  The origin of Big Bang singularity in 3+1 dimensions can be understood in an
exact string theory background obtained by an analytic continuation of a cigar
like geometry with a nontrivial dilaton. In a T-dual conformal field theory
picture there exists a closed string tachyon potential which excises the
singular space-time of a strongly coupled regime to ensure that a higher
dimensional universe has no curvature singularity. However in 3+1 dimensions
the universe exhibits all the pathology of a standard Big Bang cosmology. The
emergence of a singularity now owes to a higher dimensional orbifold
singularity which does not have a curvature singularity in higher dimensions,
suggesting that close to the compactification scale an effective description of
3+1 dimensions breaks down and bouncing universe emerges in 5 and higher
dimensions.
