Generation of metric perturbations during preheating

We consider the generation of the scalar mode of metric perturbations during the preheating stage in a two field model with the potential V(φ,χ)=m²φ²/2+g²φ²χ²/2. We discuss two possible sources of such perturbations: (a) due to the coupling between the perturbation of the matter field δχ and the background part of the matter field χ₀(t); (b) due to nonlinear fluctuations in a condensate of ``particles'' of the field χ. Both types of metric perturbations are assumed to be small, and estimated using the linear theory of metric perturbations. We estimate analytically the upper limit of the amplitude of the metric perturbations for all scales in the limit of so-called broad resonance, and show that the large scale metric perturbations are very small, and taking them into account does not influence the standard picture of the production of metric perturbations in the inflationary scenario.