Neutrino mass constraints in the context of 4-parameter dark energy equation of state and DESI DR2 observations

TL;DR

This study reexamines cosmological constraints on the total neutrino mass $\sum m_\nu$ in the context of a flexible four-parameter dark-energy equation of state (4pDE), extending beyond the CPL parametrization. Using a modified CLASS implementation and a COBAYA-MCMC analysis of Planck 2018, DESI DR2 BAO, and Pantheon+, the authors quantify how 4pDE alters degeneracies with neutrino mass and tightens constraints on late-time expansion. They find $\sum m_\nu < 0.101$ eV (95% C.L.) in the 4pDE framework, softer than the $\Lambda$CDM bound but still stronger than CPL-based DESI analyses, with the present-day EoS $w_0$ tightly constrained and a reconstructed $w(z)$ showing a possible phantom-crossing near $z\sim0.5$ in the best-fit model. Although the 4pDE model marginally improves the fit to the data ($\Delta\chi^2_{\min} = -7.3$), the Akaike Information Criterion indicates only weak evidence in its favor ($\Delta\mathrm{AIC} = +0.7$), underscoring that current data do not decisively prefer a dynamical dark energy scenario despite its impact on neutrino mass inference.

Abstract

Cosmological constraints on the total neutrino mass, $\sum m_ν$, are strongly shaped by assumptions about the dark-energy equation of state due to the well-known degeneracy between massive neutrinos and late-time cosmic acceleration. In this work, we move beyond the two-parameter Chevallier-Polarski-Linder (CPL) form adopted in recent DESI analyses and re-examine neutrino mass constraints using a flexible four-parameter dark energy equation of state (4pDE). We implement the 4pDE model in a modified version of CLASS and perform a full MCMC analysis using Planck, DESI DR2 BAO, and Pantheon+ data. Relative to our previous 4pDE study based on pre-DESI BAO datasets, the inclusion of DESI DR2 substantially tightens the constraints on the transition parameters while still yielding a relaxed neutrino-mass bound compared to $Λ$CDM, $\sum m_ν< 0.101$ eV ($95\%$ C.L.). This upper limit is more stringent than the DESI DR2 constraint obtained within the $w_0w_a$CDM framework. From the best-fit parameters, we reconstruct the evolution of the 4pDE equation of state along with both $68\%$ and $95\%$C.L. We do not find a statistically significant phantom-crossing at $z \sim 0.5$, consistent with the conclusion from the DESI collaboration; at higher redshifts, the reconstructed $w(z)$ follows the CPL evolution and deviates only at low redshift. Additionally we also find reduction in $Δχ^2_{\rm min}=-7.3$ compared to $Λ$CDM model.

Figures (3)

• Figure 1: Two-dimensional marginalized posterior distributions of the 4pDE model parameters ($\omega_{\rm cdm}$, $H_0$, $w_0$, $w_m$, $\log_{10} (a_{\rm t})$, $\log_{10} (\Delta_{\rm de})$, and $\sum m_\nu$) obtained from our MCMC analysis for the dataset combinations of Planck, DESI DR2 BAO and Pantheon+. The contours correspond to the 68% and 95% confidence regions.
• Figure 2: Dark energy equation of state plotted for the 4pDE model using the best-fit values obtained from MCMC analysis using Planck+DESI DR2+Pantheon+. The shaded regions indicate the obtained $1\sigma$ and $2\sigma$ confidence regions. The result is then compared with CPL parameterization obtained from DESI analysis DESI:2025zgx
• Figure 3: $1$D posterior distributions of the neutrino mass for the $\Lambda$CDM and 4pDE models (left), and the corresponding $2$D marginalized contours between the neutrino mass and $\Omega_m$ (right). All results are obtained from MCMC analyses using the combined Planck, DESI DR2 BAO, and Pantheon+ datasets.