Since LIGO’s Nobel-winning discovery of gravitational waves—ripples in spacetime—the U.S.-based detector has been picking up on hundreds of signals from black hole mergers. And, after a decade of studying gravitational waves, researchers believe a significant fraction of black holes may come from cosmic chain reactions.
A recent paper published in Physical Review Letters describes an analysis of 155 pairs of binary black holes, identified by LIGO and its sisters, Virgo and KAGRA, in Italy and Japan, respectively. According to the study, about 14% of merging black holes may be what’s called “second-generation black holes,” or black holes that form from previous mergers of two smaller black holes. This “hierarchical” backstory is vastly different from the textbook version of how black holes emerge from the explosive death of a star.
“Overall in the universe, black holes are merging all the time,” Cailin Plunkett, the study’s first author and a graduate student at the Massachusetts Institute of Technology, told MIT News. “Now we’re seeing a relatively consistent picture where there’s a decent percentage of black holes that are coming from this repeated pathway.”
Tracking the invisible
Gravitational waves that reach Earth’s detectors typically come from extremely intense events. Over the years, LIGO has picked up some truly perplexing signals. For example, last summer it found the most colossal black hole merger ever—and if that wasn’t wild enough, the black holes that took part in the merger lie within a cosmic “dead zone” for black holes.
This zone refers to a range of black hole masses in which, physically speaking, black holes can’t form through ordinary stellar collapse. From these discoveries, astronomers realized just how little we knew about black holes, which are challenging to investigate directly. In that sense, it was a no-brainer that the ever-growing catalog of LIGO’s gravitational signals would turn up entirely new insights about black holes.
“It is increasingly clear, both from individual events and population analyses, that massive black holes exist in [this] range,” the researchers wrote in the latest paper. “These observations have spurred further investigation into mechanisms that can populate this gap.”
A wobbly imprint
The latest research represents one such investigation. During mergers, the two black holes spiral toward each other along an orbital plane. When one or both black hole spins are misaligned, the orbital plane can wobble, or “precess,” the researchers explained to MIT News. The degree to which the disk wobbles acts as a parameter from which researchers can measure the masses and spins of the merging black holes.
One telling sign of hierarchical mergers is that they’re “lopsided,” meaning one of the pair has a much higher spin and mass than the other. For the study, the team created an analytic model to capture the kind of wobble that would have emerged from second-generation black holes. Around 14% of merging black holes followed this pattern, and the second-generation black holes identified had a very specific range of masses, at around 20 solar masses or 40 solar masses and above.
Of mysterious origins
To be fair, that might not sound like a whole lot. But it demonstrates that a sizeable portion of known black holes indeed follow this pattern. As for why, the team suspects hierarchical mergers emerge from dense stellar environments. Simply, when multiple neighboring stars die and collapse into black holes, the dense environment can make it easier for those black holes to find each other and merge. That could further lead to the formation of second-generation black holes. Theoretically, this could “repeat potentially ad infinitum, by virtue of the fact that you have a ton of stars and black holes in this really dense environment,” Plunkett said.
But an ensuing mystery concerns those black holes in the 40-and-above regime, which coincides with the aforementioned “death zones” for black hole masses. According to stellar evolution theory, black holes born of supernovas shouldn’t leave any black holes above roughly 45 solar masses, explained Plunkett.
“Yet we have seen black holes that are that massive,” she mused. “And the question is: Where did they come from?”
For now, it’s hard to say when we’ll get an answer to that question, if ever. But one thing seems to be clear: black holes are a lot weirder than we could ever imagine.
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