Cosmology with a successful Vainshtein screening in theories beyond Horndeski

We propose a dark energy model where a scalar field phi has nonlinear self-interactions in the presence of a dilatonic coupling with the Ricci scalar. This belongs to a subclass of theories beyond Horndeski, which accommodates covariant Galileons and Brans-Dicke theories as specific cases. We derive conditions under which the scalar sound speed squared c(s)(2) is positive from the radiation era to today. Since c(s)(2) remains smaller than the order of 1, the deviation from Horndeski theories does not cause heavy oscillations of gaugeinvariant gravitational potentials. In this case, the evolution of matter perturbations at low redshifts is similar to that in the coupled dark energy scenario with an enhanced gravitational interaction. On the spherically symmetric background with a matter source, the existence of field self-interactions suppresses the propagation of the fifth force inside a Vainshtein radius. We estimate an allowed parameter space in which the model can be compatible with Solar System constraints while satisfying conditions for the cosmological viability of background and perturbations.