The density conjecture for activated random walk

In the late 1990s, Dickman, Mu\~noz, Vespignani, and Zapperi explained the self-organized criticality observed by Bak, Tang, and Wiesenfeld as an external force pushing a hidden parameter toward the critical value of a traditional absorbing-state phase transition. As evidence, they observed empirically that for various sandpile models the particle density in a finite box under driven-dissipative dynamics converges to the critical density of an infinite-volume version of the model. We give the first proof of this well-known density conjecture in any setting by establishing it for activated random walk in one dimension. We prove that two other natural versions of the model have the same critical value, further establishing activated random walk as a universal model of self-organized criticality.