  We consider an eight-dimensional Einstein-Yang-Mills theory to explore
whether Yang-Mills instantons formed in extra dimensions can induce the
dynamical instability of our four-dimensional spacetime. We show that the
Yang-Mills instantons in extra dimensions can trigger the expansion of our
universe in four-dimensional spacetime as well as the dynamical
compactification of extra dimensions. We also discuss a possibility to realize
a reheating mechanism via the quantum back-reaction from the contracting tiny
internal space with a smeared instanton.
