Isocurvature perturbations in multiple inflationary models

The dynamics of long-wave isocurvature perturbations during an inflationary stage in multiple (multicomponent) inflationary models is calculated analytically for the case where scalar fields producing this stage interact among themselves through gravity only. This enables us to determine the correct amplitudes of such perturbations produced by vacuum quantum fluctuations of the scalar fields during the multiple inflationary stage. An exact matching to a post-inflationary evolution that gives the amplitude of isocurvature perturbations in the cold dark matter model with radiation is performed in the case where a massive inflaton field remains uncoupled from usual matter up to the present time. For this model, isocurvature perturbations are smaller than adiabatic ones in the region of the break in the perturbation spectrum which arises due to a transition between the two phases of inflation, but they may be much bigger and have a maximum at much shorter scales. The case of an inflaton with a quartic coupling that remains uncoupled after inflation is considered, too.