A team of scientists say it is possible to use tiny ripples in space and time, or gravitational waves, to measure the rate at which our universe is expanding. This could solve one of the greatest mysteries in physics today, an inequality in the calculation of this speed known as the “Hubble mission.”
Scientists have known since 1998 that not only is universe is expanding, but also that the rate of expansion is accelerating. “Dark energy” was introduced as a placeholder name for the mysterious force driving this acceleration, but there is an outstanding problem surrounding the universe’s expansion rate in general, even after over two and a half decades of investigation.
A key part in measuring the speed of our universe expansion is 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 supernovae for your measurements – you get one value. But when you start the calculation starting from the distant and ancient cosmos—and use a major framework in physics called the Standard Model of Cosmology to measure the answer—you get a different value. Scientists have therefore long sought a third way to measure the Hubble constant as an additional way to check its true value. And now a team of researchers from the University of Illinois Urbana-Champaign and the University of Chicago believe the answer lies with them gravitational waves.
“This result is very important – it is important to get an independent measurement of the Hubble constant to resolve the current Hubble tension,” team leader Nicolas Yunes, founder of Urbana’s Illinois Center for Advanced Studies of the Universe (ICASU), 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 history of gravitational waves begins in 1915 with Albert Einsteinhis theory of gravitation, known as general relativity. General relativity suggests that objects with mass cause the very fabric of spacetime (the four-dimensional union of space and time) to warp. What we experience as gravity arises from this twisting; the greater the mass, the greater the curvature and the stronger the gravitational effect.
However, general relativity also predicts that when objects accelerate in spacetime, this generates ripples that radiate outward at the speed of light. They are called gravitational waves. Humanity made the first discovery of gravitational waves in 2015, thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the USA The detected waves came from the collision and merger of two massive black holes is around 1.3 billion light-years away. Since then, together with its fellow detectors Virgo and the Kamioka Gravitational Wave Detector (KAGRA) in Italy and Japan respectively, LIGO has detected gravitational waves from many mergers between pairs of black holes, pairs of ultra-dense neutron stars – and even a mixed merger between a black hole and a neutron star.
Gravitational waves have been proposed as a way to measure the Hubble constant before, but the problem has been that the accuracy has not been there. This team believes their new approach has that accuracy, and says it will only increase as our gravitational wave detectors become more sensitive.
“It’s not every day you come up with a completely new tool for cosmology. We show that by using the background gravitational wave hum from merging black holes 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 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.”
To use gravitational waves to measure the Hubble constant, scientists need to measure the speed at which the events that send the waves are receding away from us, not just estimate the distance of said events. It requires astronomers to track light, or more precisely, electromagnetic radiation, from these events or even from the galaxies that host the events
By comparing these two forms of astronomy, collectively known as “multi-messenger astronomy”, researchers can obtain two values for the Hubble constant: one with electromagnetic radiation alone, one with 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 has not yet been accounted for.
What the team proposes to use in the technique they call the stochastic siren method is background gravitational waves. This can be thought of as the universe’s background hum from a series of more distant collision events that underlie the loud crash orchestra of relatively nearby massive black hole mergers.
“Because we observe individual black hole collisions, we can determine the frequency of these collisions occurring throughout the universe,” Cousins said. “Based on these rates, we expect that there will be many more events that we cannot observe, which are called the gravitational wave background.”
Cousins and colleagues reason that for lower Hubble constant values, there is a lower volume of space available for collisions, resulting in a higher collision density and thus a stronger gravitational wave background signal. So if that background cannot be detected, it suggests a higher Hubble constant.
Although the LIGO-Virgo-KAGRA conglomerate 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 implied higher Hubble constant values and thus a faster universal expansion rate.
It was just a proof of concept for the team; the stochastic siren method may really come into its own in the next six years, as sensitivity increases and scientists can tighten constraints on the Hubble constant. After this period, gravitational wave detectors should be sensitive enough to “hear” much of the gravitational wave background, and this method could have evolved enough to provide an independent measure of the Hubble constant, potentially ending the Hubble tension.
“This should pave the way for using this method in the future as we continue to increase sensitivity, better constrain the gravitational wave background, and perhaps even detect it,” Cousins said. “By including that information, we expect to get better cosmological results and be closer to resolving the Hubble tension.”
The team’s research appears in the March 11 issue of the journal Physical review letters.
