Ripples in Space

Scattered throughout the universe are mysterious dark objects millions or billions of times the mass of our sun. These are the biggest black holes in existence. They sit at the center of galaxies, giant collections of stars—including our home galaxy, the Milky Way. Sometimes galaxies collide. When that happens, their black holes spiral around one another and eventually merge. Scientists think that as these gigantic black holes spin closer and closer to each other, they send out enormous ripples that stretch and compress space.

Last summer, astronomers announced they’d found the predicted gravitational waves rippling through the universe! Researchers are still investigating the giant waves’ origins. Circling supermassive black holes are the leading hypothesis. But it may turn out that the waves come from a source astronomers have never seen before. It’s even possible they were produced 13.8 billion years ago during the big bang—the explosive beginning of our universe.

Scientists around the world are combing through the new data to learn more about these gigantic waves and where they come from. The discovery offers scientists a whole new way to study and understand the universe.

Mysterious dark objects are scattered around the universe. They’re millions or billions of times the mass of our sun. These are the biggest black holes. They sit at the center of galaxies, including our home galaxy, the Milky Way. Galaxies are giant collections of stars. Sometimes galaxies collide. When that happens, their black holes spiral around one another. Eventually, the black holes merge. Scientists think these huge black holes send out enormous ripples as they spin closer and closer to each other. These ripples stretch and compress space.

Last summer, astronomers made an announcement. They’d found the predicted gravitational waves rippling through the universe! Researchers are still studying where the giant waves come from. Those spiraling supermassive black holes are the leading idea. But the waves may come from a source astronomers have never seen. They might even have started 13.8 billion years ago during the big bang. That was the explosive beginning of our universe.

Scientists around the world are studying the new data. They’re trying to learn more about these enormous waves and their source. The discovery gives scientists a whole new way to study and understand the universe.

Astronomers first confirmed the existence of gravitational waves in 2015. They detected waves released by a collision between two smaller black holes (see A Wrinkle in Spacetime, Science World, March 2018). The waves had a wavelength, or distance between two neighboring peaks, of about 3,200 kilometers (2,000 miles). To find these waves, scientists built a sophisticated detector more than 4 km (2.5 mi) long.

The newfound gravitational waves are much bigger than those detected almost a decade ago. The scale of these waves is “mind-bendingly large,” says Mingarelli. A single wavelength can be multiple light-years long. A light-year is the distance light travels in a year, about 9.5 trillion km (5.9 trillion mi). That means one wavelength can be thousands of times the length of our solar system! It also means these waves have a much lower frequency, or number of cycles in a given amount of time (see Anatomy of a Wave). And their period—the time it takes for one full wavelength to pass by—can be decades.

To find these low-frequency gravitational waves, scientists needed a massive detector. Building a large enough one on Earth would be impossible. So NANOGrav astronomers came up with a clever way to search for the waves using 68 pulsars scattered across the Milky Way. “Pulsars are rotating stars that give off beams of radio waves [a type of invisible light energy], which sweep around like the beam from a lighthouse,” explains Vigeland. “They rotate hundreds of times per second, and they’re very precise, like the ticking of a clock,” she says.

As gravitational waves cross the galaxy, they distort spacetime. NANOGrav scientists realized that this would change the timing of when flashes of radio waves from pulsars reach Earth. The change in timing should follow a specific pattern. In 2007, NANOGrav started collecting pulsar observations. During the analysis of more than 15 years’ worth of data, the tell-tale patterns of massive gravitational waves emerged.

Astronomers first detected gravitational waves in 2015. This confirmed their existence. A collision between two smaller black holes released these waves (see A Wrinkle in Spacetime, Science World, March 2018). Their wavelength was about 3,200 kilometers (2,000 miles). Wavelength is the distance between two neighboring peaks of a wave. To find these waves, scientists built a complex detector. It was more than 4 km (2.5 mi) long. 

Those waves were detected almost a decade ago. The newfound gravitational waves are much bigger. Their size is “mind-bendingly large,” says Mingarelli. A single wavelength can be multiple light-years long. A light-year is the distance light travels in a year. That’s about 9.5 trillion km (5.9 trillion mi). So one wavelength can be thousands of times the length of our solar system! And these waves have a much lower frequency. That’s the number of cycles in a given amount of time (see Anatomy of a Wave). The time for one full wavelength to pass by is its period. The newly observed waves can have a period of decades.

These low-frequency gravitational waves weren’t easy to find. Scientists needed a massive detector. They couldn’t build one large enough on Earth. So NANOGrav astronomers found a clever way to search for the waves. They used 68 pulsars scattered across the Milky Way. These stars release a type of invisible light energy. “Pulsars are rotating stars that give off beams of radio waves, which sweep around like the beam from a lighthouse,” explains Vigeland. “They rotate hundreds of times per second, and they’re very precise, like the ticking of a clock,” she says.

Flashes of radio waves from pulsars reach Earth. NANOGrav scientists realized that gravitational waves would change the timing of when the flashes arrive. That’s because gravitational waves warp spacetime as they move across the galaxy. The change in timing should follow a specific pattern. In 2007, NANOGrav started collecting pulsar observations. They’ve studied more than 15 years’ worth of data. And they found the tell-tale patterns of massive gravitational waves.

The large gravitational waves picked up by NANOGrav are like a deep background hum, constantly rippling throughout the universe. Another way of picturing them is to imagine space as a vast ocean, says NANOGrav astrophysicist Dustin Madison of the University of the Pacific in California. “It’s as if our solar system is bobbing up and down in the middle of this sea,” he says. “Waves are coming at us from all directions. We detected them by observing how they affect distant buoys floating all around us—the pulsars. But we don’t know what created each ripple.”

The large gravitational waves that NANOGrav found are like a deep background hum. They constantly ripple through the universe. Dustin Madison is a NANOGrav astrophysicist at the University of the Pacific in California. He suggests another way to picture these gravitational waves: Imagine space as a vast ocean. “It’s as if our solar system is bobbing up and down in the middle of this sea,” he says. “Waves are coming at us from all directions. We detected them by observing how they affect distant buoys floating all around us—the pulsars. But we don’t know what created each ripple.”

That’s what scientists want to find out next. They already have a few ideas about what’s generating background gravitational waves. “The most promising sources are supermassive black holes,” says Vigeland. Spiraling pairs of supermassive black holes churning up spacetime across the universe could generate the hum.

To find out more, NANOGrav will continue to gather data. And teams around the world are sharing observations to speed things along. “The waves we’re detecting now are like the noise of many conversations happening all around us at a crowded party,” says Vigeland. “We hope that in time we’ll be able to pick out individual voices—like a specific black hole pair,” she says.

That could help scientists understand how common spiraling black holes are, how often galaxies merge, and the nature of gravity, says Mingarelli. “But we could also find something completely unexpected,” she adds. “We’ve only been observing gravitational waves for a short time. The more we look, the more likely we’ll see things we didn’t even know existed.”

Scientists want to answer that question next. What is creating background gravitational waves? They already have a few ideas. “The most promising sources are supermassive black holes,” says Vigeland. Pairs of giant black holes spiral around each other and warp spacetime across the universe. That could generate the hum.

To learn more, NANOGrav continues to gather data. And teams around the world share observations to speed things along. “The waves we’re detecting now are like the noise of many conversations happening all around us at a crowded party,” says Vigeland. “We hope that in time we’ll be able to pick out individual voices—like a specific black hole pair,” she says.

That could help scientists answer more questions, says Mingarelli. For example, how common are spiraling black holes? How often do galaxies merge? And what is the nature of gravity? “But we could also find something completely unexpected,” she adds. “We’ve only been observing gravitational waves for a short time. The more we look, the more likely we’ll see things we didn’t even know existed.”