How do cosmic voids form? What do they teach us about dark energy? What did the early universe look like? What does all this have to do with cupcakes? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Alright. Let's say I give you a cupcake. We all like cupcakes, right? So you're not gonna complain that that I gave you a cupcake and we're not it's it's a game just gonna be a vanilla cupcake. Alright?
Just you don't get to pick the flavor and this is my analogy. Alright? This is my analogy. And you're getting a vanilla cupcake with vanilla frosting and sprinkles. Okay?
Now, I give you the cupcake, and you take a bite, and you like the cupcake. You take it home, you're at home, you take a slice out of it because you slice cupcakes instead of just eating them face first like you ought to, and you take a bite and you really like it, and you want to make your own cupcake. Just like it because one wasn't enough. You want a whole dozen cupcakes. So what do you do?
You only have this one cupcake. You only have this one thing to study to help you understand how cupcakes work. So you you make some observations. You taste it again. You poke it.
You know, maybe you burn a little piece of it, you get out some instruments, a thermometer, you take some basic basic data about the cupcake, and then you start cooking. Yo, maybe you put in a bunch of random ingredients like olive oil and anchovies and ice cream and try to mix it together and make a cupcake and horribly fail, of course. But over time, maybe you get a little bit smarter. You realize maybe you need some flour, maybe an egg, and some sugar, and some sort of leavening ingredient. Yeah.
Like, you start to get some salt. Don't forget the salt. Salt is very important. You start to get some basic idea, and you start not just with playing with the ingredients, but also the recipe. When do you mix the ingredients together?
How much of each ingredient? How long do you cook it? How just etcetera, etcetera. Eventually, with enough time and enough money, enough effort, you might be able to replicate the cupcake. And you'll be able to test it because you'll take the cupcake that you made, taste it, poke it, take its you know, measure it, do all your experiments on it, and compare it to the cupcake I gave you.
And the closer you get to the cupcake I gave you, the more you can feel confident that you've identified the closest thing possible to the original recipe for my cupcake. So what do cupcakes have to do with anything? So I'm gonna talk about the universe, and I'm gonna talk about cosmology, and I'm gonna talk about how we try to understand the universe by making cupcakes. And a bunch of you been asking about my own particular research, especially Christian c, Nick c, Joel b, Steve, Neo Silver, at Brian Delight, John r, Steve d, Raymond a, Cyrilio, Campbell d, Chrissa b, at SMTR, Laura w, at Infamous, Nick c, Gary p, Fudd f, and Daniel have all asked, like, about my research and especially something that is very near and dear to my heart when it comes to my research. And this is episode 100.
Insert your own celebratory music as you see appropriate. I will not judge your choices in this moment. So I thought for once I'd give you exactly what you want, and here it is, an episode about cosmic voids. It's one of the things I've worked on in my research. I spent a few years focusing on cosmic voids, and that might just be the way I'm gonna have to say it throughout the entire episode.
Don't work on them so much anymore for various reasons, but in my prime, back in my day, I was perhaps one of the top five researchers in the world who studied Cosmic Voids. That is because there were five people in the world who were studying Cosmic Voids, and I was one of them. So automatic top five. Okay. So what is a void?
It's nothing. That's the whole point. It's in the name. It's a void of matter. It it doesn't have any matter.
There's this joke, you know, if you're a specialist, you know more and more about less and less, and I took that phrase to the extreme, and I became incredibly knowledgeable about absolutely nothing. Now what about a cosmic void? To understand a cosmic void, we have to go big. Bigger than a solar system, bigger than a local group, bigger than a spiral arm, bigger than a galaxy, bigger than a few galaxies, bigger than a bunch of galaxies. We have to go out here to millions or tens of millions of light years in distance before we get to this scale where we can really talk about cosmic voids.
We need to talk about the cosmic web. And I've done episodes on it before. I've I've brought it up in episodes before, but I'll say it again because it's so beautiful. The cosmic web is the largest pattern found in nature. The cosmic web is the largest pattern found in nature.
The cosmic web is a thing that is made of galaxies. A thing made of galaxies. It's the same way your body is made of cells if your cells were a million times smaller. It's actually kind of hard to generate analogies here of just the enormity of the cosmic web. It's a feature.
It's a thing. When we zoom out on these huge scales, we see that galaxies aren't scattered around randomly. No, they form a structure, a pattern, they form this cosmic web. There are long thin ropes of galaxies. There are dense knots.
There's broad walls. And then in between all those filaments and snots and structures and walls are vast empty regions. The voids. In fact, most of the universe, by volume, is in void. Most of the universe is nothing at all.
Most of the universe all is these giant empty gaps between the galaxies. And to get a cosmic web, you need to grow one. You need to spin it. The universe hasn't always had a cosmic web, and in fact, in the future, eventually, the cosmic web will go away. The universe a long time ago, like thirteen and a half billion years ago, was was smaller, hotter, denser, and just a soup of stuff, of dark matter, hydrogen, some helium, some lithium, and that was it.
And there was almost no structure in this soup. The universe was pretty uniform. It was pretty thin, or it's just uniform, homogenized early universe. But there were tiny, tiny little differences here and there. They're just little random pockets with a little bit more density than average.
And then as time went on, gravity just did its thing, and that extra little slightly higher than average density pocket would attract some of its neighbors, and it get a little bit more dense, a little bit bigger, which will give it more gravitational attraction, which will draw on more of its neighbors and more and more and more and more and more and soon enough and soon enough being, you know, a billion years, you start building things like stars in the first galaxies. And then those early galaxies start merging together. And then those galaxies form clusters. And then more galaxies join on to those clusters. So as time goes on, you start from microscopic variations in density from place to place, and you build and build and build and build to get the biggest structures.
So what we see as the cosmic web today is really the outgrowth of a process that was kicked in motion thirteen point eight billion years ago and started from very humble, very microscopic origins into something this grand and beautiful and complex. But to build the cosmic web, to build the filaments, to build the walls, to build the clusters, you have to remove material from somewhere else. So, as the cosmic web grew and was spun and began to form, the voids began to open up for the first time. And, as time went on and more material collapsed into the cosmic web, the voids opened up even more. And fast forwarding far into the future, eventually, the cosmic web will get ripped apart because of the accelerated expansion of the universe.
Eventually, there won't really be as such a thing as a void because the voids are defined by their boundaries, by the filaments and the walls. And if you break down all the walls, then it's either there's no void anywhere or the whole universe is a void. It's, you know, your choice. So this cosmic web has different parts. It has voids, but also has clusters and walls and filaments and, of course, the individual galaxies themselves.
And we're very interested in the cosmic web. Because the cosmic web is our cupcake. This is our universe. This is the one we get. Our universe has certain ingredients.
It has a certain recipe to make that cosmic web. What do you need to make a cosmic web? Well, we've been studying it for a while. We know you need dark matter. We know you need hydrogen for the galaxies and helium.
They'll eventually convert into heavier elements. And you need to the recipe. Our recipe right now is, gravity and time gives you a cosmic web. But that's our cupcake. This is what we can observe.
The one universe that we're allowed to observe. And we have to compare that. We have to do testing. We have to do science on this one universe. And how do you do it?
Well, you make pretend cosmic webs. Right? You do this in simulation. You follow the physics of gravity as it acts on things like galaxies. And you evolve it.
And you start changing the recipe. How much dark matter? How much dark energy? A little bit of hydrogen or maybe there's a bunch of neutrinos in the mix. Do we need to cook this for thirteen billion years?
Or is ten billion years enough? You you keep messing with the recipe, the ingredients and the instructions until you get something that resembles the actual universe that you see, until you get the actual cupcake that I gave you. The universe gave us one cosmic web, so we gotta study it. We have to compare it against our simulations and our predictions. So you gotta slice it up.
How do you characterize? If you just if I just gave you a spider web and said, here, characterize it. You know, I'd say, okay, how many little knots are there? How far away are the knots on average from each other? Or you might turn to the gaps, like, okay, on average, how big are the gaps?
How wide are they? Are they clustered together? Were they all kinda scattered randomly? Most commonly, when studying the cosmic web, scientists look at the galaxies because, you know, galaxies are big and bright, and you can see them really well. And we do various statistical tests that will not drag you through to study those galaxies.
But what about looking at the not galaxies? The not galaxies, the places where there aren't galaxies. That might be a little bit useful. It might be, and this was the area of my research, was instead of studying the properties of the galaxies and their relations to each other, it was about studying the voids, the gaps. And these voids are very interesting creatures indeed once we looked into it in more detail.
They're not totally empty. Of course, the universe would never be so simple as to do that. The interiors, the deep cores of the voids are indeed very very empty. Like, may there might be some stray hydrogen gas floating around, but no galaxies, nothing like that. But then as you get closer and closer to the cosmic web, the amount of material starts to raise.
You might see more gas, you might start seeing a few stray galaxies floating around, and then you hit the wall. Then you hit the filament, the boundary, the the actual wall, the cluster, the boundary of this void. And so the voids themselves are surrounded by these shells of material. And the voids themselves, are have a variety of sizes. So the smallest ones are around, 20,000,000 light years across, and the biggest ones, we're talking two, three hundred million light years across.
These are giant beasts. The great voids really are that big, but most of the voids are relatively small, only 20,000,000 light years across. Give you some perspective, the distance between Milky Way and Andromeda is about two and a half million light years. So you 10 times the distance between us and the nearest neighbor galaxy, that's your minimum size for a typical cosmic void. Now these voids are juicy.
Even though they're empty, they tell us something. They tell us something very powerful about the early universe. I mean, the early universe was 13,800,000,000 years ago, and there is a direct physical connection between what went on in the early universe at this you know, think of the room you're standing on. That room has been there since the early universe. That that space, that coordinate in space has been there for a very very long time.
And if you re round re round the clock, at one point that space was filled with a bunch of plasma, and then it went away, and then it was filled with a bunch of neutral hydrogen, and then it clumped together. Maybe you were a part of a star at one point or embedded inside of a galaxy, and then you ended up being on a planet in a room. That's a lot of physics. That's a quite a journey to get from the early universe with just a bunch of hydrogen floating around to your room. That's beautiful physics.
That's very cool physics, but also very complicated physics. Physics that's very difficult to untangle that full story. Like, yeah, we know the general picture, but I can't give you the exact play by play rundown of getting from the early universe to the state of your room right now. That's not true with the voids. The voids have stayed pure.
They they don't change much. They just kinda empty out over time. That's pretty straightforward physics. It's not very exciting physics, but it is very useful physics. So when we look into a void, the void looks back at us.
Sorry, I couldn't resist doing that line. When you look into the void, you're really looking through a window into the state of the early universe. The voids simply haven't changed much in thirteen point eight billion years. Yeah, they do change. Yeah, they do empty out.
But they change much, much more slowly than the galaxies. The galaxies are fantastically complicated places and have evolved through very complicated physics. They have quite the life story behind them. Voids, not so great memoirs. I was born, I emptied out, and here I am.
That's the life story of a void. So that's why voids are one of the reasons voids are so useful because they you can look in them and you can kinda see what the universe was like billions of years ago, right there in front of your very eyes. And, I keep saying voids are empty. Voids are empty. Voids are empty.
Mostly empty. Really empty in the centers. Not so empty at the edges. Okay. That's true.
They're devoid of matter, which means they're full of dark energy. Dark energy is a property of the vacuum of space time as far as we can tell. If you have a box of nothing, you have a box of vacuum, you have a box of dark energy. It's right there. It's present in the fabric of space time itself.
So if you're interested in dark energy, you wanna go to places where there's lots of dark energy. But if you look inside of a galaxy, well, there are stars and magnetic fields and different colors of crayons that didn't exist when I was a child and just all sorts of stuff. Right? All that stuff is sitting on top. It's masking the dark energy.
But you go in a void where there's nothing? Dark energy's all you got. It's it. Cosmic voids are dominated by dark energy, and a galaxy is not. So, if you really, really want to study dark energy, let's go to the places where dark energy is there and present and powerful.
Don't go to the galaxies. It's all confusing there. Don't go to the big cities. Come back out to the Midwest Cornfields where life is boring, but at least you can understand it. That's the story of cosmic voids.
They're just empty, but they're really full of dark energy. They're laboratories of dark energy. They're places in our universe where dark energy is at its full strength because there's nothing to get in its way. So let's look to the void. We've mapped, we being, like, me and a few other people, mapped a few hundred a few thousand voids in our universe.
So what we do is, of course, we do need to rely on galaxies because we're using those as the boundaries. And so there's a lot of galaxy surveys operating out there, just pinpointing locations to galaxies, gathering data about them, moving on to the next thing. And those surveys provide maps, and in the maps, you can generate computer algorithms that, again, I'm not gonna drag you through to identify the cosmic voids. And most of these voids are, you know, nothing entirely special, but there are some voids that deserve special mention. There's one void, one of the earliest voids found, called the Bootes void.
It's in the direction of Bootes, the constellation, hence the name. It's a big void. It it's large. And there's this kinda lingering question with the Bootes void and the larger voids. The voids are, like, a hundred million light years across.
Is that, you know, imagine I give you that cupcake and you start biting into it and there's, you know, a a giant gap in the cupcake. And try as you might with different recipes and ingredients and processes, you can just cannot get a cupcake with a giant hole in it. And so you start to wonder if your recipe for how the universe works might be a little bit off if you never ever seem to get a giant hole in your cupcake. Well, the Bodys void and a couple other voids are big, big voids. Are they too big?
It's on the fence, honestly. I mean, this is ongoing research right here. Could the universe as we understand it, with its recipe, with its ingredients, ever make a cosmic web with that big of a void? Maybe yes, maybe no. There's something else I wanna mention, but there's a couple other things I wanna mention about voids, but especially cosmic voids push.
They repel. They literally pull on the surroundings. So it's not only that as the cosmic web grows and congeals and forms that it pulls on nearby material from its own gravity. So if you have a dense knot, a cluster, and now it has really strong gravity, it can suck more material on and make itself even stronger. But it's not a one way street.
The voids are also pushing material away from themselves. Remember, they're full of dark energy. And what does dark energy do? It pushes. Dark energy is accelerating the expansion of the universe.
They're accelerating the expansion of voids. They are literally voids are literally pushing on the galaxies at their edges to just give me some space. They repel. It's pretty cool. Like I said, the voids aren't entirely empty, and I'm gonna use a word here.
It's an f word. This word is a four letter word in cosmological circles, and I'll explain why. But I'm gonna say it for a reason because there's something else really cool about voice I wanna tell you. The word is fractal. Fractal.
There was a sense way back in the sixties and seventies and even going into the eighties that now, what does the universe look like at the large scales? Before we had all these galaxy surveys, just like how are galaxies arranged? Is it totally random? Is there any sort of structure? And there were a lot of proponents, especially early on as we started doing these early galaxy surveys in order to get hundreds or maybe even thousands, if you were lucky, galaxies.
There was a sense that maybe the universe is a fractal. And the guy who promoted this idea was Benoit Mandelbrot, which, yeah, mister fractal himself was thinking that the universe might be a fractal. What that means is that if you zoom in on a little part of the structure of the universe, you might see some some structures, you know, some stuff going on. And then when you zoom out, you see the same pattern. It's not right repeated necessarily, but it's, like, pretty much the same.
And then you zoom out even further, and you get pretty much the same thing. And you keep going and going up or down, it doesn't matter. You always get the same kind of pattern. You get the same kind of pattern regardless of how much you contribute to Patreon. Go to patreon.com/pmsudder to learn how you can keep this show and all of my education outreach activities going.
I truly appreciate it. I really do. I can't thank you enough. Now, fractals. Our universe is not a fractal.
Our universe is not a fractal. We do not live in a fractal universe. As we did more galaxy surveys, we realized Noah had this web like structure that is very much not a fractal. Sorry, Benoit. It would have been awesome, but, you know, evidence.
But the voids do something funny. And this was only revealed first in computer simulations and later backed up with observations that if you take a void, like, take a typical void, void and you look at it, it looks pretty empty. I mean, there's galaxies in there, but not a bunch. But if you look at that cosmic void and you highlight the positions of all the galaxy, you really dig in where are those where are all those galaxies, the galaxies inside the void aren't scattered around randomly. It's a mini cosmic web inside the void.
It's a dimmer, fainter version of the full cosmic web, but it's there. And then we continued other us and other researchers continued along this line of thinking with simulations to dig even deeper. Now you have a void, and you look inside of it, and it turns out there's, like, a mini faint cosmic web inside of the void. So you go to one of those sub voids and look inside of it, and what do you see? You see an even dimmer, quieter, a faint glimmer of a cosmic web, but a cosmic web nonetheless, inside of that void.
And you can actually take this down a few levels depending on how big of a void you started with. Voids nest inside of each other. Voids are kind, sort of, fractal. Voids within voids within voids. Cosmic webs within cosmic webs within cosmic webs.
This does not mean the cosmic web itself is a fractal. There's an upper limit. There is the cosmic web, and there's nothing bigger. And there's a lower limit too. Once galaxies start interacting with each other, you get too close, then that destroys a lot of this cosmic webbiness.
But for certain scales, inside the voids themselves, you get voids within voids within voids. You can study voids directly by, you know, looking at them, but they're not just passive players. They also, you know, do stuff. And, especially, they do stuff to light that passes through them. Say a beam of light enters a void.
So it enters the void. It's now deep inside the void, and it travels through, and then it exits. If the entry is the same as the exit, then the light will return normal. It might gain some energy as it falls into the void, and then it climbs out of the void and loses the exact same amount of energy. So while it's traveling through the void, it's changed, but any price it paid to get into the void, it gets a refund on the way out.
But what if the void changed while the bit of light was making its journey? What if the bit of light paid $10 to enter the void, and then when it left, it only got $9 back? It'd be a little bit different. It would've lost some money. It would've lost some energy.
It'd be a little bit cooler, a little bit lower energy. This is something called, and this is a wonderful name, the integrated Sax Wolfe effect because, of course, that's his name, that the presence of a void can affect the light that passes through it. And what's the best light to use to shine through a void? Why, the cosmic microwave background itself. The cosmic microwave background sits surrounding the cosmic web on all sides, encompassing it as this light, this fossil light from the very, very early universe, and that light has passed through voids on its way to us.
And if you start changing the recipe of the universe, how much dark energy, how much dark matter, how long to cook it for, all that, you change the cosmic voids, and you change this effect. Probably the biggest example of this effect is there's a spot on the cosmic microwave background called the cold spot. It's a spot that's cold. By itself, the temperature of that cold spot isn't so cold, and by itself, the size of that spot isn't so big, but its combination of size and coldness taken together, it's a really, really weird spot. It's so weird that we can't get any of our recipes for the universe to replicate it.
We just can't make a universe with that big of a spot in the cosmic microwave background unless you give it a void. There was some work a couple years ago that realized, plausibly, possibly, that there is an enormous void, a gigantic void, not a very deep one, a relatively shallow void, but a very, very big one, and that the light from the Cosmic Microwave Background has to trudge through all this void to get to us, it ends up paying a very, very big price. It ends up being really, really cold by the time it gets to us. So voids aren't just cool to look at, aren't just cool to think about. They also interact with their surroundings.
Voids themselves, in some way, are living creatures. Thank you so much for listening, especially ta big thanks to my top Patreon contributors this month, Robert r, John, Evan t, Matthew k, Helga b, Justin, Matt w, Justin g, Kevin o, Duncan m, Corey d, Kirk b, Barbara k, Nuder Dude, and Chris c. It is your contributions plus everyone else's that keep this show and all my education outreach activities going. That's patreon.com/pmsutter. You can ask your questions.
Drop me a line at askaspaceman@gmail.com. Hit me up on social media. Use the hashtag askaspaceman. I will catch it. You also buy my book, pmsutter.com/book.
And I will see you next time for more complete knowledge of time and space.