An experiment that reaches across the galaxy has 'heard' an invisible sea of ripples in space-time for the first time. It's surprisingly loud.

• Scientists have evidence for a sea of ripples in space-time, called the gravitational wave background.
• Their experiment used dead, spinning stars to hack our galaxy and turn it into a wave detector.

We are all bobbing in a sea of ripples in space-time, called gravitational waves, reverberating through the fabric of the universe, scientists announced on Wednesday.

Those ripples are probably the distant thunder of countless collisions between supermassive black holes, throughout space and time. The thunder is so loud that the researchers believe there are hundreds of thousands, if not millions, of these collisions that have taken place since the dawn of our universe.

This is yet more evidence for Albert Einstein's theory of general relativity. He predicted that the intense gravity of extremely massive objects, like black holes, warps the fabric of space-time. If those objects are moving or colliding with each other, the theory goes, they should send out waves of space-time-warping energy — gravitational waves.

In this newly-discovered cosmic ocean, called the "gravitational wave background," each wave is extremely long, lasting years or decades, and has an extremely low frequency, on the order of one-billionth of a hertz. It took 15 years, and an experiment that stretches across our galaxy, to capture signs of them.

"These observations reveal a rolling, noisy universe alive with the cosmic symphony of gravitational waves," Sean Jones, assistant director for the Directorate of Mathematical and Physical Sciences at the National Science Foundation, said in a briefing on Thursday. The NSF funded the 15-year experiment, which is called the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).

The discovery could help scientists rewrite the history of the universe and what happened after the Big Bang.

Supermassive black holes and their ripples can reveal how the universe was born

Black holes are places where so much matter is condensed into such a small space that not even light can escape its gravity. They form when massive stars, up to 100 times the mass of the sun, die and collapse.

Some, however, are inexplicably giant. Supermassive black holes are millions or billions of times the mass of the sun.

"How do they get to be that big? This has a lot of bearing into understanding how structures in the universe, how galaxies evolved — all the history after the Big Bang, from the beginning to now," Manuela Campanelli, an astrophysicist at the Rochester Institute of Technology who specializes in black holes and gravitational waves, but is not involved in this experiment, told Insider.

No other objects in the universe have such a vast range of sizes like black holes — or such a potential to reveal how the universe first took shape after the Big Bang.

NASA's James Webb Space Telescope recently revealed just how little we know about what happened after the Big Bang, when it discovered galaxies 500-700 million years old, in the universe's infancy, that were far more massive than scientists thought possible.

Supermassive black holes are thought to exist at the center of every galaxy. As galaxies have merged over time to grow larger, their supermassive black holes must merge and grow larger too, scientists theorized.

Therefore, by studying the reverberations of these supermassive collisions in the gravitational wave background, including waves emanating from that earliest era, scientists can peek into the black box of those first few hundred million years.

"The holy grail of this field, in the long run, is to explore the birth of the universe and extract that information that's carried by the gravitational waves," physicist Kip Thorne said in the briefing.

Thorne won the Nobel Prize in 2017 for his contributions to the first detection of gravitational waves here on Earth. That project built the Laser Interferometer Gravitational-Wave Observatory (LIGO), which first detected gravitational waves in 2015 from collisions of smaller black holes, only a few times the mass of our sun. Those collisions were (relatively) nearby.

But LIGO wasn't sensitive enough to pick up the low-frequency waves of faraway supermassive black hole mergers — the gravitational wave background. That's where NANOGrav came in.

How scientists used spinning, dead stars to turn our galaxy into a gravitational-wave detector

To design a detector sensitive enough for the gravitational wave background, the NANOGrav researchers imitated LIGO on a galactic scale.

The LIGO instrument uses two L-shaped detectors in Louisiana and Washington, which work by shooting out a laser beam and splitting it in two.

Those beams travel down the detector's two 2.5-mile-long arms. The beams bounce off mirrors at the ends of the arms and meet back near the beam splitter, where they should arrive at the same time and cancel each other out.

But when a gravitational wave comes through, it warps space-time, making one tube slightly longer and the other shorter for a brief moment. As a result, the two beams return to the splitter at different times, making flashes of light.

Instead of mirrors, NANOGrav uses millisecond pulsars: extremely dense dead stars that spin rapidly, sending a burst of light toward Earth every millisecond. Their precise timekeeping rivals an atomic clock, so these celestial objects are ideal for detecting changes in the fabric of space-time.

Just as LIGO researchers detect gravitational waves by measuring changes in the arrival times of light beams, NANOGrav researchers do so by measuring variations in the arrival times of light from pulsars throughout the galaxy.

As gravitational waves move through the cosmos, they squeeze or stretch the distance that this light travels to reach Earth, changing its arrival time by just a few hundred nanoseconds. Some pulsars' lights arrive a little sooner than would be expected, while others arrive a little later.

"This is galaxy-scale hacking," Stephen Taylor, chair of the NANOGrav collaboration, said in the briefing.

The researchers published their findings, from 15 years of data using 68 pulsars, in a series of papers in The Astrophysical Journal Letters on Wednesday.

Other telescopes in Europe, India, Australia, and China reported similar results from their pulsar observations.

One day astronomers could watch supermassive black holes merge together

As NANOGrav continues to listen to the gravitational wave background, the scientists may be able to single out individual notes in the symphony of waves and trace them back to a particular pair of supermassive black holes.

Soon scientists may be able to pinpoint and observe a supermassive collision in visible light, X-rays, and other forms of electromagnetic radiation.

That's what Campanelli is preparing for. Her lab runs computer models of merging supermassive black holes to predict how they behave and what signals they send out into space. One of her PhD students' simulations is pictured below.

Other astronomers can use those simulations to spot the real thing out there in the universe.

Then, by observing how these mergers happen, scientists can begin to piece together the stories of the most incomprehensible objects in our universe and how galaxies formed after the Big Bang.