Two heavy spheres circle not in a neat circle but in a highly flattened oval — now approaching, now flying apart. That's roughly how pairs of black holes on elongated orbits behave. Unlike calm pairs that gradually spiral together and merge, these dancers retain a memory of their turbulent youth: of being kicked out of star clusters or made to whirl by close neighbors.
Astrophysicists built a computer model to figure out how many such pairs could be heard with the future space-based gravitational wave detector LISA. The model accounted for three main sources. First, pairs ejected from dense globular clusters. Second, pairs pumped up by passing stars in the Galactic disk. Third, pairs whose orbits were stretched by the influence of a distant third body (the Kozai–Lidov effect). It turned out that in the Milky Way, over ten years of observations, about three dozen such objects could be spotted. Beyond our Galaxy, in other galaxies, the count runs into the hundreds, but only if the signal is very weak. Under strict selection criteria, only a handful remain.
The most intriguing part is that such "chords" can fool the detector. If you analyze the signal assuming it's simple and circular, you can make big errors in estimating the pair's mass. So the authors developed a special tool, LEAP, to help correctly decipher these complex signals. They also verified that modern formulas of Newtonian physics with Einstein's corrections reliably describe such orbits for stellar-mass black holes. That means we're ready for future discoveries.
So elongated orbits are not just a cosmic curiosity but a widespread phenomenon that will become a key target for next-generation gravitational wave observatories. And when the LISA detector begins its work, it will listen not only to the steady hum of circular pairs but also to the complex chords of those very "flattened ovals" we started with.
In globular clusters, black holes can play "gravitational billiards": the more massive ones sink toward the center, while lighter ones get flung out, sometimes forming pairs with elongated orbits.
In Bruce Sterling's novel Schismatrix, an alien civilization uses gravitational waves from binary systems for interstellar communication — much as scientists hope to pick up signals from distant pairs of black holes.