The dynamics of the order parameter φ φ of a second order phase transition slows down near the critical point and can be described with a Langevin equation. The simplest case is the universality class "model A" with a non-conserved scalar order parameter, realized for instance in axial ferromagnets, ∂ ∂ ∂ ∂ t φ φ ( x , t ) = − − λ λ δ δ H δ δ φ φ + η η ( x , t ) , H = ∫ ∫ d d x [ 1 2 r 0 φ φ 2 + u φ φ 4 + 1 2 ( ∇ ∇ φ φ ) 2 ] , ⟨ η η ( x , t ) η η ( x ′ , t ′ ) ⟩ = 2 λ λ δ δ ( x − − x ′ ) δ δ ( t − − t ′ ) . Other universality classes (the nomenclature is "model A",..., "model J") contain a diffusing order parameter, order parameters with several components, other critical variables and/or contributions from Poisson brackets.