Ripples in space-time predicted by Einstein could mend our broken understanding of the universe
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An illustration showing gravitational waves rippling in the background, inspired by the European Space Agency’s upcoming LISA detector. Studying the faint hum of gravitational waves in the universe could help solve one of the biggest problems in physics: the Hubble tension. | Credit: ESA
Using spacetime waves, physicists may find a brand new way to measure the rate of expansion of the universe, one of the greatest mysteries in cosmology. Einstein predicted.
A new study shows that weak gravitational wave background Data produced by large numbers of merging black holes in the universe can be used to independently measure how fast space is expanding. Even without directly detecting this background “hum,” the researchers show that it puts limits on the Hubble constant—a key quantity at the heart of one of modern cosmology’s greatest puzzles.
If confirmed, this technique could resolve the debate over whether we need to find new physics to explain the nature of the universe.
An independent test of the Hubble constant
The expansion rate of the universe, encoded in the Hubble constant, has become the focus of science. intense debates in recent years. Measurements based on the early universe (for example, those derived from residual radiation from the universe). big bang They do not participate in measurements from closer objects such as the cosmic microwave background (known as the cosmic microwave background), pulsating supernovae, and galaxies. This discrepancy, known as the Hubble voltage, has now reached high statistical significance.
“The Hubble tension is one of the most important open problems in cosmology.” Chiara Mingarellian assistant professor of physics at Yale University who was not involved in the new study, told LiveScience via email. “Early Universe and late Universe measurements of expansion rate differ by more than 5 sigma [the “gold standard” of statistical significance in physics]and we don’t know why. Either there is an unidentified systematic error or there is new physics. “Any truly independent measurement of the rate of expansion is extremely valuable.”
The new research has been accepted for publication in the journal Physical Review Letters and is available as: preprintproposes such an independent method that relies almost entirely on gravitational waves (subtle ripples in the fabric of space-time). Predicted by Einstein’s theory of general relativity.
Co-author of the study “This result is very meaningful” Nicolas Yunesprofessor of astrophysics at the University of Illinois Urbana-Champaign expression. “Our method is an innovative way to improve the accuracy of Hubble constant inferences using gravitational waves.”
Listening to the background hum of black holes
Since 2015, detectors such as the Laser Interferometer Gravitational Wave Observatory (LIGO), the Virgo interferometer, and the Kamioka Gravitational Wave Detector (KAGRA) have observed dozens of individual black hole mergers via gravitational waves. Each union provides information about the masses. black holes their inclusion and their distance from the Earth.
Gravitational waves are created when two massive objects, such as black holes, collide (shown here). Physicists believe that the universe hums with faint background noise from countless black hole collisions that are too faint to be detected; This feature is called the gravitational wave background. | Credit: NASA Goddard
“Because we observe individual black hole collisions, we can determine the rates of these collisions occurring throughout the universe,” said the study’s lead author. Bryce Cousinsa graduate student from the University of Illinois Urbana-Champaign said in the statement. “Based on these rates, we expect there to be many more events that we cannot observe, this is called the gravitational wave background.” This gravitational wave background, sometimes described as a stochastic (or random) signal, is the weak, collective effect of many distant mergers. Its overall strength depends on how fast the universe is expanding. A slower expansion means larger cosmic volumes and thus more mergers contributing to the background.
“This is a smart idea,” Mingarelli said. “The gravitational wave background – the collective hum of distant black hole mergers too faint to be detected individually – depends on the expansion rate. A slower expansion means larger volumes, more mergers and a higher background. So even not detecting this background has a negative impact on low values of the Hubble constant.”
Using recent data from gravitational wave detectors, the team showed that the absence of a detected background already rules out some low values of the Hubble constant. Although current limitations are broad, the method creates a new framework for cosmological inference.
A new tool for cosmology
The approach is based on the concept of “standard sounders” in which individual gravitational wave events act as distance beacons. But instead of relying on a single bright event, the new method uses the entire unresolved population of colliding black holes.
“It’s not every day you uncover a completely new tool for cosmology,” study co-author said Daniel HolzA professor of physics and astronomy at the University of Chicago said in the statement. “We have shown that we can learn about the age and composition of the universe by using the background gravitational wave hum of merging black holes in distant galaxies.
An example of gravitational waves emitted by a black hole collision. | Credit: NASA/C. Henze
“This is an exciting and completely new direction, and we look forward to applying our methods to future datasets to help constrain the Hubble constant as well as other important cosmological quantities,” Holz added.
Although the new method is promising, Mingarelli also emphasized existing limitations. “The real strength is that this is an almost entirely gravitational wave-based measurement, independent of the electromagnetic distance ladder and the cosmic microwave background,” Mingarelli said. “The limitation is that the uncertainties are still large and the result depends on the assumed black hole population model. But the authors are upfront about this and show that their choice is conservative.”
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As we look ahead, detector upgrades It is expected to significantly increase sensitivity to the gravitational wave background.
“With planned detector upgrades, the background should be detected within a few years and this should be converted from the lower bound to an actual measurement,” Mingarelli said.
If successful, this stochastic siren method could become a powerful new tool for probing the expansion history of the universe and investigating whether the Hubble strain points to new physics or hidden systematic errors in existing measurements.
