Active phase for activated random walks on ${\mathbb{Z}}^d$, $d\ge 3$, with density less than one and arbitrary sleeping rate

It has been conjectured that the critical density of the Activated Random Walk model is strictly less than one for any value of the sleeping rate. We prove this conjecture on ${\mathbb{Z}}^d$ when $d\ge 3$ and, more generally, on graphs where the random walk is transient. Moreover, we establish the occurrence of a phase transition on non-amenable graphs, extending previous results which require that the graph is amenable or a regular tree.