Phd in ASTRONOMY ASTROPHYSICS AND SPACE SCIENCE

Thesis title: Cosmology from the cross-correlation of CMB lensing and Large-Scale Structure observables

Over the past 50 years, cosmology has transitioned from a data starving science to a continuously evolving field of research. Today, we are in the epoch of precision cosmology, where we can explore the Universe with unprecedented accuracy. We can employ observations which aim at mapping the Large Scale Structure (LSS) of the Universe across various wavelengths, from space and ground-based experiments such as those expected from Euclid, SKA Observatory, EMU, the Vera Rubin Observatory, the Dark Energy Spectroscopic Instrument and many more. Additionally, we can use measurements of the Cosmic Microwave Background (CMB), a relic radiation from the early stages of the Universe that has been playing a pivotal role in Cosmology since its discovery in 1964. In the past years, ground-based and balloon-borne experiments together with satellite missions like COBE, WMAP, and Planck have extensively observed the CMB temperature anisotropies. The scientific objectives of upcoming experiments like the Simons Observatory, CMB-S4, and LiteBIRD are twofold. Firstly, these experiments aim to provide new observations of the CMB polarization anisotropies, offering insights into the early phases of the Universe's history. Additionally, they seek to significantly enhance the precision of measurements related to CMB secondary anisotropies, those arising from the interplay of CMB photons with the LSS of the Universe. This is particularly the case for the CMB lensing effect, a key observable in cross-correlation analyses with the LSS. The work presented in this thesis is situated within the framework of cross-correlation analyses, with a specific focus on exploring the interplay between CMB lensing data and the distribution of two high-redshift tracers: radio galaxies and quasars. By analysing state-of-the-art data sets, from wide and deep radio surveys, and from a novel catalogue of quasars, we successfully obtained the first detection of their cross-correlation with Planck CBM lensing. Our results demonstrate that cross-correlation analyses are less prone to systematic effects than the standard auto-correlation ones and, when combined, they are effective in characterising the properties of LSS tracers. These properties include their relation with the underlying dark matter distribution and their spatial distribution. Moreover, using this approach, we obtained competitive constraints on the amplitude of matter fluctuations, achieving one of the highest redshift measurement to date. In this thesis, we analysed not only cross-correlations from current data but also forecasted the cross-correlation science achievable with future experiments, with a particular focus on primordial non-Gaussianity. Overall, this thesis demonstrates the increasingly pivotal role of cross-correlations in cosmological research, especially in anticipation of forthcoming experiments.