Limits on reheating in inflationary cosmology models

We show that the decay of a scalar particle into ordinary matter particles is restricted by the mass and by the vacuum expectation value of the scalar field. If this scalar field is responsible for inflation, difficulties with reheating the Universe may ensue. For the case of induced-gravity models, where the magnitude of the vacuum expectation value is of order the Planck mass, the scalar field decays predominantly into gravitons and adequate reheating does not occur in the usual slow rollover scenario. We also discuss the effect of the energy stored in the scalar-field oscillations