How black holes crashing together could settle an astronomical dispute

In 2019, a conference held at the Kavli Institute for Theoretical Physics in California concluded with a heavy statement: “We would not call it a tension or a problem but rather a crisis.”

David Gross, particle physicist and former director of KITP was talking about the rate at which our universe is expanding. But Gross wasn’t worried about the expansion itself. We have already known for decades that the cosmos is exploding exponentially, as the celestial bodies surrounding our planet are continually moving away from us and from each other.

No, Gross worried about math.

To determine exactly how fast 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 astronomical community was steeped in a “crisis”, but it was a dilemma that rocked innovation. Since that tense conference, experts around the world have drastically adjusted the way they view their Hubble constant equations as an attempt to restore peace among astronomers.

And on Monday, one of these teams presented a very original idea to settle the dispute, as indicated in an article published August 3 in the journal Physical Review Letters.

Basically, University of Chicago astronomers believe that when hidden black holes in deep space collide — which they sometimes do — gravitational leviathans ripple through the fabric of space and time that could leave traces of crucial information to decode the Hubble constant. .

Ultimately, if scientists can understand the true Hubble constant, they can also get answers to some really big questions about our universe like: How evolved into the breathtaking realm we see today? What is it physically made of? What might it look like billions of years from now, long after humanity has ceased to exist and therefore can no longer look into it?

Read between the lines of space-time

From time to time, two huge black holes collide. This means that a pair of the most incomprehensibly massive objects in the universe combine into an even more incomprehensibly massive object.

When this happens, the fusion sends ripples through the fabric of space and time – as invented Albert Einstein’s General Relativity – just like dropping a rock in a pond would send ripples through the water.

Animation of gravitational waves produced by a fast binary orbit.


Just four years before Gross and his fellow physicists staged their stressful debate over the constant enigma of Hubble, two powerful observatories managed to capture these black hole-induced ripples from here on Earth. They are called the US Laser Interferometer Gravitational-Wave Observatory and the Italian Virgo Observatory.

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

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

This means that if a black hole collision happened very far out in space and the signal traveled for a long (long) time, the gravitational ripples emanating from the event would have been affected by the expansion of the universe. since the incident. If you think back to ripples in a pond, for example, dropping a rock into a pond usually creates tighter ripples right at the point of contact. But if you keep watching those ripples expand outward, they kind of get wider and duller.

Therefore, if we can somehow measure changes in black hole collision ripples, we may be able to understand how fast some of these changes occur. This would help us understand how fast the expansion of the universe might have affected them and finally, how fast the universe is legitimately expanding.

“So we measure the masses of nearby black holes and understand their characteristics, and then we look further afield and see how much those others seem to have changed,” said Jose María Ezquiaga, NASA Einstein Postdoctoral Fellow, Kavli Institute for Cosmological Physics. Fellow and co-author of the new study, said in the release. “It gives you a measure of the expansion of the universe.”

Is there a catch?

But there’s a small caveat – this technique, which the researchers call the “standard siren” method, can’t quite be implemented just yet. 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 this 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 this point, it would be an incredibly powerful method of learning more about the universe.”

Although a rather promising aspect of the standard mermaid method is that it relies on Einstein’s theory of general relativity – Tried and tested rulers that are considered unbreakable by many, and therefore incredibly trustworthy.

From left to right is an illustration of how the moon could warp spacetime, then the Earth, the sun, and a black hole on the far right.

Zooey Liao/CNET

By contrast, most other scientists tackling Hubble’s constant crisis rely on stars and galaxies, the researchers said, which involves a lot of complex astrophysics and introduces an honest opportunity for error. But, it should be noted that other experts have focused on gravitational waves as measurements of the Hubble constant.

In 2019, for example, a separate team of astronomers looked at ripples in space and time resulting from a neutron star merger, which was picked up by LIGO and Virgo in 2017. They were trying to figure out how bright the collision was when it happened by back-calculating from the gravitational waves and finally arriving at a Hubble constant estimate. And the same year, another team suggested that we only need about 25 neutron star collisions readings to fix the constant with an accuracy of 3%.

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