The Rare Merger of Unrelated Black Holes

Two merging black holes.

The data from gravitational ripple observatories showed a merger of two black holes that came far away in space from each other. (Image: Alexandr Yurtchenko via Dreamstime)

For the first time, astronomers have encountered something exciting from archival data. The data from gravitational ripple observatories showed a merger of two black holes that came far away in space from each other.

Interestingly, these two black holes born in different places found a way to merge. But how do they know these two black holes are not from the same family?

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Two black holes were born at different places and time

Almost all gravitational waves that instruments like LIGO (Laser Interferometer Gravitational Observatory) observe come from collisions of black holes that are most likely related. Related black holes were once a pair of stars born at the same place and time before collapsing to form revolving black holes or neutron stars.

When two black holes are related, they usually have their spins aligned. But in this latest discovery, the two black holes did not have any correlation between their spins and orbits — suggesting they are not from the same family.

Only black holes can make gravitational waves powerful enough to be seen by instruments such as the U.S.-based LIGO and Italian-based Virgo.
Only black holes can make gravitational waves powerful enough to be seen by instruments such as the U.S.-based LIGO and Italian-based Virgo. (Image: Crystaleyestudio via Dreamstime)

What are gravitational waves or ripples?

It’s essential to know about gravitational waves to understand how we detect black hole mergers. This is because any object that has mass and moves creates ripples. For example, if you drop an object in water, it will create ripples. And these waves lose their intensity as they move farther away.

Likewise, our Sun and Earth also create waves as they move. But the waves they produce are too weak to be detected by our current technology. Only black holes can make gravitational waves powerful enough to be seen by instruments such as the US-based LIGO and Italian-based Virgo.

LIGO first got the gravitational waves of a black hole merger in 2015. And most of the previous merging events observed by LIGO and Virgo have involved black holes that are close enough to suggest family connections. 

Merging of unrelated black holes

The new detection opens a new line of investigation for astronomers. It is an exciting find as scientists try to understand our universe and its origin.

Seth Olsen, a physicist from Princeton University, said: “This is telling we’ve finally found a pair of black holes that must come from the non-grow-old-and-die-together channel.”

Olsen and his colleagues made this exciting discovery after they went through past data released by the LIGO-Virgo collaboration. And this black hole merger is different because one of the holes is spinning upside down.

Astronomers say that can’t occur unless the two black holes were born far apart in place and time. They speculate that these two black holes met later in their lives. And this is at odds with most gravitational wave observations where littermates are born and die together.

The new detection opens a new line of investigation for astronomers. It is an exciting find as scientists try to understand our universe and its origin.
The new detection opens a new line of investigation for astronomers. It is an exciting find as scientists try to understand our universe and its origin. (Image: Alexandr Yurtchenko via Dreamstime)

Apart from this exciting find, Olsen and his team also found nine other black hole mergers that past studies overlooked.

Other scientists not affiliated with the study lauded the efforts of Olsen’s team. For instance, Patrick Brady, a physicist at the University of Milwaukee-Wisconsin, believes this is the right direction. He is also the spokesperson for the LIGO scientific collaboration.

He said: “This is the nice thing about this type of analysis. We deliver data in a format that other people can use, and then [they] will have access to try out new techniques.”

Because other teams had studied data from the LIGO-Virgo collaboration, Olsen’s team had to lower the analytical threshold a little. Sometimes, merger detections are probably noise instead of a definitive black hole merger. 

Conclusion

Assuming this new detection is not among the noise category, scientists have a new line of investigation distinct from other black mergers up to this point. It may also open new avenues for studying gravitational wave astronomy.

Brady sums it up better by saying: “We don’t understand the theory well enough to predict all these things confidently. So let’s follow this clue to see if it reflects something rare. Or if not, well, we’ll learn other things.”

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