Probing the ΛCDM Universe and beyond with present and future 21cm Intensity Mapping surveys

The measurement of the large-scale distribution of neutral hydrogen in the late Universe, obtained with radio telescopes through the hydrogen 21cm line emission, has the potential to become a pivotal cosmological probe in the upcoming years. This thesis presents a comprehensive analysis focused on forecasting the constraining power of 21cm intensity mapping observations, exploring a wide set of cosmological models, from standard \lcdm to modified gravity, dark energy, and massive neutrinos. We developed a pipeline to construct mock data sets for a variety of present and future 21cm observations, with an emphasis on the SKA Observatory and its precursor MeerKAT planned and ongoing surveys. We investigated the interplay between intensity mapping and other cosmological probes, such as the cosmic microwave background, galaxy clustering, and gravitational waves, providing valuable insights into the potential synergies between these different observables. To assess the constraining power of such observations we conducted a Bayesian analysis implementing a likelihood code to work with different 21cm probes and exploring the posterior with Monte Carlo Markov Chain methods. The numerical tools we develop are integrated with widely used codes and prepared to be exploited for upcoming observations and their cosmological analyses. As a proof of concept, the analysis pipeline is validated with the recent MeerKAT intensity mapping measurements in cross-correlation with galaxy clustering data, yielding encouraging results consistent with forecasted outcomes. The main findings of this thesis suggest a promising constraining power of 21{\small cm} observations when considered both alone and combined with other probes. The joint analysis with cosmic microwave background observations significantly narrows the parameter constraints, leading to precise estimates of pivotal cosmological parameters like $\Omega_ch^2$ and $H_0$. The tomographic nature of 21cm intensity mapping measurements further improves the estimated errors, potentially offering a powerful new probe to constrain beyond-$\Lambda$CDM scenarios. In conclusion, this thesis extensively tests the value of 21cm intensity mapping as a cosmological probe in the standard model scenario and beyond. It delivers tools for the preparation and analysis of current and future cosmological observations, taking part in the scientific community's effort to pave the way for future groundbreaking observations in cosmology that will potentially provide a deeper understanding of the fundamental properties of our Universe.