Inflation and preheating in nonoscillatory models

We study inflationary models in which the effective potential of the inflaton field does not have a minimum, but rather gradually decreases at large phi. In such models the inflaton field does not oscillate after inflation, and its effective mass becomes vanishingly small, so the standard theory of reheating based on the decay of the oscillating inflaton field does not apply. For a long time the only mechanism of reheating in such nonoscillatory (NO) models was based on gravitational particle production in an expanding universe. This mechanism is very inefficient. We will show that it may lead to cosmological problems associated with large isocurvature fluctuations and overproduction of dangerous relies such as gravitinos and moduli fields. We also note that the setting of initial conditions for the stage of reheating in these models should be reconsidered. All of these problems can be resolved in the context of the recently proposed scenario of instant preheating if there exists an interaction g(2)phi(2)chi(2) of the inflaton field phi with another scalar field chi. We show that the mechanism of instant preheating in NO models is much more efficient than the usual mechanism of gravitational particle production even if the coupling constant g(2) is extremely small, 10(-14) much less than g(2) much less than 1. [S0556-2821(99)08318-6].