A General Model for Dark Energy Crossing the Phantom Divide

Understand

Within the framework of spatially covariant theories, we propose a general model for dark energy (DE) in which the cosmological background and perturbations are independently controlled by different sets of coefficients, and the equation of state of DE is directly determined by two free functions of time from the Lagrangian.

• These properties allow to realize arbitrary background evolutions while avoiding ghost and gradient instabilities in linear perturbations.
• They also enable a more direct analysis of phantom crossing without having to first solve the background equations of motion.
• In this model, the sound speed of the scalar mode is scale-dependent and approaches infinity at large scale, so that the field becomes non-dynamical in the infrared (IR) limit.