  We describe a class of holographic models that may describe the physics of
certain four-dimensional big-bang / big-crunch cosmologies. The construction
involves a pair of 3D Euclidean holographic CFTs each on a homogeneous and
isotropic space $M$ coupled at either end of an interval ${\cal I}$ to a
Euclidean 4D CFT on $M \times {\cal I}$ with many fewer local degrees of
freedom. We argue that in some cases, when the size of $M$ is much greater than
the length of ${\cal I}$, the theory flows to a gapped / confining
three-dimensional field theory on $M$ in the infrared, and this is reflected in
the dual description by the asymptotically AdS spacetimes dual to the two 3D
CFTs joining up in the IR to give a Euclidean wormhole. The Euclidean
construction can be reinterpreted as generating a state of Lorentzian 4D CFT on
$M \times {\rm time}$ whose dual includes the physics of a big-bang /
big-crunch cosmology. When $M$ is $\mathbb{R}^3$, we can alternatively
analytically continue one of the $\mathbb{R}^3$ directions to get an eternally
traversable four-dimensional planar wormhole. We suggest explicit microscopic
examples where the 4D CFT is ${\cal N}=4$ SYM theory and the 3D CFTs are
superconformal field theories with opposite orientation. In this case, the two
geometries dual to the pair of 3D SCFTs can be understood as a geometrical
version of a brane-antibrane pair, and the tendency of the geometries to
connect up is related to the standard instability of brane-antibrane systems.
