# Research Groups

## Astrophysical and Cosmological Relativity (AEI)

**Prof. Dr. A. Buonanno (Director), Dr. J. Gair (Group leader), Dr. H. Pfeiffer (Group leader), Dr. J. Steinhoff (Group leader), Dr. Miguel Zumalacarregui (Group leader)**

The observation of gravitational waves from coalescing binary black holes and binary neutron stars provides the unique opportunity to probe fundamental physics, dynamical gravity and matter under extreme conditions. Scientists in the Astrophysical and Cosmological Relativity division predict accurate waveform models of binary systems in General Relativity by combining fast, but approximate analytical methods, with exact, but time-consuming numerical simulations on high-performance computers. They employ the computed waveform models to detect gravitational wave signals, infer astrophysical and cosmological properties of the sources, and test General Relativity in the highly dynamical, strong-field regime. Waveform models developed by members of the division were employed to identify the gravitational waves observed by Advanced LIGO and Virgo. Researchers in the division are members of the LIGO Scientific Collaboration, and the Laser Interferometer Space Antenna (LISA) Consortium. The division also participates in building the science case for the next generation of gravitational-wave detectors on Earth.

## Computational Relativistic Astrophysics (AEI)

**Prof. Dr. M. Shibata (Director), Dr. K. Kiuchi (Group leader)**

In 2017 astronomers observed both gravitational and electromagnetic waves from the merger of two neutron stars for the first time. This event inaugurates a new era of multi-messenger astronomy combining gravitational-wave and electromagnetic observations. Together, the complementary methods will enhance our understanding of extreme astrophysical events. The Computational Relativistic Astrophysics division focuses on numerical relativity simulations of astrophysical events that generate gravitational waves, neutrinos and strong observable electromagnetic signals, solving Einstein’s equations of general relativity on high-performance computers. These simulations play a crucial role in predicting accurate gravitational waveforms for the search in the detector data and for exploring high-energy phenomena such as bursts. The scientists study mergers of binary NSs and mixed binaries — binary systems of a black hole and a neutron star — as well as stellar core collapse that form black holes. They investigate the merger and post-merger phases, elucidating the physical conditions to produce signals of the kind detected in association with the first binary neutron star merger observed by Advanced LIGO and Virgo.

## Quantum Field and String Theory (Humboldt University Berlin)

**Prof. Dr. J. Plefka**

The quantum field and string theory group at Humboldt University conducts research on quantum field theory beyond the standard model, string theory and quantum gravity. A focus of activities in the past years was on innovative methods for the evaluation of scattering amplitudes in gauge theory and gravity using on-shell recursive methods. Important results in this domain include the complete analytic construction of all massless QCD tree-level amplitudes, the discovery of a hidden infinite dimensional symmetry in supersymmetric gluon scattering and reduction techniques for rewriting graviton scattering amplitudes in terms of gluon and matter amplitudes employing the newest tools of the amplitude program. In the context of this IMPRS, investigations are planned to apply these innovations to the effective field theory approach of the gravitational twobody problem. Concretely, the powerful and yet still mysterious color-kinematic duality relating gravity to a double copy of Yang-Mills theory shall be applied to advance in this domain. Naturally, in this program also extensions of Einstein’s theory including dilatons, conformal gravity and higher derivative terms will be pursued.

## Mathematical Physics of Space, Time and Matter (Humboldt University Berlin)

**Prof. Dr. M. Staudacher**

The research group “Mathematical Physics of Space, Time and Matter” at Humboldt-University Berlin led by Dr. Matthias Staudacher concentrates on developing novel approaches to quantum field theory and to string theory. Staudacher is on a split appointment between the Department of Physics and the Department of Mathematics at Humboldt. A focus point are exact methods in four-dimensional gauge theories, following the discovery in the early 2000 that a certain important “model” QFT, the planar N=4 super Yang-Mills theory, is an integrable model, in contradistinction to earlier no-go theorems that QFT cannot be integrable beyond two dimensions. Dr. Staudacher and his group are also experts in the anti-de Sitter/conformal field theory correspondence (AdS/CFT) invented by Maldacena in 1997, a first example of a gauge/gravity duality, where a quantum field theory in flat four-dimensional space is holographically related to a string theory in moving in a curved five-dimensional anti-de Sitter space-time. Here the QFT description does not seem to “know” about gravity, while the string description does. Yet both systems are conjectured to be exactly equal. This leads clearly to a revolutionary new way of thinking about gravity - there is also a description as a field theory.

## Astrophysics (University of Potsdam)

**Prof. Dr. P. Richter, Prof. Dr. T. Dietrich**

Dr. Philipp Richter's research focuses on diffuse gaseous matter in the Universe. Methods of absorption spectroscopy in the UV and in the optical regime are employed to study the amount and distribution of hydrogen and metal ions inside and outside of galaxies at all redshifts and explore the role of this gas for the evolution of galaxies. The absorption-line measurements are complemented with emission observations in the radio, X-ray and optical regime and with state-of-the art numerical hydrodynamical simulations.

Dr. Tim Dietrich's research focuses on the interplay between state-of-the-art numerical relativity simulations and the modeling of gravitational waves and electromagnetic signals for binary neutron star mergers. In the past, numerical simulations have been employed to develop phenomenological descriptions of tidal effects to model the gravitational wave signal emitted during the calescence of two neutron stars. In addition, numerical relativity predictions can also be used to estimate and analyze transient electromagnetic signals detected by high-sensitivity observational telescopes. Combining gravitational wave and electromagnetic wave models and providing a common multi-messenger framework to interpret all observable phenomena of binary neutron star mergers is one of the main research goals at the University of Potsdam.

## Extragalactic Astrophysics and Cosmology (Leibniz Institute for Astrophysics)

**Prof. Dr. M. Steinmetz (Scientific Chairman and Director), Prof. Dr. C. Pfrommer (Section leader), Dr. A. Schwope (Group leader), Prof. Dr. L. Wisotzki (Section leader)**

Dr. Matthias Steinmetz has a long career in research on the formation as well as on the kinematical and chemical evolution of the Milky Way and of galaxies in general. His research includes the performance and analysis of large numerical simulations as well as the exploitation of large imaging and spectroscopic surveys like SDSS, 4MOST and LSST.

Dr. Christoph Pfrommer studies a broad range of topics in cosmology and high-energy astrophysics using methods of computational astrophysics. He is particularly interested in how supermassive black holes and high-energy processes impact the formation and evolution of galaxies and galaxy clusters. To this end he studies transport processes of cosmic rays in magnetized media and plasma processes associated with the propagation of ultra-relativistic gamma rays from blazars, a sub-population of supermassive black holes. Dr. Axel Schwope exploits X-ray surveys with ROSAT, XMM-Newton and eROSITA (launch 2019) to study Isolated Neutron Stars and Compact Binaries. Focus is put on white-dwarf accreting binaries as GW standard candles, as progenitors of SNIa and as sources of the Galactic Ridge X-ray emission. Dr. Lutz Wisotzki has been working for many years on the demographics of Active Galactic Nuclei and the relation with their host galaxies. More recently he expanded his research towards the study of galaxy formation and evolution in general, mainly by combining deep imaging with spectroscopy.