Remarks on generalized scalar-tensor models of dark energy

The generalized scalar-tensor models with Lagrangian $F(ϕ,R)-U(ϕ)(\nablaϕ)^2$ are considered. It is shown that the phantom-divide-line crossing and the deceleration to acceleration transition generally occurr in these models. Two specific examples, the coupled quintessence model and the Brans-Dicke model are considered. For the first example, it is shown that for the models with $ξ>3/16$, the $ω=-1$ transition exists. This is verified numerically for some special cases. For the Brans-Dicke model, it is shown that the transition does not occur, a result which can be verified by using the exact solution of this model. Finally the contribution of quantum effects on these phenomena is investigated. It is shown that for some special cases where the $ω=-1$ transition is classically forbidden, the quantum effects can induce transition. The $ξ=1/6$ of coupled quintessence model is an example of this. The quantum effects are described via the account of conformal anomaly.

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