Revisiting non-Gaussianity in multifield inflation with curved field
  space
  Recent studies of inflation with multiple scalar fields have highlighted the
importance of non-canonical kinetic terms in novel types of inflationary
solutions. This motivates a thorough analysis of non-Gaussianities in this
context, which we revisit here by studying the primordial bispectrum in a
general two-field model. Our main result is the complete cubic action for
inflationary fluctuations written in comoving gauge, i.e. in terms of the
curvature perturbation and the entropic mode. Although full expressions for the
cubic action have already been derived in terms of fields fluctuations in the
flat gauge, their applicability is mostly restricted to numerical evaluations.
Our form of the action is instead amenable to several analytical
approximations, as our calculation in terms of the directly observable quantity
makes manifest the scaling of every operator in terms of the slow-roll
parameters, what is essentially a generalization of Maldacena's single-field
result to non-canonical two-field models. As an important application we derive
the single-field effective field theory that is valid when the entropic mode is
heavy and may be integrated out, underlining the observable effects that derive
from a curved field space.
