Sterile Neutrino Effective Mass Rises to 0.35 eV in Non-Flat w0waCDM Fits to DESI DR2 and Planck 2018
Extended cosmological models with curvature and dynamical dark energy relax upper bounds on partially thermalized eV sterile neutrinos, allowing ms,eff ~0.35 eV and Delta Neff ~0.35. The same models improve fit quality and favor mild negative curvature at 2.8 sigma. Next-generation surveys can test whether this preference persists or is an artifact of current data combinations.
The study varies both sterile mass ms and partial thermalization parameter Delta Neff independently inside flat and non-flat LambdaCDM plus w0waCDM frameworks. Planck 2018 CMB spectra, DESI DR2 baryon acoustic oscillations, Pantheon+ supernovae, a conservative f sigma8 compilation and the H0DN prior are combined with oscillation-informed active-neutrino priors. The extra freedom in late-time expansion history weakens the usual CMB-BAO tension that otherwise forces sterile neutrinos toward very low abundance.
In the non-flat w0waCDM+(3+1)nu model the data prefer Omega_k approximately 0.0045 at 2.8 sigma, H0 near 70.9 km/s/Mpc and S8 near 0.785, improving chi-squared by 21-25 relative to flat LambdaCDM+(3+1)nu. The sterile sector shifts upward because curvature and evolving dark energy alter the sound-horizon and growth-rate degeneracy that previously capped ms,eff. The resulting Delta Neff remains below full thermalization, consistent with short-baseline oscillation hints yet still testable.
Related DESI year-1 results and the 2025 eBOSS final release already showed mild curvature preference; adding the sterile sector amplifies that signal rather than erasing it. Future CMB-S4 and Euclid weak-lensing data will break the curvature-sterile degeneracy by measuring the lensing potential and growth history to sub-percent precision.
If the open-geometry preference survives, it would require either new early-universe physics or a revision of the flatness assumption inherited from inflation, directly affecting forecasts for neutrino-mass hierarchy measurements at terrestrial experiments.
DESI: Full year-5 BAO release will tighten Omega_k uncertainty below 0.002 by mid-2028 or rule out the 2.8 sigma open-geometry preference at >3 sigma.
Sources (2)
- [1]Primary Source(https://arxiv.org/abs/2610.10585)
- [2]Supporting Source(https://arxiv.org/abs/2504.01669)