About 1.4 billion years ago, two orbiting black holes met in a colossal collision, forming a single entity—a new black hole—and sending the energy equivalent of the mass of our sun rippling out across space. These gravitational waves reached the United States on Christmas night, 2015 in a form so faint that only the most sensitive instruments could detect them.
It's an epic event, and one that probably happens all the time. The Laser Interferometer Gravitational-Wave Observatory (LIGO) recorded gravitational waves for the first time in September 2015 and made the discovery public in February 2016, confirming a 100-year-old theory about the mechanics of the universe. December's detection, announced Wednesday, offers a second proof and a taste of many more to come.
"The finding in February was a big announcement and discovery on many levels, but that was only one," says executive director David Reitze. Now that they've had two discoveries, he says, they're operating like "a real astronomical observatory."
LIGO has two detectors, one in Livingston, Louisiana, and the other in Hanford, Washington. Each interferometer has two arms, 2.5 miles long, that meet at a right angle and contain lasers that serve as rulers. As gravitational waves pass through the arms, one will stretch while the other shrinks. Scientists report this relative change in terms of "strain"—the degree to which the fabric of the universe moves under the force of the waves—and it is tiny. "We're talking about trying to measure changes that are on the scale of the ratio between a person's height and the size of the entire galaxy," says Chad Hanna, an assistant professor of physics at Penn State University and co-chair of LIGO's Compact Binary Coalescence Group, which focuses on pairs of orbiting stellar objects such as black holes and neutron stars.
The most recent gravitational waves gave astrophysicists details about the two black holes that were destroyed and the new one they formed. They had smaller masses than those in February's announcement, and one of the pair as well as the final black hole this time were spinning. Last time around, scientists couldn't tell for sure from the data whether the holes they had seen were rotating.
Details like these hint at the information the observatory will deliver in the coming months and years. "Every time we see something different, that's exciting," says Hanna. "Once we have enough events that we can catalog all the masses and spins and other differences we've observed, we can come up with a better idea of where these systems are coming from."
For example, it's possible that solo black holes form as stars collapse and then tend to draw together, slowly merging into larger ones as part of stellar evolution. Another theory suggests that primordial black holes may have existed since the Big Bang and could explain dark matter. With a census of black holes, astronomers will better equipped to sort out what's plausible and what isn't. And physicists expect the observatory will be able to detect other sources of gravitational waves such as binary neutron stars, supernovas and pulsars.
"Sorting some of these things can tell us a tremendous amount about the birth, life, and death of stars. There's a tremendous amount of physics that goes into each of those stages," says Hanna. "How big they are, what spin they have—as we start to measure these things and catalog them we can start to rule out theories about how stars live and how they die."
Already, LIGO has made a major contribution, says Reitze, and it's only observed six black holes in total. Yet that data has already increased the number of black holes with known masses by 30 percent. "That's pretty remarkable for being on the air for only a few months," he says.
LIGO can also identify which part of the sky the waveforms are coming from. In the future, the researchers hope to use that data to tell astronomers around the world to point their instruments in the same direction in the hope of catching more information.
"We're trying to find the signal immediately so that we can alert other astronomers to look for the electromagnetic signal," and for other data such as gamma rays, X-rays and radio waves, Hanna says. "The idea is that for at least some of the gravitational wave candidates there would be some electromagnetic component." Having that additional information would be "like watching a film compared to listening to the radio." As the data piles up, he adds, it's also possible that physicists will see new patterns in observations they've already made.
Researchers are currently making adjustments to LIGO to make it more sensitive and will turn it on for another few months in the fall. "We'll start observing in September and should see many, many more of these binary black holes and hopefully we'll see other phenomena too," says Reitze. Exactly what will be a surprise.
"This is a completely brand-new way of observing the universe," says Hanna. "It's inevitable that we'll see things we never expected to observe and that's what's really exciting."
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