Future and its affiliate partners may earn a commission when you purchase through links on our articles.
An illustration of the evolution of the universe from the Big Bang (left) to today (right). |Image source: NASA
Tiny ripples, or gravitational waves, in space and time can be used to measure how fast the universe is expanding, a team of scientists says. This could solve one of the biggest mysteries in physics today, which is calculating the difference in this rate, known as the “Hubble tension.”
Since 1998, scientists have known that not only universe expansion, and the pace of expansion is accelerating. “dark energy“Introduced as a placeholder name for the mysterious force driving this acceleration, but even after two and a half decades of research, a glaring question remains about the universe’s overall expansion rate.
A key part of measuring the speed of the universe expansion yes Hubble constant. The so-called “Hubble tension” arises from the fact that when you measure the Hubble constant starting from the local and modern universe, using Type 1a supernova For your measurement you get a value. However, when you start your calculations from the distant and ancient universe and measure the answer using the dominant physics framework called the Standard Model of Cosmology, you get another value. Therefore, scientists have long been looking for a third way to measure the Hubble constant as an additional way to check its true value. Now, a team of researchers from the University of Illinois at Urbana-Champaign and the University of Chicago thinks the answer lies in Gravitational waves.
“This result is very important,” said team leader Nicolas Yunes, founding director of the Illinois Center for Advanced Study of the Universe (ICASU) in Urbana. “Getting an independent measurement of the Hubble constant is important for resolving the current Hubble tension.” said in a statement. “Our method is an innovative way to improve the accuracy of Hubble constant inferences using gravitational waves.”
Why gravitational waves?
The story of gravitational waves begins in 1915 albert einsteinThe theory of gravity is called general relativity. General relativity states that objects with mass cause the fabric of spacetime (the four-dimensional unity of space and time) to distort. The gravity we experience is caused by this distortion. The greater the mass, the greater the curvature and the stronger the gravitational effect.
However, general relativity also predicts that when an object accelerates through space-time, it creates ripples that radiate outward. speed of light. These are called gravitational waves. Gravitational waves were detected for the first time in 2015, thanks to the Laser Interferometer Gravitational Wave Observatory (lidar) Waves detected in the U.S. came from the collision and merger of two massive objects black hole It is about 1.3 billion light-years away from us. Since then, LIGO, together with the Virgo detector and the Kamiokande Gravitational Wave Detector (KAGRA) in Italy and Japan respectively, have detected gravitational waves from pairs of black holes, pairs of ultra-dense neutron stars, and even hybrid mergers of black holes and neutron stars.
Gravitational waves have been proposed before as a way to measure the Hubble constant, but the problem was that they were not very accurate. The team believes their new method has this accuracy, and says it will only improve as our gravitational wave detectors become more sensitive.
“It’s not every day that you come up with a whole new cosmological tool. We show that by exploiting the background gravitational wave buzz produced by black hole mergers in distant galaxies, we can learn about the age and composition of the universe,” said Daniel Holz of the University of Chicago. “This is an exciting new direction, and we look forward to applying our method to future data sets to help constrain the Hubble constant as well as other key cosmological quantities.”
Illustration showing the emission of gravitational waves from black hole collisions. |Image credit: Deborah Ferguson, Karan Jani, Deirdre Shoemaker, Pablo Laguna, Georgia Institute of Technology, MAYA Collaboration
To use gravitational waves to measure the Hubble constant, scientists need to measure how fast the event that emits the wave is moving away from us, rather than just estimating the distance to said event. This requires astronomers to track these events and even the light, or more accurately electromagnetic radiation, in the galaxies that host them
Comparing these two forms of astronomy, unified into so-called “multimessenger astronomy,” scientists can arrive at two values for the Hubble constant: one involving electromagnetic radiation only, and one involving electromagnetic radiation and gravitational waves. If these techniques were inconsistent, the Hubble tension would persist, and scientists would know that there were some currently unexplained differences between the early universe and the modern universe.
The team proposes using background gravitational waves in a technique they call the stochastic Kraken method. This can be thought of as the cosmic background hum from a series of more distant collision events that underpin the vast orchestra of collisions of relatively recent massive black hole mergers.
“Because we are observing individual black hole collisions, we can determine the rate at which these collisions occur in the universe,” Cousins said. “Based on these rates, we expect there to be more events that we cannot observe, which is called the gravitational wave background.”
Illustration of gravitational wave background. |Image source: Carl Knox, OzGrav, Swinburne University of Technology
Cousins and colleagues reasoned that the lower the value of the Hubble constant, the smaller the volume of space available for collisions to occur, resulting in a higher density of collisions and thus a stronger gravitational wave background signal. Therefore, if this background cannot be detected, it implies that the Hubble constant is high.
Although the LIGO-Virgo-KAGRA consortium is not yet sensitive enough to detect the gravitational wave background, 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 thus a faster expansion rate of the universe.
This is just a proof-of-concept for the team; as sensitivity improves and scientists can tighten the constraints on Hubble’s constant, the stochastic Kraken approach could become truly useful within the next six years. After this period, gravitational wave detectors should be sensitive enough to “hear” much of the gravitational wave background, and the method may have evolved enough to provide an independent measurement of the Hubble constant, potentially ending the Hubble tension.
“As we continue to improve sensitivity, better constrain the gravitational wave background, and perhaps even detect it, this should pave the way for future applications of this method,” Cousins said. “By including this information, we expect to obtain better cosmological results and get closer to resolving the Hubble tension.”
The team’s research appears in the March 11 edition of the journal Physical Review Letters.