  The existence of a cosmic neutrino background can be probed indirectly by CMB
experiments, not only by measuring the background density of radiation in the
universe, but also by searching for the typical signatures of the fluctuations
of free-streaming species in the temperature and polarisation power spectrum.
Previous studies have already proposed a rather generic parametrisation of
these fluctuations, that could help to discriminate between the signature of
ordinary free-streaming neutrinos, or of more exotic dark radiation models.
Current data are compatible with standard values of these parameters, which
seems to bring further evidence for the existence of a cosmic neutrino
background. In this work, we investigate the robustness of this conclusion
under various assumptions. We generalise the definition of an effective sound
speed and viscosity speed to the case of massive neutrinos or other dark
radiation components experiencing a non-relativistic transition. We show that
current bounds on these effective parameters do not vary significantly when
considering an arbitrary value of the particle mass, or extended cosmological
models with a free effective neutrino number, dynamical dark energy or a
running of the primordial spectrum tilt. We conclude that it is possible to
make a robust statement about the detection of the cosmic neutrino background
by CMB experiments.
