  We examine how coupled dark matter and dark energy modify the development of
Zel'dovich pancakes. We study how the various effects of these theories, such
as a fifth force in the dark sector and a modified particle Hubble drag,
produce variations in the redshifts of caustic formation and the present-day
density profiles of pancakes. We compare our results in direct simulation to a
perturbation theory approach for the dark energy scalar field. We determine the
range of initial scalar field amplitudes for which perturbation theory is
accurate in describing the development of the pancakes. Notably, we find that
perturbative methods which neglect kinetic terms in the scalar field equation
of motion are not valid for arbitrarily small perturbations. We also examine
whether models that have been tuned to match the constraints of current
observations can produce new observable effects in the nonlinear structure of
pancakes. Our results suggest that a fully realistic three-dimensional
simulation will produce significant new observable features, such as
modifications to the mass function and halo radial density profile shapes, that
can be used to distinguish these models from standard concordance cosmology and
from each other.
