At the beginning of time and the center of every black hole lies a point of infinite density called a singularity. Einstein’s general theory of relativity, which is used to describe the gravity of black holes, uses a single field equation to explain how space curves and matter moves. Several scientists, including Misner, had devised useful simplifying assumptions to let them explore scenarios like the Mixmaster universe. The term “black hole” had only just come into widespread use. Earlier black hole models assumed perfect symmetries not found in nature, positing, for example, that a star was a perfect sphere before collapsing into a black hole, or that it had no net electrical charge.