  We study the evolution of a quantum scalar field in a toy universe which has
three stages of evolution, viz., (i) an early (inflationary) de Sitter phase
(ii) radiation dominated phase and (iii) late-time (cosmological constant
dominated) de Sitter phase. Using Schr\"odinger picture, the scalar field
equations are solved separately for the three stages and matched at the
transition points. The boundary conditions are chosen so that field modes in
the early de Sitter evolves from Bunch-Davies vacuum state. We determine the
(time-dependent) particle content of this quantum state for the entire
evolution of the universe and describe the various features both numerically
and analytically. We also describe the quantum to classical transition in terms
of a {\it classicality parameter} which tracks the particle creation and its
effect on phase space correlation of the quantum field.
