Gravity, thermodynamics and cosmology

There are two types of fundamental forces in Nature: those responsible for particle interactions at subatomic scales and those responsible for the large scale structure of the universe. The latter is described by Einstein's General Theory of Relativity (GR) and the former by Quantum Field Theories (QFTs) such as the Standard Model. Einstein's theory is conceptually simple, but is classical and breaksdown when the force of gravity is strong, as it is at very small scales, whereas QFTs are effective theories which ignore the gravitational interactions and which cannot be trusted at very high energies. During the last three decades, String Theory has emerged as a conceptually rich theoretical framework reconciling both GR and QFT. In particular, it is a theory of quantum gravity and my research programme is firmly focused on a wide range of quantum gravitational aspects of String Theory.String theory challenges the geometrical notions of spacetime on which GR is predicated. At very small( stringy ) scales the nature of spacetime is believed to be fundamentally different from GR---its continuous structures thought to be replaced by discrete, algebraic structures. Black holes are ideal candidates to study this transition. They are thermodynamical objects allowing a classical description in terms of Einstein's equations. Just as a gas allows a thermodynamical description in terms of temperature, pressure and volume, but it also has a description in terms of statistical properties of its constituent molecules, I look for the atoms of spacetime responsible for the entropy of the black holes. I plan to extend our current understanding on supersymmetric black holes, to less supersymmetric systems, and eventually to non-supersymmetric, and thus more realistic, ones. Any progress in the latter direction would be of extraordinary importance in our field.The expansion of the universe is incontrovertible. Since its creation after the Big Bang, the universe has been in permanent evolution. One may wonder whether a physical test of string theory will come from the realm of cosmology. I propose to study both theoretical and experimental aspects of this expectation. At a formal level, I want to develop the poorly understood string theory tools to deal with these physical situations in order to be able to answer questions such as Does string theory resolve Big-Bang-type singularities? ---a natural question in view of the fact that string theory is known to resolve other types ofsingularities. I also want to explore whether our ideas on black holes can help us understand the potential thermodynamical nature of the universe accessible to a classical observer. At an experimental level, I want to derive observable cosmological consequences of string theory in an effort to falsify it or at least to constraint the landscape of possible vacua explaining the universe we live in.The main beneficiaries from my work would be the members of the scientific community working on related topics to the ones developed in this proposal : cosmology & astrophysics, QFT, GR, stringtheory, statistical mechanics, information theory and mathematical physics. Any human being interested in knowing about the structure of the world in which we live, both from the measurable point of view, but also from a much more conceptual point of view, would find my work of interest.