Negative Masses and Spatial Curvature: Alleviating Neutrino Mass Tensions in LambdaCDM and Extended Cosmologies

TL;DR

This study explores how spatial curvature and dark energy dynamics influence neutrino mass constraints in cosmology, revealing that boundary effects and degeneracies significantly impact current bounds.

Contribution

It introduces a symmetric extension method allowing negative neutrino masses and analyzes their impact on cosmological parameter constraints.

Findings

Spatial curvature significantly alters neutrino mass posteriors.

Including curvature reduces tension with terrestrial neutrino mass limits.

Parameter flexibility degrades the precision of neutrino mass estimates.

Abstract

We investigate the impact of spatial curvature, $\Omega_k$, and dynamical dark energy on the cosmological constraints of the neutrino mass sum, $\sum m_\nu$. Using a joint analysis of the latest CMB (Planck and ACT DR6), BAO (DESI DR2) and SNe Ia (DESY5 and DES-Dovekie) datasets, we perform an exploration of the neutrino mass parameter space. To mitigate prior-driven biases near the physical boundary, we implement a symmetric extension wrapper that allows for effective negative masses. We find that the inclusion of spatial curvature significantly modifies the posterior distributions, exhibiting a smooth transition across the $\sum m_\nu = 0$ threshold. In the $\Lambda$CDM + $\Omega_k$ + $\sum m_{\nu,\mathrm{eff}}$ framework, we obtain $\sum m_{\nu,\mathrm{eff}} = -0.011^{+0.052}_{-0.050}$, reducing the tension with the terrestrial lower limit of 0.06 eV from $2.59\sigma$ for the $\Lambda$CDM + $\sum m_{\nu,\mathrm{eff}}$ model to $1.17\sigma$. For the most flexible scenario $w_0 w_a$CDM + $\Omega_k$ + $\sum m_{\nu,\mathrm{eff}}$, we find $\sum m_{\nu,\mathrm{eff}} = -0.07 \pm 0.11$ with a tension of $1.13\sigma$, illustrating how the increased parameter freedom notably degrades the precision of the mass estimate compared to simpler extensions. Our results demonstrate that current cosmological bounds on $\sum m_\nu$ are heavily influenced by boundary effects and geometric degeneracies.