Abstract
We study nonlinear cosmological perturbations during post-inflationary evolution, using
the equivalence between a perfect barotropic fluid and a derivatively coupled scalar field
with Lagrangian 
. Since this Lagrangian is just a special case of
k-inflation, this approach is analogous to the one employed in the study of non-Gaussianities
from inflation. We use this method to derive the second-order metric during matter dominance
in the comoving gauge directly as a function of the primordial inflationary perturbation
ζ. Going to Poisson gauge, we recover the metric previously derived in the literature.