Cosmic Censorship

This is a topic that was of great interest to me when I was a grad student in Physics. Einstein's theory of General Relativity (GR) is a theory of Gravity which has persisted for nearly a century. Despite being one of the best tested theories in all of physics, a fascinating thing about GR is that scientists know that it must be wrong. Early on in the history of GR, scientists realized that the theory predicts what we now call blackholes. A blackhole's gravity is so strong that it is impossible for anything to escape from it once it gets too close. There is an imaginary surface surrounding the center of a blackhole from which nothing can escape, which is called the Event Horizon.

The problem with blackholes is that GR predicts they contain singularities; this means that the strength of gravity at the center of a blackhole is infinite. Usually when physical theories predict an infinitely large value, this means that the validity of the theory has broken down. Rather than gravity being infinitely strong, it makes more sense that the theory doesn't work anymore once we enter a blackhole. Luckily, the Event Horizon saves us from this failure; if nothing, not even information, can travel from the singularity through the Event Horizon, then the singularity cannot affect physics outside of the blackhole.

This is a nice argument, but unfortunately there is no mathematical proof that singularities are always surrounded by an Event Horizon. The Hawking-Penrose theorems prove the existence for very massive blackholes, but no proof exists for the general case. The hypothesis that singularities are always surrounded, or "clothed", by an Event Horizon is called the Cosmic Censorship hypothesis, and is one of the biggest open questions in Cosmology today.