Current measurements of black holes are insufficient to determine how the universe’s invisible giants form, researchers say – Zoo House News
- December 10, 2022
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Clues to the origins of a black hole can be found in the way it spins. This is especially true in binaries, where two black holes orbit close together before merging. The spin and inclination of each black hole just before they merge can reveal whether the invisible giants formed from a quiescent galactic disk or a more dynamic star cluster.
Astronomers hope to find out which of these origin stories is more likely by analyzing the 69 confirmed binaries discovered so far. But a new study finds that the current catalog of binaries is insufficient for now to reveal anything fundamental about black hole formation.
In a study published in the journal Astronomy and Astrophysics Letters, MIT physicists show that when all known binary stars and their spins are incorporated into models of black hole formation, the conclusions can be very different, depending on the model used to interpret the data is used .
The origins of a black hole can therefore be “spun” in different ways, depending on a model’s assumptions about how the universe works.
“If you change the model and make it more flexible or make different assumptions, you’ll get a different answer to how black holes formed in the universe,” says study co-author Sylvia Biscoveanu, an MIT graduate student working in the LIGO -Lab works. “We’re showing that people have to be careful because we’re not ready with our data to believe what the model is telling us.”
Study 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 Exploration at MIT.
A story with two origins
Black holes in binary systems are thought to form in one of two ways. The first is “Field Binary Evolution,” in which two stars co-evolve and eventually explode in supernovae, leaving two black holes that continue to orbit in a binary system. In this scenario, the black holes should have relatively aligned spins since they would have had time – first as stars, then as black holes – to pull and drag each other into similar alignments. If a binary system’s black holes have roughly the same spin, scientists believe they must have evolved in a relatively quiescent environment, such as a galactic disk.
Binary black hole systems can also form through “dynamic assemblage,” in which two black holes evolve separately, each with its own tilt and spin. Eventually, through some extreme astrophysical processes, the black holes are brought together, close enough to form a binary system. Such a dynamic pairing would likely not occur in a quiescent galactic disk, but in a denser environment, such as a globular cluster, where the interaction of thousands of stars can smash two black holes together. If the black holes in a binary system have randomly aligned spins, they likely formed in a globular cluster.
But what proportion of binaries are produced by one channel compared to the other? The answer, astronomers believe, should lie in data, and more specifically in measurements of black hole spins.
To date, astronomers have deduced the spins of black holes in 69 binaries detected by a network of gravitational-wave detectors, including LIGO in the US and its Italian counterpart Virgo. Each detector is listening for signs of gravitational waves — very subtle reverberations through spacetime left over from extreme astrophysical events like massive black hole mergers.
With each binary detection, astronomers have estimated the properties of that black hole, including its mass and spin. They incorporated the spin measurements into a widely accepted model of black hole formation and found evidence that binaries may have a preferred, aligned spin as well as random spins. That means the universe could produce binaries in both galactic disks and globular clusters.
“But we wanted to know if we have enough data to make that distinction?” says Biscoveanu. “And it turns out things are messy and uncertain and it’s harder than it looks.”
spinning the data
In their new study, the MIT team tested whether the same data would lead to the same conclusions when incorporated into slightly different theoretical models of black hole formation.
The team first reproduced 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, while the rest of the binaries have random spins. They found that the data appeared to agree with this model’s assumptions, showing a peak where the model predicted there should be more black holes with similar spins.
They then adjusted the model slightly, changing its assumptions so that it predicted a slightly different orientation of the black hole’s preferred spins. When they incorporated the same data into this optimized model, they found that the data had been shifted to match the new predictions. The data also made similar shifts 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 and not on the data itself,” says Biscoveanu.
“We need more data than we thought if we want to make a claim independent of our astrophysical assumptions,” adds Vitale.
How much more data will astronomers need? Vitale estimates that once the LIGO network is back up in early 2023, the instruments will discover a new black hole binary system every few days. Over the next year, there could be hundreds more measurements to add to the data.
“The measurements of the spins that we have now are very uncertain,” says Vitale. “But if we build a lot of them, we can get better information. Then we can say that no matter how detailed my model is, the data always tells me the same story – a story that we could then believe.”
This research was supported in part by the National Science Foundation.