Black hole collisions could measure the cosmic expansion rate

Scientists may have found a way to use colliding black holes to measure how fast the universe is expanding and solve some of the mysteries surrounding dark energy.

Violent mergers of black holes create ripples in spacetime called gravitational waves. New techniques measure changes in these signals as we witness the expansion of the universe directly.

Astronomers have known since the late 1990s that the universe is expanding at an accelerating rate, and call this expansion rate the Hubble constant. But when scientists calculate the Hubble constant based on observations of the universe and current theory, they arrive at very different values. Scientists therefore want to use cosmic collisions between close pairings of black holes as what the team calls “spectral sirens” to provide an alternative measurement technique for the Hubble constant. . A final elucidation of this pressing cosmological question could reveal in greater detail how the universe evolved and what the early universe looked like.

In particular, a better understanding of how the universe evolved could help cosmologists solve some important mysteries about dark energy, which is the source of matter and energy in the universe. about 68% of the total, and scientists want to determine when this mysterious force began to take over matter and why this switch occurred.

On Earth, highly sensitive laser interferometers such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), Italy’s Virgo Observatory, and Japan’s Kamioka Gravitational-Wave Detector (KAGRA) detect these faint gravitational-wave signals. I can measure. Since gravitational waves were first detected in September 2015, LIGO and its partner instruments have collected data from about 100 distant mergers. Each discovery will give scientists an idea of ​​​​the size of the black holes involved in the merger. For example, this first gravitational wave detection was brought about by the collision of two black holes, each containing about 30 times the mass of the Sun.

At the heart of the spectral siren method are gravitational waves (ripples in the fabric of space and time) caused by powerful cosmic events such as the collisions and mergers of massive compact bodies such as neutron stars and black holes.

A new spectral siren method suggests that gravitational wave signals can encode other information as well. Because these waves travel very long distances in spacetime and reach Earth on long timescales, the properties of their signals are changing with the expansion of the universe. “For example, placing a black hole early in the universe changes the signal and makes it look like a larger black hole than it actually is,” said study and university co-author Daniel Holtz. A Chicago astrophysicist issued a statement.

To unlock the information about the expansion rate of the universe encoded in gravitational wave data, scientists need to know how the signal has changed since it passed through space. Holz and his colleagues believe that the newly discovered population of local black holes could serve as a tool for assessing these changes.

“We measure the mass of nearby black holes, understand their properties, and then study how far away black holes have traveled,” the statement said. “And it gives us a measure of the breadth of the universe.” Gravitational waves, like light, take time to travel from their source to Earth, so detecting these waves allows scientists to go back in time to more distant black hole mergers. And the authors of the study believe that as LIGO and other detectors become more powerful and collect gravitational wave signals from more distant events, researchers will one day be able to move back in time about 3.8 billion years from the Big Bang. Later, it may be possible to observe a collision that occurred 10 billion years ago. . This is also the time when researchers believe dark energy began to dominate other forms of matter and energy.

“It was around this time that we transitioned from dark matter as the dominant force in the universe to the power of dark energy, and I am very interested in studying this important transition,” Ezquiaga said. Ezquiaga and Holz say his spectral siren method, which measures the Hubble constant, may outperform other techniques, such as measuring changes in the frequency of light from distant supernovae and exploding stars. I’m here. (These approaches rely on understanding the physics of stars and galaxies, which complicates physics and astrophysics.)

However, this new technique relies less on Einstein’s well-established gravitational model, 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. Holz concludes: “At that point, it would be an incredibly powerful way to learn about the universe.”

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