Daniel and Kelly’s Extraordinary Universe - Supervoids

Episode Date: September 29, 2026

Daniel and Kelly explore the structure of the Universe and examine its emptiest places, making no effort to spare the scatological humor.See omnystudio.com/listener for privacy information.

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Much better than standing on a mountaintop or on the top floor of a building. Stretching over zillions of light years of cosmic space, you're seeing entire stars and galaxies across this black ocean. It's amazing. But there's something else for you to see up there, for us to learn about the empty bits. Between the galaxies stretch huge regions without all that flashy, glowy stuff. How is that all arranged? What is the map of galaxies in the universe? And what does that tell us about how the universe?

universe forms and the forces that shape it. Today in the pod, we're digging into the biggest maps we can make of the biggest stuff and the biggest non-stuff in the universe. Welcome to Daniel and Kelly's extraordinary, mostly empty universe. cleaner Smith, I study parasites and space and super void sounds like a sort of gothic concept, so 16-year-old Kelly is totally into it. Hi, I'm Daniel. I'm a particle physicist. who likes to think about aliens and I'm about 0% goth. So when I hear Supervoids, I don't think black lipstick.

I think Marvel characters. Oh, all right. Well, so Supervoids does make me think of gothic stuff, which reminds me of some of my favorite gothy characters, which are Frankenthurter from the Rocky Horror Picture Show. Yes. R. I.P. Tim Curry.

Exactly. And the devil in legend. Which, yes. So my question for you today is, what was Tim Curry's best role? And why is it the saddest thing in the whole world that Tim Curry is gone? Because it's the saddest thing in the world that Tim Curry is gone. It is sad. He was wonderful.

I loved him in Clue. Oh. You know, the hilarious movie, wonderful. And I love a mystery, you know. For me, science is the biggest mystery in the universe. How does this all work? Who caused the ripples in the primordia plasma? You know, was it the butler with a candlestick in the early universe?

Who knows? You are so good at keeping us on track. I'm doing my best here. Oh, my gosh. This car is going off into the weeds of Virginia. I just needed a moment to like vent and feel seen because I am just devastated about the loss of Tim Curry. Would you say that the loss of Tim Curry creates a void in your heart? A super void, Daniel.

And so, and my heart is now cold and empty. Oh, no. And let's go ahead and change the subject before I get too emotional here. and dig into supervoids. Why are we talking about super voids today? We're talking about super voids because they're super awesome. They're fascinating. They're one of the biggest things or non-things in the universe.

They tell us so much about its history, about its future, about its formation, about how it works. And it's so hard to get them in your mind because they give you a sense of scale of the universe, you know, how tiny, how small, how insignificant we are. And yet how we can cast our minds to understanding the largest scale structures in the universe, which stretch across billions and zillions of light years. It's super awesome to think about super voids. And I'm not the only one who thinks so. We got a request from two listeners, Levi and Nathan, who wrote it and said,

Hey, would you tell us why the universe has super voids? So that's why we're talking about super voids specifically today. Well, and I noted in that email that Levi also says he enjoys my poop jokes. And so this is a family that gets our podcast. Exactly. And we'll try to make as many poo jokes as we can in today's episode just for you, Levi. I am trying to connect supervoids to poo jokes. And the only thing I can think about is the prep for a colonoscopy. You are super voided at that point. But probably that joke's over Levi's head because Levi's a little bit younger, isn't he? Yeah. Yes, exactly. But maybe Levi thinks about super voids every time he's voiding himself. Okay. Well, good. Look it. We did it. We did it. We got the poop jokes in there. Yes. And, you know, a really important thing for understanding super voids and the structure of the universe is dark matter, you know, which is its own poo joke right there. Oh, man. I love the work we do together, Daniel. It's the best. We are out here doing hard-hitting science. No, really, we are proving that it's possible to do deep dives into science and also make.

poo jokes. That is our niche in the SICOM community. But before we tell you what we think about super voids and dig into the cosmic history that created them and our confusion about them, I wondered what the extraordinaire's thought about why the universe has super voids. So I reached out to our group of volunteers, which you are very, very welcome to join. Just write to us to questions at danielandkelly.org. And you can join this crew. In the meantime, think about it for a minute. Why do you think the universe has super voids? The universe is expanding, and the amount of stuff in it is not increasing. So we can fight all of those dastardly dark energy and dark matter criminals.

Initial randomness allowed some parts to get denser, and presumably some parts to get a lot sparser. It's for the same reason and my gravy has lumps in it. I don't know what that reason is, but I think it's the same reason. collects in some places, then it has to clear out in others. I think it has to do with the way Matteras created during the Big Bang. I would guess some astronomers saw some very cold or empty patch in space and could not avoid to give it a super name.

Does not have to be uniform. I have no idea what a super void is. Is it like a really, really big void? Or is it? You can't really have like a void is just void. Avoid is just void. Because of dark matter. It's because of dark matter.

Yeah. We're confused. Help us. Oh my gosh. I love the answers. Astronomers saw an empty patch and couldn't avoid giving it a super name. You are my new best friend. That's got everything you love, right?

It's got a pun. It's got a dig against astronomers. I feel like it implies that astronomers give things good names. Oh, maybe they do. No, no, they don't, actually. What is it the rings around Saturn or Jupiter or, like, A, B, C, and D? Yeah, exactly. The solar system is a disaster.

Even down planets and dwarf planets, you've got like centaurs and meteors and all sorts of stuff. It's a mess. It's a mess. But we are here to clear up confusion. And so, let's jump right in. Daniel, what the heck is the super void? So to understand super voids, we first have to understand our cosmic context, like the structure of the universe around us. So let's start here in California or Virginia.

California, by the way, feeling more like Virginia these days, you know, because it's like stinky and humid and gross and like, where do I even live anymore, man? Oh, my, I see that your handle on Riverside today is humidity hater. This is really living rent-free in your brain. I'm not a humidity fan either. I mean, since we're putting ourselves in the cosmic context, it's important to note that Los Angeles yesterday was the most humid place in the country, which is, you know, breaking all the rules if you ask me. That does not seem fair. Anyway, zoom out from the sticky issues here on planet Earth.

And of course, we have our solar system. And the sun is just one of, you know, hundreds of billions of stars in the Milky Way, swirling around in roughly a disk. And then our galaxy is not just one of zillions scattered through the universe. galaxies themselves cluster. So we have the local cluster of galaxies, a bunch of galaxies that are gravitationally bound together, which doesn't mean that there's like some big object in the center around which everything is orbiting the way it is in the solar system.

But it does mean that there is a center of mass of the local cluster and everything is swirling around that. Okay, I'm with you. All right. So we have clusters. And now clusters organize into super clusters, right? Astronomers, great names. And super clusters are not always gravitationally bound.

They're like near each other in that you can look at them and say, oh, these clusters are near each other. But it's not clear whether gravity is strong enough to hold them together. Because remember, the universe is expanding. It's creating new space between everything. And that includes new space between California and Virginia, new space between Earth and the Sun, new space between the Sun and the Center of the Milky Way. But because there's enough gravity to hold the Earth together and hold the Sun, to the earth and hold the sun to the rest of the galaxy, those distances are not increasing. So the expansion of the universe is losing to gravity. But gravity gets weaker as distances get

greater. So when you get to super voids, dark energy is starting to get powerful enough that it's going to overcome gravity. And that's right about the cutoff. So we don't think that all superclusters are technically objects in the sense that they're not gravitationally bound. We think that as time goes on, they probably will get pulled apart. But there's still sort of objects in the sense that they're near each other. And if you looked at them on a map, you'd probably draw a circle around them. Okay. That doesn't sound voidy. No, we're not at the voids yet. Okay. But now we've already zoomed out incredibly, right? Like from the earth to the sun, to the galaxy, to like clusters of galaxies to now super clusters of galaxies, it might feel like this is the biggest thing you could ever have

in your head. Okay, now collapse all those superclusters down to a point because now we're going to zoom out so far that structure inside a supercluster is irrelevant. We're just going to treat each supercluster like a dot. And we're going to ask, how are the superclusters organized across the universe? Wow. And they're not organized evenly. It's not like somebody sprinkled sand across the universe, each one being a supercluster. Instead, they form filaments. They form walls. They form bubbles. right? So there are sheets and filaments of superclusters, and inside those bubbles are the voids. That's where there are fewer galaxies and fewer superclusters. And so imagine like a foam, right, where superclusters lie on the edges of the bubbles and inside the bubbles there's comparatively less.

All right. So my brain is now wondering why you can't get super voids on the outsides of the bubbles, too. Am I just letting this analogy get stuck in my head too much? There is no outside to the bubbles. The universe. is filled with these bubbles. Oh. Yes, the bubbles are everywhere. The universe is just bubbles. The foam fills the whole universe, and everywhere you have a bubble edge where the

bubbles meet, that's where you have superclusters, walls, sheets of superclusters. And inside, those are the voids. Whoa, okay. Are they totally empty or just less empty than the outside of the bubble? Yeah, great question. They're not completely empty, right? It's a relative density. So we measure the density of stuff in the universe.

And we say anything less than like 10% of the mean density, we call that a void. And these things are pretty big. Like they have diameters like 10 to 100 megaparsecs, which is like 3 to 300 million light years across. So like these are vast structures. And these are not super voids. These are just voids. These are just normal every day. Run-of-the-mill voids.

Already these things are much, much bigger than one individual supercluster relative to a void. supercluster is so tiny, you just think of it as a dot. And remember, inside that supercluster is a bunch of clusters of galaxies. And inside each cluster of galaxies is a bunch of galaxies, each of which is unfathomably huge to the human mind. So this is like a real brain stretcher to even get this in your head, what a super void is and how big it is. Yeah, I was, so you said 30 to 300 million light years, and I was still having a little trouble wrapping my head around that number. So I just looked it up, the distance between our galaxy and Andromeda is 2.5 million light years away. So we are talking about voids that are like, I don't know, 15 times bigger than that minimum.

That is huge. Yeah. And, you know, the size of our galaxy is like 100,000 light years across. And so if you can somehow get the Milky Way into your brain, then we're talking about distances that are thousands of times bigger than the Milky Way. Oh, my gosh. And so, yeah, these are big, empty regions of space. Wow. We should put something in there. I mean, I know your house is filled to the brim with all kinds of interesting stuff. And so you're always looking for more storied space.

Yeah, I was thinking we could put all the stuff in my house into these super voids so that I don't have. But I think Zach might fill the super void pretty quick with Dorito bags. Not just Dorado bags, but you know, you've got all these projects. They're always half done. You're like, I'm storing all these cabinets in this room because I can't put them on the wall yet. They're on the wall now. Do you imagine someday in the future you will be done with projects and the house will just be like actually totally functional? No.

No. She says with resignation. I don't know when I'll have time to finish everything. Maybe when I die. All right. Well, maybe your house never will. have voids, it's always going to be filled with stuff. But the universe has voids and they're fascinating and they were kind of a surprise when we learned that the universe had this structure.

So why? Okay, so if I'm like imagining a giant space, I would imagine that if you like distributed stuff around that space, you would expect there to be some empty areas for random reasons. But you've, I think you've already explained to us that these voids aren't random because they tend to be on the inside of like bubbles. Does that tell us something about how they're formed? It does tell us something about how they're formed. And you put your finger on exactly what the discovery was. Until around the 70s, we thought that galaxies were distributed roughly equally. That, you know, any galaxy had an equal chance to be here or there. And if you took any chunk of space, you would find roughly the same number of galaxies, you know, that the density was pretty smooth. And it wasn't until the 70s that

they went out and they started measuring these things and trying to map out the structure of the universe. You know, I love when we have an idea about how the universe works, and then we develop the technology, the capacity to actually measure it, and the universe is like, no-uh, what you thought was natural, what you thought was intuitive, what you thought made sense, that's not how things work. Because those are the best moments, those are the moments when the universe is confronting your intuition and teaching you something about how things work. I mean, not like the universe is a teacher, you know, that cares what we understand, but those are the best moments of discovery. Yes. And it was in the 70s that people,

People first started measuring red shifts of these galaxies and trying to assemble them into sort of a 3D map. What you need to understand the structure of the universe is to know where is a galaxy, how far away is it? If you know those two things, you can start to build a 3D map of where all the galaxies are and you can start to see structure. Okay. And so people started doing this seriously in the 70s, the Center for Astrophysics at Harvard and the Smithsonian started to do this with the Tillinghouse Telescope. And they were surprised. They surveyed like 2,200 galaxies, and they saw the things were not smooth, right? It didn't look like galaxies were distributed randomly across the universe.

They were clustered together into these bubbles. Imagine a sheet of paper and you throw sand over it, and you expect there, as you said, to be some places where there are galaxies and some places where there are not. It's not going to be perfectly smooth. You don't expect the galaxies to be like in a grid, you know, perfectly arranged. There'd be some clustering accidentally, but what they saw was definite structure, not just random distributions. They saw, like, a bubble bath of cosmic voids and filaments.

And they discovered what they call the Great Wall, a superstructure over 500 million light years wide. Whoa. And what a moment, you know, to be the first person to map the universe, right? Like, I always imagined the joy of being like a first person to land on a new shore or to cross a land bridge or, you know, get in a boat 10,000 years ago and cross the Pacific to the South Pacific Islands or whatever, right? What a moment of discovery.

But this is just on another scale, you know? This is like, oh, my gosh, the universe has patterns, and we're part of this, and there's a huge wall over there. And like, oh, my God, what a moment. And, you know, I think that's sort of natural for us to think about now because we've known about it for 50 years. But at the time, this was a real revolution in the way people thought about, you know, our entire cosmic context.

So that was very cool. And that was in the 70s. And, again, the primary way we figured this out is that we look at, galaxies, and we measure their red shift, meaning that the light that comes from them has been shifted to lower frequencies. And we measure their red shift, which tells us how the light has been shifted, which tells us how fast they're moving away from us, because remember, the things that are moving away from us have their light shifted into the red.

And if we know how fast they're moving away from us, then we can tell how far away they are, because that's the Hubble relationship. It tells us the things that are further away are moving away faster. So if you just measure the light from one of these galaxies, you measure the red shift, you say, I know where hydrogen should be, I know where helium should be on my spectrum, and I see it shifted. You measure that shift. You can translate that to the distance to the galaxy. Whoa.

And now you know the angle in the sky, right? You know, I saw this galaxy at this angle. I saw that galaxy, the other angle. You put those together and you start to make your 3D map of the universe. Amazing. Okay. So now we know what the pattern is. Let's take a break.

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Listen to Dirty Rush on the IHRRRRRADO app, Apple Podcasts, or wherever you're you got your podcasts. And we're back. We've been talking about how, like, the universe is, like, this foamy wash full of clusters that are, like, on the edges of the bubbles. And then inside of the bubbles, we have super voids. What does that tell us about how the universe formed? And, like, have we figured that out yet?

Because that's not what we expected. Right. Yeah. So first, slight clarification. Inside the bubbles are voids. Oh, okay. Super voids, we haven't really talked about exactly yet. Supervoids are essentially like really, really big voids.

In many cases, bigger than we expect. And so that's why we have to dive into the history of the universe to think about like, do we expect to see voids? Do we expect to see super voids? Does what we see line up with what we expect. And to do that, we have to go all the way back to the earliest known thing in the universe. Which is? The cosmic microwave background, of course, which, you know, physics is always going back to you.

You might be like, oh, my gosh, these guys lean on the CMB so much. It's just such a rich and incredible. source of information. If you were trying to understand somebody's life and you had like a picture of them in kindergarten, it would tell you so much about where they grew up and what they were like. And that's basically what we have. We have like a baby picture of the universe. And so when we want to understand how the universe got to be the way it is, we look back of that baby picture. We're like, huh, did it have seven toes when it was born? Or was that surgery that came later? You know, what features of the universe can you trace back to the origin? And if they're not there, what does that tell you? So it's incredibly valuable, and that's why we look back. And for those of you who don't remember, the cosmic microwave background light is light that came from the universe the first moment that it was transparent. So we don't know how the universe began or when it began, but we do know that it was filled with a hot, dense plasma, and that plasma was glowing, and it was opaque, just like

plasma at the center of the sun. It gives off light and then is immediately reabsorbed, which, you know, you hear maybe on pop-side channels like a photon takes 50,000 years to get from the center of the sun to the surface of the sun, and that's like mostly popsine nonsense, because a photon is emitted at the center of the sun and then immediately absorbed. It never gets to the surface. Like, you know, maybe you could say that heat waves travel through the sun at some velocity or whatever, but you're not trace on a photon. Anyway, photons that were created in the very early universe no longer around, immediately absorbed because the universe they were born into was opaque. But the universe is

expanding and the universe is cooling. And at some point, the universe expanded and cooled enough that protons and electrons found each other and became neutral. And then the universe became suddenly transparent. So photons that were born after that moment are still around. And they are still here for us to measure and for us to learn about the early universe. And what we see in those photons is not a smooth plasma. Not like everything was filled with stuff equally. We see some places with more density and some places with less density. And that is not what we expected initially, and why is that not what we expected initially? Yeah, good question.

Here we have like two threads we have to follow. One is the density of galaxies as we're seeing them, like today. And that started in the 70s. And by the 70s, we had seen the cosmic microwave background light. That happened in the 60s. It was a huge confirmation that the universe had an early hot, dense state. But we had not yet seen its wiggles. It wasn't until like the 80s and the 90s that we measured the CMB light with a

precision to see, oh, there are some little hot spots and there are some little cold spots. It just seemed sort of smooth. And so it wasn't until later that we saw wiggles in the CMB and we saw structure in the universe. And you're right to ask about that because those two things are definitely connected. And the listener who made that connection is totally right. We think that all the structure in the current universe comes from that structure in the early universe, that you had places with more density and that had more gravity and that pulled in more stuff and that in those regions you now have more galaxies and more dark matter and then in places

where you didn't. And so you should be able to line up a place in the early universe where there was less dense stuff with voids today. Okay. Got it. And you might be tempted to say, oh, well, can we trace an individual like hotspot or cold spot in the C&B to something in the universe today? And you might be preparing yourself for disappointment because usually the answer to that is no. we can just do it statistically. We can say, oh, we expect this kind of distribution of galaxies in the late universe based on this kind of density in the early universe. But we can't say like this photon caused that galaxy or whatever. But there is one feature that seems to correlate. So there's a feature in the early universe, the cosmic microwave background, that seems to line up with a super void in the universe. Yeah. So the feature in the cosmic microwave background is called the CMB Cold Spot. This is a region where it's redder than everywhere else. So we expect there to be some variations in the cosmic microwave background. They vary by like micro Kelvin.

And we talk about the CMB in terms of temperature, which is equivalent to thinking about wavelength, because black body radiation rules tell us that something at a certain temperature will radiate at a certain wavelength. So when we measure light at a certain wavelength, we say, oh, that came from an object at a certain temperature. So for example, the CMB light came from a really hot plasmuth. thousands and thousands of degrees, I think 3,000k. But that light has been red shifted by the expansion of the universe down to about 2.7K. So if you read that like, oh, the C&B light is 2.7K,

that's what it means. It means that it's the wavelength that an object at 2.7 Kelvin would emit. It's a very, very cold light. And we see variations like 20 micro Kelvin in the Cmb. That's like typical expected hotspots and cold spots. But there is one spot in the C&B, one spot in the sky when we look at it, that it's like 70 micro-Calvin colder. Wow. So some places like 150 microk colder. It's in the southern hemisphere in the direction of the constellation Eridinus, which is maybe the source of a poop joke for those of you who are interested in the southern hemisphere.

If you could see how it's spelled, you would get why this could be a poop joke. Maybe all constellations in the southern hemisphere should have anus in them, you know? Absolutely. Yes. Astronomers get on that. Or synonyms of anus, yes. There you go. Because you need some options.

Yeah. So this is really interesting. You know, it could just be random. It could just be like a fluctuation. And they've done a calculation suggests it's like, there's like a 1.5% chance of this happening randomly. So it's like weird. It sticks out, but it's not insane.

But it does also correspond with a place in the late universe that there seems to be a super void. And you said at one point, well, we haven't really talked about super voids yet. And super voids are just like really big voids, right? Yes, exactly. And so if you look in the structure of the universe today in the same direction, you do see a really, really big void there. It's like a thousand times the size of a typical void. So it's like you have bubbles in your sink and they're all roughly the same size. And then there's one that's just like a thousand times bigger than all the other bubbles. This one's a billion light years across. It's like five to ten billion light years away. So is this like most mornings you eat special K, but one morning you eat poops like a champion? And that's the difference between void and super void? Yes, exactly.

You feel it about 24 hours later, exactly, when you have a super void. And here the universe has formed a void like a champion. And just to be clear, poops like a champion isn't actually. cereal you told me about the other day. It is an actual cereal, and we have boxes of it at home because some company out there that makes it heard about Katrina and her fiber journey and her effort to make everybody poop better by eating fiber and improve their gut microbiome. So they sent her a bunch of free boxes of poop like a champion, and I am not on their payroll so I can tell you that it

tastes like cardboard. But still, I think Katrina is one of the better people on this planet. That's what that story tells me. She's out there. She really does want to improve your pooping, even though she has nothing to do with super voids. I tried really hard to derail this conversation. Let's go back to super voids of the universe. Yeah. So this is really interesting because when you run the simulations in the universe, you don't get these kind of super voids.

Like, it's really unusual. And so it helps us try to understand, like, you know, we talked about in our simulating the universe episode, recently, when you see something different in your simulation and in your data, it tells you that there's a gap. There's something in your simulation that isn't describing the universe correctly, or there's an element of the universe that you're not describing correctly. And these are super fascinating, especially because they're very sensitive to the expansion of the universe. How we see these cold spots and these hotspots in the CMB tells us a lot about the expansion of the universe because photons fly through the universe as they get to us. And

there's two different effects here to disentangle. When we're looking at the CMB and we see some places are hotter, some places are colder. There's two different reasons why. One is, well, we could just be looking at a place that was denser or it was less dense. And so this like initial over density or initial underdensity, that's what we typically think about. But remember that the photons also have to fly through the universe to get to us. And those photons are redshifted by the expansion of the universe. And they fly through the universe, which means that they're sensitive to gravitational density. Because if you fly through a region with a lot of mass, you get redshifted, right? Like black holes produce gravitational redshift.

And so a photon that's flown through the universe in some senses measures the density of the universe along its path, which is super duper cool. That is super duper cool. That photo from our childhood is telling us a lot. Although, is it more like, I mean, you should never really dig into an analogy. but it's more like a video from your childhood, isn't it, than a photo? It is like a video because if you keep watching it, you see different things, right? We are looking at the cosmic microwave background of radiation, and we can't see it for the whole universe.

We see the life that's arriving right now, which left, of course, many, many, many billions of years ago, and it's arriving to us from a shell of that original plasma that's very far away and around the Earth. And as time goes on, we get life from a different shell. So it's not like we're watching the same place over time. Over time, we're scanning more and more distant shells of that original CMB. Got it. Cool. So let's trace the trajectory of a photon as it's moving through the universe.

And let's think first about over densities, places where there's like a lot of mass. So say a photon is on its way to us, but it's passing by a black hole, right? And so as it falls in towards the black hole, it gains energy. So it's getting blue shifted. And then if it doesn't get eaten by the black hole, it makes it out and it comes towards us. Then as it's leaving the black hole, it's losing energy, it's having to climb out of that gravitational well. So it gets redshifted.

Okay. And so you might think, okay, blue shifted more energy, red shifted, less energy. It all balances out. So how could a photon that's coming to us tell us anything about that dense region if it's all got canceled out, if it's basically unchanged, right? So you can't watch it as it changes. You just see like the average of what happens. happen to it over a distance? Yeah, you can't watch a photon go through the universe. You just

see one here on Earth. And you're like, oh, is it hotter than I expected or colder? Okay. Got it. And so the opposite is true for voids, right? If you go near a black hole, then you get blue shifted as you fall in and redshifted as you fall out. And the opposite is true for a void. You get red shifted as you fall in and blue shifted as you fall out. Okay. And so if we only see the average of what happened to a photon when it gets here, how do we know that? So that's our theoretical calculation, but it's missing something really important, which is as time goes on, when the photon is in that void or near that black hole, the universe is not static. The universe is expanding. And so the amount of energy the photon loses or gains when it goes in or out of that void changes because the void is changing or the black hole is changing as time goes on.

So it doesn't perfectly cancel out. And so it does leave an overall effect on these photons. So you can get cold spots in the CMB if photons have moved through a super void on their way here, if the universe is expanding. And so not only do supervoids tell us about like, hey, what is the evolution of the universe? How do you get these bubbles? Do they come from initial over densities? Also, the CMB tells us about photons moving through the universe and measuring existing voids, not from the early universe, but today. So there's like so many dimensions of information in the C&B.

It's not just a baby picture. It's a baby picture that was then like dragged through your life and distorted by the way your life turned out. And then here we are like, we can disentangle those two things in the single picture from the C&B and be like, oh, this is a primordial hotspot or this is one that appeared because of how the photons moved through the universe. Wow. So all these incredibly clever little effects we can use to try to disentangle. history of the universe. And this is why, like, people seem like they're in love with the CMB. It's just such a gift. It's such a gift. Oh, my gosh. Wait, so what if a photon went through a black, or passed a black hole and then through a super void? Like, how do you, how can you

know what a photon saw when it could have seen a bunch of things before it gets to us? You get it in that interview room. Okay. You shine the lights in its face. Oh, boy. No, you're right. You can't. And all you can do. is look for patterns. And we see in the universe, we see a big cold spot, right, in the sky. And that's in the same direction as a super void. And so that means either there was like an initial underdensity there, which created a colder region, or the photons getting here from there have passed through a super void on the way. Okay. And so it's just like, it's fascinating to see that in the CMB and also then to look at our late-time measurements of galactic structure and

see, oh, there's also a super void over there. So, like, things are lining up. They're clicking together. You know, that's exciting when you're doing science. And you see two completely separate measurements that are sensitive to the same thing, lining up and telling you the same story. That's when you feel like, ooh, we're going to crack this case. All right.

We're getting super excited over here, but we have to take a break. Supervoid, if you must, but come back in just a moment. But when we get back, we're going to talk about how super voids get made. the space ones, not the one you just made. The WNBA playoffs are about to begin. The talent is insane and the personalities are giving main character energy. And of course, the fans love it. But 30 years ago, when the league first started, it was a different story.

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Listen to Dirty Rush on the IHR Radio app, Apple Podcasts, or wherever you get your podcasts. All right, and we're back. So, all right, Daniel, how do Super Voids get made? So this is really fun to think about how you get super voids. One is that you can just have an underdense region initially, and you could just have a fluctuation. Like these things we think are seeded originally by quantum fluctuations. Why, anyway, are there regions that are higher or lower density?

One theory is you have quantum fluctuations in the very early universe, and then inflation, this pre-Big Bang theory we talked about recently, stretched those out into macroscopic actual fluctuations that matter to gravity. You know, that's just a speculative theory we don't know for sure. But that would suggest that the fluctuations themselves, the reason we have more stuff here and less stuff there, comes initially randomly, which means that, you know, lots of stuff could happen.

For example, you're flipping coins, you don't expect to get heads and the tails and heads and the tails. You'll have runs of heads and runs of tails. And so if somebody's flipping coins for the early universe, it's like, are we getting stuff here or not? They could have just come up with no stuff, this part of the universe a lot of times occasionally, right? So that kind of thing can happen.

But it's unlikely, right? The bigger, the void, the less likely it is that can happen. The same way, like, having a run of 10 heads is less likely than a run of five heads and a run of, like, 50 heads in rows essentially unheard of, though there is a probability. And so, you know, we only have this one universe. And so we can't tell, like, hey, did we just get lucky or slash unlucky? We don't know. But there are also other ways people have come up with in watching these simulations.

to see supervoids form, which is that you can get voids merging. Oh. Right? So these bubbles can, like, pop because you get gravitational disturbances between these things, and the walls can, like, fall apart, essentially. And so you can get these cells merging. Basically, you know, if you have an over-density in some walls, they can attract those filaments, and, you know,

it's not like these super voids are totally empty. And so you still have some galaxies remaining inside of them. And so you can get, like, mergers of these voids. to form super voids. And this is the kind of example of something you can learn about in simulation. You can run a simulation in the universe. You can watch you. You'd be like, look at that. I didn't expect that to happen. But you see it in simulation. You learn about emerging phenomena in simulation that you didn't predict. Super duper cool. Yay, theoretical physicists. But this allows us to ask the question, like, well, why are there super voids? Do they make sense in the universe?

And, you know, we have this cold spot, which we can partially explain using this like photons come in, photons go out. But it's not a full explanation. It explains like part of the cold spot, but not all of it. It's really very unusual. Though it's hard to say, like, how unusual is a 1% effect? You know, you see it in one out of 100 universes. And so are we just unlucky or is this a hint that there's something else going on? Because remember that these things are very sensitive to the parameters of the universe.

How much dark matter was there, dark matter forming that structure, how much dark energy was there to create this acceleration and this expansion, which is what's causing this effect, this cold spot effect. Because remember, it's only if the voids are changing as the photons are going through them that you even see an effect from the voids. And so there's a lot of questions about whether the super voids are something we expect or whether they indicate that we need something new in our theory of the universe, which of course is where the fun begins, because then we get to speculate about all the crazy ideas. Daniel has a sparkle in his eye. So at the beginning of the episode, you said supervoids remind you of like the Marvel universe. So if you had like parallel universes like you do in the Marvel universe, would you expect supervoids to be distributed about the same way or like have about the same number?

Like what I'm trying to get to parallel universes for you. Well, there's an even cooler way that parallel universe. connect to super voids, which is that maybe you had a bunch of initial universes early on. And remember, we talked about inflation, how our universe maybe is like a tiny dot in a vast landscape of inflationary matter. And in that dot, it's just like went from inflationary matter to normal matter, and then our universe is there and expanded. And there could be other dots out there. And usually those other dots are really far away. And there's a bunch of inflationary matter between us and them, which is expanding at some insane rates. So we'll never see those

other universes. But what if that's not true? What if there are other universes out there and our universe is collided? If early on our bubble bounced into another bubble, it might leave an imprint on the early universe. And according to some theories and according to some simulations that can cause like a bruise in the density, which would be like a cold spot, which would lead to a super void. And so there are some really fun speculative theories of the early universe out there that suggest that if there are these parallel bubble universes created in this inflationary landscape, that sometimes they'll bump into each other and they'll leave a mark and that mark is super voids. Well, I think I'm still having one, awesome, two, I'm having a little trouble imagining. So you get two bubbles, they like merge. And then is it like whatever was making the outside wall of the bubble where they touch just sort of like gets blown to somewhere else and now you get a big center as those two bubbles merge?

Is that how does that result in a super void? Yeah, great question. And I realized that now we're using the word bubble to mean two different things. Shee on us. Shea man me. Yeah, I'm not taking any blame for that. Fair. No, when we talked earlier about bubbles, we were talking about the structure of.

our universe and there's densities of superclusters of galaxies and under densities. And we talked about those organizing into bubbles. And those are bubbles. Now we're talking about bubbles as the whole universe is one bubble in a vast or inflationary landscape. Okay. And so if our, so let's not call them a bubble. Let's just say if our universe bumps into another universe because, remember, in this theory, universes are finite. There are little regions of this inflationary landscape that decayed into normal matter. Okay. So if our universe bumps into another universe and then bounces off, they don't merge,

they bounce off each other, but they leave a mark. You know, the way, like, if you take two apples and you smash them together, they don't become one apple, but you get a bruise on each one. Yeah. What would that bruise look like on our universe if another universe had bumped into us and, like, not even left a note, just, you know, just like hit and run universe style. Not cool. Well, we would get a bruise, and it would change the initial density of matter in the early

universe and simulations and calculations suggest that that would leave a cold spot and under density in that part of the universe, which would, if you ran the simulations forward, leave to a super void today. Wow. Okay. All right. I'm with you. Yeah.

So that's one fun theory. And, you know, there's a lot of debate about that. People say, you know, you're cherry picking, you're crafting an explanation and match something you see. It would be much more compelling if you had a prediction for this before you saw the supervoid. and you say, I predict a supervoid, and then you went out and saw it, that would be much more compelling. This is more like a post-diction. It's like, oh, we see this thing in the universe, what could explain it? Not as a criticism, but just saying it would be more powerful,

more compelling if it was a prediction and not a post-diction. Okay. I mean, I totally see that. But on the other hand, like, okay, but you've seen the thing and you can't unsee it, and now you have to try to explain it. And so, but I agree. It would have been more powerful if it had met expectations, but the universe doesn't care about our expectations. Yeah. So another potential explanation for this is a favorite alternative for dark matter. You remember that we talked about dark matter being out there. We have lots of independent lines of evidence for it, but there are alternatives and it's healthy that people are thinking about other things and try to understand if those other explanations are coherent, if they can explain multiple things and not just galactic rotation curves. One of those is modified gravity. To say maybe gravity doesn't work the way we expect, maybe. Maybe it's not that there is more invisible stuff out there causing gravity, but that gravity behaves differently than there isn't any dark matter.

This is called Mond, modified Newtonian dynamics. Remember how we talked about some of these cold spots are due to the expansion of the universe as photons fall in and out of these voids or in and out of the neighborhood of a black hole. Well, that could also be explained in some models of modified gravity. Okay. Because, you know, you're changing how gravity works. So it's going to change how these photons experience the universe. And even some theories of the universe isn't expanding.

And these things are just due their evidence that gravity doesn't work the way that we expect. And it's true that we can't fully explain these super voids. We don't know where they come from. Our current theories do definitely need something. Do you need to go all the way to modifying gravity to explain supervoids? I don't think so. But, you know, maybe I'm just not a fan of the modified gravity theory. And so I'm blind to it.

I do respect that they're trying to find other ways to support this theory, other places to look for evidence that might indicate that it's the real explanation for the universe and not just a single fudge factor. They're using to fix galactic rotation curves. Now, when we finish getting through all of the explanations, are you going to tell us which explanation is your favorite? Or are you just going to super void on this one? No, instead, I'm going to tell you about how we might learn. more about the universe rather than betting on any individual one. Because, you know, we talked about how in the 70s they started putting this together when they were measuring the cosmic structure. And they've continued to do that. There was a big survey in the 2000s called the Sloan Digital

Sky Survey mapped out a huge number of these big walls and bubbles and really giving us a broader sense of where we are in the universe. And that's continuing. We now have this telescope called Daisy D-E-E-S-I, the Dark Energy Survey instrument, which is the most powerful telescope we've ever had for understanding where things are in the universe in terms of measuring the redshift of a huge number of distant galaxies. And so, like, what's more important? What's more exciting than, like, mapping out the universe? And that's what we're doing.

And just yesterday we saw the launch of the Nancy Grace Roman Telescope, which is also going to tell us about the expansion of the universe and red shifts and the structure of the universe. So we are learning so much more. It reminds me of like, have you seen early maps? I love looking at like maps people drew of the world in like 1,200. You know, when like they didn't know about this whole continent and they misunderstood this whole piece. But you see truth in there. You see like the shape of Portugal or you see like the coast of Madagascar or whatever.

And that's what our maps are like today. And I think in a thousand years, people look back at our primitive basic maps of our neighborhood and be like, wow, they knew none. Nothing. But the information is coming. We are building those maps. We are exploring the universe. We are figuring out. And along the way, we're discovering mysteries and puzzles, which are going to clue us in to how the universe works. There are answers out there to questions we haven't thought to ask because we haven't even mapped out what the universe looks like, you know? And so we can't, we don't even know what to ask yet. Yep. We are constantly pushing the boundaries of our ignorance back farther and farther. I love it. Yeah. And supervoids are a really fascinating whole in our understanding because, you know, we don't fully understand how they form. Are we just lucky slash unlucky? Is there something else going on in the universe that makes these incredibly under dense regions? Is this a clue or is it a red herring? We don't know. And really the only way to know is to get more data, more mapping, more structure, more images, more understanding of where we are in the universe. I'm definitely for that. And also, it's cheap, you know, compared to the kind of stuff we spend money, it cost pennies. Usually physics doesn't feel super cheap, but this one's cheap, you're saying? I mean, it's a lot more expensive than, say, a week in the archives in New York City reading somebody's blood-covered notes, hypothetically. That was fun.

It's a lot more expensive than a lot of stuff. But compared to aircraft carriers or wars in the Middle East, it's definitely cheap. And then you get to enjoy that knowledge forever. Yes, you do. And you create the opportunities for a new generation. of scientists to study new puzzles and come up with new answers. So thanks very much to Nathan and Levi for asking about super voids and giving us an opportunity to talk about this incredible cosmic history, all the detective stories we've cracked along

the way, and so many open questions we have yet to even ask. Fun science, please. Thanks everybody for listening. Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us. Daniel and Kelly's extraordinary universe is edited by the amazing Matt Kesselman. He really is a wizard. You can also find us online on Blue Sky, Instagram, and X, D&K Universe.

Come engage with us. You can email us at questions at Daniel and Kelly.org. We really do want to hear from you. And you can find our website, www. Daniel and Kelly.org, where you'll also find an invitation to join our Discord, where everybody comes and talks about the amazing. amazing universe. And we also have the most amazing moderators.

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