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On May 20 2025, Stefano Schmidt has successfully defended his thesis. He accomplished his PhD at the Institute for Gravitational and Subatomic Physics (GRASP), Gravitational Waves Group. The defense has taken place in the Academiegebouw in Utrecht.

Summary PhD thesis

Searching for Precessing Black Hole Binaries in Gravitational-wave Data

When two black holes orbit each other and eventually collide, they can send out ripples in space called gravitational waves. If the black holes are spinning in different directions instead of neatly aligned, their orbits can wobble or precess—a bit like a spinning top. Studying these wobbling systems could help scientists learn more about how far away the collision happened and what the black holes were like before they merged.

However, spotting these wobbling signals in the data is very tricky. Most search methods assume the black holes are aligned, so they often miss or misinterpret precessing signals. In this study, we made improvements to a key search tool used in gravitational wave detection to better find these harder-to-detect, precessing black hole signals.

After testing the new version of the tool, we searched for this special type of black hole collision in data from the LIGO and Virgo detectors. In the end, we picked up 30 gravitational wave events that had already been found by other studies, but we didn’t discover any new ones. This result helped us estimate how often these rare, wobbling black hole mergers might happen. Based on our findings, it seems unlikely that there’s a large hidden group of these precessing systems that other searches have missed.

Supervisors: prof. dr. C.F.F. Van Den Broeck and dr. S.E. Caudill