No-hair theorem

The no-hair theorem states that an isolated black hole in general relativity is characterized externally by mass, electric charge, and angular momentum, regardless of its formation.

Kerr–Newman metric: An exact solution of Einstein–Maxwell equations describing a rotating, electrically charged black hole. It represents the stationary black-hole family specified by mass, charge, and angular momentum.
Event horizon: A boundary in spacetime beyond which events cannot send signals to distant observers. The horizon separates the black-hole interior from the exterior whose properties the theorem classifies.
John Archibald Wheeler: An American theoretical physicist who shaped modern general relativity and popularized the phrase “black hole.” Wheeler's “no hair” phrase captured the idea that collapse erases external detail.
Scalar hair: Black-hole structure carried by scalar fields outside the horizon beyond mass, charge, and angular momentum. Scalar hair can violate the classical three-parameter description in suitable theories or settings.
Gravitational-wave astronomy: The use of gravitational waves to observe astrophysical sources and test gravity. Black-hole mergers let observations test whether remnants settle into the predicted family.
Kerr metric: An exact vacuum solution describing an uncharged rotating black hole. It is the exterior geometry for the astrophysically common case of negligible charge.
Schwarzschild metric: The spherically symmetric vacuum solution describing an uncharged, nonrotating black hole. It is the simplest member of the black-hole solution family.
Werner Israel: A physicist whose uniqueness results established key properties of stationary black holes. His 1967 theorem proved uniqueness for static black holes under specified assumptions.
Black-hole hair: Independent externally measurable properties of a black hole beyond mass, electric charge, and angular momentum. The theorem's name denies such additional independent exterior parameters under its assumptions.
Black hole spectroscopy: The measurement of black-hole oscillation frequencies to infer properties of the spacetime. Multiple ringdown modes can test whether a remnant has the expected mass-and-spin geometry.

Linked from 2 pages

Rotating black holeRelated: It motivates tests of whether real rotating black holes depart from Kerr predictions.