What Does The Latest Cosmology Data Say About Neutrinos?

Cosmology data combined with neutrino oscillation data favors a sum of three neutrino masses between 0.06 and 0.087 eV (nominally ruling out an inverted mass hierarchy at the 95% confidence interval level, which oscillation data alone favor at a 2-2.7σ level), implying a lightest neutrino mass eigenstate of about 0.007 eV or less (compared to an upper bound on the lightest neutrino mass of 0.8 eV potentially reducible to 0.2 eV within a few years from direct measurements). 

These combined data sources also imply that there are exactly three neutrino types with masses low enough to count as neutrinos in the ΛCDM model at the five sigma level, which would include any light sterile neutrinos if they existed. The predicted value of N(eff) in this scenario is 3.045 which is within 0.2 sigma of the observed value.

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We present robust, model-marginalized limits on both the total neutrino mass (∑mν) and abundance (Neff) to minimize the role of parameterizations, priors and models when extracting neutrino properties from cosmology. The cosmological observations we consider are CMB temperature fluctuation and polarization measurements, Supernovae Ia luminosity distances, BAO observations and determinations of the growth rate parameter from the Data Release 16 of the Sloan Digital Sky Survey IV. 

The degenerate neutrino mass spectrum (which implies ∑mν>0) is weakly (moderately) preferred over the normal and inverted hierarchy possibilities, which imply the priors ∑mν>0.06 and ∑mν>0.1 eV respectively. Concerning the underlying cosmological model, the ΛCDM minimal scenario is almost always strongly preferred over the possible extensions explored here. The most constraining 95% CL bound on the total neutrino mass in the ΛCDM+∑mν picture is ∑mν<0.087 eV. The parameter Neff is restricted to 3.08±0.17 (68% CL) in the ΛCDM+Neff model. These limits barely change when considering the ΛCDM+∑mν+Neff scenario. 

Given the robustness and the strong constraining power of the cosmological measurements employed here, the model-marginalized posteriors obtained considering a large spectra of non-minimal cosmologies are very close to the previous bounds, obtained within the ΛCDM framework in the degenerate neutrino mass spectrum. Future cosmological measurements may improve the current Bayesian evidence favouring the degenerate neutrino mass spectra, challenging therefore the consistency between cosmological neutrino mass bounds and oscillation neutrino measurements, and potentially suggesting a more complicated cosmological model and/or neutrino sector.

Eleonora di Valentino, Stefano Gariazzo, Olga Mena, "Model marginalized constraints on neutrino properties from cosmology" arXiv:2207.05167 (July 11, 2022).