Fully non-linear equivalence of delta N and covariant formalisms
  We explicitly show the fully non-linear equivalence of the $\delta$N and the covariant formalisms for the superhorizon curvature perturbations, which enables us to safely evaluate the non-Gaussian quantities of the curvature perturbation in either formalism. We also discuss isocurvature perturbations in the covariant formalism and clarify the relation between the fully non-linear evolution of the curvature covector and that of the curvature perturbation for multiple interacting fluids. 
Teruaki Suyama, Yuki Watanabe and Masahide Yamaguchi
astro-ph.CO gr-qc hep-ph hep-th