How Are Cosmology Based Neutrino Mass Estimates Calculated?

How are cosmology based neutrino mass estimates calculated? What conditions must hold for them to be accurate? 

A new pre-print explains:

The cosmological upper bound on the total neutrino mass is the dominant limit on this fundamental parameter. Recent observations-soon to be improved-have strongly tightened it, approaching the lower limit set by oscillation data. Understanding its physical origin, robustness, and model-independence becomes pressing. 

Here, we explicitly separate for the first time the two distinct cosmological neutrino-mass effects: the impact on background evolution, related to the energy in neutrino masses; and the "kinematic" impact on perturbations, related to neutrino free-streaming. We scrutinize how they affect CMB anisotropies, introducing two effective masses enclosing background (∑mBackg.ν) and perturbations (∑mPert.ν) effects. We analyze CMB data, finding that the neutrino-mass bound is mostly a background measurement, i.e., how the neutrino energy density evolves with time. The bound on the "kinematic" variable ∑mPert.ν is largely relaxed, ∑mPert.ν<0.8eV. 

This work thus adds clarity to the physical origin of the cosmological neutrino-mass bound, which is mostly a measurement of the neutrino equation of state, providing also hints to evade such a bound.

Toni Bertólez-Martínez, Ivan Esteban, Rasmi Hajjar, Olga Mena, Jordi Salvado, "Origin of cosmological neutrino mass bounds: background versus perturbations" arXiv:2411.14524 (November 21, 2024).