Chaotic reheating

We discuss the structure of parametric resonance which occurs in the process of reheating after inflation with two interacting scalar fields. It is found that, for the case of a not too large coupling constant, a quasi-homogeneous part of the second, initially subdominant scalar field may not be neglected due to its stochastic growth during inflation. This fact has strong consequences for the reheating stage: dynamics of background fields becomes chaotic after inflation -- it consists of subsequent chaotic and regular eras, and a tachyonic instability for inhomogeneous perturbations arises in the quantum reheating problem. This instability may pose a problem for the standard reheating scenario. In order to avoid it, the coupling constant should be either sufficiently small, or very large.