On dark energy models of single scalar field

TL;DR

This paper explores single scalar field dark energy models with higher derivatives, demonstrating they can cross the cosmological constant boundary smoothly without instabilities, expanding the theoretical landscape of dark energy modeling.

Contribution

It introduces a degenerate higher derivative scalar field model that avoids ghost modes and allows the equation of state to cross -1, with a detailed perturbation analysis.

Findings

Models can cross the -1 boundary smoothly

Models are free from ghost instabilities

Equation of state can violate null energy condition

Abstract

In this paper we revisit the dynamical dark energy model building based on single scalar field involving higher derivative terms. By imposing a degenerate condition on the higher derivatives in curved spacetime, one can select the models which are free from the ghost mode and the equation of state is able to cross the cosmological constant boundary smoothly, dynamically violate the null energy condition. Generally the Lagrangian of this type of dark energy models depends on the second derivatives linearly. It behaves like an imperfect fluid, thus its cosmological perturbation theory needs to be generalized. We also study such a model with explicit form of degenerate Lagrangian and show that its equation of state may cross -1 without any instability.