  It is shown that a present acceleration with a past deceleration is a
possible solution of the Friedmann equation by considering the Universe as a
mixture of a scalar with a matter field and by including a non-equilibrium
pressure term in the energy-momentum tensor. The dark energy density decays
more slowly with respect to the time than the matter energy density does. The
inclusion of the non-equilibrium pressure leads to a less pronounced decay of
the matter field with a shorter period of past deceleration.
