Two black holes merged despite being born far apart in space

Signals buried deep in data from gravitational wave observatories imply a collision of two black holes that were clearly born in different places.

Almost all the spacetime ripples that experiments like the Laser Interferometer Gravitational-Wave Observatory, or LIGO, show collisions between black holes and neutron stars that are likely close relatives (SN: 1/21/21). Once they were pairs of stars born at the same time and place, only to eventually collapse in old age to form black holes or neutron stars.

Now, a newly noted black hole marriage, found in existing data from US-based LIGO and its sister observatory Virgo in Italy, appears to belong to an unrelated couple. Evidence for this comes from: how they revolved as they melted into oneresearchers report in a paper in the press on Physical assessment D. Black holes born in the same place tend to have their spins aligned, like a pair of toy tops spinning on a table, spinning around each other. But the pair has no correlation between their respective spins and orbits in this case, implying that they were born in different places.

“This tells us that we’ve finally found some black holes that must be coming from the channel that don’t age and die together,” said Seth Olsen, a physicist at Princeton University.

Past events that have surfaced in gravitational wave observations show that back holes are merging that aren’t perfectly aligned, but most are close enough to strongly imply family ties. The new detection, which Olsen and colleagues discovered by searching data the LIGO-Virgo collaboration made public, is different. One of the black holes is effectively turning upside down.

That can’t just happen unless the two black holes come from different places. They probably met late in their stellar life, unlike the black hole nestmates that seem to make up the bulk of gravitational wave observations.

In addition to the fusion between unrelated black holes, Olsen and his collaborators identified nine other black hole fusions that were created by the previous LIGO Virgo studies (SN: 8/4/21).

“This is actually the nice thing about this kind of analysis,” said LIGO spokesperson Patrick Brady, a physicist at the University of Wisconsin-Milwaukee who was not affiliated with the new study. “We deliver the data in a format that can be used by other people and then [they] access to try out new techniques.”

To collect so many new signals in data already looked at by other researchers, Olsen’s group lowered the analytical bar a little bit.

“Out of the 10 new ones,” Olsen says, “statistically, about three are likely to come from noise,” rather than definitive detection of black hole merger. Assuming the amalgamation of black hole aliens isn’t one of the false signals, it almost certainly tells a story of black hole histories different from the others seen so far.

“It would be [extremely] It’s unlikely to come from two black holes that have been together all their lives,” Olsen said. “This must have been a catch. That’s cool, because we can finally start exploring that region of the [black hole] population.”

Brady notes that “we don’t understand the theory” [of black hole mergers] good enough to be able to predict all these things with confidence.” But the recent study may point to new and interesting opportunities in gravitational wave astronomy. “Let’s follow this lead to see if it really reflects something rare,” he says. “Or if not, we’ll learn other things.”

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