  The nonlinear equations describing phase ordering dynamics can be closed by
assuming the existence of an underlying Gaussian stochastic field which is
nonlinearly related to the observable order parameter field. We discuss the
relation between different implementations of the Gaussian assumption and
consider the limitations of this assumption for phase ordering dynamics. The
fact that the different approaches gives different results is a sign of the
breakdown of the Gaussian assumption. We concentrate on the non-conserved order
parameter case but also touch on the conserved order parameter case. We
demonstrate that the Gaussian assumption is fundamentally flawed in the latter
case.
