Black hole collisions could help us gauge how fast the universe is expanding

A black hole is usually where information disappears, but scientists may have found a trick to using its final moments to tell us about the history of the universe.

In a new study, two astrophysicists from the University of Chicago have presented a method to use colliding pairs of black holes to measure the rate of expansion of our universe and thus understand how the universe evolved, from what it is made and where it is. Go.

In particular, the scientists believe the new technique, which they call a “spectral siren,” could tell us about the otherwise elusive “teenage” years of the universe.

A cosmic ruler

A major ongoing scientific debate centers on the exact rate at which the universe is expanding, a number called the Hubble constant. The different methods available so far give slightly different answers, and scientists are eager to find other ways to measure this rate. Verifying the accuracy of this number is particularly important because it affects our understanding of fundamental questions such as the age, history and composition of the universe.

The new study offers a way to do this calculation, using special detectors that pick up cosmic echoes from black hole collisions.

Every once in a while, two black holes collide – an event so powerful that it literally creates a ripple in spacetime that ripples through the universe. Here on Earth, the American Observatory of Laser Interferometer Gravitational Waves (LIGO) and the Italian Observatory Virgo can pick up these ripples, called gravitational waves.

Over the past few years, LIGO and Virgo have collected readings from nearly 100 pairs of colliding black holes.

The signal from each collision contains information about the mass of the black holes. But the signal has traveled through space, and during this time the universe has expanded, which changes the properties of the signal. “For example, if you took a black hole and put it earlier in the universe, the signal would change and it would look like a bigger black hole than it actually is,” the astrophysicist explained. UChicago Daniel Holz, one of the study’s two authors. paper.

If scientists can find a way to measure how this signal has changed, they can calculate the expansion rate of the universe. The problem is calibration: how do they know How many does it change from the original?

In their new paper, Holz and first author Jose María Ezquiaga suggest that they can use our new knowledge of the entire black hole population as a calibration tool. For example, current evidence suggests that most detected black holes are between five and 40 times the mass of our sun. “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 Ezquiaga, a NASA Einstein postdoctoral fellow and fellow at the Kavli Institute for Cosmological. Physics. working with Holz in UChicago. “And that gives you a measure of the expansion of the universe.”

The authors call it the “spectral mermaid” method, a new approach to the “standard mermaid” method pioneered by Holz and his collaborators. (The name refers to the “standard candle” methods also used in astronomy.)

Scientists are excited because in the future, as LIGO’s capabilities expand, the method could offer a unique window into the universe’s ‘teenage’ years – around 10 billion years ago – which are difficult to study with other methods.

Researchers can use the cosmic microwave background to observe the very earliest moments of the universe, and they can observe galaxies near our own galaxy to study the more recent history of the universe. But the intermediate period is more difficult to reach and is an area of ​​special scientific interest.

“It was during this time that we transitioned from dark matter as the predominant force in the universe to dark energy taking over, and we are very interested in studying this critical transition,” Ezquiaga said.

The other advantage of this method, according to the authors, is that there are fewer uncertainties created by gaps in our scientific knowledge. “By using the entire population of black holes, the method can calibrate itself, identifying and correcting errors directly,” Holz said. Other methods used to calculate the Hubble constant rely on our current understanding of the physics of stars and galaxies, which involves a lot of complicated physics and astrophysics. This means that the measurements can be a bit off if there is something we don’t know yet.

In contrast, this new black hole method relies almost solely on Einstein’s theory of gravity, which is well-studied and has stood up to every way scientists have tried to test it so far.

The more readings of all black holes they have, the more accurate this calibration will be. “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.”

Quote : “Spectral sirens: cosmology from the complete mass distribution of compact binaries.” Ezquiaga and Holz, Physical Review Letters, August 3, 2022.

Funding: NSF, NASA

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