Remarks on generalized scalar-tensor models of dark energy

TL;DR

This paper investigates generalized scalar-tensor dark energy models, demonstrating conditions for phantom-divide-line crossing and acceleration transition, analyzing specific models, and exploring quantum effects that can induce transitions otherwise forbidden classically.

Contribution

It provides new insights into the conditions under which dark energy models exhibit phantom crossing and acceleration, including quantum effects, with specific analysis of coupled quintessence and Brans-Dicke models.

Findings

Phantom-divide-line crossing occurs in these models.

Quantum effects can induce transitions in classically forbidden cases.

Transition behavior varies between different scalar-tensor models.

Abstract

The generalized scalar-tensor models with Lagrangian $F(\phi,R)-U(\phi)(\nabla\phi)^2$ 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 $\xi>3/16$, the $\omega=-1$ 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 $\omega=-1$ transition is classically forbidden, the quantum effects can induce transition. The $\xi=1/6$ of coupled quintessence model is an example of this. The quantum effects are described via the account of conformal anomaly.