Contact process on a 2D lattice: active sites infect neighbors at rate λ, and spontaneously die at rate 1. At the critical λc ≈ 1.65, the system transitions from absorbing (inactive) to active phase — Directed Percolation universality.
Activity density ρ
Step: 0
Active cells: —
Activity ρ: —
λ/λ_c: —
Critical point λ_c ≈ 1.649 (2D contact process).
Below: absorbing state (all inactive).
Above: active steady state ρ ~ (λ−λ_c)^β, β≈0.583.
Universality class: Directed Percolation.