G3 -interacting scalar-tensor dark energy

Abstract

We study the effect of adding an interaction in the G3 term of Horndeski theory, where the propagation of gravitational waves is not modified. We derive the background and perturbation equations of motion from the action. We also derive the no-ghost and Laplacian instability conditions for tensor modes and scalar mode propagation. Then we study the evolution of the matter perturbation in the quasistatic approximation. We find that the gravitational couplings to the baryonic and cold dark matter over density are modified in this theory. We introduce a concrete model of the free function in the theory and study the background and linear perturbation dynamics. We then use the genetic algorithm to test the model. We compare the H(z) function of the model and the H(z) curve predicted by the genetic algorithm, using the H(z) data. For the perturbation sector, we compute the fσ8 observable for the model and compare it with the predicted function from the genetic algorithm from the fσ8 data.