What is the meaning of the horizon of the universe? Is the Earth in a special place? What’s the “dark flow”? Can this explain dark energy? Why don’t we believe it anymore? I discuss these questions and more in today’s Ask a Spaceman!
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what's outside the universe. Now that's a fun question, right? And it must be a fun question, because I get it all the time. And it's just one of those questions. That is the latest iteration of the kind of question that people have been asking since basically forever. We keep wanting to know what's beyond the boundaries of what we already know, right? We want to know more than the limits of our current knowledge. We want to keep pushing boundaries. What's over that mountain range? What's across the river? What's beyond the horizon? What's the deal with this new continent? What's on those other worlds? What are inside those distant Galaxies? We are just so dang adorably curious as a species, and we keep wanting to know more. So here we are, asking the ultimate boundary pushing question about the ultimate boundary, the very edge of the universe itself. And what lays beyond that. And it's just one of those questions that invites so much curiosity because what's outside the universe could be literally anything.
It could be nothing. It could be a void. It could be more stuff. It could be weird extra dimensions. It could be normal extra dimensions, and we're the weird ones. It could be another universe and on and on on. It's just so much fun to think about. But in order to properly answer the question, what's outside the universe? We need to make sure we have proper definitions. For all the words involved in that question, uh, what's and the are pretty much under control, which leaves outside and universe. So let's start with the word universe. If we want to know what's outside of it, we need to know what we're talking about. It doesn't help, and I'll be the first to admit that astronomers and cosmologists get super sloppy with the word universe and use it to mean different things at different times for different context. Sometimes in the same paragraph, they'll have the word universe mean different things.
And then, of course, I don't blame any reporter a any blog writer to see all these fuzzy uses of the word universe and then use that inappropriately or incorrectly, because the scientists themselves are using it all over the place to mean different things. And we we we'll see what that means in a little bit. But ultimately we want to answer this question. What's outside the universe? We need to know what the word universe means. So we can, you know, get to what's on the inside and the outside. So let's start with the first possible definition of universe literally everything. There is all the things if it exists. It's a part of the universe because the word universe encompasses all the things without question. That's just how it you know. If it's a thing, it's it's part of the universe because the universe is all the things. So in this definition there is no such thing as outside the universe, because outside is still a thing right outside is still a place outside is still a location.
Outside is still an entity, and universe is by definition all the entities. And so there can't be an outside. It's like if I want to define my home to be my house, the physical dwelling with walls and a roof and all that stuff, Then obviously my backyard is outside of that house. But if I define my home to be all the things that I own, my property, then there isn't an outside to my house to my home because the backyard is also included in the list of things that are part of my home. Even if you want to pretend that there is this great infinite void somehow outside the boundaries of the universe, well, great infinite void is still on the list of things, and so that gets counted as part of the universe. And so the question becomes meaningless because you can't have a thing outside the entity that contains all the things. What I just said can either make total sense or be utterly confusing, and also both at the same time.
It does make total sense if the universe is infinitely big. If we live in an infinitely big universe, then the universe is just all the things which is the infinity of all the infinities, and that's just the way it is. This is the way a bunch of Greek philosophers like to think about it, because if you if you think about like, if there's an edge to the universe, if there's a boundary, well, what do boundaries do? What is their job? Will they separate one thing from another? Well, if you have an edge and you're separating one thing from another. Well, that other thing is also a thing. And so it's a part of the universe. And so the universe can't have a boundary because the definition of boundaries is to separate two things. But two things are still things, and hence part of the universe of the universe has to be infinitely big. So that's that's fine. Like if if the universe is infinitely big, then the word universe means all the things and we don't need to worry about what's outside of it, because there is no such thing because it takes up all spatial extent. But if the universe is finite, what I just said about the universe, not having it outside doesn't make any sense at all.
Like if the universe had a defined spatial extent. If the universe had a finite volume, then it makes sense that it would have a boundary because things have finite volumes have boundaries, they have an edge. They have an inside and an outside. I can point to the earth and say you're either inside the earth or you're not inside the earth. You're either on the surface or you're not. That is a finite thing. It has volume and therefore has an edge. And so naturally you wonder. OK, if there's a finite volume to the universe, then what's outside that finite volume again, if it's I'll say this again because I know this gets super slippery really fast. If the universe is infinite, we don't care. It's there is no such thing as outside the universe because it's infinitely big. But if it's finite, you'll say, What's outside of that? That's the next question to ask. OK, it's so big. So what's past that? But here's the thing, folks. The universe doesn't work that way, and the universe really doesn't like it when you try to use analogies to understand it, like using the analogy of looking at the Earth to try to understand the finite volume of the universe, we don't know if the universe is infinite or not.
We don't know. We do know that it's very, very large. It's larger than the observable extent of what we can see. I'll, I'll talk about the observable patch here in a little bit. Don't worry, but we really don't know if it's infinitely big. We have theories of the early universe that we call inflation that predicted that the universe is 10 to the 62 times larger than our observable patch, at least 10 to the 62 or 10 to the 52. It depends on your exact theory of inflation, but it's somewhere around. There is 10 to the something large, bigger than our current observable patch of what we can see. That's pretty big. It could be infinite, but it could also be finite. We don't really know. And it's really hard to measure one of the ways that we can measure the size of the universe, even though we're limited in what we can observe. I'll I'll tell you now. I'll tell you now. The observable patch of our universe is about 90 billion light years across. That's the limit of what we can see. We we suspect that there's more universe out there, and it's up to at least 10 to the 52 10 to the 60 ish times bigger than that, and it could be infinitely big, but we can't measure it because it's literally outside of our observable passion.
We can't measure the true size of the size of the universe because the true size of the universe is hidden beyond what we can see. But we can measure the curvature. We can measure the global geometry of our universe, and we can use that to get some sense of its size or potential size. So, for example, well, this is like this is like trying to measure the size of the earth, being limited to your backyard and trying to measure very, very, precisely the curvature of your backyard to get some sense of the overall shape and perhaps size of the earth. It sounds really hard, but we can actually do it. The easiest way to do it is with light from the cosmic microwave background. That light has been traveling to us for 13.8 billion years. If there is any grand cosmological distortions of geometry, like if our three dimensional space was what we call flat, that means parallel lines stay parallel.
That means beams of light stay parallel to each other. If they start out parallel, then all that light that's been traveling for 13.8 billion years will stay parallel if light from the distant cosmic microwave background converges on itself. That means we have a closed geometry, a closed universe that would mean our geometry is like the surface of the earth, where parallel lines start parallel but then intersect. And if the light rays diverge, that's what's called an open universe. And that means that the geometry of our universe kind of sort of looks like a horse saddle, although in three dimensions not two, which I know is hard to think about, but too bad it just means parallel lines move away from each other. So, like that's a fundamental test of our geometry. And obviously, if our geometry is closed, if parallel lines eventually end up converging on each other like they do on the surface of the earth, those lines of longitude start out parallel at the equator and then intersect at the poles. That's how you know the geometry of the universes of the earth is closed.
Then there's some hope of measuring its spatial extent because we can measure the curvature here in our local patch and then extend out to the whole universe and figure out its size if it's open. If the geometry of the universe is open, that means our universe is infinitely big. There's no way around it. And if it's flat, it could be infinite, or it could be finite. The easiest way to think about a flat universe is for it to be infinite, like just that's it, like it just goes on forever. But you can have different Toppo that are flat. And this was the subject of, like my very first research paper, which I remember very, very fondly. Uh, the classic example is the surface of a cylinder. A surface of a cylinder is geometrically flat. I know that makes no sense at all. But welcome to mathematics. It's flat because you can draw parallel lines on them on the surface of a cylinder, and they always stay parallel. That is the definition of flatness. And so, you know, the surface of a cylinder is flat. So even if we were to measure our universe to be flat, it doesn't guarantee that it is infinitely big.
It could be closed. That's the word we used for it, where one dimension or more wrap up on themselves after a long enough period of time. Same thing with a closed universe just like the surface of the earth. You head off in one direction and eventually end up where you started. In principle, if our universe is finite, it means that you can head off in one direction on your rocket ship, travel long enough and end up back on Earth. It would take you a very, very long time, but it could in principle, happen. That's what it means for us to be in a finite universe. We've measured the curvature using the cosmic microwave background. It comes out to flat or nearly flat. I mean, it's just flat, with a small level of observational uncertainty. So we have a couple options here. Either. It really, really, really is truly flat, in which case the universe could be finite or infinite. It could be like a piece of paper, or it could be rolled up like a cylinder or the curvature. It could be truly curved, like a horse saddle or a beach ball. But on a scale so large it just looks flat to our measurements, like we are trying to measure the curvature of the earth from our backyard.
And yep, I got it in the backyard. It sure is Flatt there, you know that doesn't tell you enough information about the curvature of the earth. So basically, we've measured it to be flat, but it's wild West out there. The universe could be finite or infinite. It turns out, after all these measurements of curvature, uh, we haven't really nailed down the size of the universe, whether it's finite or infinite. If it is finite, it's on a scale much bigger than we can ever observe. If it's infinite, then it you know it's by definition on a scale bigger than we can ever observe. So we don't know. This big question about what's outside the universe hinges on what you mean by universe, and it hinges on the size of the universe. So in order to answer the question, what's outside the universe? We actually have to answer the question. How big is the universe? And the answer to that is, we don't know. But but this is where it gets totally loony tuned. So stay with me here. If the universe is infinite, then case close discussion is over. It's just infinitely big, and there's no such thing as the outside because it's infinitely big.
But let's assume for the sake of argument that it is finite. Let's just just say it's finite. Let's just let's let's go down that road like OK, what if some measurement in the future determines that the universe is finite? Then how big is the universe and what's outside of it? I want you to imagine a finite universe. You would probably imagine some object, like a ball or a horse saddle or a Mobius strip or whatever. But you imagine some object, and it's full of Galaxies and cosmic webs and co and voids and all that stuff. But it's it's a ball. That's how you imagine it. Guess what? You just played a trick on yourself. When you imagine, say, the Earth, I ask you to imagine the Earth from an outside perspective. You probably imagine the view of a of an astronaut, you know, orbiting serenely above the earth and you're looking down and you can see the whole globe in front of you. That's probably how you imagine a finite universe. In order for that trick to work, then you have to embed our three dimensional universe in a higher dimensional construct.
There has to be an outside of the universe in order to envision that, and in order for there to be an outside, there has to be a higher dimensional thing, like if you want to see the entire two dimensional surface of the earth at once, you have to jump out into a third dimension. Then you gain that perspective. You actually don't have an entire perspective of the three dimensional Earth because you can't see inside of it. But you can have a perspective of the two dimensional surface by jumping into a higher dimensional, three dimensional space. So in order to make the outside of the earth work and to be able to have that all encompassing perspective, the two dimensional surface of the Earth has to be embedded in a three dimensional construct. You have to jump up into dimensions in order to get that outside perspective. I'll say it again because I know this is such a headache. In order to see a two dimensional earth surface of the earth, you have to have a three dimensional perspective.
If you want to see and have that outside global perspective of a three dimensional universe, you need 1/4 spatial dimension. You need to jump outside in order for there to be an outside there has to be an extra spatial dimension. Now some exotic theories of physics do predict 1/4 spatial dimension that our universe is embedded in. It's called the bulk for various reasons, and our three dimensional universe is embedded in a four dimensional bulk. And, yeah, then there could be an outside to the universe where this bulky thing is. I mean, if you use the word universe to mean all the things, then that also includes the bulk. And but, you know, that's just headache inducing. If you mean universe to mean all the things in our three dimensional reality with our sets of laws of physics, then yes, there can be an outside in a in a four dimensional embedding that's allowed, but that's not required. We don't know if those theories are true or not, or accurate or not.
Here's the thing, folks. The universe could be finite, not embedded in a higher dimensional construct and still not have an outside the mathematics that we use to describe our three dimensional universe, our cosmology, our models, our theories. Everything that we use does not require an outside, even if the universe is finite, even if the universe has a volume. Even if the universe has an extent, it could have an outside if it's embedded in a higher dimensional thing and you're willing to not call that thing also the universe. Because guess what? If you start playing these questions like, 00, well, what's outside the universe? Well, it's embedded in a higher, dimensional bulk, and then you say, Well, wait, wait, wait, wait. What's outside the higher dimensional bulk? And you're like, Well, either it's infinitely big or it doesn't have an outside. You run into the same problems, so you might as well address it here. If our universe is finite, it does not need to have an outside. The mathematics are totally self consistent.
The mathematics make sense. The mathematics are crystal clear. You can have a three dimensional universe with finite volume and yet no boundary and yet no edge. And yet no inside versus outside. I know it makes no sense, but that's why we have math. Mathematics is a tool that lets us study, understand, grapple with make predictions for the literally unimaginable. You cannot imagine what it's like to be on the outside of a three dimensional universe that has no outside. This statement of the outside of the universe makes no sense. It's like asking, You know what? Flavor is purple and don't say grape, don't get smart with me. You're trying to take two separate concepts, taste and color and try to merge them together and they don't fit. What's a 0.1 mile north of the North Pole? The question doesn't make sense. What's outside the universe? The question doesn't make sense. Either our universe is infinite or there is no such thing as outside.
So basically, no matter what, there is no such thing as the outside to the universe. Well, what about the multiverse? You might ask first, Please don't interrupt me because this is my show. I'm just kidding. You can interrupt me any time. If you're into this whole multiverse thing, then the word universe means this local bubble with a certain set of physics, which is really a subset of a much larger physical reality, with many such bubbles all in three dimensional space where you could penetrate the boundary of our universe, travel through some vacuum and then end the boundary of another universe, But in that case, the word multiverse should really be the universe, because the universe is all the things and our local bubble should be called. I don't know a subvert or something, but but the names were already taken. So we have universe and multiverse instead of universe and subvert. We ended up with this tangled mess. But no matter what, you're gonna run into this issue. If you want to believe in a multiverse, then what's outside the multiverse? If you want to believe in a higher dimensional bulk, then what's outside the higher dimensional bulk? No matter what, you're gonna run into this problem.
The answer to the question What's outside the universe is meaningless. It's not nothing, because nothing is still a thing. It's still vacuum. It's still an entity. The what's outside the universe is nonsense, not I. I don't mean like it's silly stuff outside the universe. I mean, the question itself is nonsense. But let's leave all that aside because one it's confusing as I'll get out and even my head hurts. And two, none of this is directly observable anyway, and so it doesn't really matter because it can't ever impact our local environment. So let's focus on our local environment before I continue. Though, folks, I want to tell you about the Amazing the great Courses plus, who are sponsoring this episode of Ask a Space Man. Now I've given you lots of recommendations. I really want you to check this one out because it's it's a little bit different. They have a lecture on the science of cooking. You know I love food, especially cheese, and this digs into all the cool chemistry and physics that are happening when you're in the kitchen.
And then when you're done with that, go check out cooking across the ages so you can learn about, like ancient Roman cooking and imperial Chinese cooking and and chickpeas and meat and beer and sugar and spice and sweets and pasta. It's like, huh? I'm getting hungry just thinking about it. I think really you'll enjoy it. Go check it out. The great courses Plus is amazing. I really do think you'll like it, and when you sign up, you support this show. That's how it works because it's an advertisement and like so if you go, you are helping this show. Uh, just be awesome. and more. Ask us space man like we all like, but I need you to go to a special URL. It's the great courses plus dot com slash spaceman. Once again, it's the great courses plus dot com slash spaceman. You get an entire month of unlimited access for free to try it out, and you can get hungry just like me. I mentioned it before the observable universe. In some times we call it just the universe because we're getting lazy.
Hence one source of all this confusion. Our observable universe is about 90 billion light years across. Another name for the boundary of the observable universe is the particle horizon. It's the furthest we can see, like in order to see something it had to emit light in the distant past and travel long ways and then reach us. You might think that this distance is simply the age of the universe, times the speed of light like OK, if our universe is 13.8 billion years old, then light has had 13.8 billion years to reach us, and so the particle horizon, the edge of our observable universe, should be a radius of 13.8 billion years. But it's not or sorry billion light years, but it's not. It's around 45 billion light years, but you're not surprised by that because you're very sophisticated, especially because you contribute to Patreon. It's patreon dot com slash PM Sutter to keep the show going. I sincerely do appreciate it. The ads are nice. Trust me. The ads help, but Patreon drives this show. It really is a joy to have this support from you, the listeners to keep the show going anyway.
The universe is 14 billion years old, which seems much too young for its current size. The thing is, we live in an expanding universe and that distance that 90 billion light year diameter. That's because our universe has expanded since it was born. So it's not just a matter of light traveling to us from distant Galaxies. Those in the meantime, since those Galaxies have sent the light, the Galaxies have moved away, and so now we're marking where those Galaxies are. Now, when the light finally reaches us, it's like Imagine if Continental Drift was like 1000 times faster than it is, and you start sending postcards across the continents and a friend of yours like sends a postcard from Europe. And a week later, by the time it actually reaches your hands, your friend is farther away from you than when they sent it. So that's how we calculate the particle horizon or the edge of the observable universe. It's where the Galaxies are today, based on when they sent the light yesterday where those boundary Galaxies are right now.
At this instant, there's more. One more wrinkle to this story. The universe is expanding faster than light. It's just that simple. And no, that's not a problem. Like the universe is allowed to expand faster than light. There's no law of physics that says the universe can't expand faster than light. They just is it just It's just doing its thing. And we actually expect that in an expanding universe, the universe is always expanding faster than light in an expanding universe. The farther away an object is, the faster it appears to recede from us because there's more space in between us and the object to do the expanding. So if you look at a nearby galaxy, it might be receding away from us at a certain distance at a certain speed, and then you look at a galaxy twice as far away. It will be receding away from us twice as fast, and you look at a galaxy four times further away, and it's receding four times faster because there's four times as much space to be doing the expanding. And eventually you reach a point where Galaxies appear to be moving away from us faster than the speed of light.
That distance is 13.8 billion light years away. At the age of the universe, this boundary between the parts of the universe that are expanding away from us, slower than the speed of light and the parts of the universe expanding away faster than the speed of light is called the Hubble horizon. The point of the Hubble horizon is that if an object beyond the Hubble horizon emits light right now, at this very second, that light will never reach us. If an alien in that galaxy passed farther away than 14 billion light years away, if it waves at us, we will never see it because the galaxy is receding away from us too quickly and light can't go. That fast. We can see the galaxy now at least four now based on the light it emitted in the past. But as time goes on, it will slowly dim and fade from view. So there are Galaxies that we can see now that we won't be able to see in the future, which, I guess if you worked hard enough, would qualify as outside the universe. But that's on you. But like I said earlier, according to inflation theory, the actual universe is much, much larger than our observable bubble.
How big is the rest of the universe? Compared to our observable bubble, it's like 10 to 50 10 to to 60 times bigger than 90 billion years. So that comes out to let me see here, let me get my calculator car that, um uh, it's just big folks. The universe is big, possibly infinite, but at least very large. And what's outside the observable universe? It's just stuff, you know, but unobserved. There are more Galaxies, stars, planets, voids, cosmic Web, doing the same thing. It does as us. But in a slightly different arrangement. We have this concept in cosmology called homogeneity. Our universe is homogenous like If you look at the Earth, it's very, very different than, say, the sun. If you look at the solar system, it's very, very different than another solar system. If you look at our galaxy, it's very, very different than the empty patch of space right next to our galaxy. This is in homogenous. But once you get to a big enough scale and that big enough scale is around 100 million light years, the universe looks pretty much the same.
It means you can take a box 100 million layers on a side and look at different random patches of the universe, and it will all be statistically the same. Yeah, there'll be a different structure there. There'll be a different cosmic Web. There'll be different arrangements of voids and arrangement of Galaxies, but but the statistics will be the same. Like you'll see, you find roughly the same number and sizes of cosmic voids. You'll find roughly the same number and sizes of galaxy clusters. You'll find roughly the same number and sizes of Galaxies themselves. All the statistics are the same. It's just like a slightly different pattern. We call that the homogeneity, scale and that happens at about 100 million light years, which is much, much, much smaller than the observable universe. Our observable universe is 90 billion light years across. So once you reach up to the scale of the observable universe, it should be homogeneous. It should be the same. So this is what allows us to state with confidence what outside the observable universe looks like, because the universe is pretty much the same from place to place, with just some interesting local color to spice things up a bit like no matter where you go in the world, you might meet people who look different or sound different.
I heard there are even people who don't eat cheese, but they're still people. If you were to somehow be able to go outside of our observable universe, you would just find more universe in a slightly different arrangement. And we know that through this homogeneity scale. So imagine our surprise a decade ago when a group of astronomers announced the discovery of something called the Dark Flow because apparently we've run out of all the non silly names in cosmology. But looking at a sample of galaxy clusters, they spotted a uniform motion in one particular direction. This should not happen in a fully homogeneous universe. If you look at a whole bunch of galaxy clusters in all sorts of directions on the sky, they should be moving, but in all sorts of random directions. And instead they seem to be moving in a uniform direction. They called this dark flow and everyone went nuts, wondering what it was. Is something tugging on us from outside the observable universe that can only work if we somehow interacted with some weird giant clump before inflation happened in this super early universe.
And we're just now feeling the effects now, Uh, because by now it's too late. Like if there's something giant, if there's a like a giant monster out there outside the observable universe, well, everything's expanding away from each other faster than light, and so it can't affect us. So this had to happen in an incredibly early epoch where it could influence us, and now we're just feeling it. The scientists who studied this and found this found it in a map of the cosmic microwave background. This light from the early universe. Sometimes this light passes through galaxy clusters and the hot gas in the galaxy cluster and the motion of the galaxy cluster itself can boost up that that light. And so you get When you look through a galaxy cluster to the cosmic microwave background, it seems a little little tiny, bit hotter in that direction. And you can use that to measure the velocity. The speed of these galaxy clusters, which is an amazing thought in itself. Like galaxy clusters weigh like 10 to the 15 times that of the sun and have thousands of Galaxies in them.
And here we are, pinpointing their location on the sky and being able to measure their movement. It's pretty wild to think about. I still get questions about dark flow today, which is unfortunate because the idea is dead, like dead, dead DEA, D dead. There's simply no confirmed measurement of the dark flow. This came out about a decade ago. People reanalyzed the original work. There have been more surveys of the cosmic microwave background, like with the plank satellite, and we get nothing. We get no measurements of dark flow. We see no evidence for it. I don't know why it has somehow still persisted in the public imagination. Probably just that's how science news works is the cool stuff gets out there, and then when people come along to refute it, it goes away. The only people still claiming to see the dark flow signal are the original authors of the study. No one else in cosmology believes them, because in order to get this result, you have to really stretch the statistics you have to. Really. It's one of these things I see crop up every once in a while where the researchers believe that they have this or want to have it.
And so they twist the analysis it enough to get that result out. But when you do a more careful analysis or any other kind of analysis, that signal just goes away. Real signals. Real evidence should be independent of your analysis technique, and if the only way to get it is by following the way that the people who believe it already say it is that that's not a good sign. But that's that's a different episode. We have absolutely no evidence for a non homogeneous universe. We have no evidence for dark flow as far as we can tell the outside of the observable universe is just like the inside, only different. But you know, also the same, and the outside to the universe itself is not a question at all. And I'm sorry if that's unsatisfying. But you know what? It's our universe, and we got to live in it. Thank you to Phil C and at PK her on Twitter for the questions that led to today's episode and thank you to my top patreon contributors This month, we've got Matthew K, Justin Z, Justin G, Kevin Duncan, M Coy, D, Barbara K, No Duke, Robert MNH and F Chris L Cameron, NAIA Aarones, Tom B, Scott M and Rob H is their contribution, and everyone else's over on patreon dot com slash PM.
Sutter, go read my book. How to Die in Space. Go leave a review on iTunes or Spotify or wherever. Go tell a friend. I really do appreciate all your support. All your love, all your questions. Hashtag ask a spaceman. Ask spaceman at gmail dot com. Ask us spaceman dot com at Paul Matt Sutter on all social channels, and I will see you next time for more complete knowledge of time and space