  We examine the renormalization of an effective theory description of a
general initial state set in an isotropically expanding space-time, which is
done to understand how to include the effects of new physics in the calculation
of the cosmic microwave background power spectrum. The divergences that arise
in a perturbative treatment of the theory are of two forms: those associated
with the properties of a field propagating through the bulk of space-time,
which are unaffected by the choice of the initial state, and those that result
from summing over the short-distance structure of the initial state. We show
that the former have the same renormalization and produce the same subsequent
scale dependence as for the standard vacuum state, while the latter correspond
to divergences that are localized at precisely the initial time hypersurface on
which the state is defined. This class of divergences is therefore renormalized
by adding initial-boundary counterterms, which render all of the perturbative
corrections small and finite. Initial states that approach the standard vacuum
at short distances require, at worst, relevant or marginal boundary
counterterms. States that differ from the vacuum at distances below that at
which any new, potentially trans-Planckian, physics becomes important are
renormalized with irrelevant boundary counterterms.
