| String theory is to date the best candidate for the "theory of everything" which would describe all four fundamental interactions, including gravity, on the same footing. The general description of black holes and other spacetimes with horizons remains a key aspect of quantum gravity that must be addressed by a consistent theory, such as string theory. Also, the resolution of singularities is a fundamental problem in a quantum theory of gravity. At the same time, the recent revolution in observational cosmology creates a pressing need to accommodate fundamental issues such as cosmic acceleration and cosmological horizons within the framework of string theory. Therefore, the focus of this thesis is on new aspects of gravitation from a point of view of string theory. In the first part of this thesis, after an introduction in the subject, we deal with the resolution of singularities in string theory. In particular, we describe one of the intrinsically stringy mechanisms to resolve singularities, namely "Enhancon mechanism". We generalize the basic enhancon solution by constructing solutions without spherical symmetry.; The second part of the thesis is devoted to investigating different aspects of holography. The AdS/CFT correspondence is a concrete realization of the holographic principle. Such correspondence is referred to as duality in the sense that the supergravity (closed string) description of D-branes and the field theory (open string) description are different formulations of the same physics. This way, the infrared (IR) divergences of quantum gravity in bulk are equivalent to ultraviolet (UV) divergences of dual field theory living on the boundary. A novel method to renormalize the stress-energy of gravity and provide a measure of gravitational mass was proposed. We used this method to study locally asymptotically anti-de Sitter spacetimes with nonzero NUT charge and charged black holes configurations in de Sitter spacetime, respectively. |
