The research program of the present proposal is at the overlap of Particle Physics, Cosmology, General Relativity and String Theory. The unifying concept is the accelerated expansion of the Universe and the problem of the vacuum energy in Particle Physics and Gravitation. The PI proposes detailed studies within a new paradigm according to which the observed accelerated expansion of the Universe is the first evidence of a new physical scale at which laws of gravity and cosmology are modified. Models that enable one to modify gravity at large cosmological distance scales present great intellectual challenges for Field Theory as well as String Theory. Successful resolution of these challenges is likely to lead, among many other exciting applications, to a solution of the long-standing problem of the vacuum energy. The PI will develop and study models that modify gravity at large distances while keeping all the successes of General Relativity at shorter scales. General questions that will be addressed by the PI fall under the following three categories: (1) Studies of the models of large distance modification of gravity as consistent field theories; (2) Cosmological and astrophysical consequences of these models; (3) Embeddings of these models into String Theory.
