When One-Parameter Dark Energy Makes Neutrinos Physical Again

Abstract

A puzzling implication of current data interpreted in the $Λ$-Cold Dark Matter cosmology is the preference for a negative sum of neutrino masses. Moving to $w_0w_a$CDM brings an appreciable fraction of the neutrino mass posterior back to positive values, while the constant equation-of-state dark energy case $w$CDM does not. We investigate a variety of one-parameter dynamical dark energy equations of state (DE EoS) whose redshift evolution is controlled by a single free parameter, each representing distinct physical properties, to understand whether a two-parameter DE EoS is required to bring the neutrino mass into the positive region. The conclusion is that certain one-parameter DE EoS can suffice, implying that the data are pointing toward physical characteristics rather than a broad degeneracy. This behavior effectively lowers the dark energy density at high redshift, allowing the sum of neutrino masses to shift toward the physical region. The required characteristics are identified as phantom dark energy at high redshift, crossing $w=-1$ at lower redshift.