Colliding black holes could clock universe's expansion rate

Scientists may have found a way of using the collisions of black holes to measure the rate at which the universe is expanding and solve some of the mysteries surrounding dark energy, the mysterious force that drives accelerated cosmic expansion.

The violent mergers of black holes launch ripples in spacetime called gravitational waves, and the new technique measures changes in these signals that occur as they experience the universe's expansion firsthand.

Astronomers have understood since the late 1990s that the cosmos is expanding at an accelerating rate, and they call the speed of this expansion the Hubble constant. But when scientists calculate the Hubble constant based on observations of the universe and current theories, they end up with vastly different values. So scientists hope to use cosmic collisions between tight binary black hole pairings as what the team term 'spectral sirens' to provide an alternative measurement technique for the Hubble constant. Finally settling this pressing cosmological concern could reveal in greater detail how the universe has evolved and how it looked in its early years. 

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In particular, a better understanding of the universe's evolution could help cosmologists solve some key puzzles about dark energy. Dark energy makes up about 68% of the matter and energy content of the universe, and scientists want to determine when this mysterious force began to rule over matter and why this switch occurred. 

At the heart of the spectral siren method are gravitational waves — ripples in the very fabric of space and time — that are launched by powerful cosmic events like the collision and merger of massive compact objects like neutron stars and black holes.

On Earth, incredibly sensitive laser interferometers like the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Italian observatory Virgo and Japan's Kamioka Gravitational Wave Detector (KAGRA), can measure these faint gravitational wave signals. 

Since the first detection of gravitational waves in September 2015, LIGO and its partner instruments have collected data from around 100 distant mergers. Each detection gives scientists a hint at the size of the black holes involved in the merger. For instance, that first gravitational wave detection stemmed from the collision of two black holes each containing roughly 30 times the mass of the sun. 

The new spectral siren method suggests that gravitational wave signals may encode other information as well. Specifically, as these ripples in spacetime travel across tremendous distances and over long timescales to reach Earth, the properties of their signals are changed by the expansion of the universe.

"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 really is," research co-author and University of Chicago astrophysicist Daniel Holz said in a statement. 

In order to unlock information about the expansion rate of the universe coded in gravitational wave data, scientists will need to know how the signal has changed since it was launched through space. Holz and his colleague think that a newly discovered population of local black holes could be used as a tool to assess these changes.

"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, co-author and an astrophysicist also at the University of Chicago, said in the statement. "And this gives you a measure of the expansion of the universe."

Because gravitational waves, like light, take time to travel from their source to Earth, detecting these ripples from more distant black hole mergers lets scientists look back in time. And the study authors say that as LIGO and other detectors become even more powerful and collect gravitational wave signals from more distant events, researchers could perhaps one day observe collisions that occurred as long as 10 billion years ago — around 3.8 billion years after the Big Bang. This is also when researchers believe dark energy began to dominate other forms of matter and energy.

"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," Ezquiaga said. 

Ezquiaga and Holz say that the spectral siren method of measuring the Hubble constant could have advantages over other techniques, such as measuring the change in frequency of light from distant supernovas, or exploding stars. (These approaches hinge on understanding the physics of stars and galaxies, and thus, complicated physics and astrophysics.) 

This new technique, however, depends on little more than Einstein's well-established model of gravity — the theory of general relativity — and uses local black holes as a built-in calibration tool. This calibration will improve as more gravitational wave data is collected from colliding black holes. 

"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 concluded. "At that point, it would be an incredibly powerful method to learn about the universe."

The duo's research is discussed in a paper published Aug. 3 in the Physical Review Letters.

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