Asymptotic Behavior of Aldous’ Gossip Process

Aldous (2007) defined a gossip process in which space is a discrete N ×N torus, and the state of the process at time t is the set of individuals who know the information. Information spreads from a site to its nearest neighbors at rate 1/4 each and at rate N−α to a site chosen at random from the torus. We will be interested in the case in which α < 3, where the long range transmission significantly accelerates the time at which everyone knows the information. We prove three results that precisely describe the spread of information in a slightly simplified model on the real torus. The time until everyone knows the information is asymptotically T = (2 − 2α/3)Nα/3 logN . If ρs is the fraction of the population who know the information at time s and ε is small then, for large N , the time until ρs reaches ε is T (ε) ≈ T+Nα/3 log(3ε/M), whereM is a random variable determined by the early spread of the information. The value of ρs at time s = T (1/3) + tNα/3 is almost a deterministic function h(t) which satisfies an odd looking integro-differential equation. The last result confirms a heuristic calculation of Aldous.