Dark energy perturbations and the robustness of cosmological neutrino-mass constraints

Abstract

Cosmological observations are placing increasingly stringent bounds on the sum of neutrino masses, approaching the lower limits implied by neutrino oscillation experiments. Recent studies have suggested that dynamical dark energy may alleviate this apparent tension. However, these conclusions generally rely on the assumption that dark energy remains smooth, neglecting its perturbations. In this work we investigate the robustness of cosmological neutrino-mass constraints by consistently incorporating dark-energy perturbations. Using CMB, BAO, RSD, and supernova data, we show that the commonly reported alleviation of the neutrino-mass tension in dynamical dark-energy models is not generic. While smooth dark energy substantially relaxes the neutrino-mass bounds, allowing dark energy to cluster shifts the preferred neutrino mass toward smaller, and even more negative, effective values. We demonstrate that this behavior originates from a degeneracy between neutrino free-streaming and dark-energy perturbations in structure-growth observables. Different combinations of neutrino mass and dark-energy clustering can provide similarly good fits to current data while yielding significantly different neutrino-mass constraints. Our results show that cosmological neutrino-mass measurements are inherently model dependent and that reliable neutrino-mass inference requires a consistent treatment of dark-energy perturbations.