Interacting radiation after Planck and its implications for the Hubble
  Tension
  Standard cosmology predicts that prior to matter-radiation equality about 41%
of the energy density was in free-streaming neutrinos. In many beyond Standard
Model scenarios, however, the amount and free-streaming nature of this
component is modified. For example, this occurs in models with new neutrino
self-interactions or an additional dark sector with interacting light
particles. We consider several extensions of the standard cosmology that
include a non-free-streaming radiation component as motivated by such particle
physics models and use the final Planck data release to constrain them. This
release contains significant improvements in the polarization likelihood which
plays an important role in distinguishing free-streaming from interacting
radiation species. Fixing the total amount of energy in radiation to match the
expectation from standard neutrino decoupling we find that the fraction of
free-streaming radiation must be $f_\mathrm{fs} > 0.8$ at 95% CL (combining
temperature, polarization and baryon acoustic oscillation data). Allowing for
arbitrary contributions of free-streaming and interacting radiation, the
effective number of new non-free-streaming degrees of freedom is constrained to
be $N_\mathrm{fld} < 0.6$ at 95% CL. Cosmologies with additional radiation are
also known to ease the discrepancy between the local measurement and CMB
inference of the current expansion rate $H_0$. We show that including a
non-free-streaming radiation component allows for a larger amount of total
energy density in radiation, leading to a mild improvement of the fit to
cosmological data compared to previously discussed models with only a
free-streaming component.
