Singularity

A singularity is a point where the quantities physics deals with, such as density and the curvature of space and time, become infinite, and the known laws of physics stop giving sensible answers. It is less a "thing" than a signal that a theory has reached the limit of what it can describe.

Singularities are predicted in two places. One sits at the centre of every black hole, where collapsing matter is crushed to a point. The other is at the very beginning of the universe: run the expansion of the cosmos backwards and everything converges on an initial singularity, the Big Bang.

Why it mattered to Hawking

Singularities are at the heart of Hawking's early fame. With Roger Penrose he proved the singularity theorems, showing that, under general relativity, singularities are not flukes of tidy models but an unavoidable prediction of the theory, both inside black holes and at the universe's origin.

That result carried a deep implication. Because a singularity is exactly where general relativity breaks down, proving they must exist proved that general relativity is incomplete, and that a deeper theory of quantum gravity is needed. Much of Hawking's later career, including the no-boundary proposal, was an attempt to describe the universe's beginning without a singularity at all.