Coupled dark energy model inspired from general conformal transformation

Abstract

We study the coupled dark energy model constructed from the general conformal transformation in which the coefficient of the conformal transformation depends on both the scalar field and its kinetic term. Under this conformal transformation, the action for subclass of Degenerate Higher-Order Scalar-Tensor (DHOST) theories is related to the Einstein-Hilbert action. The evolution of the background universe has the scaling fixed point which corresponds to acceleration of the universe at late time. For the choices of parameters which make the late-time scaling point stable, the fixed point corresponding to $ϕ$-matter-dominated-era ($ϕ$MDE) is a saddle point, and the universe can evolve from radiation dominated epoch through $ϕ$MDE before reaching the scaling point at late time with the cosmological parameters which satisfy the observational bound. During the $ϕ$MDE, the effective equation of state parameter is slightly positive, so that one of possible mechanisms for alleviating the $H_0$ tension can be achieved. In this coupled dark energy model, the effective gravitational coupling for dark matter perturbations on small scales can be smaller than that in the $Λ$CDM model. Therefore a growth rate of the dark matter perturbations is suppressed compare with the $Λ$CDM model, which implies that the $σ_8$ tension could be alleviated.