We see that if A < 0 , small fluctuations around ϕ ϕ = 0 have a negative effective diffusive mobility and will grow rather than shrink. To understand the growth dynamics, we disregard the fluctuating currents due to ξ ξ , linearize the equation of motion around ϕ ϕ = ϕ ϕ i n and perform a Fourier transform into k -space. This leads to