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Why Black Holes Smashing Together Could Settle an Astronomical Dispute

Violent ripples reverberating across space-time might finally reveal how quickly our universe is expanding.

Two black holes are about to collide in the center of the image, thus warping surrounding spacetime with sort of swirly, circular patterns.
Two University of Chicago astrophysicists laid out a method for how to use pairs of colliding black holes (shown as an artist's rendition above) to measure how fast our universe is expanding.
Simulating eXtreme Spacetimes (SXS) Project

In 2019, a conference held at the Kavli Institute for Theoretical Physics in California concluded with a fraught statement: "We wouldn't call it a tension or a problem but rather a crisis."

David Gross, a particle physicist and former director of the KITP was talking about the rate at which our universe is expanding. But Gross wasn't worried about the expansion itself. We've already known for decades that the cosmos is exponentially blasting apart, because celestial bodies surrounding our planet continuously drift farther away from us and from each other. 

No, Gross was worried about mathematics.

To determine exactly how quickly this cosmic shift is happening, scientists must calculate an important value called the Hubble constant -- yet, even today, no one can agree on the answer. 

Thus, the astronomy community was permeated with a "crisis," but it was a dilemma that cradled innovation. Since that tense conference, experts everywhere have starkly adjusted the way they look at their Hubble constant equations as an attempt to restore peace among stargazers. 

And on Monday, one such team presented a very out-of-the box idea to settle the dispute, as outlined in a paper published Aug. 3 in the journal Physical Review Letters.

Basically, astronomers from the University of Chicago believe when black holes lurking in deep space smash into one another – which they do sometimes – the gravitational leviathans reverberate ripples across the fabric of space and time that might leave traces of information crucial to decoding the Hubble constant. 

In the end, if scientists can figure out the true Hubble constant, they can also derive answers to some really big questions about our universe like: How did it evolve to the stunning realm we see today? What is it physically made out of? What might it look like billions of years from now, long after humanity ceases to exist and therefore can't cast an eye on it?

Reading between the lines of space-time

Every so often, two enormous black holes collide. This means that a pair of the universe's most incomprehensibly massive objects combine into an even more incomprehensibly massive object. 

When this happens, the merger sends ripples across the fabric of space and time -- as coined by Albert Einstein's general relativity -- just like dropping a rock in a pond would send ripples across the water. 

Animation of gravitational waves produced by a fast binary orbit.

NASA

Just four years before Gross and fellow physicists hosted their stressful debate over the Hubble constant conundrum, two powerful observatories managed to capture those black hole-induced ripples from down here on Earth. They're called the US Laser Interferometer Gravitational-Wave Observatory and the Italian Virgo observatory. 

Over the past few years, both LIGO and Virgo have detected the ripples from almost 100 pairs of black hole collisions, and those readings might help us calculate the rate at which the universe is expanding, according to Daniel Holz, an astrophysicist at the University of Chicago and co-author of the new study. They might shed light on the Hubble constant. 

"If you took a black hole and put it earlier in the universe," Holz said in a press release, "the signal would change, and it would look like a bigger black hole than it really is."

What this means is that if a black hole collision happened way (way) out in space, and the signal has been traveling for a long (long) time, the gravitational ripples emanating from the event would've been affected by the universe expanding since the incident. If you think about pond ripples again, for instance, dropping a rock in a pond usually creates tighter ripples right at the point of contact. But if you keep watching those ripples extend outward, they get sort of wider and blunter.

Therefore, if we can somehow measure the changes in black hole collision ripples, perhaps we can understand the rate at which some of those changes occur. That would help us understand the rate at which the universe's expansion might've affected them and finally, the rate at which the universe is legitimately expanding. 

"So we measure the masses of the nearby black holes and understand their features, and then we look further away and see how much those further ones appear to have shifted," Jose María Ezquiaga, a NASA Einstein Postdoctoral Fellow, Kavli Institute for Cosmological Physics Fellow and co-author of the new study, said in the release. "This gives you a measure of the expansion of the universe."

Is there a catch?

But there is a bit of a caveat -- this technique, which the researchers call the "standard siren" method, can't quite be implemented right now. In truth, LIGO and Virgo are going to have to really buckle down and get to work for us to even imagine a future where it becomes commonplace. 

"We need preferably thousands of these signals, which we should have in a few years, and even more in the next decade or two," Holz said. "At that point, it would be an incredibly powerful method to learn about the universe."

Though a pretty promising aspect of the standard siren method is that it relies on Einstein's general relativity theory -- tried and tested rules that are considered unbreakable by many, and thus incredibly trustworthy. 

From left, an illustration of how relative amounts that the moon might warp space-time, then the Earth, the sun, and a black hole all the way on the right.

Zooey Liao/CNET

By contrast, most other scientists tackling the Hubble constant crisis rely on stars and galaxies, the researchers said, which involves a lot of complex astrophysics and introduces an honest possibility of error. But, of note, there have been some other experts zeroing-in on gravitational waves as measurements of the Hubble constant. 

In 2019, for example, a separate crew of astronomers looked at ripples across space and time stemming from a neutron star merger, which was picked up by LIGO and Virgo in 2017. They were trying to understand how bright the collision was when it happened by reverse calculating from the gravitational waves and eventually arriving at a Hubble constant estimate. And in the same year, another team suggested that we need only about 25 neutron star collision readings to nail down the constant to within an accuracy of 3%.