What’s the difference between the universe and the multiverse? How can cosmic inflation make bubble universes? Do those universes get different physics? If I travel far enough away, will I meet…myself? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
Is this the only universe? That seems like kind of an important question, doesn't it? Like, we live in a universe. It exists. It is all of existence, but is that it?
It's it's fun to imagine other universes, isn't it? With with another you, another life. You know? Think of all the choices you've made all over the years, big and small, like who to marry you, where to live, what to have for dinner, what to put on your salad. Imagine a whole other universe where the only thing is different is that you got the Italian dressing instead of the ranch, and it totally changed your life.
Like, what if you turn left instead of right last Tuesday at that intersection? How would your life be different? What if you moved to Buffalo or or moved away from Buffalo for that job? You run that movie forward and imagine you running in parallel. Right?
Like, a slightly different version of you just vaguely over there somewhere else in another universe, a parallel universe, a a perpendicular universe, an alternate universe. And if these different universes are possible, these different options that represent all sorts of different choices, could you, like, hop between them? Could you jump? Could you mix them, so to speak? Could you grab one universe and bring it over?
These are important questions. Right? But I suppose first, we need to define universe. Right? The usual definition if I say universe, you probably think all the things, all there is, which if we take the word universe to mean all the things, it means there's no such thing as multiverse, which is the subject of this episode because there's only one universe, which is all the things, which includes all the possible options.
That's not quite a useful definition, and that's why the word multiverse comes in because when a cosmologist like me or an astronomer uses the word universe, they usually mean something else. They usually mean the observable universe. It's the limit of what we can see. So the universe is 13,800,000,000 years old, and light can only travel so fast. The universe has only expanded in so much in that time, and so there's a bubble of what we can see.
It's about 92,000,000,000 light years across. And, yes, that is far larger than 13,800,000,000 years times the speed of light. That's because the universe can expand faster than the speed of light. No. That's not a problem.
Typically, when you hear a scientist say universe, they mean observable universe, what we can see in our cosmos. We know that past that bubble of what we can see, there's more stuff. There's more galaxies. There's more stars. There's more more just junk floating around out there.
So all that stuff, the stuff that we can see in our observable universe and the rest of it that we can't is properly called the universe. All the stuff, even if we can't see it. But we'll never ever see past our observable bubble or know it or test it or smell it, so it doesn't really matter in terms of our understanding of everything that's going on. What happens outside our observable bubble happens outside our observable bubble and that's it. We're never ever gonna see it, so it doesn't matter when it comes to science, But there's just this knowledge that the universe is far bigger than what we can see.
By how big? Well, large. If I had to pick a word, I would pick the word large. It's at least you know, and these are based on models of the very early universe. If you remember the episodes I did on inflation, this is going back to that.
Our entire universe is about 10 to the 52 times bigger in diameter than our observable bubble. So if you take 92,000,000,000 light years and you multiply by 10 to the 52, you get the minimum diameter of our total universe. It could be much larger, of course. It could be infinitely large if that even makes sense. It doesn't matter.
It's big. It's very, very big. But then but but then there's, like, a limit. Like, you there's a finite size to the universe, or at least there's a finite size to this universe of what we call everything, our observable bubble, the unobservable parts. There's a sense that, like, there that's it.
But what if you were to go farther than that? It's pretty easy to imagine stepping outside of our observable bubble, and there's just more universe out there. Is there more to the more? Is there another layer? Right?
Is there another there's the observable universe, the whole universe. Is there a multiverse? Is there another layer to that story? Now the word multiverse, I I exhausted a lot of words to describe define the word universe. Now I need to define the word multiverse.
The the word crops up in a few different areas of physics and philosophy and religion and other less interesting lines of thought. Well, that's unfair. Other very interesting lines of thought that are not subject to this podcast. Multiverse. Perhaps the most general definition I can give to the multiverse is other universes, just other batches of stuff that are unique and different and special and more varied than our own batch of stuff.
Okay. So maybe that's not the most helpful definition, so maybe some examples will help in various kinds of physics. One is something we call the many worlds interpretation of quantum mechanics, and I haven't really dug into this yet in the episode or in the show, and I'm definitely not gonna dig it up today. But this is where, you know, quantum mechanics is based on probabilities. You run an experiment on an electron or a beam of light, and if there's a fifty fifty chance of the electron pointing up or pointing down, then half the time your experiment will get up pointing electrons and half the time you'll get down pointing electrons.
And so it makes you wonder, maybe we get one universe, we run that experiment, and the universe splits somehow, Where there's one set of experiences that get to enjoy the up electron and there's another set of experiences that get to enjoy the down electron, and they both get to do it. Now, I mean, this produces quite a lot of multiverses very, very quickly. Like, you go out to cut the grass and there's so many quantum mechanical interactions, you just spawned like a quadrillion quadrillion quadrillion universes so that it seems a little excessive. But you know what? Nature's been weird before, so we can't quite rule it out.
But this interpretation is just a way of of navigating the thorny mathematics of quantum mechanics and try to putting putting some human story on top of it to make sense of what the math of quantum mechanics are telling us. But, again, I'm gonna save the bulk of that for another show. Feel free to ask, but that's one place where the multiverse kicks in. Another place where the multiverse kicks in is in string theory. This is the nice and tidy, except it doesn't work, theory of merging all the forces of nature together and explaining all the particles and the constants.
Like, it's like the super theory. It's like the the theory to rule them all. It's the one ring of theories where it's just gonna explain everything. It's gonna explain physics, period. Why there are certain constants have the numbers they do?
Why there are four forces of nature? Why there are four dimension three spatial dimensions, one time dimension? Why you know, etcetera, etcetera. All the it's just gonna explain physics. Slight thorny issue cropped up in the explorations of string theory, which is that, of course, you had need to add extra dimensions.
I can get into this if you wanna ask, but these dimensions are all curled up in on themselves very, very tightly. And the way these extra dimensions in our universe can fold in on themselves is not unique. There's actually a bunch of different ways that the dimensions can fold in on themselves. And interestingly enough, each way that the dimensions fold in on themselves leads to a different set of physics. So if they twist this way or weave this way, you get one set of physics and constants and everything.
And if they twist another way, you get another set of physics and constants. And there's a lot of ways to fold up these dimensions, something like 10 to the 60, maybe infinite, just a lot. And so the concept of the multiverse comes in because we have one universe with one set of physics and one set of constants and one set of forces, and it's just is this it? Are there other universes where the dimensions folded up differently and led to different kinds of physics? Not just different parts of the universe with the same set of physics, but different experiences, but totally different physics.
This is called the landscape in string theory. And in this idea, our universe is one particular little nugget of the landscape and surrounding us on all sides are other nuggets with their own sets of physics. Again, I can dig into that more if you want. Just feel free to ask. But instead, I wanna focus on the most plausible candidate for a multiverse idea.
Because at the end of the day, the multi world's interpretation of quantum mechanics is just a language to describe the mathematics. It might be completely untestable. In the string theory landscape, well, string theory is on shaky ground anyway. But so so, like, what is there any way to get to a multiverse with the physics that we know or at least kinda sorta know? The answer is yes, and the answer is through inflation.
You remember my two part series on inflation. If you don't, now would be a good time to pause the episode and backtrack to that, but I'll give you the short recap here. When our universe was very, very, very, very young, like, 10 to the minus thirty five seconds old, it underwent, we think, it underwent a period of inflation, hence the name. It got really big, really fast, like, ten to the minus ten seconds. No.
Not even that. It's shorter than that. I forget all the numbers that I had to dig up for that episode. It just got really big, really fast. Okay?
And, eventually, the universe expands very rapidly, gets bigger and bigger and bigger, and then it stops, and it cools off, and it becomes sedate. And we get the universe that we know and love today. So this happened very, very early on. Inflation did its thing. The universe expanded rapidly then stopped expanding so rapidly and just kind of expanded from there.
And then, you know, particles came in, you know, etcetera, etcetera, etcetera, the big bang. But here's the thing with inflation. That story I just told of the universe inflating, getting bigger, and then stop getting bigger assumes that inflation ends equally everywhere all the time. And that doesn't necessarily have to be the case now, does it? What if some parts of the universe just kept on inflating, ignoring what the rest of the universe was doing?
And what if some part of that little pocket of the universe, which kept inflating regardless of what everyone else was doing, what if some part of that kept inflating while the rest of it calmed down? This isn't so crazy an idea in inflation because what drives inflation is something we call a quantum field, this entity, this thing that permeates all of space time. And in the early universe, there was one particular kind of quantum field that we call the inflaton because the inflaton inflates. And this field that permeated all of space time was doing the work of inflation. It was doing the thing that made inflation happen, but this is a quantum mechanical idea.
The inflaton or any other quantum field is a quantum thing. It has no single super well defined value across the entire universe. It fluctuates. It bubbles. It foams.
And so this quantum mechanical field in the early universe that's doing the work of inflating the universe, some parts of it are naturally gonna be a little bit more aggressive than others, and other parts of it are gonna be a little less aggressive, aren't they? That's just how quantum mechanics works. Everything's always fuzzy. Everything's always vibrating. Everything's always a little bit wibbly wobbly in the quantum world.
So it's not so crazy to suggest that maybe inflation didn't end equally everywhere all the time, that there were some fluctuations and some parts just randomly got a little bit faster than average. So it could be that our observable universe, the the patch that we get to see, pinched off because inflation ended in our little pocket while the rest of the bigger universe keeps on inflating faster than our own expansion. So we we got a little pocket that pinched off and just returned to, like, kind of sedate expansion that we know and love today, but while the rest of the bigger universe just kept on inflating. This scenario of generating different pockets of observable bubbles of the universe is generally like, a good analogy is a bunch of bubbles in a big foam with the foam constantly expanding. So you imagine this foam expanding, and then there's these teeny tiny little bubbles inside of it.
Each one of these little bubbles is its own little universe, its own little observable pocket, and it just keeps going and going and going. Each bubble has its own big bang, its own observable limit, its own set of galaxies, but the inflating universe keeps going on forever. There is always in this view, because quantum mechanics is quantum mechanics, some parts of this field that are driving inflation are always gonna be a little bit more aggressive than normal. That's just the way statistics works. So there will always be some part of the universe that is always inflating, and some pieces of that inflating universe will always be pinching off to make their own little bubbles while the rest of the universe continues to inflate.
And if this is true, you can ask what those other bubbles might look like and how far away they might be. How are they arranged? And so this tells you, if this story is correct, and I should emphasize we're not exactly sure because we don't know a lot about inflation, that these other bubbles, for their own little pockets of the universe, their own little observable limits, are a finite distance away from us. That you could, if you you know, if the universe froze in a moment and stopped inflating, stopped expanding, you could get in a spaceship and travel far enough and you would get to reach one of these other pocket universes. It's a physical distance away.
What would these other universes look like? Well, for sure, the other universes would have more people contributing to Patreon. Go to patreon.com/pmsutter to learn how you can contribute to keep this show going. I cannot tell you how much. I am grateful for all of your support over all these years.
I'm just kidding. I think I'm living in the best possible universe when it comes to Patreon. Thank you so much. If this idea of the multiverse is right, that's based out of inflation where there's constantly spawning new universes somewhere out there, you get a lot of universes really quick, a lot of observable bubbles really, really quick, that our observable bubble isn't the only one. And it may be true.
We're not exactly sure about this. It may be true that there's only a finite way set of ways to arrange matter. Like, if you think of all the electrons and protons in the universe and you took a snapshot and you start mixed them around and and that gave you a different universe, you know, a star over here, galaxy up there instead. Okay. And then do it again.
Mix it up again. You get another universe. Mix it up again. You get another universe. Mix it up again.
You get you get the point. But what if there are only so many different ways of arranging matter? What if you're not allowed to have any possible combination of this electrons over here and going this fast? Okay. That neutrino is gonna be like this.
That photon is gonna be doing this. What if there's only a finite number of ways of arranging matter? If there are, that means you have a copy, which is kind of uncomfortable to think about. That if you raced out if you froze the expansion and inflation of the universe and you raced out of our observable bubble, you would eventually encounter another little pocket, another little observable universe with its own arrangement of stuff, and then you'd go from there to somewhere else to somewhere else to somewhere else. And eventually, if you traveled far enough, which in far is very, very far, don't get me wrong, you would find a universe identical to your own.
Why? Because this this idea of what we call eternal inflation, where there's always inflation happening somewhere, very, very, very quickly spawns enough bubble universes that there's more universes than there are ways of arranging matter. And that means there will be a copy of this universe somewhere out there where I am doing the exact same thing and you are doing the exact same thing. And it's a finite distance away it's a very large distance but it's there and that the universe would just take a break from expanding and inflating you could go there and visit your doppelganger your copy This idea of an eternally inflating universe spawning universe after universe, bubble after bubble, possibly with doppelgangers if there's a finite way of arranging matter, doesn't just come out in the early universe in this inflationary epoch in the very, very young big bang. This idea also comes out when we look at the far, far future of the universe.
And I'm not talking a billion years or trillion years or a hundred trillion years. I'm talking deep future of the universe. And in the deep future of the universe, again, quantum mechanics is in charge. There are events that are perfectly possible but are so rare that they basically never happened. Quantum fields still exist just like the inflaton, the quantum field that drove inflation, was a big player in the early universe, today's universe, we still have quantum fields.
There's a quantum field for the photon. We call it the electromagnetic field, by the way. There's a quantum field for the electron. There's a quantum field for the neutrino. There's a quantum field for everything.
And all these quantum fields permeate all of space time all the time, constantly overlapping and juggling back and forth, and this is what we call high energy particle physics. And most of the time, essentially all the time, these quantum fields are very, very well behaved. Technically, because they're wiggly quantum things, they could randomly become very, very excited in a very high energy state. But the higher the energy state, the the more rare it is, and so it basically never happens. What if in the deep, deep, deep, dark future of our universe, one of these quantum fields popped in some little patch, pop a new big bang?
Randomly fluctuated so that there is enough energy concentrated in a small enough volume that new big bang. In this view, the multiverse isn't coming about in the early universe. It's coming out in the late universe. As our universe dies, there could just be that random lucky fluctuation that spawns a new Big Bang, and it could happen in other places simultaneously. And so the universe gets to keep going.
And, again, every time you spawn a new Big Bang, every time you get a new arrangement of matter, if there's a finite number of ways of arranging matter, you'll get doppelgangers again. You'll get a copy of yourself. And so far, with this eternal inflation and all that, I haven't gotten to the weird part yet. Yeah. I haven't gotten to the weird part yet.
The weird part is that when inflation ended in our early universe, there was an event that we call in physics circles, feel free to copy, spontaneous symmetry breaking. Spontaneous symmetry breaking. What a weird phrase. Here's what's going on. In the very early universe, preinflation, the energies and temperatures and densities were high enough that all the forces of nature were unified, we think, in a single force called the force.
Nowadays, we have four forces of nature. We know at higher energies, electromagnetism and weak nuclear combined together, and we're pretty dang sure that even at higher energies, the strong nuclear force gets folded in. And then at extremely high energies, we think all four forces of nature are unified. So they were very early on in this very intimate state. They're intertwined with each other.
They they were unified. It was a very symmetric state until it wasn't when inflation ended or maybe when it began. We're a little bit sketchy on the details, so just work with me here. At some point involving this thing called inflation, the forces of nature split away from each other. But there are many different ways to split away from each other.
Here's a metaphor. It's not the best metaphor, but I'm just gonna go with it. Imagine a pencil balanced on its tip. This is a perfectly symmetric situation. You can look at it from any direction.
It still looks like a pencil balanced on its tip. And imagine knocking that pencil over. You have now broken the symmetry because now it's landed on the table in a certain direction, and different points of view give you different views of that same pencil. This symmetry has been broken. And when our unified forces when the force split apart and fell down into its different configurations that we have today, it didn't necessarily have to land this way.
The pencil could have landed pointing forward towards you, away from you, sideways at some weird angle. There's many, many, many choices of how that pencil would fall. There are many, many choices of how the forces would split in this event. And the way the forces split give you different numbers of forces, give you different fundamental particles, gives you different masses of those particles, gives you different strengths of the forces. When inflation ended and the force split apart, it could have split apart in a bajillion different ways.
It picked one for our universe. But if this eternal inflation idea is legit and there's always somewhere in the universe where this end of inflation is happening, do those other universes get different physics? Do those other universes get different fundamental forces, particle masses, interaction strengths, all sorts of stuff? Well, we don't know. We just don't know.
Like, seriously, we don't know because it's kind of hard to test this because we only have the one universe. You can't just go looking for the elder multiverses because we can't ever see them most of the time. There's a chance. There was a chance in our very early universe when our universe could have intersected with one of its neighbors, if this story is right. It has to happen in the early universe because there's always a part of the universe that is inflating, and inflating is very, very rapid expansion, the most rapid expansion the universe will ever experience, way faster than the expansion that we see today or has ever been in our universe.
So very very quickly as soon as your little pocket, your little bubble pinches off, like, you're gone. All the rest of the other potential bubbles just get scattered away from you, pulled away from you very very quickly. However, if, you know, conditions are lucky, if, you know, surrounding our little bubble, it's not so bad, you know, that particular value of the inflation rate is, you know, a little bit milder, and another universe just happened to nucleate right next to us, maybe we could have intersected briefly before getting carried away. Maybe our bubbles overlapped, and there's an imprint of it in, you know, data of the early universe. And our prime candidate for studying the early universe is the cosmic microwave background, the the fossil radiation from the Big Bang itself.
If our universe intersected with another universe, like two soap bubbles pressed up against each other, we should see you Okay. So we didn't haven't seen anything, and we we've looked really, really, really, really, really hard for circles in the cosmic microwave background. We can't find a dang one, so we just basically have to move on. Like, it's nice. Like, even if the multiverse idea was right, it had to be a very lucky set of, circumstances to give us circles in the cosmic microwave background.
So it doesn't necessarily rule out the multiverse idea, but there's no positive evidence for it. So our testing, that's one direct test and it, you know, got nothing. So all other tests of the multiverse are circumstantial. Instead of testing the multiverse idea directly, we have to test the specific inflation model. And inflation, the story I gave is just a generic picture.
There's a bunch of different ideas of actually how this proceeds and who's responsible and how hard they're working. But all this testing is circumstantial because a lot of the inflation models are generic. There's a lot of inflation models that produce very, very similar universes with very, very similar results. And we're not sure if this idea of inflation automatically gives you a multiverse. Some people argue yes.
Some people argue no. We're not sure if inflation always gives you a multiverse or if there's cases where it doesn't. And maybe there's some models of inflation that do give you a multiverse, and if you can demonstrate that those best explain the data, then that gives you a hint that maybe the multiverse exists, but that's it. You only get hints because this is a very thorny problem and we're not exactly sure, and we can't ever likely directly test it. All the other little bubbles are way too far away by now, and we'll never be able to reach them.
So absent any direct evidence or indirect evidence, we start talking. We start philosophizing, which philosophy is no bad thing. I will say it a hundred times just for the two philosophers who listen to this so you don't get mad and unsubscribe. I love you. We have to ask something very, very serious here.
We exist in this universe. We are alive in this universe, and it appears like the forces of nature and the fundamental constants conspired to make life possible. Because if you change, like, the mass on the electron, life as we know it is gone. If you change, like, how strong the strong nuclear force is, like, okay, stars don't work. If you change the value of dark energy, then, okay, universe accelerates expansion before it even gets a chance to get going.
It seems like life can only exist in a relatively narrow set of physics. Most combinations of physics are incompatible with life. So how did we happen to find ourselves in a universe that is compatible with life? If this multiverse idea is right and there's spontaneous symmetry breaking and when the force splits off into its smaller forces, does so in a bajillion different ways, and there's a bajillion different universes. Why are we in this one?
Why not another one? Well, this is the anthropic principle. How did we find ourselves in a universe compatible with life? Because it's the only one we're capable of observing. Life didn't arise in the other universes of the multiverse because life can't work in those other variations of physics.
It's only in the variations of physics that are compatible with life that you'll get something. You'll get observers that can start asking these kinds of questions about the universe. This idea of the anthropic principle. And so this is like an argument because people say, like, okay. You want a multiverse, you want eternal inflation, blah blah blah, and all these universes end up with different physics.
Why are we here? This is a big sticking point with the multiverse idea because we can't test it directly. You can argue about it, say, yes. In the multiverse, you're likely to find life on the bubbles that are compatible with life, that have just the right physics. And that's, you know, not a bad argument, but is it a scientific argument?
There's a big difference. Not all questions are meant to be answered with the tools of science. Our discussions of the multiverse because we can't directly test it, or we tried and we failed, we can't indirectly test it, or at least it's very, very hard. Will we ever know for sure if we live in a multiverse? Will we ever know for sure if there's another me out there with an oddly high pitched voice.
We'll never know. Is this even science? Is the multiverse concept even science? One side says yes. This is a natural extension of inflation, and we're trying to dig out what inflation looks like, and it's a very solid theory that's been around for decades.
And, no, we don't understand anything, but we know understand some things, and we feel like we're on the right track. And multiverse is a natural extension of it. Maybe it's not. We can never test it. We can never prove it.
We can never disprove it. We can't know the answer through scientific means. If you can't know the answer to a question through scientific means, it means you can't use science to talk about it, which means you can talk about it, but not in science circles. It's a big sticking point for a lot of people, and there's a lot of arguments in the community. And I'll just leave it.
What whether you think multiverse is a scientific concept or not, I'll leave that up to you. And maybe there's another universe where you disagree with yourself. Thank you so much to all the people who asked questions. We got at producer Evie on Twitter, Matthew a and Ruthie over on Facebook, Tom s, Taha on email, Oliver h on YouTube, Wally h on email, Christian d on Facebook, Keith g on email, Alex d on YouTube, Murtaza p on email, Ken l on email, Gabby p on email, and Slinkerdier on YouTube for asking the questions that led to this wonderful discussion on the multiverse. And, of course, please go to patreon.com/pmsutter to learn how you can support the show.
I'd like to give a shout out to my top Patreon contributors this month, Robert r, John, Evan t, Matthew k, Haugabee, Justin, Matt w, Justin g, Kevin o, Duncan m, Corey d, Kirk b, Barbara k, Neuter Dude, and Chris c. We are taking reservations for Astro Tours all over the world. Go to AstroTours.co. You can also buy my book where I talk something about the multiverse. Go to pmsutter.com/mook.
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