Relaxing Cosmological Neutrino Mass Bounds with Unstable Neutrinos

Understand

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.