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An investigation is undertaken into the properties and effects of apreinflationary era during at least part of which semiclassical gravity wasvalid. It is argued that if the Universe (or our part of it) was approximatelyhomogeneous and isotropic during that era, then the Universe was likely to havebeen radiation dominated. A simple model in which the Universe containsclassical radiation and a cosmological constant is used to investigatepotential effects of such a preinflationary era on the cosmic microwavebackground. The power spectrum is computed using the mode functions of aquantized massless minimally coupled scalar field. Various choices of state forthis field are considered, including adiabatic vacuum states of various ordersand the vacuum state that would naturally occur if the Universe made a suddentransition from being radiation dominated to de Sitter space. In all casesinvestigated, there is a suppression of the power spectrum at large angles,and, when plotted as a function of the momentum parameter, there are alwaysoscillations with state-dependent amplitudes.

Semiclassical predictions regarding a pre-inflationary era and its effects on the power spectrum