Gravitational wave background could repair our broken understanding of the universe

Physicists may have an entirely new way to measure the rate of expansion of the universe, one of the biggest unsolved mysteries in cosmology, by harnessing the ripples in space-time predicted by Einstein.

A new study suggests that the weak gravitational wave background produced by the mergers of large numbers of black holes across the universe can be used to independently measure how fast the universe is expanding. Even without directly detecting this background “hum,” the researchers show that this background already places limits on the Hubble constant. The Hubble constant is an important quantity at the heart of one of the biggest puzzles in modern cosmology.

If confirmed, this technique could settle the debate over whether we need to invent new physics to explain the nature of the universe.

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An independent test of the Hubble constant

The expansion rate of the universe, encoded by the Hubble constant, has been the focus of intense debate in recent years. Measurements based on the early Universe, such as those inferred from residual radiation from the Big Bang (known as the cosmic microwave background radiation), do not match measurements from closer objects, such as flickering supernovae or galaxies. This discrepancy, known as the Hubble tension, has now reached high statistical significance.

“The Hubble tension is one of the most important unsolved problems in cosmology,” Chiara Mingarelli, an assistant professor of physics at Yale University who was not involved in the study, told Live Science in an email. “Measurements of the expansion rates in the early and late Universe do not agree beyond 5 sigma.” [the “gold standard” of statistical significance in physics]I don’t know why. Either there is an unidentified systematic error or new physics exists. A truly independent measurement of expansion rate would be of great value. ”

The new study, which has been accepted for publication in the journal Physical Review Letters and is available as a preprint, proposes such an independent method based almost entirely on gravitational waves, subtle waves in the fabric of space-time predicted by Einstein’s theory of general relativity.

“This result is extremely important,” study co-author Nicholas Younes, a professor of astrophysics at the University of Illinois at Urbana-Champaign, said in a statement. “Our method is an innovative way to use gravitational waves to improve the accuracy of Hubble constant inference.”

Hear the background noise of a black hole

Since 2015, detectors such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Virgo Interferometer, and the Kamioka Gravitational-Wave Detector (KAGRA) have observed the merger of dozens of individual black holes via gravitational waves. Each merger provides information about the mass and distance from Earth of the black holes involved.

“Because we are observing individual black hole collisions, we can determine the rate of those collisions occurring throughout the universe,” study lead author Bryce Cousins, a graduate student at the University of Illinois at Urbana-Champaign, said in a statement. “Based on their speed, we expect there to be many more events that we cannot observe, called the gravitational wave background.” This gravitational wave background, sometimes described as a stochastic (or random) signal, is the faint collective effect of many distant mergers. Its overall strength is determined by how fast the universe is expanding. Slower expansion means that the volume of the Universe is larger, and therefore more mergers are contributing to the background.

“This is a smart idea,” Mingarelli said. “The gravitational wave background (the sound of distant black hole mergers so faint that they cannot be detected individually) depends on the expansion rate. Slower expansion means larger volume, more mergers, and a larger background. So even if this background is not detected, a low value of the Hubble constant is a disadvantage.”

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Using current data from gravitational wave detectors, the research team showed that some lower values ​​of the Hubble constant have already been ruled out by the lack of detected background. Although current limitations are extensive, this method establishes a new framework for cosmological reasoning.

This approach is based on the concept of a “standard siren” in which individual gravitational wave events act as distance markers. But rather than relying on a single bright event, the new method takes advantage of an entire unresolved population of colliding black holes.

“It’s not every day that you come up with a completely new tool for cosmology,” study co-author Daniel Holtz, a professor of physics and astronomy at the University of Chicago, said in a statement. “We have shown that we can learn about the age and composition of the universe by harnessing the background sound of gravitational waves resulting from the merger of black holes in distant galaxies.

“This is an exciting and entirely new direction, and we look forward to applying our method to future datasets to constrain the Hubble constant and other important cosmological quantities,” Holtz added.

While the new method is promising, Mingarelli also highlighted current limitations. “The main strength is that this is an almost entirely gravitational wave-based measurement, independent of the electromagnetic distance ladder or the cosmic microwave background,” Mingarelli said. “The limitations are that uncertainties are still large and the results depend on the assumed black hole population model. However, the authors are upfront about this, indicating that their choice is conservative.”

Future detector upgrades are expected to significantly increase sensitivity to the gravitational wave background.

“Planned detector upgrades should detect the background within a few years, turning this from a lower limit into an actual measurement,” Mingarelli said.

If successful, the stochastic siren method could become a powerful new tool for investigating the history of the universe’s expansion and investigating whether the Hubble tension indicates new physics or hidden systematic errors in existing measurements.

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