Scientists have for the first time found traces of the unusual orbit of a pair of colliding black holes. Analysis of data from the LIGO and Virgo gravitational-wave observatories showed that these objects were not moving in a perfect circle, as usual, but along a flattened, oval trajectory. This type of orbit is extremely rare and can reveal a lot about how black hole systems form in the first place. The results of the work were published in Physical Review D.

When the orbit is not circular

Most binary stars and black holes that orbit each other for a long time eventually settle into almost perfect circular orbits. But if the orbit remains elongated, it means that the pair formed relatively recently – perhaps after a collision or “acquaintance” in a close stellar environment.

“This is a clue to where and why the pairs of black holes we see using gravitational waves are born,” explains Dr Isobel Romero-Shaw from Cardiff University’s Institute for Gravitational Research.

The story of one collision

Detected event that received an index GW200208_222617became one of the few where signs of eccentricity are visible – that is, an orbit that does not resemble a circle. The international team simulated several possible scenarios and came to the conclusion: this pair of black holes most likely did not arise alone, but in a close stellar environment – e.g. dense cluster or in triple systemwhere they could be influenced by a third star.

“If the orbit is oblate, this means that the binary system has recently formed or has been impacted by something external—gas, a nearby star, or another black hole,” explains Romero-Shaw.

Such collisions provide rare clues about the life of black holes. They help us understand how these massive objects find each other and why they merge.

Why is this discovery important?

The discovery of elongated orbits changes our view of the origin of many binary systems. If at least one pair of black holes formed in a dense cluster, then a significant portion of the others could appear in the same place.

“If one eccentric event is reliably recorded, it suggests that many other black holes may have gone through the same evolutionary path,” Romero-Shaw said.

In other words, the discovery could help astronomers figure out where in the Universe such catastrophic collisions most often occur – in single star systems or in “multi-story” regions of space where stars live crowded together.

similar observations are needed to definitively confirm the rare orbital shape. But if the findings are confirmed, This will be one of the first direct evidence that not all black holes are born and live alone.

“Eccentricity is like a fingerprint that shows that these black holes did not live alone. It shows that the Universe is not a quiet place, but an environment full of collisions and interactions,” explains Romero-Shaw.

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