Ordering-Independent Wheeler-DeWitt Equation for Flat Minisuperspace Models

We consider minisuperspace models with quadratic kinetic terms, assuming a flat target space and a closed Universe. We show that, upon canonical quantization of the Hamiltonian, only a restricted subset of operator orderings is consistent with the path-integral viewpoint. Remarkably, all consistent orderings are physically equivalent to all orders in hbar. Specifically, each choice of path-integral measure in the definition of the wavefunction path integral uniquely determines an operator ordering and hence a corresponding Wheeler-DeWitt equation. The consistent orderings are in one-to-one correspondence with the Jacobians associated with all field redefinitions of a set of canonical degrees of freedom. For each admissible operator ordering--or equivalently, each path-integral measure--we identify a definite, positive Hilbert-space inner product. All such prescriptions define the same quantum theory, in the sense that they lead to identical physical observables. We illustrate our formalism by applying it to de Sitter Jackiw-Teitelboim gravity and to the Starobinsky model.