No.40 IF Colloquium – Crossing the Phantom Divide: Difficulties and Solutions

Abstract

The nature of cosmic acceleration remains one of the central open questions in cosmology. Recent observations have renewed interest in dynamical dark-energy scenarios in which the equation of state evolves across the cosmological-constant boundary, w_DE = -1, known as the phantom divide. Realizing such a crossing consistently is theoretically nontrivial, since it can be difficult to achieve in simple scalar-field models without introducing instabilities.

In this talk, I will discuss a scalar–vector–tensor dark-energy model in which scalar and vector degrees of freedom work together to produce a stable crossing of the phantom divide. The vector sector drives the dark-energy equation of state into the phantom regime at earlier times, while a canonical scalar field subsequently drives it toward w_DE > -1 at late times. I will then discuss observational constraints from Type Ia supernovae, DESI baryon acoustic oscillations, cosmic microwave background information, and measurements of structure growth. Finally, I will highlight what these results tell us about the possibility of dynamical dark energy beyond the standard LambdaCDM cosmology.

Keywords

Dynamical dark energy, Phantom divide, Scalar-vector-tensor models