The notions of time and evolution in quantum cosmology

Abstract

We re-examine the notions of time and evolution in the light of the mathematical properties of the solutions of the Wheeler-DeWitt equation which are revealed by an extended adiabatic treatment. The main advantage of this treatment is to organize the solutions in series that make explicit both the connections with the corresponding Schr\"odinger equation as well as the modifications introduced by the quantum character of gravity. When the universe is macroscopic, the ordered character of the expansion leads to connections with the Schr\"odinger equation so precise that the interpretation of the solutions of the Wheeler-DeWitt equation is unequivocally determined. On the contrary, when the expansion behaves quantum mechanically, i.e. in the presence of backscattering, major difficulties concerning the interpretation persist.