Constraints on the sum of the neutrino masses in dynamical dark energy models with w (z )≥-1 are tighter than those obtained in Λ CDM

We explore cosmological constraints on the sum of the three active neutrino masses M_ν in the context of dynamical dark energy (DDE) models with equation of state (EoS) parametrized as a function of redshift z by w (z )=w₀+w_az /(1 +z ) , and satisfying w (z )≥-1 for all z . We make use of cosmic microwave background data from the Planck satellite, baryon acoustic oscillation measurements, and supernovae Ia luminosity distance measurements, and perform a Bayesian analysis. We show that, within these models, the bounds on M_ν do not degrade with respect to those obtained in the Λ CDM case; in fact, the bounds are slightly tighter, despite the enlarged parameter space. We explain our results based on the observation that, for fixed choices of w₀ , w_a such that w (z )≥-1 (but not w =-1 for all z ), the upper limit on M_ν is tighter than the Λ CDM limit because of the well-known degeneracy between w and M_ν. The Bayesian analysis we have carried out then integrates over the possible values of w₀-w_a such that w (z )≥-1 , all of which correspond to tighter limits on M_ν than the Λ CDM limit. We find a 95% credible interval (C.I.) upper bound of M_ν<0.13 eV . This bound can be compared with the 95% C.I. upper bounds of M_ν<0.16 eV , obtained within the Λ CDM model, and M_ν<0.41 eV , obtained in a DDE model with arbitrary EoS (which allows values of w <-1 ). Contrary to the results derived for DDE models with arbitrary EoS, we find that a dark energy component with w (z )≥-1 is unable to alleviate the tension between high-redshift observables and direct measurements of the Hubble constant H₀. Finally, in light of the results of this analysis, we also discuss the implications for DDE models of a possible determination of the neutrino mass ordering by laboratory searches.