At present , cosmological observations set the most stringent bound on the neutrino mass scale .
Within the standard cosmological model ( \Lambda CDM ) , the Planck collaboration reports \sum m _ { \nu } < 0.12 \text { eV } at 95 % CL .
This bound , taken at face value , excludes many neutrino mass models .
However , unstable neutrinos , with lifetimes shorter than the age of the universe \tau _ { \nu } \lesssim t _ { U } , represent a particle physics avenue to relax this constraint .
Motivated by this fact , we present a taxonomy of neutrino decay modes , categorizing them in terms of particle content and final decay products .
Taking into account the relevant phenomenological bounds , our analysis shows that 2-body decaying neutrinos into BSM particles are a promising option to relax cosmological neutrino mass bounds .
We then build a simple extension of the type I seesaw scenario by adding one sterile state \nu _ { 4 } and a Goldstone boson \phi , in which \nu _ { i } \to \nu _ { 4 } \phi decays can loosen the neutrino mass bounds up to \sum m _ { \nu } \sim 1 \text { eV } , without spoiling the light neutrino mass generation mechanism .
Remarkably , this is possible for a large range of the right-handed neutrino masses , from the electroweak up to the GUT scale .
We successfully implement this idea in the context of minimal neutrino mass models based on a U ( 1 ) _ { \mu - \tau } flavor symmetry , which are otherwise in tension with the current bound on \sum m _ { \nu } .