Topological defect formation during a symmetry-breaking phase transition quench
The Kibble-Zurek mechanism: a quench through a second-order phase transition at rate τ_Q freezes order-parameter fluctuations when the relaxation time ξ_τ ≈ τ_Q^(ν/(1+νz)) at the "freeze-out" time. Topological defects (domain walls, vortices) form with density n_d ~ τ_Q^{−ν/(1+νz)}. Faster quenches → more defects. First proposed for cosmic string formation (Kibble 1976), then for condensed matter (Zurek 1985).