  We consider the proposition that multiple universes exist by reviewing the
various manifestations. In recent years, this idea has been elevated from
science fiction and introduced in separate guises as an explanation for
coincidence problems in cosmology, the prediction of dark energy, gravitational
anomalies, a consequence of string theory, an extension of inflation, and
conceptual issues in quantum mechanics. However, there appears to be no single
consistent formalism that addresses all the issues - it is not even clear if
the multiple universes interact or are accessible. Because of the absence of
clear evidence, it is easy to dismiss claims for multiple Universes, but it is
not that simple: Space-time geometries in a variety of forms are an established
aspect of physics, and history has shown that things which are not expressly
forbidden often appear at a later date. With this in mind, a new example of
possible multiple geometries is introduced to explain a difficult problem in
cosmology: Why are distant galaxies subject to the Hubble expansion but rulers
within our galaxy (and presumably, by the Cosmological Principle, all other
galaxies) do not appear to expand? There has actually been much debate in
recent years about whether local systems really are subject to the cosmological
expansion; we adopt the established view that local systems are not expanding
and investigate the transition condition where a mass may be part of the static
galactic system or participate in the expansion. We show that a remarkably
simple model based on coexisting spacetimes clarifies the situation and makes
quantative, testable cosmological predictions. The model is also testable
through Earth-based experiments.
