'Dark Sirens' In Space Can Solve a Huge Mystery In Spacetime, Scientists Propose

We live in an expanding universe, in which objects like galaxies are carried away from each other like boats in a widening ocean. Still, clocking the exact speed of its movement has become a thorny open question for scientists, because our two best measurements of this value—known as the Hubble constant—do not match.

Now, scientists have proposed that this frustrating discrepancy could be eased by a truly mind-boggling source: “Dark sirens,” which are tumultuous collisions between black holes that create ripples in spacetime called gravitational waves.    

Gravitational waves were first detected in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO), a breakthrough that opened up an entirely new window into our universe. LIGO and its Italian counterpart, Virgo, are due to undergo upgrades to their sensitivity that raise the “tantalizing possibility” of measuring the Hubble constant using dark sirens, according to a recent paper in The Astrophysical Journal Letters. 

The idea of using black hole mergers to measure the expansion of the universe is not new, said Ssohrab Borhanian, a graduate student at Pennsylvania State University who led the study, in a call. “What we really want to show is that now, with updates coming, there will be definitely a population [of dark sirens] that we can see” in unprecedented detail, he said.  

The expansion of the universe has been measured with two main methods: standard candles and the cosmic microwave background. The first makes use of bright events, like stars and certain supernovae, which are called “standard candles” because scientists know their luminosities. The second method measures the constant using temperature fluctuations in the cosmic microwave background, the oldest light in the universe. 

The standard candle method gives scientists a Hubble constant value of about 50,400 miles per hour per million light-years, while the cosmic microwave background method returns a value of 46,200 mph per million light-years. It may seem like a small difference, but it has big consequences for understanding one of the fundamental properties of our universe.

Borhanian and his colleagues are optimistic that dark sirens can provide another means to test this important question that would be analogous to the standard candle method. Scientists use known luminosities of standard candles to measure their precise distances from Earth across billions of light years, which can then be used to clock the expansion of space. 

Dark sirens get their name from a similar concept; instead of looking for light to measure the Hubble concept, sirens are a way to measure the expansion of the universe through the vibrations of spacetime. One of the big upsides to this approach is that gravitational waves naturally contain information about the distance to the events that created them, thereby easily filling in one of the big values needed to calculate the Hubble constant.

“Candles are a good measurement for distances for light,” said Borhanian, who is presenting his research this Sunday at a meeting of the American Physical Society. “Sirens do this with gravitational waves.

“The only difference is we replace the distance measurement from the candle with a siren measurement,” he continued. “Because gravitational waves just intrinsically and directly give a distance estimate, in the future they will be much better than what we can do, at least currently, with candles anyway.” 

LIGO and Virgo have captured dozens of dark sirens since 2015, which have provided a new way to study black holes without the use of light-based astronomy. Scientists have even captured a “bright siren” caused by the collision of two neutron stars that produced both gravitational waves and a radiant flash that enabled scientists to trace the event back to a specific galaxy.

As their name implies, dark sirens don’t produce these bright flashes, but Borhanian and his colleagues outline how the soon-to-be upgraded versions of LIGO and Virgo will be able to detect “golden dark sirens” that can be localized to a specific host galaxy, even without a transient flash. 

These golden sirens are likely to be extremely rare because they’d have to meet several parameters, such as high signal quality, a distance within about 1.5 billion light years of Earth, and a clear galactic origin beyond the Milky Way. But because gravitational waves provide such precise distance measurements, it might take only one detection of a golden dark siren to generate an estimate of the Hubble constant that is accurate enough to resolve the tension between the standard candle and CMB measurements. 

Borhanian and his colleagues expect golden dark sirens to be detected within the next five years, thanks both to the LIGO and Virgo upgrades and new detectors that are expected to come online in India and Japan. However, Borhanian emphasized that dark sirens are just one of many possible means of constraining the expansion of our universe, and that bright sirens will eventually help to shed light on this question as well.

“Let's gather as much information as possible from the universe” and “see what happens,” he concluded.