Towards optimal calibration in 21-cm cosmology with LOFAR
In her dissertation, Stefanie Brackenhoff investigates 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. This spectral line can tell us how the first stars and galaxies illuminated and ionised the Universe about 13 billion years ago. Although LOFAR is extremely sensitive and has collected thousands of hours of data, the expected 21-cm signal remains buried beneath by “excess variance”: contaminating signals from other astronomical sources, the atmosphere, and the telescope itself. Through extensive simulations, Brackenhoff examines which of these disturbances are the main culprits and how we can better correct for them.
Brackenhoff's key results are:- The ionosphere (atmosphere) does not have a major impact on the final outcome.- Bright radio sources far from the pointing direction can cause problems because they are difficult to model accurately.- A new calibration method developed in this dissertation improves the removal of these contaminating sources in simulations and offers leads for further optimisation. The improvement on real data is limited.
Brackenhoff shows that the search for the 21-cm signal still faces challenges, but also that we are progressing: a new generation of telescopes and new analysis techniques are bringing us closer to a full understanding of the early Universe.