Clues to a black hole’s origins can be found in the way it spins. This is especially true for binaries, in which two black holes approach each other before merging. The rotation and tilt of the respective black holes just before they merge can reveal whether the invisible giants originated from a silent galactic disk or from a more dynamic star cluster.
Astronomers hope to determine which of these origin stories is more likely by analyzing the 69 confirmed binaries detected so far. But a new study reveals that for now, the current catalog of binaries is not enough to reveal anything fundamental about the formation of black holes.
In a study published today in the journal Astronomy and astrophysicsMIT physicists show that when all known binaries and their spins are fitted into models of black hole formation, the conclusions can look very different, depending on the particular model used to interpret the data.
The origins of a black hole can therefore be “rotated” in different ways, depending on the assumptions of a model on the functioning of the universe.
“When you change the model and make it more flexible or make different assumptions, you get a different answer about how black holes formed in the universe,” says study co-author Sylvia Biscoveanu, MIT graduate student working at the LIGO lab. “We are showing that people have to be careful because we are not yet at the stage with our data where we can believe what the model is telling us.”
The study’s co-authors include Colm Talbot, an MIT postdoc; and Salvatore Vitale, associate professor of physics and member of the Kavli Institute for Astrophysics and Space Research at MIT.
A story with two origins
Black holes in binary systems are thought to arise through one of two pathways. The first is through “binary field evolution”, in which two stars evolve together and eventually explode as supernovae, leaving behind two black holes that continue to spin in a binary system. In this scenario, the black holes should have relatively aligned spins, because they would have had time — first as stars, then as black holes — to pull and pull each other in similar orientations. If black holes in a binary have roughly the same spin, scientists believe they must have evolved in a relatively quiet environment, like a galactic disk.
Black hole binaries can also form by “dynamic assembly”, where two black holes evolve separately, each with its own tilt and rotation. By some extreme astrophysical process, the black holes are finally brought together, close enough to form a binary system. Such dynamic pairing would likely occur not in a silent galactic disk, but in a denser environment, such as a globular cluster, where the interaction of thousands of stars can cause two black holes to collide. If black holes in a binary have randomly oriented spins, they probably formed in a globular cluster.
But what fraction of binaries is formed through one channel versus the other? The answer, astronomers say, should lie in data, and in particular in measurements of black hole spins.
To date, astronomers have derived the spins of black holes in 69 binaries, which have been discovered by a network of gravitational wave detectors including LIGO in the United States and its Italian counterpart Virgo. Each detector listens for signs of gravitational waves – very subtle reverberations through spacetime that are left behind by extreme astrophysical events such as the merger of massive black holes.
With each binary detection, astronomers estimated the black hole’s respective properties, including their mass and spin. They worked spin measurements into a generally accepted model of black hole formation and found signs that binaries might have both preferred and aligned spin, as well as random spins. In other words, the universe could produce binaries in both galactic disks and globular clusters.
“But we wanted to know, do we have enough data to make that distinction?” said Biscoveanu. “And it turns out things are messy and uncertain, and it’s harder than it looks.”
In their new study, the MIT team tested whether the same data would yield the same conclusions when used in slightly different theoretical models of black hole formation.
The team first replicated LIGO’s spin measurements in a widely used model of black hole formation. This model assumes that a fraction of the binaries in the universe prefer to produce black holes with aligned spins, where the rest of the binaries have random spins. They found that the data appeared to agree with this model’s assumptions and showed a spike where the model predicted there should be more black holes with similar spins.
They then tweaked the model slightly, changing its assumptions so that it predicted a slightly different orientation from the black holes’ preferred rotations. When they worked the same data in this modified model, they found that the data shifted to align with the new predictions. The data also made similar changes in 10 other models, each with a different assumption about how black holes prefer to spin.
“Our paper shows that your result depends entirely on how you model your astrophysics, rather than the data itself,” says Biscoveanu.
“We need more data than we thought, if we’re going to make independent assertion of the astrophysical assumptions we’re making,” adds Vitale.
How much more data will astronomers need? Vitale estimates that once the LIGO network restarts in early 2023, the instruments will detect a new black hole binary every few days. Over the next year, this could add hundreds more measurements to add to the data.
“The measurements of spins that we have now are very uncertain,” says Vitale. “But as we build a lot of it, we can get better information. Then we can say, no matter how detailed my model is, the data always tells me the same story, a story that we could then believe.”
Salvatore Vitale et al, Spin it as you like: The (lack of one) measurement of spin-tilt distribution with LIGO-Virgo-KAGRA binary black holes, Astronomy & Astrophysics (2022). DOI: 10.1051/0004-6361/202245084
Provided by Massachusetts Institute of Technology
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