What Does it Take to Make Black Holes Collide?

In a recent study published in Astronomy and Astrophysical Letters, a team of researchers at the Massachusetts Institute of Technology (MIT) used various computer models to examine 69 confirmed binary black holes to help determine their origin, and found their data results changed based on the model’s configurations, and the researchers wish to better understand both how and why this occurs and what steps can be taken to have more consistent results.

“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,” Sylvia Biscoveanu, an MIT graduate student working in the LIGO Laboratory, and a co-author on the study, said in a statement. “We show that people need to be careful because we are not yet at the stage with our data where we can believe what the model tells us.”

Like binary stars, binary black holes are two massive objects orbiting each other, with both having the ability to potentially collide—or merge—together, with another shared characteristic being black holes are sometimes born from the collapse of dying massive stars, also known as a supernova. But how binary black holes originated remains a mystery, as there are two current hypotheses regarding their formation: “field binary evolution” and “dynamical assembly”.

Field binary evolution involves when a pair of binary stars explode resulting in two black holes in their place, which continue orbiting each other the same as before. Since they initially orbited each other as binary stars, it is believed their spins and tilts should be aligned, as well. Scientists also hypothesize that their aligned spins indicate they originated from a galactic disk, given its relatively peaceful environment.

Dynamical assembly involves when two individual black holes, each with their own unique tilt and spin, are eventually brought together by extreme astrophysical processes, to form their own binary black hole system. It is currently hypothesized that this pairing would likely happen in a dense environment such as a globular cluster, where thousands of stars in close proximity could force two black holes together.

The real question is what fraction of binary black holes originate from each respective method? Astronomers believe this answer lies in the data, specifically black hole spin measurements. Using the 69 confirmed binary black holes, astronomers have determined these massive objects could originate from both globular clusters and galactic disks. The LIGO Laboratory in the United States has worked with its Italian counterpart, Virgo, to ascertain the spins (rotational periods) of the 69 confirmed binary black holes.

“But we wanted to know, do we have enough data to make this distinction?” said Biscoveanu. “And it turns out, things are messy and uncertain, and it’s harder than it looks.”

For the study, the researchers continuously tweaked a series of computer models to ascertain whether their results agreed with each model’s predictions. One such model was configured to assume only a fraction of binary black holes were produced with aligned spins, where the remainder have random spins. Another model was configured to predict a moderately contrasting spin orientation. In the end, their findings indicated the results consistently changed in accordance with the tweaked models, meaning more data than the 69 confirmed binary black holes is likely needed to have more consistent results.

“Our paper shows that your result depends entirely on how you model your astrophysics, rather than the data itself,” said Biscoveanu.

“We need more data than we thought, if we want to make a claim that is independent of the astrophysical assumptions we make,” said Dr. Salvatore Vitale, who is an associate professor of physics, a member of the Kavli Institute of Astrophysics and Space Research at MIT, and lead author of the study.

But how much more data will the astronomers require? Dr. Vitale estimates the LIGO network will be able to detect one new binary black hole every few days, once the network returns to service in early 2023.

“The measurements of the spins we have now are very uncertain,” said Dr. Vitale. “But as we build up a lot of them, we can gain better information. Then we can say, no matter the detail of my model, the data always tells me the same story — a story that we could then believe.”

As always, keep doing science & keep looking up!