Space: Observing black hole collisions could answer a big question


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

David Gross, particle physicist and former director of KITP, spoke about the rate at which our universe is expanding. But Gross wasn’t worried about the expansion itself. We have known for decades that the cosmos is moving apart exponentially, as the celestial bodies that surround our planet are constantly moving away from us and from each other. No, Gross is worried about math.

To determine the exact rate at which this cosmic change is taking place, scientists must calculate an important value called the Hubble constant—but even today, no one can agree on the answer.

So the astronomical community was steeped in a “crisis,” but it was a dilemma that rocked innovation. Since the tense conference, experts around the world have radically changed the way they look at their Hubble constant equations in an attempt to restore peace among astronomers.

On Monday, one of those teams presented a highly original idea to settle the dispute, as highlighted in an article published on August 3 in the journal Physical Review Letters.

Basically, astronomers at the University of Chicago believe that when black holes lurking in deep space collide with each other — which sometimes happens — these gravitational monsters cause ripples in the fabric of space and time that could leave traces of crucial information to decode Hubble constant.

Ultimately, if scientists can determine Hubble’s true constant, they can also answer some fundamental questions about our universe, such as: how did it evolve into the fantastic world 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 can therefore no longer monitor it?

Read between the lines in space-time

From time to time, two huge black holes collide. This means that two of the most massive and incomprehensible objects in the universe combine to form an even more massive and incomprehensible object.

When this happens, the fusion causes ripples in the fabric of space and time (according to Albert Einstein’s term for general relativity), just as throwing a stone into a pond causes ripples in the water.

Over the past few years, LIGO and Virgo have detected ripples from nearly 100 pairs of black hole collisions, and these readings could help calculate the expansion rate of the universe, according to Daniel Holz, an astrophysicist at the University of Chicago and co-author of the new study. They could also 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 the black hole is bigger than it is.” it really isn’t. »

What this means is that if a black hole collision happened very (very) far out in space and the signal traveled for a very (very) long time, the gravitational waves emanating from the event would have been affected by the expansion of the universe since the event. If you think back to the ripples in a pond, for example, dropping a rock in a pond usually creates tighter ripples. But if you keep watching these ripples expand outward, they become wider and duller.

So if we can somehow measure changes in black hole collisional waves, we might be able to understand how fast some of these changes are happening. This would help us understand how fast the universe’s expansion may have affected them, and ultimately how fast the universe is legitimately expanding.

“So we measure the masses of nearby black holes and understand their characteristics, and then we look farther away and see how much further they appear to have moved,” NASA Einstein postdoc Jose María Ezquiaga said in the statement. researcher at the Kavli Institute for Cosmological Physics and co-author of the new study. “It provides a measure of the universe’s expansion.”

Will everything go as planned?

There is (necessarily) a small drawback: this technique, which the researchers call the “standard siren” method, cannot be fully implemented at the moment. In fact, LIGO and Virgo will have to get to work so we can imagine a future where this method becomes mainstream.

“We need thousands of these signals, which we should have in a few years, and even more in a decade or two,” Holz said. “At that point, it could be an incredibly powerful method for learning more about the universe.”

The advantage of the standard mermaid method is that it is based on Einstein’s theory of general relativity, proven rules that are considered unbreakable by many, and therefore incredibly reliable.


From the left, an illustration of how the Moon can distort spacetime, then the Earth, the Sun, and a black hole on the far right. Zooey Liao/CNET

However, others rely on stars and galaxies, which involves many complex tasks in astrophysics that can introduce a significant margin of error.

In 2019, for example, another team of astronomers looked at the ripples in space and time resulting from the merger of a neutron star, which was discovered by LIGO and Virgo in 2017. They have been trying to figure out how bright the collision was when it was done by doing an inverse calculation from the gravitational waves and finally arriving at an estimate of the Hubble constant. In the same year, another team suggested that we only need about 25 neutron star collision readings to determine the constant with 3% accuracy.

CNET.com article adapted by CNETFrance

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