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 analyzed assuming a noninteracting 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 $c_{\mathrm{eff}}^2={0.34}_{-0.003}^{+0.006}$ and $c_{\mathrm{vis}}^2={0.29}_{-0.001}^{+0.002}$ at 95% C.L. for the future COrE mission data.

• Received 7 January 2013

DOI:https://doi.org/10.1103/PhysRevD.87.063509