Negative neutrino masses as a mirage of dark energy

Understand

The latest cosmological constraints on the sum of the neutrino masses depend on prior physical assumptions about the mass spectrum.

• To test the accordance of cosmological and laboratory constraints in the absence of such priors, we introduce an effective neutrino mass parameter that extends consistently to negative values.
• For the $\Lambda$CDM model, we analyze data from Planck, ACT, and DESI and find a $2.8-3.3\sigma$ tension with the constraints from oscillation experiments.
• Motivated by recent hints of evolving dark energy, we analyze the $w_0w_a$ and mirage dark energy models, showing that they favour larger masses consistent with laboratory data, respectively $\sum m_{\nu,\mathrm{eff}} = 0.06_{-0.10}^{+0.15}\,\mathrm{eV}$ and $\sum m_{\nu,\mathrm{eff}} = 0.04_{-0.11}^{+0.15}\,\mathrm{eV}$ (both at 68%).