This SciShow video is supported by Linode!

You can get a $100 60-day credit on a new Linode account at linode.com/scishow. Thanks to state-of-the-art  telescopes like the James Webb, we can look so deep into time  and space that we can see some of the very first galaxies,  and hunt for the very first stars.

And other telescopes can take  us even farther back in time, to a point 380,000 years  after the universe was born, when light began to shine through space. For a universe that’s roughly  13.8 billion years old, that’s a pretty solid baby picture. But for scientists, it’s not good enough.

They want to study just slivers of a  second after the universe was born, when everything we know was packed  into an area the size of a marble, space was opaque, and subatomic  particles like protons and neutrons were a thing of the future. The problem is, because the  universe was opaque back then, we will never see this moment in time. But we might be able to observe it anyway, using a completely different kind of signal.

Gravitational waves. Astronomers think that this moment in time created wrinkles in the fabric of space. And if it did, those wrinkles are still out there.

If we manage to find them, it would  clarify how the universe came to be, and exactly what happened  in the fractions of a second that defined the entire course of the universe. [♪ INTRO] For nearly a century, gravitational  waves were purely hypothetical. Einstein published his theory  of general relativity in 1915, and a completely mind-bending  idea soon popped out of the math. The math showed that any object with mass that was accelerating through  spacetime would warp spacetime.

And that warping would ripple out into the greater universe at the speed of light. As spacetime squeezed and stretched, the distances between objects would change. Of course, we don’t see the world around us warp every time a truck passes  by or a plane takes off.

Gravitational waves are weak. But according to general relativity,  the more massive an object, and the faster it is moving, the  stronger its gravitational waves will be. So scientists eventually started  to think that gravitational waves from the most extreme events in the  universe might just be detectable, if you could build a machine capable of measuring changes smaller than a proton.

Enter, LIGO. the Laser Interferometer  Gravitational Wave Observatory. Actually, it is a pair of  observatories that work together. One’s in Washington state  and the other’s in Louisiana.

Each has two arms that are four kilometers long and meet to form an L-shape. And they’re shaped that way because when a gravitational wave passes through space, that space will stretch in one direction and squeeze in the perpendicular direction. So if a gravitational wave were  to ripple through the observatory, one arm would get longer  while the other got shorter.

In 2015, almost exactly 100 years after Einstein’s theory of general relativity dropped, LIGO picked up a brief little chirp,  made by the gravitational waves of two merging black holes, each  roughly 30 times the mass of our Sun. The discovery was not only a major  confirmation of general relativity. It opened up a whole new way  of exploring the universe, using gravity as a probe instead of light.

And once it was clear that  any kind of gravitational wave was definitely real and definitely detectable, the idea of finding primordial  gravitational waves, the ones created just after the  dawn of reality as we know it didn’t sound that far-fetched. But if any of these primordial  waves were out there, we couldn’t rely on LIGO to pick them up. Just like human ears can only  pick up sound waves within a certain frequency range, LIGO  can’t detect all gravitational waves.

It’s limited to frequencies between roughly ten and 10,000 peaks per second. And the objects that churn out  those frequencies are objects like your garden-variety  black hole or neutron star, spiraling ever closer together until they smush. Primordial gravitational waves, on the other hand, are an entirely different beast.

These would have been made during, or right after, a hypothetical period known  as inflation that happened just after the birth of the universe. During inflation, the observable  universe swelled exponentially, going from a subatomic speck  to about the size of a marble. It was all over in a fraction of  a second, like this many seconds, but during this flash of expansion,  our universe was dominated by constant, subatomically-small  fluctuations in energy that warped spacetime just like  an object with mass does, today.

And it’s impossible for our  Earthling brains to truly envision this baby universe, so  for now, let’s just imagine a lake. Plus a bunch of raindrops  falling down onto that lake, creating ripples in the smooth surface. According to our current  knowledge of how physics works, as spacetime writhed in this early universe, gravitational waves rippled outward.

And as the universe doubled  in size many times over during this split second, those hypothetical  ripples turned into longer waves. So, in our metaphor we’d also  need our lake to be growing, but that’s okay. It’s all inside of our minds, we can totally make that happen.  Imagine the lake growing!

Currently, cosmologists don’t know  exactly how inflation played out. They’re not even 100% sure it happened. And that’s not too surprising  since we’ve never been able to observe that far back in time.

There could be multiple sources of these inflation-era gravitational waves. Some may have even been  generated by inflation ending. But no matter how this split  second of cosmic history went down, it was likely a time when space  was getting stretched and contorted in extreme ways and this was bound  to produce gravitational waves.

Then, as our tiny baby universe swelled to its current size, those waves stretched out. What started as microscopic ripples  would be light-years long, now. And that’s why finding these  ancient waves is not a job for LIGO.

Each one is billions of times  longer, the frequencies are billions of times lower than  the waves LIGO can detect. In fact, there is no way to build  a LIGO-type detector on Earth that is long enough to pick up  primordial gravitational waves. The arms would need to  stretch beyond Earth itself.

Fortunately, astronomers  have a way to do just that. It’s called a pulsar timing array. A pulsar is a spinning neutron star that shoots electromagnetic radiation from its poles.

And from Earth’s vantage point, that spinning makes the radiation appear to flash. So basically, pulsars are cosmic lighthouses. And some of these lighthouses  are flashing so steadily– the pulsars are spinning at such a precise speed– that their signals run like clockwork.

Or, they do until something disturbs  the space between them and us. And sometimes, that something  is a gravitational wave. Just like when a gravitational  wave rolls through the Earth and ever-so-slightly stretches  and compresses the arms of LIGO, it can do the same thing to interstellar space.

It changes the distance that light  travels from a pulsar to the Earth. And that tweaks the arrival time. A flash either arrives too soon, or too late.

Now, gravitational waves aren’t  the only thing that can mess with a pulsar’s timing, so astronomers  have to account for a lot of other possibilities before  declaring that a gravitational wave is behind any wonky signal. But in principle, an array  of extremely regular pulsars can work a lot like LIGO,  just with a bunch more arms, and blown up to an interstellar scale. And this is how astronomers  finally discovered evidence of low-frequency gravitational waves in 2023.

As part of a collaboration called  NANOGrav, researchers around the world compiled 15 years  of data from 67 pulsars. And what they found wasn’t one  isolated wave rolling through, like what LIGO spots. Instead, the signal was  sort of a background noise, a pattern of irregularities coming  from all directions that suggests all of spacetime is subtly and slowly quivering.

But unfortunately, the NANOGrav team doesn’t know what’s creating this  gravitational wave background. It’s like if you had a bunch  of rowboats and swimmers and gusts of wind creating waves in a lake and you wanted to figure out  which wave came from which source, but you could only look at the water,  and not the things on the water. And when a bunch of waves  interfere with each other, it’s not easy to tease them apart.

The leading hypothesis is that these low-frequency waves are coming from supermassive black holes at the  centers of merging galaxies. Unlike the smaller black holes LIGO detects, these behemoths spend millions  of years slowly spiraling around each other before they finally collide. Based on computer simulations,  spiraling supermassive black holes could produce the signal that  the NANOGrav team observed.

And if a supermassive black hole  merger is what caused these waves, it would be the first direct evidence  of such an event ever happening. But for now, there are still a few  other possibilities on the table. One of them is that these gravitational  waves, or at least some of them, are primordial—created in the first  second of the universe’s existence.

Scientists on the NANOGrav team also simulated what a low-frequency gravitational wave background would look like if it was created by inflation. And those simulations reproduced  what the researchers observed. As their pulsar timing array  provides more data, the NANOGrav team hopes they’ll be able to pin down  the true origin of this signal.

Something that’ll really help is  figuring out how to tease apart different sources of gravitational waves based  on the frequencies they generate. Going back to the lake analogy,  you can imagine that waves from a passing boat might look  different than waves stirred up by the wind or by a person doing a belly flop. So if scientists can figure out the  typical profile of a gravitational wave produced by a supermassive black hole merger, they'll be able to filter out those signals and just look for primordial waves.

If they ever find them, we’d have some of the strongest evidence yet that  inflation really did happen. And hopefully, the details  of those primordial waves would teach us what exactly went down back then. By studying a signal that’s basically  as old as the known universe, we’d finally be able to confirm  whether or not some fundamental laws of physics work the way theoretical  physicists think they do.

Which would be a lot better  than looking at a picture from when the universe was  380,000 thousand years old and trying to extrapolate backward. But first, we need more data.  Astronomers need to find more pulsars that are reliable enough  to add to NANOGrav’s array, and watch the array for even longer to see if meaningful patterns emerge. Humanity will have to wait a  while before anyone announces they’ve definitely detected a  primordial gravitational wave.

But we might not have to wait  as long as you might think, because NANOGrav isn’t the only  collaboration doing this work! Teams in China and Australia are also monitoring their own pulsar timing arrays. And all these teams are  working to combine their data to try and get an even clearer picture.

And even though it’s using gravity  instead of light to take that picture, making it a little hard to print out, I’m ready to add an even older baby picture to the universe’s photo album. Thank you to Linode for  supporting this SciShow video and so many other SciShow videos! Linode is a cloud computing company from Akamai that keeps some of the best stuff on the internet running with data centers across the world.

Like, if you’re part of a giant business, Linode can handle your storage needs. With a petabyte of object storage,  even the big dogs are covered. And Linode is constantly  improving.

Just this year, they’ve reduced latency, increased storage, and added global servers to  their already fast, vast, and far-reaching service. But just because Linode works  great for large businesses doesn’t mean your company has to be  a giant international corporation to get the most out of Linode. You can scale your services and corresponding payments to only what you need.

You can get going by clicking the  link in the description down below or heading to linode.com/scishow  for a $100 60-day credit on a new Linode account. Thanks for watching and thanks for learning with us here at SciShow! [♪ OUTRO]