FLUCTUATION EFFECTS ON WAVE-PROPAGATION IN A REACTION-DIFFUSION PROCESS

The reaction-diffusion process corresponding to the Fisher-Kolmogorov equation is studied by means of a discrete multivariate master equation. For travelling wave fronts the stability criterion necessary for the applicability of a system-size expansion is shown to be violated due to the existence of a zero mode of the first variational equation. This zero mode is connected to the translational invariance of the system. Performing stochastic simulations of the master equation in a wide range of parameters it is demonstrated that for finite size of the system (up to about 107 particles in the frontal region) a rather large fluctuation effect on the wave propagation speed results: in general, the asymptotic wave speed lies below the stable, minimal speed which is given by a theorem of Kolmogorov for the macroscopic equation. The wave front position exhibits a diffusion-type behaviour associated with translative fluctuations along the propagation direction.