Scientists can use black holes to finally discover how fast the universe is expanding

The Hubble constant, the rate at which the universe expands, has been debated since it was discovered 100 years ago

Adam Smith
Thursday 18 August 2022 11:38 BST

Related: Astronomers Discover a Black Hole Devouring the Equivalent of One Earth Per Second

Scientists believe they can understand how much the universe is expanding by utilizing a strange ruler – black holes.

Astrophysicists from the University of Chicago have developed a new method that uses colliding black holes to measure how fast our universe is growing.

The rate of expansion – known as the Hubble constant – has been debated since 1929, when it was discovered by Edwin Hubble. Scientists have been measuring and calculating the growth for nearly a century, however, because the constant does not match up with real-world observations.

The fundamental physics of the universe suggest that the universe should expand by 68 kilometres per second measuring a galaxy one megaparsec away from Earth – but unfortunately, that does not match up with the observations of actual stars, which seem to be moving at a much faster rate and implies that we do not understand a significant part of our universe.

As such, scientists are keen to find new ways to measure the rate of expansion. This new test uses black holes merging with each other, an event so powerful that it creates a ripple in space-time that travels across the universe. These ripples – also known as gravitational waves - can be measured on Earth using technology such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) or the Virgo observatory.

Using measurements from over 100 pairs of black holes colliding, scientists can see how the waves change as they travel. “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 really is,” University of Chicago astrophysicist Daniel Holz said.

The challenge then becomes how to calculate the expansion rate from the original signal, but current evidence suggests that most of the detected black holes have between five and 40 times the mass of our Sun.

“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 and Kavli Institute for Cosmological Physics Fellow also working on the problem, says. “This gives you a measure of the expansion of the universe.”

With more work, this method could help scientists discover more information about the universe when it was only 10 billion years old — a time that is hard to study with other methods.

“It’s around that time that we switched from dark matter being the predominant force in the universe to dark energy taking over, and we are very interested in studying this critical transition,” said Dr Ezquiaga.

The other advantage of this method, the authors said, 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, directly identifying and correcting for errors,” Dr Holz said. Other methods rely on our current understanding of the physics of stars and galaxies, but if there are aspects of the universe we do not understand it could have a severe impact on the measurements. This method, by contrast, relies almost entirely on Einstein’s theory of gravity which is more consistent.

With more data from black holes, the more accurate future calculations will become. “We need preferably thousands of these signals, which we should have in a few years, and even more in the next decade or two,” said Dr Holz. “At that point it would be an incredibly powerful method to learn about the universe.”

The research, “Spectral sirens: cosmology from the full mass distribution of compact binaries”, is published in Physical Review Letters.

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