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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.
Essentially, the input consistently altered the
output, 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.
Essentially, results were consistently altered based
on the model's tweaks, 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 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?
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 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."
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