  We study inflationary dynamics using a recently introduced classical-quantum
correspondence for investigating the backreaction of a quantum mechanical
degree of freedom to a classical background. Using specifically a coupled
Einstein-Klein-Gordon system, an approximation that holds well during the very
early inflationary era when modes are very deep inside the Hubble horizon, we
show that the backreaction of a mode of the quantum field will renormalize the
Hubble parameter only if the mode's wavelength is longer than some threshold
Planckian length scale. Otherwise, the mode will destabilize the inflationary
era. We also present an approximate analytical solution that supports the
existence of such short-wavelength threshold and compare the results of the
classical-quantum correspondence with the traditional perturbative-iterative
method in semiclassical gravity.
