What are some sources of gravitational waves that aren’t black holes? How did inflation create gravitational waves? How can we possibly detect them? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPT (AUTO-GENERATED)

Trust me, you don't want to ever get near a gravitational wave. Well, I should add some caveats to that, as is the usual ask a spaceman way of going about things. You are technically near gravitational waves pretty much all the time. They are waves of gravity, hence the name gravitational waves. But if you've ever wondered why they're not just called gravity waves, because that seems to make a lot more sense, that's because by the time we figured out gravitational waves, Einstein discovered them as soon as he developed relativity, like 1917 or so, he discovered that there can be waves of gravity. The name gravity waves was already taken. These are waves in the atmosphere that respond to the force of gravity, and they were called gravity waves, so that name was already taken, so it had to be gravitational waves. But anyway, gravitational waves are waves of gravity. They are ripples in the fabric of space-time itself, the same way you might imagine waves on the surface of an ocean.

And as the waves pass over you, you move up and down. As a gravitational wave passes through you, it doesn't move you up and down, but it squeezes you and pulls you in alternating directions. I know that's a tiny bit harder to imagine. We know that waves can move in three dimensions. It's just really, really hard to visualize it. But do your best and you'll get there at least most of the way. Anyway, these waves, these gravitational waves, slosh and crash through all of us every single moment of every single day. But we don't notice. And that's because gravity is by far the weakest of the forces of nature. Even if it were a trillion times stronger than it is, it would still be the weakest force. It takes the whole entire Earth to hold down a cup of coffee. And all that might of the entire planet Earth can be counteracted by picking up the cup of coffee. It takes something like LIGO, the Laser Interferometer Gravitational Wave Observatory, I guess the W is silent.

It's one of the most precise instruments ever built. It takes something like that to even get a hint of some of the tiny fraction of the waves hitting the earth. They have to take a laser and bounce it back and forth down mile-long tunnels and watch for vibrations at the scale of less than the width of an atomic nucleus to detect these gravitational waves and the influence that gravitational waves have on the Earth. Those gravitational waves are so weak that they are swamped out by the vibrations caused by people talking in the break room. That's why it took LIGO over a quarter of a century of refinement to get their first detection of a gravitational wave because there are all these sources of noise, all these sources of interference, all these things that can wiggle and jiggle those mile-long lasers. better than any, even the strongest gravitational waves passing over the Earth.

And then even their first detection of a gravitational wave in the fall of 2016 was stronger than they expected. They were doing an upgrade of the instrument, they had just developed, put in upgrades, more precision, more accuracy, and they were still not expecting to get gravitational waves, but it turns out the gravitational waves that they detected were stronger than they had predicted. That's the only way they were able to get some. And what LIGO discovered was the gravitational waves generated by two merging black holes. And those waves, even though they were less than a whisper, they could barely nudge an atom more than the width of its own nucleus, we're talking 10 to the minus 15 meters here, that's how much a gravitational wave is able to squeeze matter. Those gravitational waves, even though they were barely detectable here on the Earth, when they were created, well let's just say you don't want to get near gravitational waves.

When black holes merge, they release a crazy and unfathomable amount of energy. Something like 3 to 5 solar masses converted into pure energy. Like if you were to take a sun, multiply it by 3 or 5, so 3 to 5, so you have a sun that is 3 to 5 times heavier. and then convert all of that mass into pure energy as in E equals mc squared taking the mass of five suns multiplying by the speed of light squared that's the kind of energy you get released by black hole mergers and all that energy goes into gravitational waves which is one of the wildest things for me to imagine out there happening in the universe. And there's a lot of candidates. There are a lot of wild things happening out there in the universe. One of those is gravitational wave mergers. They are completely silent. They are totally invisible. There is no flash. There is no explosion. There's no deafening roar, nothing that we would associate with this kind of release of energy.

A black hole merger will release more energy than a galaxy's worth of stars. They are one of the most energetic events in the entire universe, but they are totally invisible because they release all of their energies in the form of gravitational waves. The gravitational waves are so powerful that they contribute to Patreon. That's patreon.com slash pmstutter. Even here, millions of light years away, they nudge atoms in just the right way to hit that contribute button. Isn't that remarkable? That's patreon.com slash pmstutter. And I'm eternally grateful for all of your contributions. If you were to be close to one of these gravitational wave events near a black hole merger, if you were say a light year away. which is pretty far away, but a light year away from the collision, the gravitational waves would be so strong that you would literally get torn apart. You would be stretched and squeezed so much that the bonds that hold your molecules together to make your body would stop bonding.

And LIGO is able to know that these are gravitational waves made by emerging black hole and not, say, people chatting over lunch because of a special fingerprint. People chatting over lunch, yeah, they cause vibrations, but it's all random. It's noise. When black holes merge, the gravitational wave signature that they emit is very brief. It's very sharp. It's very loud. It lasts less than a second. And it has a very specific pattern where it gets really high really quickly and then decays. It gets weaker and weaker and weaker with every passing wave. And then there's a little hum that lasts for a little bit longer and then it goes away completely. We know that this is the signature of black holes merging because this is all happening in general relativity, the ripples of space time. This is stuff we can calculate, stuff we can compute. I remember once I had a professor in undergrad who was a specialist in general relativity, which is kind of funny.

And he said, oh, yeah, when he was a postdoc, when he got his Ph.D. This is back in the 90s. Everyone Ian, who was working on relativity, was coming up with templates of different masses of black holes, different ways that they can merge together, different angles of impact, and coming up with what the gravitational wave fingerprint would look like. He said that's what everyone was doing, that's all they did. And he didn't want to do that, so he ended up in a more teaching-oriented career, because he didn't want to sit down and calculate templates all the time. But it's a good thing somebody did that template, because decades later, when we have LIGO, we know exactly what we're looking for. But as powerful as those gravitational waves are, powerful enough that if you were a light year away, they would tear you apart, they are not nearly the strongest gravitational waves in the universe. Oh no, on the gravitational wave menu, folks, there's way more than black holes colliding together.

There are supermassive black holes merging together. Imagine the release of energy when two black holes, say 10, 20, 30 solar masses merge together. You can imagine, oh yeah, that's a lot of energy. Now, repeat that performance, but with objects weighing millions of solar masses. and the energy those things release. Think about the energies released when stars wander too closely to black holes and they get torn to shreds. Think about the energies released in a supernova. All of these kinds of events release enormous and very powerful gravitational waves. But we don't have direct observations of these kinds of gravitational waves because they're at the wrong frequency. LIGO, using its mile-long lasers, is very sensitive to essentially what are high-pitched gravitational waves. Short frequency stuff. Little chirps and blurps. I know that's not a word, but it is now. of a quick spark, like a weep, of gravitational waves passing through.

Very short, very bursty, very bright and very loud and very brief. Things like giant black holes burgeoning. Things like supernova. Things like stars getting torn to shreds. They're not little bleeps. They're not short, quick bursts that quickly rise above the noise and then disappear. They're much longer. They're like low rumbles as the waves come through. When a black hole merger wave passes over the earth, it's over and done with in less than a second. Some of these events, the gravitational waves, last for hours from high to low. And this incredibly low frequency, this long wavelength, this deep rumble is much harder for something like LIGO to pick out. Because LIGO is sitting here on the earth and there's noise everywhere. There's not just people chatting in the lunchroom, there are trucks driving by a few miles down the road. There are earthquakes constantly, tectonic shifts happening constantly. There are changes in air pressure and wind against the tunnels.

All of this makes noise. But when there's a brief burst of a black hole merger coming by and we get a whoop, it just stands above the noise. You can hear that whoop above all the noise. You can pick it out. But if you're trying to target a long, slow rumble that slowly rises up and then slowly descends, It's harder to pick that out of the noise because even though there's more energy involved, that energy is spread out over much longer timescales. And so you can't pick out that slow rise of the wave coming in and then it and then it leaving because there's just too much jitter in your instrument. So to tackle. These kinds of ultra-powerful gravitational waves, but waves that are so powerful that they extend over such long times that we can't really hear them above the noise here on the Earth, we need to go to space, and that's motivation for LISA. LISA is the Laser Interferometer Space Antenna, and it's like LIGO, but in space.

Instead of lasers going down a tunnel, sitting on the ground, lasers, well, they go through You have the idea is to put three satellites in orbit around the Sun flying in formation and then they bounce lasers back and forth to each other and then as the gravitational wave you can imagine a gravitational wave sloshing through the solar system. As the wave passes, these satellites will get a little bit closer together and then a little bit further apart, a little bit closer together, a little bit further apart. This is what waves of gravity look like. They change the distances between objects. And if these satellites are free-flying, which is the point, then they will pick up these distance changes. Now, on the one hand, this is amazing because there's no trucks rolling by, there are no people in the lunchroom, there's no wind to bother them, so it totally eliminates all earthly noise.

The downside is that there are all sorts of nasty gravitational interactions, like G Jupiter's over here and Saturn's over there. And then as they go in the orbits, the gravity of those planets is going to slightly tweak these instruments. Even things like if there's a solar flare or a coronal mass ejection coming from the sun that changes the balance of weight in the solar system and that these satellites are sensitive enough. to pick that up. So there's a lot of work to be done. ELISA isn't scheduled to launch until the mid 2030s and as is the case with all space missions, don't hold your breath. There's already been a Pathfinder mission to test the fundamental technologies to make sure that we can get the precision we need with our laser measurements. And so it's on track. Like, I strongly suspect in my lifetime that Lisa is going to fly. And Lisa because these satellites. are going to be very, very far away from each other. They're not going to be a mile apart.

They're going to be a few million kilometers apart. They'll be able to pick out these long, slow waves that breathe through the solar system. that carry ungodly amounts of energy but do it in a very slow and ponderous way and because there won't be all these sources of earthly noise and because they have these very very long arms it's like a different kind of antenna that can pick up that is designed to pick up these very low frequency waves that LIGO simply can't see. But there's one source of gravitational waves that stands above the rest. These gravitational waves are perhaps so powerful but so quiet that maybe even Lisa won't be able to see it. Lisa should be able to see giant black holes merging or stars getting torn to shreds or supernova going off. But there's another source of gravitational waves that even Lisa might not be able to get. These are the most powerful gravitational waves ever made in the entire history of the universe.

They're so powerful that these gravitational waves didn't destroy a star or even warp a galaxy. These waves were the universe itself. They're called primordial gravitational waves and they're amazing. We're going to go way back in time to the wee hours of the Big Bang. We're talking less than a second of total existence here. And we're talking about that oh-so-mysterious, oh-so-frustrating, but oh-so-necessary event of inflation. We strongly suspect, we have no proof whatsoever, that when the universe was incredibly young, we're talking 10 to the minus 30 seconds, but who's keeping score? Let's just call it less than a second. that when the universe was in this extreme and exotic state, it underwent a radical phase transition where it inflated, hence the name inflation. where the universe got really, really big, really, really quick, where it got many orders of magnitude bigger, like 10 to the 60 times bigger.

To give you a sense of scale, that's like taking an atom and inflating it to be, you know, bigger than the size of the observable universe. If we were to inflate you by an order of 10 to the 60, you would be bigger than the observable universe. So in this event, the universe got really big and it did it really quickly when by the time the first second was done, inflation had already completed. It already started, done its thing, and then went away. We strongly believe inflation happened. It solves a lot of problems with our Big Bang picture of the early universe, like the fact that the universe is so amazingly flat, and the fact that the universe seems to have the same temperature everywhere we look. One of the only ways we can think of to do that is to have this ridiculous event of inflation happening. We also have some evidence that something like inflation happened when we look at the cosmic microwave background, which was emitted when the universe was 380,000 years old.

That's like, by this time, by the time the CMB is created, the universe is ancient compared to when inflation happened. But inflation left behind little ripples in space. It took the quantum foam that's operating at the very smallest scales in the universe and inflated it to be just kind of small. Instead of the very smallest things, they were just kind of small things. This left behind an imprint like a pattern in spacetime itself, regions of slightly higher density, regions of slightly lower density. These would eventually grow up to become entire stars and galaxies. But more directly observationally, they leave behind an imprint that shows up in the cosmic microwave background as regions of slightly higher temperature and slightly colder temperature. And the statistics of those temperature variations, those differences in temperature, exactly match what we expect from inflation.

Now that's not ironclad proof because there are other ways that you could plausibly or kinda sorta plausibly generate those patterns. And inflation has a major weakness which is we have no idea what powered it, how long it lasted, or why it stopped when it stopped giving us the universe instead of just a vast empty plane of existence. So inflation has some major weaknesses, which we all know and we all acknowledge, and we would love to get a direct picture of inflation. But we can't take a picture of the event of inflation because any light that was generated back then got absorbed by the cosmic microwave background. It got all mixed up and blocked. We can't see directly past the Cosmic Microwave Background. The Cosmic Microwave Background is the most distant thing that we can see with light, but not gravitational waves. Even though inflation happened in the earliest waking moments of the universe itself, it did leave behind something we could potentially, perhaps, maybe detect.

And what I'm talking about are gravitational waves, specifically primordial gravitational waves, which it sounds really, really cool. Like in the earliest moments of the universe when inflation triggers this gigantic phase transition and the universe is getting bigger, it's inflating everything, it's amplifying everything, it's stretching out everything. You have these random quantum jitters in space in the quantum foam that's always there from the uncertainty principle. It gets inflated. Some of this jitter grows up to become the seeds of structure, the stars and galaxies of the universe, and some of it stays as jitter. And that jitter starts wiggling. It starts waving. And what is waving, jiggling, wiggling space-time? It's gravitational waves. It's like an earthquake that shook the entire universe.

A cosmoquake triggered by this event of inflation that transformed the entire cosmos, brought the present-day cosmos into being through this event of inflation, laid down the ripples that would someday grow up to be the largest structures in the universe, but some of those ripples persisted and traveled and moved and they kept waving. I can't even begin to describe how powerful these gravitational waves would have been. It's not like you could have felt them. I can't transport you back to this epoch when they were generated because when they were generated, the entire observable universe was smaller than an atomic nucleus. And that makes it kind of cramped. So you can't even be inside the universe to feel these gravitational waves. But these gravitational waves, even though the entire cosmos was smaller than an atom at this point, the gravitational waves filled up. The entire universe, they release more energy in this epoch, I don't know, it just dwarfs anything that would come after.

You can imagine the entire universe vibrating like a bell, quaking to its very subatomic foundations from the release of these gravitational waves. And in June of 2014, we thought we had our first signs of seeing them. I don't know if I've told this story on Ask a Spaceman before, but if I have, it's a good one. So back in 2014, I was a postdoctoral researcher. I was living in Paris. I was doing lots of void stuff, but I was also working with the Planck Collaboration. This is a satellite launched by the European Space Agency to map the cosmic microwave background to very high detail. and across the entire sky, like a global map of the cosmic microwave background. We're doing our thing, doing our analysis. We hadn't made our results public yet. We are still in the throes of analysis, our first round of papers, and it's a giant collaboration. There were like 400 people joining this effort.

And at the same time, there was a smaller ground-based telescope, a telescope based out of Antarctica at the South Pole, actually, called BICEP. BICEP wasn't meant to scan the whole entire sky of the Cosmic Microwave Background. It was going to zoom in into one tiny little patch. And what they were looking for was the signature of gravitational waves. Just like inflation itself left an imprint on the Cosmic Microwave Background through temperature differences. In the polarization of the light, gravitational waves leave their own slight imprint, this little, how should I say, this little twisting of the cosmic microwave background light. And they were hunting for that twisting that were caused by not the inflation itself, but by the gravitational waves released by inflation. As these ripples go through, they still persisted to a small degree, 380,000 years after they were laid down. When the cosmic microwave background was generated, they left this tiny little twisting imprint on them.

BICEP was looking for it. But this signal, this twisting signal, could also be mimicked by dust. The light emitted by the Cosmic Microwave Background has traveled through billions of light years of cosmic distance to reach us here on the Earth. It has traveled through billions of light years of just dust, and that dust absorbs some of the Cosmic Microwave Background light and then re-emits it, but with its own little twist. And the twist it gives it is very similar to the twist generated by the gravitational waves. It's pure coincidence, just bad cosmic luck that this is how it shakes out, that these two wildly different things give the same imprint. They dirty up the signal in the exact same way. So, for BICEP, to hunt for gravitational waves, they had to know what the imprint of the dust was like. They had to say they had to get their raw image and they say, okay, well, we know what the dust is like. We're going to subtract that.

And then if there's anything left in this little twisty signal that we're after, then that must be due to primordial gravitational waves. But they couldn't get the dust itself. They needed a global map of the whole entire cosmic microwave background to get an understanding of what the dust was like, how the dust was interfering with the signal, then they can go in their tiny little patch and make that subtraction. And that's what Planck, one of the things that Planck What it did, and still does, is provide that global all-sky map of the dust so that you can subtract the dust from the signal and look for anything interesting underneath there. So in June of 2014, or in early 2014. The BICEP collaboration announced the discovery of primordial gravitational waves. Huge press release, people in tears, talk of Nobel Prizes. This is a major discovery. This is ironclad evidence for inflation because the only way to make these gravitational waves happen is through inflation.

We know exactly how it proceeds. We hadn't had our direct observations of a direct evidence for but here it was in the imprint left in the cosmic microwave background this like twistiness this tiny little twistiness in the signal in the images and they made an announcement that they had found it. But I was over in the Planck collaboration at the time and we hadn't made our data public yet. And they needed our data to understand how the dust was, how the dust was contaminating the signal to subtract it out and go searching for gravitational waves. We hadn't made that data public yet. And we had our own internal teams. I wasn't on one of these teams, but we did have teams that were doing searches for primordial gravitational waves and they saw nothing like the bicep signal. As the months went by, The BICEP team got more and more recognition, they were doing interviews, headline news, the usual wave of big science news, and everyone in the Planck Collaboration knew that they were wrong.

We knew that they were wrong because what they had done, and this is a true story, someone from the Planck Collaboration gave a talk at a conference. And they showed some of our maps of dust, some of our analysis of what the dust was like between us and the cosmic microwave background. And they had labeled the graph preliminary. This is just, you know, we're not finished with our analysis. You know, don't take this as gospel truth. It's just, you know, just giving you an example. They weren't technically didn't even have permission to show that, but it just slipped through the cracks because, you know, when you have 400 people, especially scientists, they're not going to listen to all the rules. They're at a conference, they gave this talk, they said, oh yeah, this is a preliminary dust map, you know, I just want to give you a sense.

Someone from the BICEP team took a picture of that slide, of that presentation, and used that to calibrate their results for the BICEP team to get this result in their massive discovery of primordial gravitational waves. But we knew in the collaboration that that was wrong, the preliminary map was not complete, that the way that the BICEP team had tried to eliminate dust from their signal to get primordial gravitational waves, we knew it was wrong. But we couldn't say because we were still under embargo. which means we weren't allowed to share our results until our first round of papers. So as the months went by, I remember conversations within the collaboration, we as Bicep was gaining more and more steam, more and more attention, and we're sitting here silent knowing that they were wrong. So what do we do? We emailed them. We reached out to them and said, hey, by the way, we can't quite share our results. We can't make it public yet, but we know you're wrong.

You need to Maybe provide a correction to your paper. And it was interesting. I, I just happened to be in the right place at the right time. I did not play a major role in playing whatsoever, but I was a member of the collaboration doing some corner of the analysis, not related to this at all, but I happened to overhear all these conversations, which was pretty amusing at the time. Anyway, we have no direct observations of primordial gravitational waves. We do not have a, we have not been able to detect an imprint of them in the cosmic microwave background, but maybe we can get them directly. We can't see the event of inflation, no radiation persists from that epoch, but the gravitational waves do. These gravitational waves that shook the entire universe, that filled up the entire volume of the universe, that released more energy than like all the stars in the rest of the futurists of the universe ever would, they're still here today. They're still passing over us right now.

They filled up the universe, they're still here. Lisa MIGHT be able to detect these gravitational waves from the Big Bang, from the event of inflation. We're not sure yet. It depends on how strong they are, what their wavelengths are, the exact properties, and how precise Lisa can get. There is a successor to Lisa, just like Lisa is a successor to LIGO. There's one that's planned to come after it. It's called the Big Bang Observer. The idea from the Big Bang Observer is to make Lisa, but make it even bigger. Lisa is going to be three satellites flying in tandem. Big Bang Observer will be three sets of satellites, each with its own handful of stations coordinating together both within each set and then literally lasers going across the solar system to other sets. giving it access to a wide variety of gravitational wave frequencies. It would be like a gravitational wave super observatory.

It would capture everything that LIGO can get, that LISA can get, plus more, to greater precision, greater accuracy, and across all sorts of frequency ranges. It would basically be able to see like every gravitational wave passing through the solar system, including ones generated by inflation. It would be the ultimate end-all be-all for gravitational wave observatories. Right now, it's just a concept. There's no funding. There's no development path. I wish I could say there was hope on the horizon, but that's just not in the cards right now. Right now, we are pouring our funding and our work and our development into LISA. The Big Bang Observer is just an idea and a cool name. And we have to either hope that Lisa just works out and we can always get lucky and see these hints of primordial gravitational waves, the most powerful gravitational waves ever released in the history of the universe.

Or failing that, if we know we're on the right path, at least maybe give us motivation to keep going and build something like the Big Bang Observer. In the meantime, we'll just have to close our eyes and imagine the gentle whisper of gravitational waves passing over us and through us, knowing that some of those gravitational waves were generated in the first instant of the Big Bang itself. and that will have to be good enough. You can also help the show by dropping a review on your favorite podcasting platform or by sharing about this show to anyone you meet, even random people in the street. It's totally fine. It's not weird at all. And you can also contribute to Patreon. That's patreon.com slash PM Sutter. I would like to thank my top Patreon contributors this month.

They're Justin G, Chris L, Alberto M, Duncan M, Corey D, Michael P, Nyla, Sam R, John S, Joshua, Scott M, Rob H, Scott M, Louis M, John W, Alexis, Gilbert M, Rob W, Jessica M, Jules R, Jim L, David S, Scott R, Heather, Mike S, Pete H, Steve S, Lisa R, Kevin B, Michael B, Eileen G, Don T, Steven W, Deborah A, Michael J, and Phillip L. It is all your contributions that make this show possible. I can't thank you enough, and I will see you next time for more Complete Knowledge of Time and Space.

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