Speaker: Shuntaro Mizuno

Abstract:    We investigate linear cosmological perturbations in multiple field inflationary models where some of the directions are light while others are heavy (with respect to the Hubble parameter). By integrating out the massive degrees of freedom, we determine the multi-dimensional effective theory for the light degrees of freedom and give explicitly the propagation matrix that replaces the effective sound speed of the one-dimensional case. We then examine in detail the consequences of a sudden turn along the inflationary trajectory, in particular the possible breakdown of the low energy effective theory in case the heavy modes are excited. Resorting to the mass basis, i.e. in the basis spanned by the eigenvectors of the effective mass matrix for the perturbations, instead of the usual adiabatic/entropic basis, we study the evolution of the perturbations during the turn. In particular, we compute the power spectrum and compare with the result obtained from the low energy effective theory. We also consider the resonant effect due to the background oscillations following a sharp turn. Working in the mass basis, we provide an analytical estimate of the resonant effect. In comparison with earlier estimates, we find the same the spectral dependence but a smaller amplitude.