Gravitation theory: perturbations of gravitational systems in string theories

Abstract

General relativity is the best suited theory describing gravitational phenomena. It is successfully used in a vast class of situations, from the description of isolated stars, at distances typical of the solar system, all the way up until cosmology, at the largest observable distances. The study of perturbations in General Relativity is a formidable instrument to study actual problems and comprehend the cosmos from the structure of astrophysical objects (where, among other things, gravitational waves are predicted) until the possibility of describing physical processes at the beginning of the cosmic history. Quantum Gravity, on the other hand, allows knowledge about a unified theory of all interactions, thus rendering possible the understanding of the initial explosion itself, what cannot be done in the context of General Relativity only. In this case, String Theory is the correct formulation, such that we have to test its validity. One of the most important problems in this context is that of the cosmological constant, extremely small, but explain the accelerating universe, as experimentally seen by means of the supernovae observations. Thus, we search the understanding of questions involving Einstein Gravity and its quantization, as well as its applications, together with Quantum Field Theory, in the cosmological context. The study of gravitational waves is closed related to the temporal evolution of small perturbations in the gravitational field of a star (or a black hole, or else another large astrophysical object). The so called Quasi Normal Modes are, solutions of the wave equations in the presence of a background gravitational field, and dominate the propagation and evolution of small perturbations. It is thus essential to handle them with confidence in order to be able to foresee the caracteristical signals that passed by, or originated in far astrophysical objects. In order to probe string theory as a result of facts it foresees, which are observationally important, is the most important aim to be achieved. Thus, it is important to understand string theory solutions that play an important role in cosmology and in the formation of the universe, as well as in its description as a whole. This is done by means of the Theory of Branes. A whole series of questions are posed in such a context, in particular that of the existence of shortcuts for gravitational waves by means of passages through the extra dimensions, outside of the brane itself. Beside that, one of the most important questions is that of the description of a very small but positive cosmological constant. This implies that it is crucial to understand the de Sitter space and the Conformal Field Theory at the border of the universe. Theoretical as well as observational cosmology present some important and difficult challenges, in spite of the recent progress. A good example is the question of the inflationary theory, that solves some problems while bringing new ones, such as the fact that there are many different inflationary scenarios, or the question of physical theories at extremely large energies. We are thus searching quantum effects and observational consequences in the very early universe. Another area of interest of our group, closed related to the above mentioned question of String theory and the Cosmological Constant, is the question of Dark Energy, or Dark Matter. Recently, the observation that the universe passes by a period of accelerated expansion caused an uproar in the theoretical community. The confirmation of such an observation means that the major part of the universe is built out of a strange kind of matter with negative pressure, nicknamed "quintessence". Theories describing such matter are still quite rough, and the -corresponding models very rudimentary. Much labour has to be done obtaining more natural theories that naturally describe such a new dark matter, and to foresee the observational consequences. (AU)