Researchers just announced the discovery of the gravitational wave background.
After a 15-year-long study, the scientists have finally “heard” what they were looking for: the gravitational waves that should be filling the universe.
This will open up a whole new way to explore our universe, allowing us to probe everything from galaxy formation to supermassive black holes in a whole new way—and, just maybe, fill in some of the gaps in the Standard Model.
If you’ve been extremely tuned into physics and astronomy Twitter over the last few days, you’ve probably seen just about every relevant researcher and science communicator freaking out about a world-changing announcement coming just around the corner.
Well, here it is. And it really is the big one.
Scientists from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have officially made the first detections of the gravitational wave background.
They made the first detections of what? Fair question, especially if you haven’t, in fact, been extremely tuned in to physics and astronomy Twitter. So let’s back up.
We’ve known about gravitational waves for a while. First discovered in 2015 by the Laser Interferometer Gravitational-Wave Observatory, or LIGO, gravitational waves are physical ripples in spacetime caused by objects with mass moving through space, and they’re one of our best potential windows into mysteries like dark matter and supermassive black holes that we can’t see through more “traditional” means of space observation.
Everything creates gravitational waves—even humans—but we can only detect the really, really strong ones. And the stronger the source of gravity, the stronger the wave. The first gravitational waves ever detected were generated by the merger of two supermassive black holes.
Even though we’ve managed to spot these things, they’re still incredibly hard to detect—there were almost exactly 100 years between Einstein’s initial proposal of gravitational waves and their discovery—and that’s largely due to their wavelength.
So far, we’ve only been able to detect gravitational waves when they were at their shortest wavelength, right before and during the merging of the objects creating those waves. As two massive objects orbit closer and closer in the lead up to a merge, they start orbiting faster and faster. And as they orbit faster, the gravitational waves they emit also increase in frequency (vibrate faster) and shorten in wavelength.
But we want to be able to detect the rest of the wavelengths, too. A single wavelength of a gravitational wave can stretch lightyears in length, and we just haven’t been able to pick those up yet. We haven’t found most of the gravitational waves, and those big, long ones that would come from every gravitational wave source not quite ready to merge should be just all over the place.
“These are by far the most powerful gravitational waves known to exist,” codirector of the NANOgrav Physics Frontiers Center Maura McLaughlin said in a press release. “Detecting such gargantuan gravitational waves requires a similarly massive detector, and patience.”
Given that no human-made detector could ever possibly be that massive, scientists turned to the stars. Specifically, they turned to pulsars, a type of neutron star that spins incredibly fast. That spinning basically makes the stars act like lighthouses, and creates flashes of light that make it to Earth in incredibly consistent intervals. Until just a few years ago, you could keep time with a pulsar better than you could with an atomic clock. (That only changed because we got better at making atomic clocks.)
And researchers were able to use that consistency to their advantage. The NANOGrav team took 67 metronome-like pulsars and basically turned them into a galaxy-wide telescope. The team kept careful note of how frequently they sent its lighthouse beams our way. Then, the scientists kept watching. For 15 years.
The researchers were looking for those pulses to change. Remember the description of gravitational waves as “physical ripples in spacetime?” That’s literal. When gravitational waves pass through a region, they physically stretch and compress space. This means that if a gravitational wave passed between us and a pulsar, the actual amount of space between the two objects will change. The pulsar will be closer to or farther away from us as the gravitational wave passes through without either the star or Earth having moved at all.
And here’s the key: light travels through space at a finite speed. The flashes from a pulsar at a certain distance from Earth will always hit Earth at the same intervals, because it will always take the light the same amount of time to travel from there to here. But if there’s suddenly, say, more space between the pulsar and Earth, it will take the light longer to travel from there to here, and the clock-like timing will be thrown off.
Watching their pulsar clock for 15 years gave the scientists the chance to let those incredibly long gravitational waves pass through and leave their mark on the pulsars’ timing. And they found what they were looking for. Through the changes in the timing of their pulsars, the researchers were finally able to “hear” the gravitational wave background they had been looking for.
“It’s like a choir, with all these supermassive black hole pairs chiming in at different frequencies,” Chiara Mingarelli, a NANOGrav scientist who worked on the new findings, said in a press release. “This is the first-ever evidence for the gravitational wave background. We’ve opened a new window of observation on the universe.”
And it turns out, the universe is loud. Not only is the background there, it’s stronger than anyone anticipated. “The gravitational wave background is about twice as loud as what I expected,” Mingarelli said. “It’s really at the upper end of what our models can create from just supermassive black holes.”
There are a few potential reasons for that excessive “volume.” For one, there could just be many more merging black holes than we thought there were. For another, String Theory suggests that there might be deformations in spacetime called “cosmic strings” producing the waves. If you believe in the Big Bounce instead of the Big Bang, the waves could be coming from that as well. And there’s always the chance that pulsars just aren’t quite as perfect as we thought.
There are plans to investigate all of this, and more, now that this data is out in the world. But as it stands, it seems like this is the real deal. And it’s not just NANOGrav. There are papers just now being released from multiple collaborations spanning multiple countries that all point to same thing: Yes, this is real, and yes, this is out there.
The detection of the gravitational wave background is going to open up an entirely new avenue of study for scientists to probe the biggest mysteries in our universe, from neutron stars to galaxy formation. It may even—if we’re as lucky as researchers think we will be—allow us to probe entirely new physics, and fill in some of the gaps in the Standard Model.
In a recent podcast, Mingarelli explained that we do all of our universe-exploring through the use of packets of light called photons. Every time we made use of a new wavelength of light (infrared, radio, etc.), we’ve opened up a whole avenue of exploration. But the first time we realized that different wavelengths carried different information, it opened up not just a new avenue, but a whole new world. Mingarelli described the importance of this discovery as akin to that first multi-wavelength discovery. We’re going to be able to explore the universe like literally never before.
“What’s next is everything,” she said in a press release. “This is just the beginning.”
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