Origin of cosmological neutrino mass bounds: background <i>versus</i> perturbations
Toni Bertólez-Martínez, Ivan Esteban, Rasmi Hajjar, Olga Mena, Jordi Salvadó
Abstract
Abstract 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 (∑ m ν Backg. ) and perturbations (∑ m ν Pert. ) 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 ∑ m ν Pert. . is largely relaxed, ∑ m ν Pert. < 0.8 eV. 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.