Congratulations to Dr. Markus Rexroth for obtaining his PhD

According to Albert Einstein's General Theory of Relativity, spacetime is strongly distorted by massive objects like galaxies and galaxy clusters. As a result, these objects act like a lens and background galaxies can be strongly deformed or even multiply imaged. This effect is called gravitational lensing. It is a pillar of modern cosmology, as it permits astronomers to map the distribution of Dark Matter in the cosmos and to study Dark Energy and the early universe.

In his PhD thesis, Dr. Rexroth develops advanced techniques to further push the limits of gravitational lensing. The techniques cover theory, observation, and computation. On the theoretical side, he proposes a new technique to address a persistent challenge in gravitational lensing, the mass-sheet degeneracy. On the observational side, he develops a software pipeline for ESO's (http://www.eso.org/public/ ) new MUSE instrument (http://www.eso.org/sci/facilities/paranal/instruments/muse.html ). This pipeline permits the determination of the redshifts and thus of the distances of astronomical sources. Subsequently Dr. Rexroth uses this tool to measure the distances to lensed galaxies which are several billion light years away from us. He combines these results with data from the Hubble Space Telescope (make link: http://hubblesite.org/ ) to improve the mass map of the galaxy cluster MACSJ1931, which acts as a strong gravitational lens. On the computational side, the thesis investigates High Performance Computing techniques like Graphics Card acceleration and Mixed Precision algorithms and demonstrates that they greatly improve current gravitational lensing software. The resulting speedup will be necessary to rapidly process high quality data from upcoming telescopes like the James Webb Space Telescope (https://www.jwst.nasa.gov/ ). Finally, Dr. Rexroth investigates the measurement of weak lensing flexion in Hubble Space Telescope images, which could lead to higher resolution maps of the mass distribution in galaxy clusters.

The public PhD thesis defense and the awarding of the PhD diploma will take place on January 26 2018 at 15h00 at the Observatoire de Genève.