Phase transition of the contact process on random regular graphs

Abstract

We consider the contact process with infection rate $\lambda$ on a random $(d+1)$-regular graph with $n$ vertices, $G_n$. We study the extinction time $\tau_{G_n}$ (that is, the random amount of time until the infection disappears) as $n$ is taken to infinity. We establish a phase transition depending on whether $\lambda$ is smaller or larger than $\lambda_1(\mathbb{T}^d)$, the lower critical value for the contact process on the infinite, $(d+1)$-regular tree: if $\lambda < \lambda_1(\mathbb{T}^d)$, $\tau_{G_n}$ grows logarithmically with $n$, while if $\lambda > \lambda_1(\mathbb{T}^d)$, it grows exponentially with $n$. This result differs from the situation where, instead of $G_n$, the contact process is considered on the $d$-ary tree of finite height, since in this case, the transition is known to happen instead at the _upper_ critical value for the contact process on $\mathbb{T}^d$.