Exactly how shy is the universe? Does the universe permit naked singularities? It seems that there may now be an observational means of answering this question.
To understand what a naked singularity is, it is first necessary to recognise that space-time singularities come in two forms: spacelike singularities and timelike singularities. The former are consistent with determinism, whilst the latter are not. Even if one specifies the state of the entire 3-dimensional universe at a moment in time, if timelike singularities exist, then the state of the universe at a moment in time is insufficient to predict what will emerge in the future from the timelike singularity. Note that the Schwarzschild space-time for a non-rotating, electrically neutral black hole, and the corresponding space-time for a white hole, both possess spacelike singularities.
However, the maximally extended Kerr space-time for a rotating black hole contains a timelike singularity. Whilst this is strictly inconsistent with determinism, those Kerr space-times which possess a physically realistic angular momentum, contain an event horizon which prevents the unpredictable output from the singularity reaching the outside world. In contrast, a naked singularity is a timelike singularity which is not confined within an event horizon.
The family of Kerr space-time solutions to the Einstein field equations are such that the event horizon disappears if the angular momentum-to-mass ratio exceeds a certain extremal value. Arguments have thus raged for some years over whether a naked singularity could form by a physically realistic process, or whether it is merely a theoretical artifact.
Arlie Petters (Duke University) and Marcus Werner (University of Cambridge) now suggest that the space-time of such a rapidly spinning, naked singularity, would form a distinctive gravitational lens, enabling astronomers to spot such an object, if it exists, in our own galaxy.