Cosmic neutral hydrogen as tracer of the large scale structure of the Universe

The nature of the most abundant components of the Universe, dark energy and dark matter, is still to be uncovered. The study of the matter distribution in the Universe and of its clustering properties may reveal key insight on the origin of these phenomena. However, matter is not directly observable, but can be mapped through tracers. In this study we consider neutral hydrogen, which pervades space from the time of recombination up to present day. It has a characteristic line emission at around 21cm that being redshifted gives a measure of comic distance and makes it possible to reconstruct the three-dimensional density field over a wide range of redshift and scales. In the upcoming years a new generation of experiments will map the distribution of neutral hydrogen from unresolved galaxies up to the high-redshift Universe, by employing a technique called intensity mapping. In this thesis we assess the potential of the 21cm intensity maps for constraining cosmological models. We analyse competitive and realistic dark energy and dark matter models with state-of-the-art numerical simulations and show how different cosmologies produce distinctive and detectable effects in the 21cm signal. We compute radio telescope forecasts showing how these models will be distinguishable in an unprecedented way. The drawback of 21cm observations is the intrinsic weakness of the signal compared to the expected large foregrounds. Motivated by this, we study the cross-correlation signal between 21cm intensity maps and the transmitted Lyman-alpha forest flux, representing a powerful way to extract more information, by isolating instrumental and astrophysical systematics.