Turtles All The Way Down

From this physics preprint.

Making Sense Of Cosmological Neutrino Mass Bounds

A new paper tries to make sense of the DESI results that suggest a sum of the three neutrino masses that is equal to or smaller than the minimum value suggested by neutrino oscillation experiments (which is roughly 0.06 eV).

An outlier data point in the DESI data and a known methodological issue in the Planck 18 data that a later version of the analysis of the Planck data has corrected largely resolve this issue. A non-constant "cosmological constant" would also help resolve the discrepancy and is increasingly favored by observation.

Even with relaxed bounds, however, the cosmological constraints on the sum of the three neutrino masses are still considerably tighter than those derived from direct measurements of the lightest neutrino mass. And, cosmological bounds, along with other data sources, continue to favor a "normal hierarchy" of neutrino masses over an "inverted hierarchy."

Cosmological neutrino mass bounds are becoming increasingly stringent. The latest limit within ΛCDM from Planck 2018+ACT lensing+DESI is ∑mν < 0.072eV at 95% CL, very close to the minimum possible sum of neutrino masses (∑mν > 0.06eV), hinting at vanishing or even ''negative'' cosmological neutrino masses. 

In this context, it is urgent to carefully evaluate the origin of these cosmological constraints. In this paper, we investigate the robustness of these results in three ways: i) we check the role of potential anomalies in Planck CMB and DESI BAO data; ii) we compare the results for frequentist and Bayesian techniques, as very close to physical boundaries subtleties in the derivation and interpretation of constraints can arise; iii) we investigate how deviations from ΛCDM, potentially alleviating these anomalies, can alter the constraints. 

From a profile likelihood analysis, we derive constraints in agreement at the ∼10% level with Bayesian posteriors. We find that the weak preference for negative neutrino masses is mostly present for Planck 18 data, affected by the well-known "lensing anomaly". It disappears when the new Planck 2020 HiLLiPoP is used, leading to significantly weaker constraints. Additionally, the pull towards negative masses in DESI data stems from the z=0.7 bin, which is in ∼3σ tension with Planck expectations. Without these outliers, and in combination with HiLLiPoP, the bound relaxes to ∑mν<0.11eV at 95% CL. The recent preference for dynamical dark energy alleviates this tension and further weakens the bound. As we are at the dawn of a neutrino mass discovery from cosmology, it will be very exciting to see if this trend is confirmed by future data.

Daniel Naredo-Tuero, et al., "Living at the Edge: A Critical Look at the Cosmological Neutrino Mass Bound" arXiv:2407.13831 (July 18, 2024).