Böcker av Oliver Piattella

Cosmology has become a very active research field in the last decades thanks to the impressing improvement of our observational techniques which have led to landmark discoveries such as the accelerated expansion of the universe, and have put physicists in front of new mysteries to unveil, such as the quest after the nature of dark matter and dark energy. These notes offer an approach to cosmology, covering fundamental topics in the field: the expansion of the universe, the thermal history, the evolution of small cosmological perturbations and the anisotropies in the cosmic microwave background radiation. Some extra topics are presented in the penultimate chapter and some standard results of physics and mathematics are available in the last chapter in order to provide a self-contained treatment. These notes offer an in-depth account of the above-mentioned topics and are aimed to graduate students who want to build an expertise in cosmology.

Unified Dark Matter models aim to describe thedynamical properties of Dark Matter and Dark Energyas different aspects of the same entity which is, inthe simplest case, embodied in a perfect barotropicfluid.Such models, though successful in passingobservational tests on the expansion rate of theUniverse, may present drawbacks in their perturbativestability. Indeed, to be in agreement withobservation, they must possess a sufficiently smallspeed of sound. In this thesis, two counterexamples to this generalconclusion are put forward. First, addressing the generalised Chaplygin gas, onefinds strong hints on the viability of the model alsowhen its asymptotic speed of sound is sufficiently large.Second, a new class of Unified Dark Matter models isintroduced. They are characterised by a fasttransition from a matter-dominated era to a phasewith constant negative pressure. It turns out that, if the transition is fast enough,these models may predict a satisfactory structureformation process and cosmic background radiationanisotropies pattern.