Singularities occur at the center of black holes. Because the General Theory of Relativity is a theory of space-time as well as of gravity, the consequences of the unbounded energy densities predicted by that theory at the end of gravitational collapse and at the start of the universe are catastrophic, for they imply an end to space-time itself. The possible history of an observer or particle simply comes to an end; physics breaks down, and space-time ceases to exist. It is difficult even to begin talking about this situation, for even the word exist ceases to have meaning. It is unclear if quantum gravity theories will avoid this implication.
An unresolved problem pertaining to singularities is whether gravitational collapse can lead to a naked singularity, that is, one that will be visible from far away and so can influence events in the outside world. The contrary of this possibility is that a naked singularity can only lead to a black hole, where a singularity occurs but is hidden from the outside world by an event horizon.
See also Black Hole; Cosmology, Physical Aspects; Gravitation; Relativity, General Theory of; Space and Time
george f. r. ellis
sin·gu·lar·i·ty / ˌsinggyəˈlaritē/ • n. (pl. -ties) 1. the state, fact, quality, or condition of being singular: he believed in the singularity of all cultures. ∎ a peculiarity or odd trait.2. Physics & Math. a point at which a function takes an infinite value, esp. in space-time when matter is infinitely dense, as at the center of a black hole.