Primordial Perturbations from Multifield Inflation with Nonminimal
  Couplings
  Realistic models of particle physics include many scalar fields. These fields
generically have nonminimal couplings to the Ricci curvature scalar, either as
part of a generalized Einstein theory or as necessary counterterms for
renormalization in curved background spacetimes. We develop a gauge-invariant
formalism for calculating primordial perturbations in models with multiple
nonminimally coupled fields. We work in the Jordan frame (in which the
nonminimal couplings remain explicit) and identify two distinct sources of
entropy perturbations for such models. One set of entropy perturbations arises
from interactions among the multiple fields. The second set arises from the
presence of nonminimal couplings. Neither of these varieties of entropy
perturbations will necessarily be suppressed in the long-wavelength limit, and
hence they can amplify the curvature perturbation, $\zeta$, even for modes that
have crossed outside the Hubble radius. Models that overproduce long-wavelength
entropy perturbations endanger the close fit between predicted inflationary
spectra and empirical observations.
