Without additional data, the origin of a black hole can be spun in any direction

(ORDO NEWS) — Hints to the origin of a black hole can be found in the way it spins. This is especially true for binary systems, in which two black holes orbit close to each other before merging.

The rotation and tilt of the corresponding black holes just before the merger may reveal whether the invisible giants have emerged from a quiet galactic disk or from a more dynamic cluster of stars.

Astronomers hope to find out which of these origin stories is more likely by analyzing the 69 confirmed binary objects discovered to date.

But a new study has shown that, for now, the current catalog of binary systems is not enough to reveal anything fundamental about the formation of black holes.

MIT physicists show that when all known binary systems and their spins are converted to models of black hole formation, the conclusions can look quite different, depending on the particular model used to interpret the data.

Thus, the origin of a black hole can be “spun” in different ways, depending on the model’s assumptions about how the universe works.

“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 Silvia Biscoveanu, an MIT graduate student who works at the LIGO lab.

“We show that people need to be careful because our data is not yet at a stage where we can trust what the model tells us.”

It is believed that black holes in binary systems arise in one of two ways. First, when two stars develop together and eventually explode in supernovae, leaving behind two black holes that continue to spin in a binary system.

In this scenario, the black holes would have to have relatively aligned spins because they had time (first like stars and then black holes) to attract and pull each other into similar orientations.

Scientists believe that if black holes in binary systems have roughly the same spin, they must have evolved in a relatively quiet environment, such as the galactic disk.

Binary black holes can also form through “dynamic assembly,” where two black holes develop separately, each with its own tilt and spin.

Through some extreme astrophysical processes, black holes eventually come close enough to form a binary system.

Such a dynamic connection is likely to occur not in a quiet galactic disk, but in a denser environment such as a globular cluster, where the interaction of thousands of stars can push two black holes together. If the black holes of a binary system have randomly oriented spins, they formed in a globular cluster.

But what fraction of binary systems are formed one way versus the other? Astronomers believe the answer must lie in the data, and especially in measurements of the spins of black holes.

To date, astronomers have determined the spins of black holes in 69 binary systems that have been detected by a network of gravitational wave detectors, including LIGO in the US and its Italian counterpart Virgo.

Each detector listens for signs of gravitational waves – very subtle reflections in space-time left over from extreme astrophysical events such as massive black hole mergers.

With each double detection, astronomers estimated the corresponding properties of the black hole, including its mass and rotation.

They incorporated spin measurements into the generally accepted model for black hole formation and found indications that binary systems could have either a preferred, aligned spin or a random one. That is, the Universe could produce binary systems both in galactic disks and in globular clusters.

But scientists wanted to know if we have enough data to make that distinction? And it turns out that everything is confusing, uncertain and more complicated than it seems.

Without additional data the origin of a black hole can be spun in any direction 2

In a new study, the MIT team tested whether the same data would lead to the same conclusions when running slightly different theoretical models of black hole formation.

The team reproduced for the first time measurements of LIGO’s rotation in a widely used model of black hole formation.

This model suggests that some of the binaries in the universe prefer to create black holes with aligned spins, while the rest of the binaries have random spins.

They found that the data was consistent with that model’s assumptions and showed a peak 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 of the black hole’s preferred spins.

When they ran the same data on this modified model, they found that the data shifted to match the new predictions.

The data also produced similar shifts in 10 other models, each with different assumptions 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 are to make a statement that is independent of the astrophysical assumptions we have made.”

How much more data will astronomers need? It is estimated that once the LIGO network is back online in early 2023, the instruments will detect one new binary black hole every few days. Over the next year, hundreds more measurements could be added to the data.

“The rotation measurements we have now are very uncertain,” the scientists say. “But since we create a lot of them, we can get better information. Then we can say, regardless of the details of my model, the data always tells me the same story – a story that can be believed.

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