Maximal displacement of simple random walk bridge on Galton-Watson trees

Abstract

We analyze simple random walk on a supercritical Galton-Watson tree, where the walk is conditioned to return to the root at time $2n$. Specifically, we establish the asymptotic order (up to a constant factor) as $n\to\infty$, of the maximal displacement from the root. Our results, which are shown to hold for almost surely every surviving tree $T$ (subject to some mild moment conditions), are broken up into two cases. When the offspring distribution takes a value less than or equal to $1$ with positive probability, the maximal displacement of the bridge is shown to be on the order of $n^{1/3}$. Conversely, when the offspring distribution has minimum possible value equal to at least $2$ (and is non-constant), the maximal displacement is shown to be of order less than $n$, but greater than $n^{\gamma}$ (for any $\gamma<1$). Each of these cases is in contrast to the case of a regular tree, on which the bridge is known to be diffusive. To obtain our results, we show how the walk tends to gravitate towards large clusters of vertices of minimal degree, where it then proceeds to spend most of its time. The size and frequency of such clusters is generally dependent on the minimum possible value attainable by the offspring distribution, and it is this fact which largely accounts for the existence of the two regimes.