'Dark sirens' could solve one of the greatest mysteries in cosmology

In recent years, cosmologists have been faced with a crisis: The universe is expanding, but no one can agree on how fast it's moving away from us.

That's because different ways of measuring the Hubble constant, a fundamental parameter that describes this expansion, have produced conflicting results. 

But a single, lucky observation of what are known as dark sirens — black holes or neutron stars whose crashes can be picked up by gravitational wave detectors on Earth but remain invisible to ordinary telescopes — could help resolve this tension. 

As the cosmos expands, galaxies in the universe move away from Earth at a speed that depends on their distance from us. The relationship between speed and distance is called the Hubble constant, after American astronomer Edwin Hubble, who first calculated its value in the 1920s. 

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By looking at flickering stars known as Cepheids in the local universe, some researchers have produced modern, highly precise measurements of the Hubble constant. But a rival method relying on a relic of light from 380,000 years after the Big Bang, known as the cosmic microwave background (CMB), yields a completely different answer, leaving cosmologists scratching their heads about what's going on.

"Gravitational waves can give you a different handle on the Hubble constant," Ssohrab Borhanian, a physicist at Pennsylvania State University, told Live Science.

When massive objects such as black holes or neutron stars smash together, they warp the fabric of space-time, sending out gravitational waves. Since 2015, the U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European counterpart Virgo have been listening for such massive crashes, which ring in their detectors like little bells. 

Depending on their distance from Earth, these events will sound louder or quieter to LIGO, enabling scientists to calculate how far away they happened. In some cases, the clatter of these heavy entities also results in a flash of light that astronomers may catch in their telescopes, encoding information about how fast they are traveling away from us. 

So far, researchers have only observed one such event with both gravitational wave and light signals, a pair of neutron stars that astronomers observed in 2017 in both LIGO's detectors as well as other telescopes. From this, physicists have calculated a value for the Hubble constant, though the error bars on the measurement are large enough to overlap with both the results coming from flickering stars and those from the CMB,  Borhanian said. 

Prior work showed that cosmologists would need to see about 50 events like this, which are fairly rare, in order to get a more precise Hubble constant calculation, he added. 

Dark sirens offer a potentially quicker route. Such crashes are not associated with flashes of light, which contain the all-important information on speed. These events, which are invisible except through gravitational waves, are the most common signals picked up at LIGO and other gravitational wave facilities.

Over the next five years, LIGO's detectors are expected to receive upgrades that will enable them to unpack many more details of gravitational wave signals and pick up far more events, including more dark sirens. The U.S. and European facilities have recently been joined by the Kamioka Gravitational Wave Detector (KAGRA) in Japan, and an Indian detector should come online around 2024. 

One day, the network should be able to pinpoint where in the sky a dark siren crash happened 400 times better than scientists currently can, Borhanian said. With this information, astronomers can identify a galaxy in the exact location where the smash took place, and then determine how quickly that galaxy is speeding away from Earth. There will be no need to also find an associated flash of light.

Borhanian and his team have shown that crashes between objects that are particularly loud, heavy, or unequal in mass, which they call golden dark sirens, will be particularly information-rich, producing data that might pin down a gravitational wave crash so well they can calculate the Hubble constant with high precision. 

"We can do this with a single event, instead of 50," he said, and it will perhaps be enough to sway the cosmology community towards one measurement or the other. Borhanian will present his group's findings at the American Physical Society's April meeting on April 18. 

Because dark sirens can potentially provide such excellent distance measurements from pure physics alone, they are "extremely unique, and extremely clean and appealing," said Maya Fishbach, a gravitational wave astronomer and LIGO team member at Northwestern University in Evanston, Illinois, who was not associated with the work. 

The group's results suggest that LIGO and its counterparts around the world should start seeing many more well-localized events in the near future, she said. But it's possible that other measurements may resolve the crisis over the Hubble constant before dark sirens will, Fishbach told Live Science. 

Still, Fishbach is excited about the potential for the field of gravitational wave cosmology to answer other fundamental questions in the future, such as the nature and details of dark energy, the mysterious substance driving an accelerated expansion of the universe.

Editor's Note: This story was updated on Friday, April 9 at 1:40 p.m. E.T. to note that dark sirens could pinpoint the location of gravitational wave sources 400 times, not 40,000 times, better than scientists currently can.

Originally published on Live Science.