A complete convergence theorem for the q-voter model and other voter model perturbations in two dimensions

Abstract

The q-voter model is a spin-flip system in which the rate of flipping to type i is given by the qth power of the proportion of nearest neighbours in type i for $i=0,1$. If $q=1$ it reduces to the classical voter model. We show that in the critical 2-dimensional case, for $q<1$ and close enough to 1,for any initial state as $t\to\infty$, the system converges weakly to a mixture of all 0's and all 1's, and a unique invariant law which contains infinitely many sites of both types. This follows as a special case of a general theorem which proves a similar "complete convergence theorem" for cancellative, monotone, finite range voter model perturbations on $\mathbb{Z}^2$ providing a certain parameter, $\Theta_3$, is strictly positive. Similar results follow for the affine and geometric voter models and Lotka-Volterra models, all for parameter values close to the giving the voter model. This kind of asymptotic behavior is quite different from that of the 2-dimensional voter model itself, which undergoes clustering, and converges to a mixture of all 0's and all 1's. The above parameter $\Theta_3$ has an explicit expression in terms of asymptotic coalescing probabilities of 2-dimensional random walk, and we give a rather simple sufficient condition for it to be strictly positive. An important step in the proof is to establish weak convergence of the rescaled spin-flip systems to super-Brownian motion with drift $\Theta_3$. In fact, a convergence result is proved under weaker hypotheses which includes all known such results for 2-dimensional voter model perturbations and a number of new ones, including a rescaled limit theorem for the q-voter model where $q\uparrow $1 with the rescaling.