Effective Phantom Divide Crossing with Standard and Negative Quintessence

Abstract: Cosmic microwave background data from the Planck satellite, combined with baryon acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument and Type Ia supernovae from various samples, provide hints of dynamical dark energy (DE). These results indicate a peak in the DE density around $z\sim 0.4-0.5$, with the highest significance observed when using the supernovae from the Dark Energy Survey. In this Letter, we show that this peak does not necessarily imply a true crossing of the phantom divide if the measured effective DE is not a single component, but a combination of standard and negative quintessence. The latter is characterized by negative energy density and positive pressure, both decreasing in absolute value and tending to 0 in the future. For appropriate values of the parameters, negative quintessence is relevant at intermediate redshifts and becomes subdominant in front of standard quintessence around $z\sim 0.4-0.5$, giving rise to the aforementioned peak in the DE density. We find that our model is preferred over $\Lambda$CDM at a $3.27\sigma$ CL, which is comparable to the level of exclusion found with the Chevallier-Polarski-Linder parametrization. Our analysis leaves open the possibility of negative quintessence and other exotic fields existing in the low-energy universe, potentially playing a significant role in cosmic dynamics.