Phantom matters

TL;DR

This paper explores a coupling mechanism in quintessence models that naturally produces a phantom dark energy regime, aligning with recent observations and potentially addressing the Hubble tension.

Contribution

It introduces a coupling-based approach allowing steep scalar potentials to generate phantom regimes consistent with observations.

Findings

Models can reach $w<-1$ without instabilities.

String-inspired potentials fit observational data well.

Early Dark Energy features emerge naturally in the models.

Abstract

Cosmological observations of the recent universe suggest that dark energy equation of state parameter $w$ is growing with time, departing from a cosmological constant for which $w=-1$. Standard quintessence models allow for a varying $w\geq-1$, but observations report that a phantom regime, $w<-1$, is quickly reached in the past. Often discarded because of uncertainties or parametrisation, we rather propose here to embrace the reality of this phantom regime. We revisit an elegant mechanism that accounts for it, thanks to a coupling of quintessence field(s) to matter (and possibly radiation). We show that this allows for steep scalar potentials, and illustrate this with string-inspired models, where $V=V_0\, e^{-\lambda\, \varphi}$ and $\lambda \geq \sqrt{2}$. Those provide solutions in very good agreement with observations, including the phantom regime. We then discuss poles that can appear in $w$, making it diverge at recent times ($z\leq 4$), and that could be detected by observations. We finally comment on an Early Dark Energy-like feature, that systematically appears for free from the models considered, and could be of interest for the Hubble tension.