Dark energy scenario consistent with GW170817 in theories beyond Horndeski gravity

The Gleyzes-Langlois-Piazza-Vernizzi (GLPV) theories up to quartic order are the general scheme of scalar-tensor theories allowing the possibility for realizing the tensor propagation speed cₓ equivalent to 1 on the isotropic cosmological background. We propose a dark energy model in which the late-time cosmic acceleration occurs by a simple k-essence Lagrangian analogous to the ghost condensate with cubic and quartic Galileons in the framework of GLPV theories. We show that a wide variety of the variation of the dark energy equation of state w₃₄ including the entry to the region w₃₄1. For the case in which the deviation of w₃₄ from -1 is significant at a later cosmological epoch, the values of and tend to be larger at low redshifts. We also find that our dark energy model can be consistent with the bounds on the deviation parameter ₇ from Horndeski theories arising from the modification of gravitational law inside massive objects.