Cosmic Censorship

Cosmic censorship is a conjecture, proposed by Roger Penrose in 1969, asserting that the singularities produced by gravitational collapse are always hidden from distant observers behind an event horizon — never 'naked.' The singularity theorems guarantee that a singularity forms, but say nothing about whether it is visible. A naked singularity, exposed to the outside universe, would be a region where the laws of physics are undefined yet able to broadcast unpredictable signals into the rest of spacetime, destroying the deterministic, predictable evolution that general relativity otherwise enjoys.

Two versions are distinguished. Weak cosmic censorship holds that no singularity formed from generic, regular initial data is ever visible from future null infinity — the singularity stays sealed behind a horizon, so observers far away are never affected. Strong cosmic censorship is the more sweeping claim that, for generic initial data, the spacetime is globally hyperbolic: physics remains predictable for every observer, even one who falls in, because no Cauchy horizon survives perturbation. The two are logically independent.

Cosmic censorship remains one of the most important open problems in classical general relativity. It is known to fail for finely-tuned, non-generic initial data — Choptuik's 1993 numerical study of critical gravitational collapse found arbitrarily small black holes shading into marginally naked singularities — but no proof or disproof exists for generic collapse. Hawking conceded a famous 1991 bet with Preskill and Thorne on a technicality and reformulated it; the underlying question is still open.

Conjectured by Roger Penrose in his 1969 paper 'Gravitational Collapse: The Role of General Relativity'; the weak and strong forms were later distinguished, and the subject of well-known Hawking–Preskill–Thorne wagers in 1991 and after.