On the choice of entropy variables in multifield inflation
  We discuss the usefulness and theoretical consistency of different entropy
variables used in the literature to describe isocurvature perturbations in
multifield inflationary models with a generic curved field space. We clarify
which is the proper entropy variable to be used to match the evolution of
isocurvature modes during inflation to the one after the reheating epoch in
order to compare with observational constraints. In particular, we find that
commonly used variables, as the relative entropy perturbation or the one
associated to the decomposition in tangent and normal perturbations with
respect to the inflationary trajectory, even if more useful to perform
numerical studies, can lead to results which are wrong by several orders of
magnitude, or even to apparent destabilisation effects which are unphysical for
cases with light kinetically coupled spectator fields.
