Scientists say they can use tiny ripples in spacetime, or gravitational waves, to measure the rate of expansion of the universe. This could solve one of the biggest mysteries in physics today: the discrepancy in calculating this velocity, known as the “Hubble tension.”
Since 1998, scientists have universe It is expanding, but it can also be said that the speed of expansion is accelerating. ”dark energy” was introduced as an alternative name for the mysterious force causing this acceleration, but even after more than two and a half years of research, there are still unresolved questions about the universe’s expansion rate in general.
“This result is very important. Obtaining independent measurements of the Hubble constant is critical to resolving the current Hubble tension,” said team leader Nicolas Younes, founding director of the Illinois Center for Advanced Studies in Space (ICASU) in Urbana. said in a statement. “Our method is an innovative way to use gravitational waves to improve the accuracy of Hubble constant inference.”
Why gravitational waves?
The story of gravitational waves begins in 1915. albert einsteingravitational theory known as general relativity. General relativity suggests that objects with mass cause distortions in the very fabric of space-time (the four-dimensional unity of space and time). What we experience as gravity results from this distortion. The greater the mass, the greater the curvature and the stronger the effect of gravity.
But general relativity also predicts that when an object accelerates in spacetime, it creates ripples that radiate outward. speed of light. They are called gravitational waves. Humankind has developed a laser interferometer gravitational wave observatory (Lygo) Waves detected in the US arose from the collision and merging of two giants black hole It is located approximately 1.3 billion light years away. Since then, LIGO, along with other detectors in Italy and Japan, Virgo and the Kamioka Gravitational Wave Detector (KAGRA), have detected gravitational waves from many mergers between pairs of black holes, pairs of ultra-dense neutron stars, and even mixed mergers of black holes and neutron stars.
Gravitational waves have been proposed as a method to measure the Hubble constant, but the problem was that the accuracy was not very high. The team believes their new approach has that precision, and says it will become even more accurate as gravitational wave detectors become more sensitive.
“It’s not every day that you come up with a completely new tool for cosmology,” said Daniel Holtz of the University of Chicago. “We’ve shown that by harnessing the background gravitational wave hums from merging black holes in distant galaxies, we can learn about the age and composition of the universe.” “This is an exciting and entirely new direction, and we look forward to applying our method to future datasets to help constrain the Hubble constant and other important cosmological quantities.”
To use gravitational waves to measure the Hubble constant, scientists not only need to estimate the distance to the event that emits the gravitational waves, but also measure the speed at which the event that emits the gravitational waves is moving away from us. To do so, astronomers need to track the light, or more precisely the electromagnetic radiation, from these phenomena or from the galaxies that host them.
Comparing these two forms of astronomy, unified as so-called “multimessenger astronomy,” provides scientists with two values for the Hubble constant. One uses only electromagnetic radiation and the other uses electromagnetic radiation and gravitational waves. If these techniques do not match, the Hubble tension persists, and scientists know that there is something different about the early universe and the modern universe that is currently unexplained.
It is the background gravitational waves that the research team proposes to use in a technique they call the stochastic siren method. You can think of this as the cosmic background hum of a host of more distant collision events underlying that loud, crashing orchestra of relatively nearby massive black hole mergers.
“Because we are observing individual black hole collisions, we can determine the proportion of those collisions occurring throughout the universe,” Cousins said. “Based on their speed, we expect there to be many more events that we cannot observe, called the gravitational wave background.”
Cousins et al. reason that a lower value of the Hubble constant makes less volume of space available for collisions to occur, resulting in a higher collision density and therefore a stronger gravitational wave background signal. Therefore, if that background is not detectable, it suggests a higher Hubble constant.
Although the LIGO-Virgo-KAGRA conglomerate is not yet sensitive enough to detect gravitational wave backgrounds, the team was still able to apply the stochastic siren method to the data collected by these detectors. They found that this meant a higher value for the Hubble constant, and therefore a more rapid rate of expansion of the universe.
It was just a proof of concept for the team. The stochastic siren method could come into its own over the next six years as sensitivity improves and scientists can tighten the constraints on the Hubble constant. After this period, gravitational wave detectors should be sensitive enough to “hear” much of the gravitational wave background, and the method could be developed enough to provide an independent measurement of the Hubble constant, potentially ending the Hubble tension.
“This should pave the way for future applications of this method, as we continue to increase sensitivity and be able to better suppress and perhaps even detect the gravitational wave background,” Cousins said. “We hope that including that information will yield better cosmological results and move us closer to resolving the Hubble tension.”
The research team’s research will be published in the March 11 issue of the same journal. Physical review letter.