FEDERICO PIAZZA, APC Paris. The discovery of the accelerating expansion of the Universe is motivating an impressive amount of theoretical and observational activity. After briefly reviewing the problem and challenges of "Dark Energy", I will focus on recent and ongoing works where a unifying description of dark energy and modified gravity is proposed, that is inspired by the Effective Field Theory (EFT) of Inflation formalism. By extending such a framework to late time cosmology one can write the most general action for cosmological perturbations in the presence of an additional scalar degree of freedom. I will focus on a few operators that are quadratic in the perturbations and which appear in non-minimally coupled scalar-tensor gravity and "Galileon" theories and describe the mixing between gravity and the scalar degree of freedom that such operators produce.

DATA: 08-11-2013