Two black holes spiraling toward each other resemble figure skaters in a pair spin: each rotates about its own axis, and together they weave a shared dance. This collective twist, the so-called effective spin, can be measured by astronomers from the gravitational waves rippling outward from the pair, like ripples on a pond. In the new study, scientists analyzed the latest catalog of such events, compiled by the LIGO, Virgo, and KAGRA detectors.
It turns out that most pairs spin very weakly: their effective spin clusters near zero, as if the skaters barely rotate. But some pairs show a noticeably stronger twist, and it more often points in the same direction as their orbital motion. Previously, researchers thought the distribution of these twists was simply skewed to one side, but the new analysis reveals a central "cloud" of weakly spinning pairs, with distinct "outliers" beyond it that have strong spins.
Particularly intriguing is that among the most massive pairs (from 47 to 65 solar masses), those with a twist opposite to the orbital motion are slightly more common. However, this asymmetry is not very confident: the odds in its favor are about 13 to 1, and if one suspicious event with data glitches is removed, only 5 to 1. Nevertheless, such an excess of "negative" spins could point to a special birth pathway: black holes that themselves formed from previous mergers of other black holes, and then merged again. Such "hierarchical" mergers are thought to occur more often in dense gas disks around supermassive black holes at the centers of galaxies.
In other words, by measuring the hidden twist of colliding black holes, astronomers can peer into the corners of the Universe where these pairs were born. A weak twist likely indicates quiet stellar outskirts; a strong, unidirectional twist points to turbulent gas disks in galactic cores. Each new merger adds a stroke to the map of cosmic "maternity wards."
The effective spin of a black hole pair can range from –1 to +1. Zero means that the spin axes are either nearly not tilted or their contributions cancel each other out.
In Carl Sagan's novel "Contact," an alien signal contains blueprints for a machine that uses rotating black holes to travel through spacetime.