How fast is the universe actually expanding? Ripples in spacetime could finally solve 'Hubble tension'

A team of scientists says it's 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 biggest mysteries in physics today, a disparity in calculating this rate known as the "Hubble tension."

Scientists have known since 1998 that not only is the universe expanding, but also that the expansion rate is accelerating. "Dark energy" was introduced as a placeholder name for the mysterious force driving this acceleration, but there's an outstanding issue surrounding the universe's expansion rate in general, even after over two decades and a half of investigation.

A key part in measuring the rate of our universe's expansion is the Hubble constant. The so-called "Hubble tension" arises from the fact that when you measure the Hubble constant starting from the local and modern-day universe — using Type 1a supernovas for your measurements — you get one value. However, when you begin 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 another value. Scientists have therefore long been hunting for a third way to measure the Hubble constant as an extra way of checking its true value. And now, a team of researchers from the University of Illinois Urbana-Champaign and the University of Chicago thinks the answer lies with gravitational waves.

"This result is very significant — it's important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension," team leader Nicolas Yunes, the founding director of Urbana's Illinois Center for Advanced Studies of the Universe (ICASU), said in a statement. "Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves."

Why gravitational waves?

The story of gravitational waves begins in 1915 with Albert Einstein's theory of gravity, known as general relativity. General relativity suggests that objects with mass cause the very fabric of spacetime (the four-dimensional unification of space and time) to warp. What we experience as gravity arises from this warping; the larger 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. Those are called gravitational waves. Humanity made the first detection of gravitational waves in 2015, thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S. The detected waves came from the collision and merger of two massive black holes located around 1.3 billion light-years away. Since then, along 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 of gauging the Hubble constant before, but the issue has been that the accuracy hasn't been there. This team thinks their novel approach has that accuracy, and says it will only increase as our gravitational wave detectors become more sensitive.

"It's not every day that you come up with an entirely 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," Daniel Holz of the University of Chicago said. "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 key cosmological quantities."

To use gravitational waves to measure the Hubble constant, scientists need to measure the speed at which the events that launch the waves are receding away from us, not just estimate the distance to said events. That requires astronomers tracking down light, or more precisely, electromagnetic radiation, from these events or even from the galaxies hosting the events

Comparing these two forms of astronomy, unified as so-called "multi-messenger astronomy," scientists can get two values for the Hubble constant: one with electromagnetic radiation alone, one with electromagnetic radiation and gravitational waves. If these techniques don't agree, the Hubble tension persists, and scientists know there is something different about the early universe and the modern universe that is currently unaccounted for.

What the team proposes to use in the technique they call the stochastic siren method are background gravitational waves. This can be thought of as the background hum of the universe from a host of more distant collision events underlying that loud crashing orchestra of relatively close massive black hole mergers.

"Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe," Cousins said. "Based on those rates, we expect there to be a lot more events that we can’t observe, which is 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 to occur, resulting in a higher collision density and thus a stronger gravitational wave background signal. So, if that background can't be detected, that hints at a higher Hubble constant.

Though the LIGO-Virgo-KAGRA conglomerate isn't yet sensitive to detect the gravitational wave background, the team was still able to apply the stochastic siren method to the data gathered by these detectors. They found that this implied higher Hubble constant values and thus a more rapid universal expansion rate.

That was just a proof of concept for the team; the stochastic siren method could really come into its own over the next six years, as sensitivity increases 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 this method could have developed enough to provide an independent measure of the Hubble constant, potentially ending the Hubble tension.

"This should pave the way for applying this method in the future as we continue to increase the sensitivity, better constrain the gravitational-wave background, and maybe 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 edition of the journal Physical Review Letters.