A nonlinear quantum model of the Friedmann universe

A discussion is given of the quantization of a physical system with finite degrees of freedom subject to a Hamiltonian constraint by treating time as a constrained classical variable interacting with an unconstrained quantum state. This leads to a quantization scheme that yields a Schrodinger-type equation which is in general nonlinear in evolution. Nevertheless, it is compatible with a probabilistic interpretation of quantum mechanics and in particular the construction of a Hilbert space with a Euclidean norm is possible. The new scheme is applied to the quantization of a Friedmann universe with a massive scalar field whose dynamical behaviour is investigated numerically.