Mars et al. make compelling arguments in gr-qc: 0710.0820 that the Universe's accelerated approach towards a future singularity may be due to a change in signature of our braneworld in a bulk 5D Anti de Sitter space (AdS_5).
The authors argue that by choosing hypersurfaces in AdS_5 appropriately, it is possible to construct signature changing branes, with well behaved properties. They explain that hypothetical scientists living on such a hypersurface in AdS_5 might not realize that they are living on a brane, much less a brane that can undergo a Lorentzian -> Euclidean signature change. If the scientists living on the brane use General Relativity as their gravitational theory, they will find that the eigenvalues of the brane Einstein tensor diverge at a 'singularity' placed on the signature changing set. This type of singularity has recently been called a sudden singularity, as well as 'type III' singularity and 'big freeze'. Sudden singularities violate certain energy conditions, signaling the existence of phantom or dark energy components, an illusion caused by an imminent signature change on the brane.
The paper is a nice 8-page read for anyone interested in braneworld cosmology.
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