One-dimensional topological defects formed during cosmological phase transitions
Cosmic strings are one-dimensional topological defects — stable field configurations that form when a spontaneously broken symmetry has a π₁(M) ≠ 0 vacuum manifold. They have enormous tension μ ~ η² (where η is the symmetry-breaking scale). The Kibble mechanism predicts a scaling network where strings reconnect with probability p≈1, producing loops that decay by gravitational radiation. Current CMB bounds: Gμ < 1.5×10⁻⁷ (PLANCK). Strings cause gravitational lensing and may generate gravitational wave backgrounds.