  We construct 4D flat Big Bang-Big Crunch cosmologies and Anti-de Sitter (AdS)
planar eternally traversable wormholes using braneworlds embedded in
asymptotically AdS${}_5$ spacetimes. The background geometries are the
AdS${}_5$ magnetic black brane and the magnetically charged AdS${}_5$ soliton,
respectively. The two setups arise from different analytic continuations of the
same saddle of the gravitational Euclidean path integral, in which the
braneworld takes the form of a Maldacena-Maoz Euclidean wormhole. We show the
existence of a holographic dual description of this setup in terms of a
microscopic Euclidean boundary conformal field theory (BCFT) on a strip. By
analyzing the BCFT Euclidean path integral, we show that the braneworld
cosmology is encoded in a pure excited state of a CFT dual to a black brane
microstate, whereas the braneworld wormhole is encoded in the ground state of
the BCFT. The latter confines in the IR, and we study its confining properties
using holography. We also comment on the properties of bulk reconstruction in
the two Lorentzian pictures and their relationship via double analytic
continuation. This work can be interpreted as an explicit, doubly-holographic
realization of the relationship between cosmology, traversable wormholes, and
confinement in holography, first proposed in arXiv:2102.05057,
arXiv:2203.11220.
