How do we actually define voids? Are there regions within them that are truly empty? What would it be like to be inside one? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

I have to admit, I would have never thought when I started this podcast that we'd end up here at episode 250. I mean, I suppose I understood that fact in an abstract sense that, yes, if I kept the show going, then eventually the numbers would tick up and and I would reach 250. But I suppose also that I never thought of it in concrete terms. That someday, years after I started, we'd still be exploring complete knowledge of time and space. And I'm perfectly aware that this is technically episode 251, and I meant to do this last episode, but I got carried away with the constants of nature stuff, so here we are.

And on these landmark episodes, I like to be a little self indulgent. Not entirely self indulgent because I have several perfectly real invalid questions regarding today's topic, but it's a little self indulgent because that topic is indeed a subject near and dear to my heart, the vast emptiness that dominates our universe. You know it. Everybody's favorite, the cosmic voids. Believe it or not, I've been working off and on with voids since the inception of this podcast.

Our first episode launched in January of twenty fifteen, and at that time, I had just finished a research stint at the Paris Institute of Astrophysics, where I was working almost exclusively on voids. As the years have gone on, I've put down void research, picked up other kinds of research, dove head first into science communication, done all sorts of wild things I would never thought possible, like writing books and getting exotic cheese recommendations, but I never quite let go of voids. Even if my primary interests, research and otherwise, pulled me in different directions. But even though the universe may be flat, our experience of it can be curved. So here I am in my own future, roughly back where I started, holding a research position focusing on voids, this time based at Johns Hopkins University, and recording an episode of Ask a Spaceman.

There's a certain symmetry there that I appreciate. So the question for today is, what's inside the voids? Like, really inside? Are they actually empty? And what would it be like to live inside of one?

To answer that question, we have to first decide what a void is. I know it's easy enough to describe in big, broad, vague terms. It's it's the places in the universe where there isn't much stuff. When we look out at the very largest scales in the universe, we see that galaxies aren't just scattered around randomly, which we thought that was the case for a very long time. No.

There's a pattern there when we go out to tens of millions of light years, hundreds of millions of light years when we conduct these massive surveys of galaxies where we just locate distant galaxies, and all we can see in them is a point or a smudge of light. That's how far away the galaxies are even with our most powerful instruments. And we simply record their position. Okay. They're in that part of the sky, and they're that far away.

So we're gonna put a little pin on a map there. And then we do this again and again and again with galaxy after galaxy. When we do this enough and get enough faraway galaxies, enough broad sample across the sky of galaxies, this pattern emerges that we call the cosmic web. And it's a very evocative name because it it kinda sorta looks like a spider's web. There are long, thin filaments, which are made of galaxies.

These things are millions of light years long. There are the clusters of galaxies where the filaments intersect each other. There are these broad two dimensional walls. And then there's all the knot stuff, the knot web, the anti web. And these are the cosmic voids.

And it's easy enough to look at a galaxy survey. You can pick out the voids by eye, which is how we first identify when we started doing galaxy surveys. They're like, well, what about that batch right over there? There are no galaxies there. And turns out, those are real things, real empty places called the voids.

And it's easy enough to describe in big, broad, vague terms how the voids got there. Long time ago, the universe was much more uniform. It was much more evenly spread out. The densities from place to place were pretty much the same. But then as time went on, tiny little clumps grew into bigger clumps, grew into stars and galaxies and clusters, and that stuff had to come from somewhere.

And so the voids are like the quarries that we use to dig out the rocket to build our buildings. One of my favorite examples was, Paris where I did a lot of work on voids. And, yeah, Paris, the city of light is this gorgeous stone that they use in almost all the buildings. Underneath Paris is a giant void where they dug out those rocks and put them above ground to build the city. In fact, the Paris Institute of Astrophysics, we had our own secret entrance to the Paris Catacombs.

If you've been to Paris in the Catacombs, you're actually not far from the observatory. And the Paris Institute of Astrophysics, we had our own secret entrance. We also kept our computers down there. Long story for another day. The voids are the places where the universe mines the material to build the cosmic web, and that leaves these big giant holes.

But that's big broad vague terms. If we're going to be specific, and this is science, so being specific is a big plus. We need more than these, yeah, yeah, yeah. There's a void over there, and I kinda sorta know how it got there. If I want to really understand the voids, I need to measure them.

I need to quantify them. I need to ask and answer very specific questions like, what is the edge of a void? What's the boundary of a void? Where is the center of the void? How can we categorically point to one place in the universe and say, that's inside of a void and another and say, nope.

Not that. That's not inside a void. How do we make that distinction? How do we make that decision? Well, we have an answer for you.

It's called VEED, which comes from the French word for empty. Our team developed this in Paris, which is how I thought of the name. And of course, it's a silly acronym for void identification and examination toolkit. The toolkit isn't part of it. That's just it's the v you get the idea, v I d e.

And it's just sort of nerdy thing that us scientists get excited about. It's also free and available to use if you ever want to go void hunting yourself, if you ever want to if you do your own galaxy survey or computer simulation of the universe and you want to know where the voids are, or you can download and install Veed and and run it, sadly, I will not be devoting any Ask a Spaceman episodes to the technical details of how to use it. You're going to have to read the manual like everybody else. So there are a lot of ways that you could potentially define a void, where you can have an algorithm, where you can have a methodological procedure for deciding where the voids are, how big they are, where they're located, what their edges are, where their centers are. There there are a million choices you could make.

For our approach, we chose one particular method because it seemed like a good idea at the time. And as the years have gone on, it has proven to be a rather good idea indeed. That's due to part dedicated effort on behalf of our team and part just sheer luck. In fact, today, Veed are this tool we made for finding voids in the universe. It is the number one void finder in the world.

Chances are that if you catch a new research paper or you see an article or news story about voids, and I know you subscribe to all the hot cosmology gossip, If the research is about voids, there's a very solid chance that research used Veed, our methods and tools, as its backbone, which is pretty neat. So here's how we decided to define a void. If if I just give you the matter in the universe, here you go, cosmic web, how do you define a void? How do you find it? How do you extract it?

You start with a collection of matter. It could be a galaxy survey where you've pinpointed the locations to a bunch of galaxies. We know that galaxies are not the only kind of matter in the universe. There's also the dark matter, which is a little bit harder to see in galaxy surveys. So you could also start with a computer simulation of the universe, of the cosmic web and how it got there that includes information about the hidden dark matter.

It, you know, it doesn't matter. You just start with your collection of where the matter is in the universe. Then you imagine that this map is a topographic landscape. Like like if you instead of looking at a galaxy survey in three dimensions, you were just looking at, like say, you know, a continent. And you saw mountains and valleys.

And we're gonna draw an analogy here where the mountains in our imaginary map are the places of high density in the universe. So if there's a cluster of galaxies, that's like a mountain peak. And if there's a filament, that's a mountain ridge. And if there's a void, that will be a broad valley. So we're going to imagine we're we're instead looking at a topographic landscape map.

Yes. This is in three dimensions, technically, for void finding, and it's kind of hard to imagine mountains and valleys in three dimensions. That's why it's just an analogy. The math takes care of all of that. We can just imagine we're looking at a topographic map, and if we see a a ridge of mountains that corresponds to a filament of galaxies.

And we will look at that river plain down there, like, okay, that's a void. Now we're going to imagine dropping rain onto that map. A cloud comes in, drops rain over the continent, and the raindrops are going to flow in certain directions. They're going to collect in the valleys, in the basins. Where those rain drops collect, those basins, those low density valleys are going to be the void.

So we follow the raindrops as they flow down into the valleys and we just search. We ask where do these raindrops go? Oh, they go in these basins. Those are the voids. So this is how we pick out the voids from our topographic map of the universe.

And then to get the edges of the voids, we look for places that split the flow of rain. Like, if you imagine a a mountain range and you're dropping rain all around the mountain range, some of the drops will flow in one direction and head to the valleys on that side of the mountain And other raindrops will hit the other side of the mountain and go to the valley on that side of the mountain. That ridge line, that top of the ridge that defines the mountain range, that is the separation point between raindrops between separate raindrops and defines where water flows. In topography, we call this a continental divide. If you're on one side of the continental divide, then all water flows eventually goes into one direction.

And if you're on the other side of the continental divide, then water goes in the opposite direction. So those ridge lines are the boundaries of the voids, and we can identify them. We can mathematically, algorithmically, computationally identify those ridgelines by following where the raindrops could go. This exact same mathematical technique is used in a lot of different applications. It is used in image processing.

It's a very easy way for computers to chop images up into different sections. It's also used in topography and map making. This is if you've ever encountered the term watershed or watershed basins. This is how those are defined. This is, like, if, like, oh, yeah.

Like, you're not allowed to pour, like, raw sewage into this sewer grate because the watershed flows into this protected area or reservoir area. That's the same idea that if you identify where water flows, you're finding the low level basins. And that has real life applications. And it also has cosmological applications where we use the exact same techniques, the exact same mathematics and computational algorithm not to find where water flows, but to find the low density regions of the universe and where their edges are. This method is really really nice because it automatically works for any kind of input.

It doesn't matter if you're doing a galaxy survey. It doesn't matter if you're doing a computer simulation of dark matter. It just works. It's the exact same technology that just gets applied over and over again. There aren't any, like, tunable parameters that you need to adjust and fiddle with to get things right.

This is one of the reasons we cooked up this idea is because it was so flexible and so powerful. You'd once you have the algorithm in place, it just runs. It just finds the voids. It's like the dumbest, simplest, most elegant solution we could think of to simply find the voids. And it helps us pick out voids within voids within voids.

We know that there is this substructure inside of voids where if you look at a giant void in the universe, if you look carefully, especially in computer simulations, you can see these threads of material, these dwarf galaxies inside of the void like a faint echo of the cosmic web. And so you can chop up that big void into sub voids. This algorithm finds it all. So that was really attractive to us when we first started developing this right before I started this podcast. And it turns out to be a very powerful set of ideas even today.

And this is what Veed, our tool, does. It takes in a collection of matter and spits out where all the voids are, how deep they are, how far they extend, where their centers on, and so on. It gives you a catalog of voids, a collection of voids. You say, okay. Here's my survey.

Here's what I've mapped of the universe so far. Can you tell me where the voids are? Here you go. That's what v does. It Chews it up for a little bit and says, here you go.

Here are here are all the voids. So now that we have a collection of voids and we've defined what a void is, we are now defining voids to be the low density valleys, the basins in this watershed raindrop approach where we turn a galaxy survey or dark matter simulation into a three-dimensional topographical map and find where water flows. That is how we are categorizing a void. We can finally ask now that we have a bunch of voids, if we crack them open, what do we find inside? And yes, my friends, what we find inside like a gigantic cosmic Kinder egg is a Patreon ad.

That's patreon.com/pmsutter. I am so grateful for all of your contributions truly. That's patreon.com/pmsutter. Thank you so much for making 250 episodes possible so far and at least 250 more to go. No.

The real answer is a whole bunch of nothing. That's the whole point of all this technology, all those algorithms to go from what is visually obvious, like, it's a hole in the galaxies. It's there's no galaxies there. That's easy and obvious. And then this tool says, yes.

You were right. Your intuition is correct. And now here is the mathematically defined boundary and center and everything you need to know about the void. It's shape where it it it extends everything you need to know matches up with your intuition but provides a rigorous mathematical treatment for it. But the thing is they're not completely empty.

I like to think of voids as deserts. And in fact, I am advocating for a naming campaign. We've we've identified, like, tens of thousands of voids in in the universe so far. And most of them just have catalog designations, you know, just numbers and letters piled together. But some of the nearby voids have names, and because they were discovered in the seventies, eighties, and nineties.

And no almost always, they're named for the constellation that you have to look through to see them, but we're starting to get more and more nearby voids, and I and I my campaign is we should start naming them after deserts because that's kind of appropriate. But that's that's that's on me. I'll you know, I don't get to be in charge of naming a lot of things, so I'm trying to do that one. But like deserts, they're not totally empty. There's not a lot of rain or life or cheese co ops, but there are still succulents and small mammals, desert adapted reptiles and insects.

There's still stuff in a desert, just not a lot of it. And there's still stuff inside of a void, just not a lot of it. In voids, you see dim dwarf galaxies with threads of gas connecting them. We can't directly see the dark matter, but when we build computer simulations and find the voids in there, we see these mini cosmic webs. This hints of a somewhat fractal nature to the cosmic web.

But it's not a lot. Just like a desert, don't you you don't walk into a desert expecting, you know, a lot of life and a lot of action. You don't go walking into a void expecting a lot of matter and a lot of galaxies. If you to give you a sense of the typical density inside of a void. If you average out the entire volume of a void, you get typically around the density of the material that's in the void is around 20% of the cosmic mean density.

So a typical void on average across its entire extent is about one fifth the density of the whole entire universe on average. But that's not a lot. Because the vast majority of matter in the universe is crammed into clusters, filaments, walls, galaxies. The vast majority of the matter is crammed into a very very small volume. If you were to smooth everything out throughout the entire universe, you know, you plucked apart every cluster, every galaxy, every filament, you took every atom, everything, and smoothed it out across the entire volume of the universe, which is 92,000,000,000 light years across.

If you smooth it all out, the average density of the universe is around one hydrogen atom per cubic meter. So spread your arms out about yay big, Imagine drawing a cube that big. That's about a meter. Cube on the side and place one single hydrogen atom in that box. That is the average density of the universe.

Inside of a void, the average density of a void is one fifth of that. That's pretty dang low. And that's on average throughout the entire extent of the void. That includes the borders. That includes the boundaries of the void that butt up against the walls and clusters and filaments.

That still get included in the definition of the void. That's like the entry way into the void where the density has started dropping. Just like you can enter a desert, and it's not like there's like a line right there where there's like lush jungle or ocean or whatever biome you feel like, and you and you just take one step and immediately you're in in a desert. No. There's a gradual transition into the desert.

And then depending on how you define the boundary of that desert, it might be really far in where it's only the real deep desert or it might be somewhere in the middle. This watershed ved technique that we've developed puts it somewhere in the middle, which seems which seems kind of appropriate, works for us. And so that includes that slight not deserty stuff at the very edges. And so the average density of of a void includes some of the the not so the slightly more dense stuff at the very, very boundaries of the voids. But it is possible to reach deep into the voids.

Just like it's possible to reach deep into the deserts and find truly lifeless portions. Like the Katara Depression in the Deep Sahara or the Empty Quarter of the Arabian Desert or Badwater Basin in Death Valley. There are deep regions of the void, every void, that are essentially devoid of matter where there's nothing. There are no dwarf galaxies. There are no threads of material.

There are no hydrogen atoms for a 100 light years, a thousand light years, maybe a million light years in some of the deepest regions of the voids. If we were to place our solar system there, honestly, the entire trajectory of modern civilization would be different. There'd be no stars in the sky. There'd be the sun, the moon, the planets. That's it.

No stars, no nebula, no Andromeda galaxy, no nearby galaxies at all? We may if if the Earth or the solar system was placed inside of one of these deep voids, why would we ever invent the telescope? Or even if we did, say Galileo did his thing, looked at the planets, saw, you know, the Galilean moons, the phases of Venus, all the usual stuff. Why would we build a bigger telescope than that? Once you can pick out those details, if we were to place the solar system deep in a void where the nearest galaxy is a million or more light years away.

If we were to place the Earth and the solar system in the deepest part of a void, where the nearest galaxy is tens of millions of light years away. We may never have the urge or knowledge to try building a bigger telescope to try to see one of those galaxies. And without advances in telescope, you know, you you may not get this something like the scientific revolution. Certainly, you don't get modern cosmology. You would have no idea.

Maybe if you developed microwaves, you might discover the cosmic microwave background. I'll get to that in a little bit because that's still there. But if you have no desire to explore scientifically the electromagnetic spectrum, then you would never build a microwave antenna, and so you'd never see the cosmic microwave background. You would never discover that there are galaxies very far away from you. You would never discover that the universe is expanding.

Your whole universe would just be your solar system. The voids are empty. We're lucky that we're surrounded by stars, that we live inside of a galaxy, that we have giant galaxies relatively nearby. Because this fuels advances in our understanding of the universe which turn into technological marvels. Sure.

You can imagine a hypothetical scenario where we develop advanced detectors if we were to live inside of a void. And we would catch what would we see. We may not see distant galaxies. We may never have the desire or need or justification for building large enough telescopes to to see the nearest galaxies. But we maybe we build a particle detector.

I mean, or maybe we still figure out quantum mechanics, so we start building bubble chambers. What we would occasionally get cosmic rays, neutrinos, particles blasted out of cosmic collisions that are just flying through the universe. You know, in a galaxy, we're we're pretty well soaked in cosmic rays, pretty well soaked in neutrinos. That's because these are all sourced from galaxies and nearby galaxies, from active galactic nuclei, from supernova, from magnetars glitching, all the good juicy stuff that blows up in the universe and creates lots of energetic events. Those are still happening.

But if you're in the middle of a void, they're all happening far away. And so you just don't cross paths with those particles as much. They're still there. Like, if if you look in the deepest region of a void, yes. Cosmic rays are passing through.

Neutrinos are passing through. Dark matter, occasional dark matter particles. Oh, and oh, yeah. Don't worry. The deepest parts of the void are empty of dark matter as well.

There there really isn't very much dark matter in the centers of the voids. We know this. We've measured it. We looked. We checked.

But occasionally, once every few millennia, dark matter particle swings by. Once every century, a cosmic ray would zip through. But of course, nothing is completely empty, and that's because there's always your constant companion. No, not me. I only visit twice a month.

It's the cosmic microwave background. Completely soaks the universe. It's the leftover light from when our universe is 380,000 years old, cooled off to go from a plasma to a neutral state that radiation hung around. It's responsible for something like 99.999 something something percent of all the light in the universe. And you can't escape it even in the deepest voids.

The loneliest spot farthest from any bit of matter. If you were to build a microwave antenna, you would detect the cosmic microwave background. It's there. I mean, it's energy density of the radiation. Radiation doesn't carry a lot of energy, especially compared to matter.

And then it's been so redshift and so diluted. The energy density of the cosmic microwave background is, like, a millionth of the average density of the universe. Used to be a big player back in the day, you know, thirteen billion years ago. Now it's just in the literal background. So the cosmic microwave background, it will be there.

It is present in the void. The the those photons, that radiation, it is present in the deepest reaches of the voids. It's not gonna do much for you. It's gonna prevent you from cooling below three Kelvin or so, which is nice, but that that's about it. Besides the cosmic microwave background, besides the wandering neutrinos, cosmic rays, dark matter that occasionally make their way through these deep deserts, there are some hints of magnetic fields that were either created in the early universe or created inside of galaxies through energetic events and then blown out into the voids.

We're not exactly sure how far those magnetic fields persist into the voids. It's not clear. We don't know yet if the deepest regions of the voids have no magnetic fields at all or they have something left. It is a pretty lonely place inside the voids. But there is something.

There's the vacuum. Dark energy is always present even in empty spaces. In fact, the voids are where dark energy calls its home. Here in a high density environment like the galaxy, dark energy is in the background. The the amount of matter, the amount of radiation, the amount of energies here in a galaxy completely swamp anything that dark energy is capable of.

Go out in the void to get rid of all the stuff. All that's left is dark energy. The accelerated expansion of the universe is happening in the voids. That's where that work is taking place to accelerate the expansion of the universe. And that's always present.

The very next episode is gonna be about vacuum and vacuum energy, so that's perfect segue into that. So even though the voids can be a very lonely place, and I should know I've stared into them enough in my life, it is comforting that even in the deepest reaches of the voids, the deepest deserts, the farthest from the lights of the galaxies, no matter where you go in the universe, far from the fading lights of the galaxies, or even the scent of matter and radiation, you're never quite alone. Thank you for joining me on these 250 episodes of Ask a Spaceman, and there is so much more to go. Thank you to Sue t, Richard p, and Stefan h for the questions that led to today's episode. Keep those questions coming to askaspaceman@gmail.com, or go to the website askaspaceman.com.

Please keep dropping reviews on your favorite podcasting platform that really helps the show visibility. But number one is questions. Number two, Patreon. Hey. Patreon.com/pm.

Sorry. It's how this show keeps going. It's how we've gotten to episode two fifty and how we will keep going. I'd like to thank my top contributors this month. They're Justin g, Chris l, Alberto m, Duncan m, Corey d, Michael p, Nylas m r, John s, Joshua, Scott m, Rob h, Scott m, Louis m, John w, Alexis, Gilbert m, Rob w, Jessica m, Jim l, David s, Scott r, Heather, Mike s, Pete, h, c, vest, what what bird, Lisa, r, koozie, Kevin b, Michael b, Eileen g, Dante, Steven w, Brian o, Deborah h, Deborah a, and Michael j.

Thank you again to everyone for all of your support, all of your downloads, and especially all of your questions. And I will see you next time for more complete knowledge of time and space.

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