Self-interacting Dark Radiation

Understand

We consider a simple class of models where dark radiation has self-interactions and therefore does not free stream.

• Such dark radiation has no anisotropic stress (or viscosity), leaving a distinct signature on the CMB angular power spectrum.
• Specifically we study a possibility that hidden gauge bosons and/or chiral fermions account for the excess of the effective number of neutrino species.
• They have gauge interactions and remain light due to the unbroken hidden gauge symmetry, leading to \Delta N_{\rm eff} \simeq 0.29 in some case.