Interacting photon-baryon fluid, warm dark matter and the first acoustic
  peak
  The Reduced Relativistic Gas (RRG) model was introduced by A. Sakharov in
1965 for deriving the cosmic microwave background (CMB) spectrum. It was
recently reinvented by some of us to achieve an interpolation between the
radiation and dust epochs in the evolution of the Universe. This model
circumvents the complicated structure of the Boltzmann-Einstein system of
equations and admits a transparent description of warm-dark-matter effects. It
is extended here to include, on a phenomenological basis, an out-of-equilibrium
interaction between radiation and baryons which is supposed to account for
relevant aspects of pre-recombination physics in a simplified manner.
Furthermore, we use the tight-coupling approximation to explore the influence
of both this interaction and of the RRG warmness parameter on the anisotropy
spectrum of the CMB. The predictions of the model are very similar to those of
the {\Lambda}CDM model if both the interaction and the dark-matter warmness
parameters are of the order of $10^{-4}$ or smaller. As far as the warmness
parameter is concerned, this is in good agreement with previous estimations on
the basis of results from structure formation.
