  An investigation is undertaken into the properties and effects of a
preinflationary era during at least part of which semiclassical gravity was
valid. It is argued that if the Universe (or our part of it) was approximately
homogeneous and isotropic during that era, then the Universe was likely to have
been radiation dominated. A simple model in which the Universe contains
classical radiation and a cosmological constant is used to investigate
potential effects of such a preinflationary era on the cosmic microwave
background. The power spectrum is computed using the mode functions of a
quantized massless minimally coupled scalar field. Various choices of state for
this field are considered, including adiabatic vacuum states of various orders
and the vacuum state that would naturally occur if the Universe made a sudden
transition from being radiation dominated to de Sitter space. In all cases
investigated, there is a suppression of the power spectrum at large angles,
and, when plotted as a function of the momentum parameter, there are always
oscillations with state-dependent amplitudes.
