In the dense core of a star cluster, three black holes converge in a dangerous dance. The motion is chaotic — trajectories tangle like balls of yarn. Then one hole is ejected, while the other two remain on an elongated orbit. Such a pair continues to spiral inward and eventually merges, producing a burst of gravitational waves.
The authors revisited the old problem of triple encounters. Previously, two parameters were used to describe the outcome, but now they reduced it to a single one — the total angular momentum. It's like replacing two tuning knobs on a complex device with one, without losing precision. Then they built a probabilistic model: how many pairs will remain on elongated orbits after the ejection of the third body. Such orbits matter — if black holes approach along a highly elongated ellipse, their merger can be detected even from great distances.
The results were applied to a model of a star cluster with a mass ranging from one hundred thousand to ten million suns. It turned out that the fraction of noticeably elongated mergers (with eccentricity greater than 0.1) ranges from 1.6% to 3.1% for current detectors. If sensitivity improves to 10 Hz, the fraction rises to 2.6–4.4%. The more massive the cluster, the more pairs are ejected into intergalactic space without merging inside.
If three black holes collide in a cluster, the lightest one almost always flies out first, like a tennis ball struck by two bowling balls.
In Liu Cixin's novel "The Three-Body Problem," the chaos of a triple star system threatens a civilization — here, the same chaos helps birth gravitational waves.