Partial creation in de Sitter spacetime is studied in a complex-time WKB approximation. It is found that the particle production parallels the description of particle creation by a collapsing spherical body. The appropriate vacuum for an inflationary early universe is the Hartle--Hawking vacuum; this is also a general feature of expanding spacetimes. The effect of particle production back on the metric is treated in a way that differs from the standard semiclassical approach.

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We show that the Dirac equation in Robertson--Walker (RW) spacetime can be written in a two-dimensional equivalent form that parallels the motion of an electron in a time-varying electric field. Using the effect of particle production, a RW spacetime that describes a universe which contracts and re-expands almost symmetrically and becomes asymptotically static is simulated. The back-reaction problem is considered only through the quantum behaviour of particle production without any reference to Einstein field equations.

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