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

Evolution of quantum field, particle content and classicality in the three stage universe