Cosmological Constraints on Invisible Neutrino Decays Revisited
  Invisible neutrino decay modes are difficult to target at laboratory
experiments, and current bounds on such decays from solar neutrino and neutrino
oscillation experiments are somewhat weak. It has been known for some time that
Cosmology can serve as a powerful probe of invisible neutrino decays. In this
work, we show that in order for Big Bang Nucleosynthesis to be successful, the
invisible neutrino decay lifetime is bounded to be $\tau_\nu >
10^{-3}\,\text{s}$ at 95\% CL. We revisit Cosmic Microwave Background
constraints on invisible neutrino decays, and by using Planck2018 observations
we find the following bound on the neutrino lifetime: $\tau_\nu >
(1.3-0.3)\times 10^{9}\,\text{s} \, \left({m_\nu}/{ 0.05\,\text{eV} }\right)^3$
at $95\%$ CL. We show that this bound is robust to modifications of the
cosmological model, in particular that it is independent of the presence of
dark radiation. We find that lifetimes relevant for Supernova observations
($\tau_\nu \sim 10^{5}\,\text{s}\, \left({m_\nu}/{ 0.05\,\text{eV} }\right)^3$)
are disfavoured at more than $5\,\sigma$ with respect to $\Lambda$CDM given the
latest Planck CMB observations. Finally, we show that when including
high-$\ell$ Planck polarization data, neutrino lifetimes $\tau_\nu =
(2-16)\times 10^{9}\,\text{s} \, \left({m_\nu}/{ 0.05\,\text{eV} }\right)^3$
are mildly preferred -- with a 1-2 $\sigma$ significance -- over neutrinos
being stable.
