Born–Oppenheimer meets Wigner–Weyl in quantum gravity

Starting from a Born–Oppenheimer decomposition of the Wheeler-DeWitt equation for the quantum cosmology of the matter–gravity system, we have performed a Wigner–Weyl transformation and obtained equations involving a Wigner function for the scale factor and its conjugate momentum. This has allowed us to study in more detail than previously the approach to the classical limit of gravitation and the way time emerges in such a limit. To lowest order we reproduce the Friedmann equation and the previously obtained equation for the evolution of matter. We also obtain expressions for higher order corrections to the semi-classical limit. (paper)