  Popular extensions of the standard model of particle physics feature new
fields and symmetries which could, for example, dynamically generate neutrino
masses from $B-L$ spontaneous symmetry breaking. If a new light scalar that
decays into dark radiation appears in the spectrum of the theory, it could
significantly modify the cosmological observables. In this case, cold dark
matter could have a stable and a decaying component and limits on its decay
rate $\Gamma_{\rm dcdm}$ can be used to put constraints on the new energy
scales of a given model. We illustrate this idea using a gauged $B-L$ model
where the dark radiation is in the form of light neutrinos.
