Semiclassical instability of compactification

It is shown that several schemes for compactification of the extra dimensions in Kaluza-Klein theories are unstable to a quantum gravitational process of barrier penetration: The universe can tunnel from a state with static extra dimensions to a de Sitter expansion of all dimensions. The tunneling rate is estimated, and it is found that the present state of the universe is probably long-lived (in good agreement with observation).