  We construct a model for the universe based on the existence of quantum
fields at finite temperature in the background of Robertson-Walker spacetime in
presence of a non-zero cosmological constant. We discuss the vacuum regime in
the light of the results obtained through previous studies of the back-reaction
of massless quantum fields in the static Einstein universe, and we argue that
an adiabatic vacuum state and thermal equilibrium is achieved throughout this
regime. Results shows that such a model can explain many features of the early
universe as well as the present universe. The model is free from the basic
problems of the standard Friedmann cosmology, and is non-singular but involves
a continuous creation of energy at a rate proportional to the size of the
universe, which is lower than that suggested by the steady-state cosmology.
