  We show that the quantum dynamics of a real scalar field for a large class of
potentials in the symmetric Gaussian state, where the nonperturbative quantum
contributions are taken into account, can be described equivalently by a
two-dimensional nonlinear dynamical system with a definite angular momentum
(U(1) charge of a complex theory). It is found that the Gaussian state with a
nearly minimal uncertainty and a large quantum fluctuation, as an initial
condition, naturally explains the most of the essential features of the early
stage of the inflationary Universe.
