A precise measurement of the Hubble constant, the value that describes how fast the universe is expanding, has eluded scientists for decades. Pinning this number down would put a long-simmering debate among astronomers to rest and bring us one step closer to understanding the evolution and fate of the universe. Now researchers have used recent detections of gravitational waves to present a proof of concept for an entirely new method of determining the constant.
Until now astronomers have taken two approaches to reckoning the constant’s value. One method uses objects of known brightness, called standard candles, such as Cepheid variable stars. A Cepheid star’s light fluctuates at regular intervals, and the interval is related to how much luminosity it puts out. Deriving the star’s actual brightness from its rate of fluctuation and comparing that with how bright it appears to Earth observers is how astronomers determine its distance. The scientists then measure the redshift of the same objects—that is, how much their light has been shifted toward the red end of the electromagnetic spectrum. Redshift occurs when a light source moves away from an observer; light waves emitted from it will be stretched. This is similar to how the sound of a car horn drops in pitch as the vehicle drives away. By measuring a distant star’s redshift, astronomers can calculate how fast it is receding from Earth. When they combine that information with its distance, they obtain a value for the Hubble constant.
The second technique for figuring out the expansion rate of space relies on the cosmic microwave background (CMB), the ghostly radiation left over from the big bang that permeates deep space. Precise measurements of temperature variations in the CMB from the Planck Space Telescope, when plugged into the standard model of the big bang’s cosmology, allow astronomers to derive the constant.
The problem is, the values obtained from these methods do not agree—a discrepancy cosmologists call “tension.” Calculations from redshift place the figure at about 73 (in units of kilometers per second per megaparsec); the CMB estimates are closer to 68. Most researchers first thought this divergence could be due to errors in measurements (known among astrophysicists as “systematics”). But despite years of investigation, scientists can find no source of error large enough to explain the gap.
A more exciting possibility is the tension reflects a real difference between the Hubble constant at the distance Planck is looking at, the faraway early universe, and that of the standard candle method, the nearby, recent universe. Of course, scientists already know the universe’s expansion is accelerating—although they do not know exactly why, and name the mysterious cause “dark energy.”
But even accounting for the known acceleration, the tension suggests something strange may be happening to dark energy to cause the Hubble constant to diverge this much. It indicates the rate of expansion during the cosmic epoch that followed the big bang, which the CMB would reflect, was radically different from what cosmologists currently believe it to be. If a dark energy anomaly is not to blame, it is possible some unknown particle such as an undiscovered flavor of neutrino, the nearly massless particles that pervade the cosmos, may be affecting the calculations. “This tension can hide the solution to the description that we have of the universe—its evolution, the sources of energy which are in it,” says Valeria Pettorino, an astrophysicist and research engineer at CEA Saclay in France who was not involved in the study. “And in practice, this decides the past, the present and the future of our universe, whether or not it’s going to be expanding forever, whether or not it’s going to re-collapse and rebound.”
Waves in Spacetime
Now, using gravitational wave signals from the merger of two black holes and redshift data from one of the most ambitious sky surveys ever conducted, researchers have developed an entirely new way to calculate the Hubble constant. They described the method in a study they submitted to The Astrophysical Journal Letters and posted on the preprint site arXiv on January 6. In it they report a value of 75.2 for the constant, albeit with a large margin of error (+39.5, –32.4, meaning the actual number could range up to 114.7 or go as low as 42.8). This large uncertainty reflects the fact the calculation comes from a single measurement, and thus does not yet help clear up the tension between the original two calculation methods. But as a proof of concept, the technique is groundbreaking. Only one other measurement, from October 2017, has attempted to calculate the Hubble constant using gravitational waves. Scientists hope future gravitational wave detections will help them improve the precision of their calculation.
Gravitational waves are ripples in the fabric of spacetime. Einstein’s general theory of relativity predicted their existence in 1915, and astronomers had been looking for ways to detect them since. Not surprisingly, collisions of massive objects create a significant splash of gravitational waves. In 1986 physicist Bernard Schutz first proposed these so-called binary systems could be used to determine the Hubble constant. He argued observatories would very likely detect them in the near future; in fact it took nearly 30 years before observatories saw the signals.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington State made the world’s first gravitational wave detection in September 2015, and has seen fewer than a dozen more events since then, along with its European counterpart, Virgo. The experiments look for minuscule alterations in spacetime caused by passing gravitational waves.
A burst of gravitational waves from the merger of two black holes is one piece of the new method for calculating the Hubble constant. Not unlike standard candles, binary black hole systems oscillate. As they spiral into each other, the frequency of the gravitational waves they spew out changes at a rate correlated to the system’s size. From this, astronomers derive the waves’ intrinsic amplitude. And by comparing that with their apparent amplitude (similar to a comparison of the actual brightness of a Cepheid with its apparent brightness), they compute how far away the system is. Astronomers call these “standard sirens.” The measured the distance to this particular collision as some 540 megaparsecs, or about 1.8 billion light-years, from Earth.
An associated redshift, such as that of the sirens’ host galaxy, provides the second piece of the new method. The researchers used redshift data from the Dark Energy Survey, which just finished mapping a portion of the southern sky more broadly and deeply than any previous survey. The redshift data combined with the distance measurement provided researchers with their new figure for the constant.
Antonella Palmese, a research associate at Fermilab and co-author of the study, says the method holds promise in part because black hole mergers are relatively plentiful. Although it is still a proof of concept, she says that as more gravitational events from LIGO/VIRGO become available, the statistics will improve. University of Oxford astronomer Elisa Chisari, who was not involved in the study, agrees. “The level of constraints that they obtained on the Hubble rate is not competitive at the moment compared to other measurements,” she says. “But as LIGO builds up its catalogue of gravitational wave events in the coming years, then by combining multiple events, this will really become a competitive method.”