Abstract: The cosmological upper bound on the total neutrino mass is the dominant limit on this fundamental parameter. Recent observations have strongly tightened it, approaching the lower limit set by oscillation data. Understanding its physical origin, robustness, and model-independence becomes pressing. In this seminar I will introduce a novel framework to explicitly separate for the first time the two distinct cosmological neutrino-mass effects. On the one hand, the impact on background evolution, related to the energy in neutrino masses; on the other hand, the "kinematic" impact on perturbations, related to neutrino free-streaming. I will didactically explain how these effects separately modify CMB anisotropies, by introducing two effective masses enclosing background and perturbations effects. I will explain the results of analyzing CMB data in this framework, which show that the neutrino-mass bound is mostly a background measurement, i.e., how the neutrino energy density evolves with time. The results also quantify the difficulty of measuring the neutrino "kinematical" mass, whose bound is relaxed by 3 orders of magnitude. 
The seminar follows 2411.14524 and, most importantly, no deep knowledge of neutrino physics or cosmology will be necessary to follow the seminar!