Black hole collisions could help estimate how fast the universe is expanding

Spectral siren.

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How fast is our universe expanding?

The Hubble constant is one of the most critical numbers in cosmology because it tells us how fast the universe is expanding. There are different methods exist to measure this rate. However, determining this number’s accuracy is essential to understand better fundamental questions like the universe’s age, history, and makeup.

The new study by two University of Chicago astrophysicists offers a way to make this calculation: using pairs of colliding black holes and thereby understanding universe evolution, what it is made out of, and where it’s going.

According to scientists, the new technique dubbed a ‘spectral siren’ could offer information about the otherwise elusive ‘teenage’ years of the universe.

Occasionally, two black holes collide. This event is so powerful that it creates a space-time ripple that travels across the universe. These ripples are also known as gravitational waves.

The U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Italian observatory Virgo can pick up those ripples here on Earth. Over the past few years, LIGO and Virgo have collected the readings from almost 100 pairs of black holes colliding.

The signal from each collision contains information about how massive the black holes were. But the signal has been traveling across space, and during that time, the universe has expanded, which changes the properties of the signal.

UChicago astrophysicist Daniel Holz, one of the two authors on the paper, said, “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 is.”

Determining a way to estimate how that signal changed could help scientists calculate the expansion rate of the universe. However, the problem is calibration: How do they know how much it changed from the original?

In this new study, scientists suggest that they can use the new knowledge about the whole population of black holes as a calibration tool. For example, current evidence indicates that most of the detected black holes have between five and 40 times the mass of our sun.

First author Jose María Ezquiaga, a NASA Einstein Postdoctoral Fellow and Kavli Institute for Cosmological Physics Fellow working with Holz at UChicago, said, “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. And this gives you a measure of the expansion of the universe.”

The scientists are excited because in the future, as LIGO’s capabilities expand, the method may provide a unique window into the “teenage” years of the universe—about 10 billion years ago—that are hard to study with other methods.

Authors noted, “The other advantage of this method is that gaps in our scientific knowledge create fewer uncertainties. The method can calibrate itself by using the entire population of black holes, directly identifying and correcting for errors. The 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 the measurements might be thrown off quite a bit if there’s something we don’t yet know.”

“By contrast, this new black hole method relies almost purely on Einstein’s theory of gravity, which is well-studied and has stood up against all the ways scientists have tried to test it so far.”

Holz said“The more readings they have from all black holes, 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. At that point, it would be an incredibly powerful method to learn about the universe.”

Journal Reference:

  1. Jose María Ezquiaga and Daniel E. Holz. Spectral Sirens: Cosmology from the Full Mass Distribution of Compact Binaries. Phys. Rev. Lett. 129, 061102 – Published 3 August 2022. DOI: 10.1103/PhysRevLett.129.061102
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