Cosmological Probes of Modified Gravity

The most dramatic discovery in science in the past few years has been the revelation that the expansion of the Universe is accelerating. Predictions based on General Relativity plus the Standard Model of particle physics fail to explain this. A possibl e explanation to the observations is to include an exotic and unknown dark energy into the Universe matter content. Alternatively, instead of attributing the accelerated expansion to our lack of understanding of the constituents of the Universe, one can a ttribute it to our lack of understanding of gravity on cosmological scales. The objective of this proposal is to investigate whether present days observations could be explained by a Modification of General Relativity. This objective can be achieved b y confronting the theoretical predictions of the different Modified Gravity models, on the expansion of the Universe and the formation of galaxies and clusters, against cosmological observations. We will investigate four different theoretical alternati ves to General Relativity: Scalar-Tensor gravities, Vector-Tensor theories, Brane World models and Tensor-Vector-Scalar theories. The models will be chosen so that they pass local constraints from gravity experiments. For instance via chameleon type mecha nisms. We will make use of measurements of galaxies distribution, clusters number counts, weak lensing and Cosmic Microwave Background anisotropies to map the evolution of matter perturbations and gravitational potentials from the radiation dominated epo ch until today. Such observations will tighten the constraints on the allowed expansion histories, and test the relationships between matter overdensities, local curvature, and the gravitational potentials. These relationships, given by Einstein's equatio ns of General Relativity, are modified in alternative gravity theories. Hence, the combination of all these measurements provide a powerful probe on the properties and viability of Modified Gravity.