Speaker: Jérôme Gleyzes
“We propose a minimal description of single field dark energy/modified gravity within the effective field theory formalism for cosmological perturbations, which encompasses most scalar field based models. Focusing on linear perturbations, we identify seven Lagrangian operators that lead to equations of motion containing at most two (space or time) derivatives, the time-dependent coefficients of three of these operators determining the background evolution. We then establish a dictionary that translates most existing or future model whose Lagrangian can be written in the above form into our parametrized framework. As an illustration, we show that Horndeski’s—or generalized Galileon—theories can be described, up to linear order, by only six free functions of time. Finally, in order to make the link with observations, we provide the entire set of linear perturbation equations in Newtonian gauge, the effective Newton constant in the quasi-static approximation and the ratio of the two gravitational potentials, in terms of the time-dependent coefficients of our Lagrangian.”