Directed percolation universality class — absorbing phase transition at λ_c ≈ 1.6488
Active fraction ρ: –
Time: 0
Phase: –
The contact process is a simple model of infection spreading on a lattice.
Each infected site recovers at rate 1, and infects each neighbor at rate λ.
Below the critical point λ_c ≈ 1.6488 (2D directed percolation), activity always dies out.
Above it, a nonzero stationary density ρ ~ (λ-λ_c)^β with β ≈ 0.584 persists.
This is the paradigmatic absorbing-state phase transition.