosmological model are discussed. In this context, the local-global H_{0} discrepancy is reduced to the common use of a U(1) model for CMB photons in parameter fits to the extremely well measured CMB power spectra. |

The full announcement can be downloaded here.

07. November 2018

**RTG ColloquiumJacobs University Bremen**

Where: Jacobs University Bremen

When: 07.11.2018

Confirmed speakers:

- Zahra Altaha Motahar (University of Oldenburg) - "Neutron stars in massive scalar tensor theory with self interaction" - Abstract
- Athanasios Chatzistavrakidis (Ruđer Bošković Institute, Zagreb, Croatia) - "Aspects of gerneralized geometry for closed strings"
- Christian Knoll (University of Oldenburg) - "Invariante Variationsprobleme" - Abstract
- Ruben Manvelyan (Yerevan Physics Institute, Armenia) - "Weyl Invariance and Higher Spin Gauge Theory" - Abstract
- Frederic Schuller (Friedrich-Alexander-Universität Erlangen-Nürnberg) - "To which question precisely is gravity actually the answer?"

The program can be found here.

# June 2018

27. June 2018

**RTG ColloquiumZARM, University of Bremen**

Where: ZARM, University of Bremen

When: 27.06.2018

Confirmed Speaker:

- Pratika Dayal (Groningen) - "First light: the rise of galaxies in the first billion years"
- Kai Flathmann (Oldenburg) - Students’ Seminar - "Gravitational waves from inspiralling compact binaries in EGBd theory"
- Michèle Heurs (AEI, Hannover) - "Gravitational wave detection using laser interferometry beyond the standard quantum limit"
- Matheus C. Teodoro (Oldenburg) - Journals’ Club - "Constraints on cosmic strings using data from the first Advanced LIGO observing run"

The program can be found here.

# May 2018

24. May 2018**COLLOQUIUMTheory Colloquium**

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

When: | 24.05.2018, 14:15 |

Speaker: | Dr. Ziri Younsi (Frankfurt) |

Titel: | Shadows, Accretion and Outflows |

Abstract: | Understanding the Emissions from Black Holes It is now widely-believed that Active Galactic Nuclei (AGN) are powered at their cores by supermassive black holes (SMBHs). The prodigious outflows from these systems, most commonly manifesting as large-scale relativistic jets, e.g. M87, are driven by accretion onto the SMBH from its host environment. There are also lower-luminosity AGN, in particular our Galactic Centre BH candidate Sgr A*, which although being significantly less luminous than many of its feeding cousins, provides an ideal testbed for BH science, in particular for studying the near-environment of black holes, from the accretion flow all the way down to the event horizon-scale. However, the issue as to whether astrophysical black holes actually exist is by no means a closed matter. Recent technological advancements in very-long-baseline-interferometry (VLBI) have enabled astrophysical sources, in particular Sgr A* and M87, to be imaged on angular scales of a few micro-arseconds, thereby providing the potential to resolve the event horizon. Indeed, recent observations of Sgr A* with the Event Horizon Telescope (EHT) have revealed structure on horizon-scales. The detection and measurement of the back hole "shadow" is expected to enable the existence of astrophysical black holes to be verified directly. Whilst in theory the mathematical description of this shadow is straightforward, in reality its observational appearance is strongly-dependent on the (thermo)dynamics of the surrounding accretion flow, which on event horizon-scales is highly turbulent. In this talk I will begin with an overview of the motivation and observational efforts regarding resolving the Galactic Centre region and Sgr A*. Next, I will discuss some of our recent theoretical efforts within the BlackHoleCam and EHT collaborations to model the black hole shadow and the electromagnetic emissions emanating from the surrounding accretion flow. By exploiting recent developments in numerical simulations of general-relativistic (GR) magnetohydrodynamics (MHD) and polarised GR radiation-transport (GRRT), we can now model very accurately the observed emission from accreting SMBHs, providing a foundation on which to both compare with and interpret astronomical observations and understand the physical properties of the black hole and its environment. Furthermore, whilst it is anticipated that Sgr A* is a spinning Kerr BH, other BH solutions exist, both within GR and in alternative theories of gravity, which cannot presently be ruled out. I will also present recent results from GRRT calculations of GRMHD simulations in such alternative theories, with the aim of helping interpret upcoming observations of Sgr A*, testing the Kerr BH hypothesis and potentially excluding (or at least constraining) other BH solutions and theories of gravity. |

The program announcement can be found here.

**RTG Colloquium**

**University of Oldenburg**

Where: University of Oldenburg

When: 16.05.2018

Confirmed speaker:

**Eugenia Buffo**(Bremen) - Why do Things Fall?**Sourabh Nampalliwar**(Uni Tübingen) - Testing Einstein's gravity with X-rays - Abstract**Daniela Pugliese**(Opava, Czech Republic) - RADs-Ringed accretion disks and the influence of magnetic field in multi-accreting events - Abstract**Roberto Tanzi**(Bremen) - Quantum Signatures Of Area-Metric Deviations From A Metric

A preliminary program can be found here.

A map highlighting the bus stop (*Universität*), the building A14 where the talks are being held, the Mensa and the restaurant for the dinner can be found here.

14. May 2018

**COLLOQUIUMPhysical Colloquium**

Location: | University of Oldenburg, Campus Wechloy, W2 1-148 |

When: | 14.05.2018, 16:15 - 18:00 |

Speaker: | PD Dr. Tanja Mehlstäubler (Physikalisch-Technische Bundesanstalt, Braunschweig) |

Title: | The Many-Body-Physics of Ion Coulomb Crystals |

Abstract: | Ion traps are a versatile tool for a broad range of applications, such as quantum information and optical clocks. They offer a well-controlled experimental environment in which single ions can be stored and manipulated. If the ions are cooled to energies lower than the potential energy of the Coulomb system, they form crystals, which can be used as quantum simulators or emulators for non-equilibrium statistical physics. A great advantage of trapped ion crystals is the in-situ access to the dynamics of the atomic particles, which are often not accessible in the emulated system. We emulate the boundary of two atomically flat solids with a self-assembled ion Coulomb crystal in the zigzag phase and study the nanofriction between these back-acting ion chains. In this system, we study second-order phase transitions and the formation and dynamics of topological defects. |

The full announcement can be downloaded here.

# April 2018

18. April 2018

**RTG Colloquium**

**University of Bielefeld**

Where: University of Bielefeld

When: 18.04.2018

Confirmed Speaker:

- Patric Hölscher (Bielefeld) - Journals’ Club - "Astrophysical Gravitational Waves in Conformal Gravity"
- David Langlois (Paris) - "Dark energy and modified gravity in generalised scalar-tensor theories"
- Antoine Petiteau (Paris) - "LISA: exploring the Universe using low frequency gravitational waves"
- Dennis Stock (ZARM) - Students’ Seminar - "Asymptotic safety scenario of QG and quantum improved BH spacetimes"

The program can be found here and a map of the *Building X* can be downloaded here.

12. April 2018

**COLLOQUIUMPhysical Colloquium**

Location: | University of Bremen, Building NW 1, Hörsaal H2 |

When: | 12.04.2018, 16:15 - 17:45 |

Speaker: | Prof. Dr. Joachim Ullrich (Präsident der Physikalisch-Technischen Bundesanstalt (PTB) Braunschweig) |

Title: | Physical Units based on Fundamental Constants – Changing with Time? |

Abstract: | In 2018, on the occasion of the 25^{th} meeting of the General Conference on Weights and Measures, CGPM, of the Metre Convention founded in 1875, it is envisaged to redefine the International System of Units (SI). In the future, as outlined by Max-Planck in his famous paper of 1900 postulating the “Planck constant”, it shall be based on fundamental constants of nature, the “defining constants”: the velocity of light, the charge of the electron, the Boltzmann, Avogadro and the Planck constants, the Cs hyperfine clock transition and the luminous efficacy.In the talk I will provide an overview on the progress, challenges and future perspectives of the new “Quantum SI”, illustrated in Fig. 1, and discuss the question on whether or not the fundamental constants are indeed constant in time. New experiments are presently being devised, one of them based on next-generation optical clocks using transitions in highly charged ions that are read out via quantum-logic schemes. They bear the chance to trace potential changes in the fine structure constant α on the level of Δα/α 10-20 per year. |

The full announcement can be downloaded here.

12. April 2018

**COLLOQUIUMPhysical Colloquium**

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

When: | 12.04.2018, 14:15 - 16:00 |

Speaker: | Prof. Dr. Nick Manton (Cambridge) |

Title: | Skyrmions: Fields, Geometry and Nuclear Physics |

Abstract: | The Skyrme model is a field theory of interacting pions. Its nonlinearity and topological features give rise to solitons - Skyrmions – that can be identified with baryons. The basic Skyrmion of baryon number 1, quantized to have spin half, models a proton or neutron. Skyrmions with larger baryon numbers model larger nuclei. A wide variety of geometrically interesting Skyrmion solutions are now known, up to baryon number over 100. Some, but not all, resemble clusters of the Skyrmion version of an alpha particle. I will discuss in some detail research to understand the ground and excited states of Carbon-12, using Skyrmion solutions of baryon number 12. |

The full announcement can be downloaded here.

# February 2018

6. February 2018

**RTG ColloquiumZARM, University of Bremen**

Where: ZARM, University of Bremen

When: 06.02.2018

Confirmed Speakers:

- Eugenia Boffo (Jacobs University Bremen) - "Introduction to Massive Bigravity" - Abstract
- Xiao Yan Chew (University of Oldenburg) - "A Matrix Method for Quasinormal Modes: Schwarzschild Black Holes in Asymptotically Flat and (Anti-) de Sitter Spacetimes" - Abstract
- Tobias Illenseer (CAU, University of Kiel) - "Self-Similar Models of Accretion Disks" - Abstract
- Audrey Trova, ZARM, Bremen - "Equilibrium of charged perfect fluids in rotation around a rotating compact object"

Please find a preliminary program here.

# January 2018

29. January

**COLLOQUIUMPhysical Colloquium**

Location: | University of Oldenburg, Campus Wechloy, W2-1-148 |

When: | 29.01.2018, 16:15 - 17:45 |

Speaker: | Prof. Dr. Helene Götschel, Fachgebiet Gender MINT, Hochschule Hannover |

Title: | Looking at Physics from a Gender Studies Perspective |

Abstract: | Gender Studies not only look at the situation of women in science and technology. Moreover, they analyze the social constructions and cultural representations of gender in STEM fields. They focus on human actors and workplace cultures in physics as much as on the image of physics and the production of knowledge in science. Using methods and tools from the humanities and social science, gender studies look e.g. at communication in research labs, educational settings at universities, behavior of physicists at conferences, physics textbooks and the representation of physics in TV series or theatre plays. After a short introduction to gender studies and the concept of gender, the main focus will be the presentation of selected research findings on gender and physics. Finally we discuss, why it could be helpful to know these research results to develop contemporary physics. |

# 2017

# December 2017

13. Dezember 2017

**RTG COLLOQUIUMOldenburg**

Where: University of Oldenburg, W30 00-033

Confirmed Speakers:

**Pasquale Bosso**(University of Lethbridge) - “Quantum Gravity Phenomenology from the Generalized Uncertainty Principle”**Matthias Hanauske**(University of Frankfurt) - "Gravitational-Wave Astronomy and the Internal Properties of Hypermassive Neutron Stars in the Context of the Long-Awaited Event GW170817"**Petya Nedkova**(Sofia University) - Distorted black holes: exact solutions in an external magnetic field and local shadows**Vojtech Witzany**(ZARM, University of Bremen) - “An informal introduction to black hole accretion - part 2”

Please find the program here.

The dinner will take place at the restaurant Ali Baba (Ammerländer Heerstraße 120). It is about 1.8 km away from the W30 building where the colloquium will take place. You can take the bus 306 at 17:52 and at 18:07 from the University stop 'Carl-von-Ossietzky-Straße' to the stop 'Universität'. It is also possible to walk, it would take about 20 minutes, a map with directions can be found here.

Here you can find a very nice video of a Neutron Star Merger Dance: Link

# November 2017

15. November 2017

**RTG COLLOQUIUMBremen**

Where: Jacobs University Bremen

When: 15.11.2017, 10:00 - 17:30

Confirmed Speakers:

**Dr. Olga Kichakova**, ZARM - Center of Applied Space Technology and Microgravity -*"Test of General Relativity with Galileo satellites 5 and 6"***Marvin Pinkwart**, Jacobs University Bremen -*"Mathematical Gauge Theory"***Dr. Harold Steinacker**, University of Vienna -*"From matrix models to quantized geometry, gravity and cosmology”***Dr. Alexander Westphal,**DESY, Hamburg -*"Towards Dark Energy & Inflation in String Theory"***Vojtech Witzany,**ZARM, Bremen -*"Instability, turbulence and enhanced transport in accretion disks"*

A preliminary program can be found here.

# July 2017

05. July 2017

**RTG COLLOQIUMBremen**

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

Student's Seminar: **Zahra Motahar** (University of Oldenburg) "Neutron stars in scalar tensor theory"

Journals’ Club:** Dennis Philipp** (ZARM, University of Bremen) Paper by H. Quevedo, "Exterior and interior metrics with quadrupole moment", arXiv:1003.4344 [gr-qc] - Abstract

A preliminary program can be found here.

# May 2017

31. May 2017

**RTG COLLOQUIUMHannover**

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 COLLOQUIUMBielefeld**

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 |