String Cosmology: New Tools to connect String Theory with Cosmological Observations

Cosmology studies the origin of the Universe, and the main features of its evolution until today. The Universe expansion is mainly controlled by gravity, the force described by Einstein's General Relativity. This theory is extremely successful for describing gravitational phenomena that occur at large distances. However, when attempting to describe phenomena occurring at small scales, General Relativity apparently fails. Indeed, the best theories we have to describe short distance physics are based on Quantum Mechanics. But its principles lead to inconsistent results, when applied to gravitational phenomena following the rules of General Relativity. This problem is relevant for Cosmology, when trying to investigate epochs in which the Universe was very young, and then very small, in which quantum mechanical effects are important. For example, in the early epochs of our Universe, it is believed that cosmological expansion has experienced a phase of extremely fast acceleration, called Inflation. During this epoch, quantum effects are decisive to produce small inhomogeneities in the structure of the space-time, that seed large scale structures in the subsequent evolution of the Universe. The aim of consistently conjugate General Relativity with Quantum Mechanics is the main goal that motivated the development of String Theory. This theory introduces very unusual concepts: its consistency requires that our Universe has six dimensions more than the observed ones, that are wrapped on a small space, and unobservable by everyday experience. Also, it predicts the existence of membrane objects, fluctuating in a higher dimensional background, where the particle constituting our Universe can be localized. In order to check these beautiful ideas, however, we need to probe phenomena happening at extremely small scales. I find all these ideas extremely exciting, so I decided to study the relation between string theory and cosmology, with the main aim to find new ways to test string theory by cosmological observations. As explained before, phenomena occurring at early epochs, when the Universe was extremely small, are affected by short distance physics, and then possibly by String Theory. Inflation is the most important example: it is a process that leaves clear imprints in the pattern of the observed CMB radiation, and provides initial conditions affecting the subsequent evolution of the Universe. For these reasons part of my research is devoted to study models of inflation in String Theory. They can have peculiar properties, since they use many of the ingredients that the theory offers. The study of string models of inflation, then, offers unique opportunities to reveal effects that are characteristic of String Theory, and allows to test its ideas by means of cosmological observations. More specifically, the main objectives of my research activity can by summarized in the following. First, I plan to develop the theoretical tools allowing to compare observational results with string motivated cosmological models. This is a broad subject that requires the elaboration of existing techniques aimed to analyze the evolution of cosmological fluctuations, in order to apply them to string motivated cosmological models. Then, I will try to individuate the most promising cosmological models, based on string theory, that offer the best opportunities for being testable with future cosmological observations. Once this has been done, I will try to elaborate those models, in order to render them fully satisfactory from a theoretical point of view, and study in detail their predictions for cosmological quantities that will be tested by observations.