  In this paper we present a study of the early stages of unstable state
evolution of systems with spatial symmetry changes. In contrast to the early
time linear theory of unstable evolution described by Cahn, Hilliard, and Cook,
we develop a generalized theory that predicts two distinct stages of the early
evolution for symmetry breaking phase transitions. In the first stage the
dynamics is dominated by symmetry preserving evolution. In the second stage,
which shares some characteristics with the Cahn-Hilliard-Cook theory, noise
driven fluctuations break the symmetry of the initial phase on a time scale
which is large compared to the first stage for systems with long interaction
ranges. To test the theory we present the results of numerical simulations of
the initial evolution of a long-range antiferromagnetic Ising model quenched
into an unstable region. We investigate two types of symmetry breaking
transitions in this system: disorder-to-order and order-to-order transitions.
For the order-to-order case, the Fourier modes evolve as a linear combination
of exponentially growing or decaying terms with different time scales.
