Two black holes fly toward each other and collide head-on. In the familiar picture, their horizons (the surfaces from which not even light can escape) stretch toward each other like water droplets and merge into one smooth shape. But calculations show otherwise: before merging, a sharp beak, a cusp, extends from each horizon. These beaks pierce space and connect the holes, like two hooks latching onto each other.
The authors traced on a computer how such a beak grows during a head-on collision of two non-rotating holes. It turns out that the mass and other properties of the horizon change along the beak according to a simple law — roughly like the tension in a rope being twisted from both ends. The closer to the tip, the stronger the curvature, and the more mass gathers at that point.
Previously, the connection between the initial holes and the final one was judged mainly by gravitational waves — the ripples that spread from the merger. Here, the authors propose to look directly at the horizons themselves: how their shape transitions from two separate ones into a single common one. Such a view promises to more accurately predict what oscillations (quasinormal modes) will arise in the newborn hole and how loudly it will sound.
The beaks on the horizons turned out to be central details without which the picture of the merger does not come together. Now, knowing how they are structured, one can build simpler models without going through the entire complex computer simulation each time. It is like finding a universal shape for a bridge's curvature without recalculating every bolt.
If you were falling into a black hole, an outside observer would see your image freeze forever on its horizon, reddening and dimming, but for you, the fall would take a finite time.
In Carl Sagan's novel "Contact," humans travel through a network of black holes connected by wormholes — similar to how cusps link horizons before a merger.