Emergence of classical geometry in quantum cosmology

Abstract

How the picture of time evolution, typical for classical cosmology, emerges from quantum cosmology? The question is difficult because the Wheeler-DeWitt equation of quantum cosmology leads to trivial dependence on time. However time dependence arises in a natural way if continuous measurement of quantum Universe is taken into account. More precisely, some degrees of freedom of the Universe (such as its scale factor) should be considered as a quantum system while all the rest degrees of freedom play role of the measuring medium. In the Restricted-Path-Integral approach the influence of these measuring degrees of freedom can be taken into account without explicit consideration of them.