Dark Energy with Constant Inertial Mass Density: Updated Constraints and Curvature-Induced Sign Transitions in $\rho_{\rm DE}$ and $\rho_{\rm DE}+p_{\rm DE}$

TL;DR

This study updates constraints on a dark energy model with constant inertial mass density, exploring its implications with recent data, and finds that spatial curvature can induce sign transitions in dark energy density, revealing new dynamical features.

Contribution

The paper introduces and constrains a simple-gDE model with constant IMD, highlighting the effects of spatial curvature on dark energy dynamics beyond standard models.

Findings

Data favor a small positive IMD

No sign transition in flat models

Curvature allows sign transitions and dynamical features

Abstract

We present updated observational constraints on the simple-gDE model, characterized by a constant inertial mass density (IMD) $\rho_{\rm DE}+p_{\rm DE}$,which belongs to the broader graduated dark energy family, and compare its cosmological implications with those of the $w$CDM and the $\Lambda$CDM models. This parametrization provides a physically motivated, one-parameter extension of $\Lambda$CDM, perspective on DE dynamics beyond the usual equation-of-state approach. We use the newly released DESI DR2 BAO data in combination with either CMB measurements from Planck 2018 or late-time probes, CC and the Pantheon+ SNe Ia sample, considered both with and without SH0ES calibration in this analysis. The data favor a small positive IMD, and Bayesian evidence indicates that the models remain statistically indistinguishable within spatially flat scenarios. Consequently, none of these models exhibits a sign transition in the DE energy density, and no improvement in $H_0$ tension. Allowing spatial curvature qualitatively enlarges the phenomenology of the dark sector. In particular, the interplay between spatial curvature and a nonzero IMD permits sign transitions in both the effective dark-energy density and the IMD during cosmic evolution. For the BAO+CC+SN+SH0ES dataset, the $o$Simple-gDE model yields a transition redshift $z^\dagger = 1.51^{+0.68}_{-0.34}$, while the crossing of the Null Energy Condition boundary (NECB), defined by $\rho_{\rm DE}+p_{\rm DE}=0$, occurs at $z_{\rm NECB}=2.36^{+1.48}_{-1.48}$. The model is statistically favored over $o\Lambda$CDM and $ow$CDM. These results highlight the potential role of IMD as a fundamental parameter in DE phenomenology and demonstrate that geometric effects, such as spatial curvature, can reveal dynamical features of the dark sector that remain hidden within the spatially flat $\Lambda$CDM framework.