Dark Radiation and interacting scenarios
  An extra dark radiation component can be present in the universe in the form
of sterile neutrinos, axions or other very light degrees of freedom which may
interact with the dark matter sector. We derive here the cosmological
constraints on the dark radiation abundance, on its effective velocity and on
its viscosity parameter from current data in dark radiation-dark matter coupled
models. The cosmological bounds on the number of extra dark radiation species
do not change significantly when considering interacting schemes. We also find
that the constraints on the dark radiation effective velocity are degraded by
an order of magnitude while the errors on the viscosity parameter are a factor
of two larger when considering interacting scenarios. If future Cosmic
Microwave Background data are analysed assuming a non interacting model but the
dark radiation and the dark matter sectors interact in nature, the
reconstructed values for the effective velocity and for the viscosity parameter
will be shifted from their standard 1/3 expectation, namely ceff=0.34 (+0.006
-0.003) and cvis=0.29 (+0.002 -0.001) at 95% CL for the future COrE mission
data.
