Searching for our origins in the Cosmic Microwave Background polarisation

This Thesis is the final product of the work I have carried out in the last three years of my PhD journey. The central topic of this manuscript is the astrophysical observable known as the Cosmic Microwave Background (CMB) radiation, one of the pillars on which our knowledge of the Universe is built. I will describe some of the huge collective effort being carried out every day by the scientific community in order to improve our understanding of the CMB and shed light on the mysteries of Nature. Nowadays, the frontier of CMB science is represented by its polarised signal, which contains crucial information about the very first phases of the history of the Universe. In particular, current and future CMB experiments are focusing on the hunt for primordial B-modes, a peculiar pattern thought to be induced by gravitational waves in the primordial plasma, possibly generated by an inflationary phase happened immediately after the birth of the Universe. Usually, this signal is parametrised through the tensor-to-scalar ratio r. The extraction of this very faint signal requires the crafting of extremely sensitive instrumentation, with a complete control over experimental systematic, as well as a full characterisation of the contamination due to the polarised emission of our Galaxy. In this field, the instrumental state-of-art is represented by the Simons Observatory (SO), a newly built CMB experiment located in Chile which is currently taking data of the sky. I have been a member of the SO collaboration since 2023; in these years, I participated to the development of the data analysis pipeline, whose main task is to measure the primordial B-mode signal from the observed multi-frequency sky maps. I contributed to the implementation of cosmological datasets from other CMB experiments (Planck, WMAP, S-PASS) in the pipeline, in order to combine SO data with external probes for an improved characterisation of the Galactic foregrounds. Within the SO project, I have also contributed to the production of several sets of simulations, which have been used both for the investigation of instrumental systematics, and for the quantification of the uncertainties in the measurement of the statistical properties of the CMB and of cosmological parameters. The continuous improvement in experimental sensitivity will bring the community closer to a potential detection of primordial B-modes. The measurement is extremely challenging and the contaminating contributions could bias the inference of the cosmological signal, possibly giving rise to a false detection. To avoid this scenario, we should be prepared for it with a set of procedures aimed at rejecting or confirming an eventual detection of primordial B-modes. Therefore, this kind of null tests is becoming more important every year as our experiments get better and better. In the last two years of my PhD, I worked on the development of robustness tests for the detection of the tensor-to-scalar ratio from CMB observations, to be applied in current and future CMB experiments. I studied the possibility of using higher-order statistics, like Minkowski functionals and scattering transform statistics, to determine if a potential B-modes detection is contaminated by Galactic foregrounds or systematics. I used scattering transforms to build and provide to the community a working robustness test methodology applicable to current and future experiments, which represents a step forward in the measurement of primordial inflation.