Science1 publisher3 min readPublished
Two teams find signs of an earlier merger among black hole pairs; one study puts the share at about 14 percent
Two Physical Review Letters studies read published LIGO data with different methods and both picked out binaries whose larger member spins fast and sits tilted. One of the estimates puts that group near 14 percent of pairs.
The Scientist · Science desk
What happened
- Two studies published in Physical Review Letters used published LIGO data, plus other observatories, to look for a subpopulation among the binary black holes already on the books.
- Both picked out a distinct group with strong signatures of hierarchical merging, meaning one member of the pair is much larger, spins faster and orbits out of alignment with its partner.
- The study led by Cailin Plunkett, a PhD candidate at MIT, puts the share of black hole duos that may be hierarchical mergers at about 14 percent.
- The two teams reached that overlap with different techniques for tracking the mass, spin and tilt of the black holes in each detected pair.
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Why it matters
- constraint Anyone fitting supernova physics to the detected mass and spin distribution has to strip out second-generation black holes first, because those objects report on a previous merger instead of a stellar collapse.
- capability Spin magnitude and orbital tilt now work as a sorting tool, letting one gravitational-wave catalogue be split by formation channel instead of treated as a single population.
- contradiction Scientific American frames the pairing as more common than researchers expected, while Isi describes results that broadly match theory, so a reader has to decide whether this is a surprise or a measurement of something already forecast.
- precedent Two methods agreeing on the same archive sets the standard for the next claim about formation channels: show the signature survives a second, differently built analysis.
The denominator matters here. Plunkett's roughly 14 percent applies to the black hole duos inside the published gravitational-wave data the teams analysed [4][1], not to black holes in the universe. Turned around, that is about one pair in seven [2], and some 86 percent of the sampled pairs with no such signature [1].
The tag both teams looked for is a combination of three properties in one binary: a member much larger than its partner, spinning faster, and orbiting out of alignment with it [3]. The standard account of these systems is two stars that lived together and died together, leaving cores that stay bound [10]. When such a pair finally merges, the product is a single faster-spinning black hole that sometimes shoots off into space [11].
"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," said Cailin Plunkett, a PhD candidate at MIT and lead author of one of the two studies [5][4].
Physicists had already predicted that the offspring of mergers in dense stellar environments would go on to find new partners [6]. Paul Lasky, an astrophysicist at Monash University and a co-author of the other study, said such objects, with their different astrophysical history, would operate "in weird and wonderful ways" [7].
What the two studies share is the data. Both drew on published LIGO results and other observatories [1], and the methods differed while the events largely overlapped [2]. So the agreement is a cross-check on two ways of reading one catalogue. Maximiliano Isi, a Columbia astrophysicist who worked with LIGO data before but was not involved in either study, called it "a concrete example of us moving astrophysics forward" [8]. "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," Isi said [9].
Where the second pairing happened is the open question. Dense stellar environments are the theoretical prediction [6]; what the two teams report is a spin and tilt signature in the waveforms. A 14 percent subpopulation also leaves the collapse picture standing. Anyone fitting supernova physics to the detected mass and spin distribution now has a named subset to remove first, and the researchers say this origin story could help clarify how supernovae work and how galaxies evolve [12]. Scientific American's own framing is that the odd-couple pairing is more common than researchers expected [13]. That sits a little oddly beside Isi's description of a picture that matches theory [9].
What to watch
- Whether the hierarchical fraction stays near 14 percent as the number of detected binaries in the next catalogue grows.
- Whether anyone ties one of these high-spin, tilted events to a specific dense stellar environment such as a globular cluster.
- Whether supernova and galaxy-evolution models refit after excluding the flagged subset shift their predictions measurably.