  We propose to describe the acceleration of the universe by introducing a
model of two coupled fluids. We focus on the accelerated expansion at the early
stages. The inflationary expansion is described in terms of a van der Waals
equation of state for the cosmic fluid, when account is taken of bulk
viscosity. We assume that there is a weak interaction between the van der Waals
fluid and the second component (matter). The gravitational equations for the
energy densities of the two components are solved for a homogeneous and
isotropic Friedmann-Robertson-Walker universe, and analytic expressions for the
Hubble parameter are obtained. The slow-roll parameters, the spectral index,
and the tensor-to-scalar ratio are calculated and compared with the most recent
astronomical data from the Planck satellite. Given reasonable restriction on
the parameters, the agreement with observations is favorable.
