Two Black Hole Mergers Emit Gravitational Waves, Upholding Einstein's Theory of Relativity

When two black holes merge into one, they sometimes leave behind ripples in space. These ripples were first predicted by Albert Einstein over a century ago, and now, they’re helping scientists piece together the process that encourages black hole mergers.

A new study published in The Astrophysical Journal Letters has investigated ripples, or gravitational waves, from two black hole mergers that occurred in late 2024, each one featuring a pair of black holes that collided. The waves emitted from the events have allowed researchers to infer that one black hole in each pair may was a “second generation” black hole, or one that had already originated from a previous merger. With this insight, they’re beginning to understand how black hole systems grow in dense cosmic environments.

Read More: This is What a Black Hole Sounds Like

A Good Signal From Black Hole Mergers

On October 11, 2024, a black hole merger (GW241011) took place roughly 700 million light-years away; the two black holes weighed around 17 and 7 times the mass of our sun, respectively, and the larger of the two had one of the fastest rotations observed to date.

Then, on November 10, 2024, another merger (GW241110) brought a separate pair of black holes together, this time 2.4 billion light-years away; these two weighed around 16 and 8 times the mass of our sun, respectively, and the primary black hole was the first to be observed spinning in a direction opposite its orbit.

Using algorithmic techniques and mathematical models, researchers were able to interpret gravitational signals from both mergers and observe key features of the detected black holes, like their speed and spin.

Hierarchies of Black Holes

Binary Black Hole Merger

(Image Credit: Carl Knox, OzGrav, Swinburne University of Technology)

What do these two mergers last year have to do with each other, though? The link, researchers found, is that both may confirm the presence of “second-generation” black holes.

This is because one black hole in each merger was larger and had distinct spin orientations compared to their smaller partners. Taking note of the size difference in both mergers, the researchers have proposed that the larger black holes likely originated from previous mergers themselves — a phenomenon called a “hierarchical merger.”

According to a statement on the new study, this happens when black hole systems reside in dense environments where black holes are more likely to run into each other and merge again and again.

“The unusual spin configurations observed in GW241011 and GW241110 not only challenge our understanding of black hole formation but also offer compelling evidence for hierarchical mergers in dense cosmic environments: they teach us that some black holes exist not just as isolated partners but likely as members of a dense and dynamic crowd,” said Gianluca Gemme, a spokesperson of the Virgo Collaboration, which oversees the development of the Virgo gravitational wave detector in Italy.

Putting Einstein's Theories to the Test

In observing the black hole mergers from October and November 2024, the researchers have been able to verify predictions related to Einstein’s theory of general relativity and mathematician Roy Kerr’s work on the rotation of black holes.

According to the researchers, when a black hole’s rapid rotation slightly deforms it, a characteristic fingerprint is left in the gravitational waves it emits. The vast difference in mass between the black holes also creates a “hum” of a higher harmonic, much like overtones in musical instruments. One of these harmonics was observed in the October merger (GW241011), aligning with Einstein’s theory.

Another way that rapidly rotating black holes help scientists is that they can be used to test whether hypothesized lightweight elementary particles exist (called “ultralight bosons”) and how large they may be. If they exist, ultralight bosons could extract rotational energy from black holes, although there are still several unknown factors related to these particles that are yet to be deciphered.

Read More: Here’s What Would Happen If You Walked Through a Black Hole

Article Sources

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• This article references information from the recent study published in The Astrophysical Journal Letters: GW241011 and GW241110: Exploring Binary Formation and Fundamental Physics with Asymmetric, High-spin Black Hole Coalescences