Nonlinear Effective Theory of Dark Energy

Understand

We develop an approach to parametrize cosmological perturbations beyond linear order for general dark energy and modified gravity models characterized by a single scalar degree of freedom.

• We derive the full nonlinear action, focusing on Horndeski theories.
• In the quasi-static, non-relativistic limit, there are a total of six independent relevant operators, three of which start at nonlinear order.
• The new nonlinear couplings modify, beyond linear order, the generalized Poisson equation relating the Newtonian potential to the matter density contrast.