  The dark matter in the universe can be in the form of a superheavy matter
species (WIMPZILLA). Several mechanisms have been proposed for the production
of WIMPZILLA particles during or immediately following the inflationary epoch.
Perhaps the most attractive mechanism is through gravitational particle
production, where particles are produced simply as a result of the expansion of
the universe. In this paper we present a detailed numerical calculation of
WIMPZILLA gravitational production in hybrid-inflation models and
natural-inflation models. Generalizing these findings, we also explore the
dependence of the gravitational production mechanism on various models of
inflation. We show that superheavy dark matter production seems to be robust,
with Omega_X h^2 ~ (M_X / (10^11 GeV))^2 (T_RH / (10^9 GeV)), so long as M_X <
H_I, where M_X is the WIMPZILLA mass, T_RH is the reheat temperature, and H_I
is the expansion rate of the universe during inflation.
