Phantom-Crossing Dark Energy and the $Ω_m$ Tug-of-War

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 $Ω_m$ tensions that arise when these datasets are individually fit to $Λ$CDM, specifically because of the ordering $Ω_m^\mathrm{BAO} < Ω_m^\mathrm{CMB} < Ω_m^\mathrm{SN}$. We show both theoretically and empirically that models with phantom crossing can shift all of these inferred $Ω_m$ 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.