Probing neutrino decays with the cosmic microwave background

We investigate in detail the possibility of constraining neutrino decays with data from the cosmic microwave background radiation (CMBR). Two generic decays are considered ${\nu }_H\to {\nu }_L\phi$ and ${\nu }_H\to {\nu }_L{\overline{\nu }}_L{\nu }_L.$ We have solved the momentum dependent Boltzmann equation in order to account for possible relativistic decays. Doing this, we estimate that any neutrino with mass $m\gtrsim 1$ eV decaying before the present should be detectable with future CMBR data. Combining this result with other results on stable neutrinos, any neutrino mass of the order 1 eV should be detectable.