Searching for new physics with CMB precision cosmology

With the advent of large astronomical surveys coming from ground- and space- based telescopes, the field of cosmology has undergone a scientific revolution in the last decades. It is said that we have entered in the era of precision cosmology: not only do we have plenty of measurements from the early and late universe, but also we can use them to test theoretical models with astonishing accuracy. One of such models, describing the universe with a flat Friedman-Lemaitre metric and containing around 5 % baryons, 25 % Cold Dark Matter (CDM) and 70 % dark energy in the form of a cosmological constant (Λ), has shown to provide a remarkable fit to a wide variety of observables. Unfortunately, the nature of its main constituents -dark matter and dark energy - lacks identification. In addition, several experimental discrepancies have emerged in recent years, possibly shedding light on the properties of these mysterious components. On the other hand, the discovery of neutrino oscillations has provided convincing evidence that neutrinos possess non-zero masses, a phenomenon that cannot be explained within the Standard Model of Particle Physics. In my work, I have put new and robust constraints on many different extensions of the ΛCDM paradigm, aiming at explaining some of the present anomalies in cosmology, or to better characterize the properties of the elusive neutrinos. I start confronting a 1-parameter model of Early Dark Energy against a wide variety of data sets, showing that it can resolve the H₀ tension, and that the addition of weak-lensing data doesn't affect this conclusion. In addition, I have made a systematic comparison of seventeen different models which have been proposed to resolve the H₀ tension (including Early Dark Energy), and quantified the relative success of each model using a series of metrics and a wide array of data combinations. Secondly, I have performed a detailed cosmological analysis of a scenario in which dark matter can decay into dark radiation and warm dark matter species, including for the first time a full treatment of perturbations. This study has shown that this model, while unable to ease the Hubble tension, can fully explain the low-S₈ measurement from recent weak lensing surveys. I end discussing updated cosmic constraints on a scenario in which neutrinos can decay into dark radiation while being non-relativistic. I show that this allows to significantly relax the bounds on the neutrino masses, making a potential detection in the laboratory compatible with cosmological constraints. (author)