## Bernd Brügmann

• Group: Numerical Relativity
• Office: Abb. 211
• Phone: +49 3641 947120
• Email: Bernd.Bruegmann(at)uni-jena.de

#### Curriculum Vitae:

• 1987. M.S. in Mathematics, Syracuse University, USA
• 1993. Ph.D. in Physics, Syracuse University, USA
• 1993 - 1995. MPI for Physics, Munich, Germany
• 1995 - 2002. MPI for Gravitational Physics, Potsdam, Germany
• 2002 - 2004. Associate Professor, Penn State University, USA
• 2004 - ... Professor, Chair for Gravitational Theory, University of Jena, Germany

#### Publications and Teaching:

Guide to the M.Sc. in Physics with focus on "Gravitational- and Quantum Field Theory" at the University of Jena

Teaching
Papers

#### Scientific Interests:

My main scientific interest is currently the two body problem for two black holes. The classical two body problem in physics is formulated for two point-like masses. In Newtonian physics this problem has the well-known solution in terms of Kepler orbits, but do we really understand how two masses move in their mutual gravitational field? As surprising as it may seem at first, the answer to this question is not really known, even if we restrict ourselves to classical, non-quantum physics and to no more than two bodies.

A modern treatment of the two body problem must be founded on Einstein's theory of general relativity, which by all accounts is an extremely successful description of the gravitational interaction in the classical regime. In the limit of velocities much below the speed of light and for weak gravitational fields, Newton's theory of gravity is an excellent approximation and with post-Newtonian approximations we can obtain good approximations for about one tenth of the speed of light. However, the question has to be asked how for example two black holes or neutron stars, which are prime examples for extreme gravity, move around each other when they approach each other at relativistic velocities.

Is there perhaps in general relativity a solution to the equations of motion for two black holes which is as simple and simultaneously as astrophysically relevant as the Kepler orbits of Newtonian physics? The answer is, in a rather satisfying manner, no! The motion of two masses generates gravitational waves, which remove energy and momentum from the system, such that a Kepler ellipse is no longer a stable solution for an orbit. Two black holes will rather move on an inward spiral, first slowly, then faster and faster, until they collide and merge to a single black hole.

This loss of stability in the Einstein equations is by no means tragic. Quite to the contrary, a growing international community of gravitational wave researchers hopes that gravitational waves can be detected in order to establish an entirely new branch of astronomy, namely gravitational wave astronomy. In fact, the very first direct detection of gravitational waves happened on September 14, 2015, and there is strong evidence that the source was a binary black hole inspiral and merger.

My own work has been focussed on numerical simulations of black hole and neutron star space times. The two-body problem of general relativity in the strong field regime is still not satisfactorily solved, although there has been significant progress in the last few years. The numerical solution of the full Einstein equations (in their standard form ten non-linear, coupled partial differential equations) is a very complex problem, and for black holes there is the additional challenge to deal with the spacetime singularities that are encountered in the interior of black holes. Given initial data for configurations of two black holes or neutron stars, the time evolution of the system is computed.

Astrophysical considerations make it likely that the last few orbits of two compact objects like black holes or neutron stars and the ensuing collision are the source of particularly strong gravitational waves. The final phase of a binary system of two black holes with a total mass of about 30 solar masses constitutes one of the most likely sources for the gravitational wave detectors GEO and LIGO based on the frequency dependent sensitivity of these interferometric detectors. The planned space based detector LISA will detect gravitational waves from the mergers of supermassive black holes at the center of galaxies, to which our numerical simulations apply equally well. This area of numerical relativity requires the development of new analytical and numerical methods, as well as their implementation on supercomputers.

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