Pancakes as opposed to Swiss Cheese
  We examine a novel class of toy models of cosmological inhomogeneities by
smoothly matching along a suitable hypersurface an arbitrary number of sections
of "quasi flat" inhomogeous and anisotropic Szekeres-II models to sections of
any spatially flat cosmology that can be described by the Robertson-Waker
metric (including de Sitter, anti de Sitter and Minkowski spacetimes). The
resulting "pancake" models are quasi-flat analogues to the well known spherical
"Swiss-cheese" models found in the literature. Since Szekeres-II models can be,
in general, compatible with a wide range of sources (dissipative fluids,
mixtures of non-comoving fluids, mixtures of fluids with scalar or magnetic
fields or gravitational waves), the pancake configurations we present allow for
a description of a wide collection of localized sources embedded in a
Robertson-Waker geometry. We provide various simple examples of arbitrary
numbers of Szekeres-II regions (whose sources are comoving dust and energy flux
interpreted as a field of peculiar velocities) matched with Einstein de Sitter,
$\Lambda$CDM and de Sitter backgrounds. We also prove that the Szekeres-II
regions can be rigorously regarded as "exact" perturbations on a background
defined by the matching discussed above. We believe that these models can be
useful to test ideas on averaging and backreaction and on the effect of
inhomogeneities on cosmic evolution and observations.
