  Quantum cosmology based on the Wheeler De Witt equation represents a simple
way to implement plausible quantum effects in a gravitational setup. In its
minisuperspace version wherein one restricts attention to FLRW metrics with a
single scale factor and only a few degrees of freedom describing matter, one
can obtain exact solutions and thus acquire full knowledge of the wave
function. Although this is the usual way to treat a quantum mechanical system,
it turns out however to be essentially meaningless in a cosmological framework.
Turning to a trajectory approach then provides an effective means of deriving
physical consequences.
