  We study the Langevin dynamics of a d-dimensional Ginzburg-Landau Hamiltonian
with isotropic long range repulsive interactions. We show that, once the
symmetry is broken, there is a coupling between the mean value of the local
field and its fluctuations, generating an anisotropic effective fluctuation
spectrum. This anisotropy has many interesting dynamical consequences. In the
infinite time limit, static results are recovered which can be compared with
the well known Brazovskii's transition to a state with modulated order. Our
study reveals that the modulated solution appears continuously in d = 3
contrary to what is found in the usual approach neglecting coupling of modes,
while for d < 3 transitions are still discontinuous. Analytical results for
positional and orientational order parameters are also obtained and interpreted
in the context of recent discussions.
