Gravitational waves are distortions in spacetime caused by super violent cosmic events. Something so cataclysmic may seem like it would be hard to miss, but in reality, these events happen so far away that detecting gravitational waves is quite a challenge. Three massive gravitational wave detection facilities were built in the mid-1990s, but for decades, scientists searched for faint signals with no avail. After the first gravitational wave events were recorded in 2015, it took four years for researchers to spot only 11 more faint signals.
Now, thanks to ever more sensitive equipment, gravitational wave observations have gone from an astronomical rarity to a near-weekly occurrence. In 2019, scientists measured 39 new wave events in just six months, Emily Conover reports for Science News.
Each wave event was caused by a massive cosmological collision: 37 of the waves rippled out from crashing pairs of black holes; one wave likely came from two colliding neutron stars; and one may have came from the collision of a black hole and a neutron star, Adrian Cho reports for Science magazine. Researchers at Laser Interferometer Gravitational-Wave Observatory (LIGO) sites in Washington and Louisiana as well as Italy's observatory, Virgo, in Pisa documented the new events in four papers published on the preprint server arXiv.
The catalog is a resource for scientists to dig into the trends across several wave events, which can be more illuminating than analyzing a solo black hole collision. By studying the whole collection of all 50 gravitational wave events measured so far, researchers could learn more about why two black holes pair up and collide.
“There will definitely be a flurry of papers that are rushing to take the first stabs at the data,” says Harvard University astrophysicist Selma de Mink to Science magazine, adding that she and her colleagues plan to do their own analyses of the data.
LIGO made its first measurement of a gravitational wave in 2015 as it was running operational tests of the wave detection equipment, Liz Kruesi reported for Smithsonian magazine in 2016, when the discovery was announced. To measure the waves, LIGO shoots laser beams down two tunnels, each about 2.5 miles long with no air inside and a hyper-smooth mirror at the end. Virgo, which came online in 2017, uses a similar pair of almost two-mile-long arms.
If a gravitational wave is present, it will stretch or compress space by just an infinitesimal amount, disrupting the laser's path to the mirror. These interruptions in spacetime are recorded as high-pitched chirps. When scientists dig into the details of each chirp, they can uncover details about the collision that sent the gravitational wave into the universe.
"It’s all about the sounds,” says gravitational wave astronomer Frank Ohme, of the Max Planck Institute for Gravitational Physics, to Inverse’s Passant Rabie. “How do I know that I'm speaking to my wife rather than a stranger on the phone? I can do this because I've learned the frequencies of people’s voices.”
As LIGO and Virgo’s detection equipment has been improved, scientists have been able to pick out more, fainter gravitational wave chirps that originate from more distant collision events. That led to a number of exciting discoveries, some of which made headlines in their own right, like the black hole that collided with a mystery object—possibly a collapsed star, or neutron star, as Alex Fox reported for Smithsonian in June.
Gathering all of the gravitational wave events together shows scientists what is common and uncommon in the universe, and helps physicists being to see patterns or test theoretical rules. For example, physicists expected to find a “mass gap,” where no black holes would weigh between 45 and 135 times the mass of our sun.
But Virgo and LIGO have now observed black holes within that gap, including one with a mass about 85 times the mass of our sun, per Science magazine. Scientists also expected that the minimum size of a black hole is about five times the size of our sun, but one black hole in the catalog is about three solar masses, Meghan Bartels reports for Space.com.
“How do you describe the boundaries of this population?” says Ohme to Science magazine. “It’s not such a clear picture anymore.”
Scientists have already used the collection of wave data to confirm Einstein’s theory of general relativity, per Science News. And researchers hope to use the data to study whether two black holes usually collide because they come from a pair of already-connected stars, or because something else brings them together after forming independently, reports Science magazine.
There’s also another six months of data yet to be analyzed, gathered between November 2019 and March 2020, when scientific staff returned home for safety amid the Covid-19 pandemic, reports Space.com.
The new studies are “super important,” Carnegie Mellon University astrophysicist Carl Rodriguez, who wasn’t involved in the research, tells Science Magazine. “With an individual event, there’s only so much you can do in comparing to astrophysics models. But with a catalog you can not only begin to constrain the theory, you can start to understand the landscape.”