  Bubble universes and traversable wormholes in general relativity can be
realized as two sides of the same concept. To exemplify, we find, display, and
study in a unified manner a Minkowski-Minkowski closed universe and a
Minkowski-Minkowski traversable wormhole. By joining two 3-dimensional flat
balls along a thin shell two-sphere of matter, i.e., a spherical domain wall,
into a single spacetime one gets a Minkowski-Minkowski static closed universe,
i.e., a bubble universe. By joining two 3-dimensional complements of flat balls
along a thin shell two-sphere of matter, i.e., a spherical throat, into a
single spacetime one gets a Minkowski-Minkowski static open universe which is a
traversable wormhole. Thus, Minkowski-Minkowski bubble universes and wormholes
can be seen as complementary. It is also striking that these two spacetimes
have resemblances with two well-known static universes. The Minkowski-Minkowski
static closed universe resembles the Einstein universe, a static closed
spherical universe homogeneously filled with dust matter and with a
cosmological constant. The Minkowski-Minkowski static open universe resembles
the Friedmann static universe, a static open hyperbolic universe homogeneously
filled with negative energy density dust and with a negative cosmological, a
universe with two disjoint branes that can be considered a failed wormhole. In
this light, the Einstein and Friedmann universes are also two sides of the same
concept. A linear stability analysis for all these spacetimes is performed. The
complementarity between bubble universes and traversable wormholes, that exists
for these static spacetimes, can be can carried out for dynamical spacetimes,
indicating that such a complementarity is general. The study suggests that
bubble universes and traversable wormholes can be seen as coming out of the
same concept, and thus, if ones exist the others should also exist.
