Phantom-Crossing Dark Energy and the $\Omega_m$ Tug-of-War

TL;DR

This paper investigates the observational preference for phantom crossing in dark energy models, showing it arises from dataset tensions and that models allowing crossing can better align inferred parameters from different cosmological measurements.

Contribution

It demonstrates that phantom crossing models naturally reconcile dataset tensions, challenging the prior interpretation that data favor quintessence-like dark energy.

Findings

Phantom crossing models align $\Omega_m$ values from different datasets.

Quintessence models worsen BAO-CMB tensions.

SN data prefer phantom crossing less when combined with other datasets.

Abstract

Recent analyses combining data from the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), and Type Ia supernovae (SN) have revealed a tentative observational preference for phantom crossing in the dark energy equation of state $w$. We argue that this preference is a natural consequence of the $\Omega_m$ tensions that arise when these datasets are individually fit to $\Lambda$CDM, specifically because of the ordering $\Omega_m^\mathrm{BAO} < \Omega_m^\mathrm{CMB} < \Omega_m^\mathrm{SN}$. We show both theoretically and empirically that models with phantom crossing can shift all of these inferred $\Omega_m$ values toward mutual alignment. In contrast, quintessence theories restricted to $w \geq -1$ can alleviate the tensions with SN data but only at the cost of exacerbating the BAO-CMB discrepancy. We therefore conclude that it is the BAO and CMB measurements - not the SN data - that drive the preference for phantom crossing over quintessence in joint analyses. Moreover, we point out that SN data exhibit greater tensions with the other datasets when fit to phantom-crossing models than when fit to quintessence, causing the preference for phantom crossing to be weaker in joint CMB+BAO+SN analyses than in analyses of CMB+BAO data alone.