Interacting viscous matter with a dark energy fluid
  We study a cosmological model composed of a dark energy fluid interacting
with a viscous matter fluid in a spatially flat Universe. The matter component
represents the baryon and dark matter and it is taken into account, through a
bulk viscosity, the irreversible process that the matter fluid undergoes
because of the accelerated expansion of the universe. The bulk viscous
coefficient is assumed to be proportional to the Hubble parameter. The
radiation component is also taken into account in the model. The model is
constrained using the type Ia supernova observations, the shift parameter of
the CMB, the acoustic peak of the BAO and the Hubble expansion rate, to
constrain the values of the barotropic index of dark energy and the bulk
viscous coefficient. It is found that the bulk viscosity is constrained to be
negligible (around zero) from the observations and that the barotropic index
for the dark energy to be negative and close to zero too, indicating a phantom
energy.
