The large-scale distribution of matter in the universe is used to test our cosmological models. To do this, we primarily use baryon acoustic oscillations (BAO) measured in the two-point correlation function of this distribution. However, the entire matter field contains information at various scales, which allows us to place tighter constraints on our models than BAO alone. At redshifts z > 2, the best probe of this matter distribution is the Lyman-alpha forest, a set of absorption lines measured in the spectra of distant sources. The large-scale DESI spectroscopic survey has collected approximately one million of these spectra. Using the partial data release “DR2”, we have already measured the BAO with a precision of 0.7%, thereby strongly constraining the expansion rate of the universe during the first billion years of its evolution.

The goal of this thesis is to analyze the complete set of DESI large-scale Lyman-alpha data to obtain the best possible constraints on cosmological models. To this end, in the first stage, the student will apply, for the first time, a method known as “reconstruction”, which improves the precision of the BAO measurement by utilizing information from the matter density field. Later in the thesis, building on similar efforts conducted in our group using DESI galaxies, the student will implement a new method known as simulation-based inference: in this approach, the entire matter field is used directly to estimate cosmological parameters, particularly dark energy. The student will thus make a significant contribution to DESI’s final cosmological measurements using Lyman-alpha.

This thesis should preferably be preceded by an internship.