When two black holes find each other in the vastness of space, they can form a binary odd couple—and it’s more common than researchers expected.
Scientists think most black holes are born when a star dies and its core collapses. This is also the case for binary black holes, pairs of black holes that orbit around each other and slowly move closer together. These pairs are often the remains of stars that lived together, died together and continued to coexist following their collapse. Eventually these two draw close enough together to merge into one faster-spinning black hole that sometimes shoots off into space.
Now research is suggesting that these whirling loners can find love again. New work suggests many black hole pairs are so-called hierarchical mergers: one large, fast-spinning black hole born from a previous merger and the partner it somehow found in the cosmos. This picture of black holes’ origin stories could help clarify how supernova physics work and how galaxies evolve, scientists say. Knowing how black holes form and change is also crucial for understanding general relativity and the history of the universe.
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In two studies released recently in Physical Review Letters, scientists describe how they used published data from the Laser Interferometer Gravitational-Wave Observatory, which tracks merging black holes through the spacetime ripples they release, and other observatories to identify this subpopulation. Each team used a different technique to track the black holes’ mass, spin and tilt, and both captured a distinct group of binary black holes that have “strong signatures” of hierarchical merging—one of the pair is much larger, is spinning faster and orbits out of alignment with its partner.
“At minimum, we’re finding evidence that there are these highly spinning and weirdly tilted black holes in the dataset, and the most straightforward way to interpret that, the Occam’s razor answer, is that these black holes probably came from previous mergers,” says Cailin Plunkett, lead author on one of the recent studies and a Ph.D. candidate at the Massachusetts Institute of Technology. Her study suggests about 14 percent of black hole duos may be hierarchical mergers.
Physicists had previously predicted that the offspring of black hole mergers in dense stellar environments would find future partners to merge with. And their different astrophysical history would make them operate “in weird and wonderful ways”—a sign that researchers have now spotted, says Paul Lasky, co-author of the other recent study and an astrophysicist at Monash University in Melbourne, Australia.
The fact that two different technical processes both found evidence of these black hole origins is “a concrete example of us moving astrophysics forward,” says Maximiliano Isi, an astrophysicist at Columbia University, who was not involved in either study but has worked with LIGO data in the past. “It’s kind of like trying to see on the road when there’s fog,” Isi says. “We’re all squinting in different ways and looking from different angles and trying to determine what is the landscape of what’s out there, and we are converging into a very nice picture that somewhat aligns with theoretical expectations.”
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