Modified Gravity as an alternative to Dark Energy.

The combination of Einstein's General Relativity and ordinary matter, as described by the Standard Model of particle physics, cannot explain the current cosmological observations. In order to account for all the astronomical observations within General Re lativity, two exotic components are required in the matter-energy budget of the Universe. These are dark matter and dark energy. The origin and nature of the dark sector is however still a mystery, since there is no well established and consistent theoret ical framework to explain them. My work focused on the dark-energy problem. There is a very important issue which one should always bear in mind when interpreting the observations : the existence of the dark sector is in fact only inferred from the motion of ordinary matter in a gravitational field. Hence, one could then ask whether the existence of the dark energy is just a sign of our lack of understanding of gravitational physics? That is, could General Relativity break down at certain cosmological/a strophysical scales? Such possibility lead to the formulation of Modified Gravities as alternatives to the dark energy. The planned research project will be to study the nature of Dark Energy with David Mota (University of Oslo). In particular to use the growth of matter perturbations in order to discriminate between Dark Energy models. We will work and test the following models. * Chameleon models where a scalar field has an effective mass that depends on its environment. * Scalar-Tensor of gravities tha t appears as low-energy effectivetheories of gravity. * The non-minimal coupled Dirac-Born-Infeld model. * The Dvali, Gabadadze and Porrati (DGP) model.