species by non-vanishing squared mass differences between the mass eigenstates and the non-vanishing corresponding mixing angles. The most recent results are $\Delta m_{12}^2\sim 8\times 10^{-5}$ eV$^2$, $\Delta m_{23}^2\sim
2\times 10^{-3}$ eV$^2$, and mixing angles of $\theta_{12} \sim 30^\circ$, $\theta_{23}\sim 45^\circ$, and $\theta_{13} \lesssim 11^\circ$. These values yield a lower bound on the heavier mass of order $0.06 \,(0.1)$ eV for normal (inverted) hierarchy, but cannot give the absolute mass scale. Forthcoming experiments based on other mechanisms, such as SuperNEMO (conceived to detect the neutrinoless double $\beta$ decay) and KATRIN (designed to probe the tritium $\beta$ decay) are designed to measure directly the *absolute* mass of the electron neutrino with a sensitivity of 0.05 and 0.2 eV, respectively.

Cosmological observations are compelling in providing an independent way to probe the absolute neutrino mass. The effect of light massive fermions in cosmology is well understood [see @NuCosmoReview for a review]. The standard cosmological model predicts the existence of relic neutrinos that decouple from the primeval plasma while ultra-relativistic and produce a cosmic neutrino background, which cannot yet be detected directly. Their contribution to the total radiation energy density is a fraction $7/8(T_\nu/T_\gamma)^4{N_\mathrm{eff}}$ of the contribution of photons, $T_\nu$ and $T_\gamma$ being the 