  We investigate possible cosmological effects of interacting scalar radiation
and dark matter. After its decoupling, scalar radiation can stream freely as
neutrinos or self-interact strongly as perfect fluid, highly depending on the
magnitude of its self-couplings. We obtain the general and novel structure for
self-scattering rate and compare it with the expansion rate of our Universe. If
its trilinear/cubic coupling is non-zero, scalar radiation can be eventually
treated as perfect fluid. Possible effects on CMB are also discussed. When this
scalar also mediates interaction among dark matter particles, the linear matter
power spectrum for large scale structure can be modified differently from other
models. We propose to use Debye shielding to avoid the singularity appearing in
the scattering between scalar radiation and dark matter.
