Coexisting Spacetimes in the Solar Neighborhood
  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. 
Alasdair Macleod