Relics from the early universe: primordial rgavitational waves, axions and neutrinos

Over the last decades our knowledge of the Universe has reached an unprecedented level of accuracy. The observations of the Cosmic Microwave Background and the large scale structure of the Universe opened the so-called epoch of precise cosmology, enabling us to test with increasing precision several aspects of fundamental physics, from the first principles of the cosmological model to global theoretical scenarios beyond General Relativity and the Standard Model of elementary particles. The research subject of this PhD thesis goes exactly in this direction: working at the interface of cosmology, gravitation and (astro)particle physics, I analyze cosmological and astrophysical observations to identify, characterize and constrain possible hints for new physics in light of their implications for the Early Universe