Reconstruction of the neutrino mass as a function of redshift

We reconstruct the neutrino mass as a function of redshift, z, from current cosmological data using both standard binned priors and linear spline priors with variable knots. Using cosmic microwave background temperature, polarization and lensing data, in combination with distance measurements from baryonic acoustic oscillations and supernovae, we find that the neutrino mass is consistent with $\sum m_{\nu }(z)$ = const. We obtain a larger bound on the neutrino mass at low redshifts coinciding with the onset of dark energy domination, $\sum m_{\nu }(z=0)$ < 1.46 eV (95% CL). This result can be explained either by the well-known degeneracy between $\sum m_{\nu }$ and ${\mathrm{\Omega }}_{\mathrm{\Lambda }}$ at low redshifts, or by models in which neutrino masses are generated very late in the Universe. We finally convert our results into cosmological limits for models with non-relativistic neutrino decay and find $\sum m_{\nu }$ < 0.21 eV (95% CL), which would be out of reach for the KATRIN experiment.