PhD ceremony Stefanie Brackenhoff

On Tuesday 16 december 2025, Stefanie Brackenhoff defends her thesis called

Towards optimal calibration in 21-cm cosmology with LOFAR

Summary of her thesis:
In this dissertation, I investigate why, despite years of effort, it has still not been possible to measure one of the weakest yet most promising signals from the early Universe with the Dutch radio telescope LOFAR: the 21-cm line of neutral hydrogen. Although LOFAR is extremely sensitive and has collected thousands of hours of data, the expected 21-cm signal remains buried beneath by “excess variance”: systematic errors that obscure the signal of interest. Through extensive simulations, this dissertation examines which of these disturbances are the main culprits and how we can better correct for them.

I exclude the ionosphere as a primary contributor to excess variance, test and develop a new spectral regularisation method for calibration of off-axis sources, and outline a framework for suppressing such sources during calibration. Furthermore, I developed a new open-source simulation package, SHIMMERR, which enables bottom-up, physically-motivated modelling of beam deviations for any hierarchical interferometer.

Overall, the search for the 21-cm signal still faces challenges, but we are also progressing. The imperfect modelling of primary beams remains an obstacle to detecting the 21-cm signal from the epoch of reionization, but with calibration schemes that explicitly account for beam-modelling errors, we will be able to take a big step forward.

Link to Stefanie's thesis

If available, you can follow the PhD ceremony via this livestream