Cosmological perturbations from multi-field inflation in generalized Einstein theories

We study cosmological perturbations generated from quantum fluctuations in multi-field inflationary scenarios in generalized Einstein theories, taking both adiabatic and isocurvature modes into account. In the slow-roll appro ximation, explicit closed-form long-wave solutions for field and metric per turbations are obtained by the analysis in the Einstein frame. Since the ev olution of fluctuations depends on specific gravity theories, we make detai led investigations based on analytic and numerical approaches in four gener alized Einstein theories: the Jordan-Brans-Dicke (JBD) theory, the Einstein gravity with a non-minimally coupled scalar field, the higher-dimensional Kaluza-Klein theory, and the R + R-2 theory with a non-minimally coupled sc alar field. We find that solutions obtained in the slow-roll approximation show good agreement with full numerical results except around the end of in flation. Due to the presence of isocurvature perturbations., the gravitatio nal potential Phi and the curvature perturbation zeta do not remain constan t on super-horizon scales. In particular, we find that negative non-minimal coupling can lead to strong enhancement of in both the Einstein and higher derivative gravity, in which case it is difficult to unambiguously decompo se scalar perturbations into adiabatic and isocurvature modes during the wh ole stage of inflation. (C) 2001 Elsevier Science B.V. All rights reserved.