The Joy of Why - Are There Reasons to Believe in a Multiverse?

Episode Date: May 17, 2023

Several areas of physics suggest reasons to think that unobservable universes with different natural laws could lie beyond ours. The theoretical physicist David Kaplan talks with Steven Strogatz about the mysteries that a multiverse would solve. The post Are There Reasons to Believe in a Multiverse? first appeared on Quanta Magazine

Hello, I'm Brian Cox. I'm Robin Ince, and this is the Infinite Monkey Cage trailer for our brand new series. We've got mummies, we've got magic, we've got asteroids. Mummies, magic and asteroids. What's the link? That it was an asteroid that magically went over the world that led to Imhotep the mummy coming back to life? That's correct. I thought it would be weird scientific as ever. But the most important thing to know is that we are going to deal with the biggest scientific question we finally ask. What is better, cats or dogs?

Listen wherever you get your podcasts. I'm Steve Strogatz, and this is The Joy of Why, a podcast from Quantum Magazine that takes you into some of the biggest unanswered questions in math and science today. In this episode, we're going to ask, do we live in a multiverse? We know that we all live in our own little bubbles, whether it be our family, our friends, our hometown, even our workplace. And if you think about it, animals live in their own little bubbles too. Fish live in certain parts of the ocean or different lakes or rivers. You won't find them in ice with a microbe population or flying around in the sky with birds, even though ice and water vapor in the sky are also forms of water.

Could the universe be the same way? Maybe we're not alone, and what we can see with the help of telescopes and infrared cameras isn't all there is. Maybe space is infinite. Maybe there are multiple universes beyond our own, perhaps made up of some of the same components or possibly even breaking the very laws of physics that allow us to call our universe home. It's a mind-blowing concept, the idea that we could live in a multiverse. And it's an idea that David Kaplan thinks could be possible.

Kaplan is a theoretical physicist at Johns Hopkins University in Baltimore. He looks at the theoretical possibilities that apply to the standard model of particle physics and cosmology. He also produced and appeared in the 2013 documentary, Particle Fever, about the first experiments at the Large Hadron Collider. David, thanks for joining us today. My pleasure. Well, this is great. I'm very curious to hear your take on the multiverse. Before we get to the multiverse, let's talk about the good old-fashioned universe.

Sure. You know, I mean, I grew up hearing about the universe. That was sort of by definition, I think, all there is. Do I have that right? How would we define the universe before we define the multiverse? Wow. Well, I think to be a little bit more useful or practical or even rigorous, we often talk about the observable universe. And the observable universe is a part of the universe that we have any sort of access to. And there's a very simple fact which limits our ability to see the entirety of whatever the universe might be, which is the fact that the universe appears at least to be finite, or at the very least, that in the early stages of what we would call our universe,

it was so dense and hot, light couldn't penetrate it. And so the observable universe is really the distance we can look back to which a place where the light leaving that part of the universe at a much earlier time than today couldn't pass through the universe. In other words, the early universe, if it was smaller and hotter and denser, light didn't travel in a straight line. It was stuck in the plasma, the soup that was the early universe. And when the universe expanded and cooled enough that light could then find room to travel from one place to another and across the universe, it left what we now call the surface of last scattering. And so there is a surface, in a sense, in all directions when we look out,

and we call it the cosmic microwave background, that's a surface at which we are seeing the universe at a very early stage, at a time when it was not transparent to light. So in that sense, we have a very rigid access to what we can see in the universe, and therefore we call it the observable universe. There certainly could be universe beyond that, but if the universe has a finite lifetime, and it takes light a finite time to get to us, we just mathematically cannot see beyond a certain point. This is great. I love the rigor that you just applied to that. Maybe we should move along

then to this notion of a multiverse. When it comes to the multiverse or the initial start of the universe or all of those things, they're all very speculative. So you do hear standard things like at the Big Bang, you create time and space. And there is some notion where you could say that is probably right in some sense. But really what's going on is when you get to densities of order, it's called the Planck energy, the description of gravity in terms of a geometry or a geometric theory of the universe breaks down. It doesn't mean that there isn't time and space of some other kind at earlier times. It's just the general relativity no longer applies in that time. And so could it be that time goes back infinitely far? A different label of what we call time? Absolutely. We don't know. We don't know what this at very low energy

densities turns into when you get to an energy scale where general relativity is not the correct description. So people pop in and say, well, time and space get created sort of instantaneously out of nothing. But that's not a mathematically well-defined description. That's a sort of hopeful compactification of space and time. And if you really have nothing, it's hard to make predictions about when something appears because there's nothing there. Now, what is the multiverse? The multiverse as it appeared in Particle Fever was a use of the multiverse.

It was the idea that in an almost mundane way, if the universe is infinite or much, much bigger, at least, than the part that we see, the observable part, then it's possible that the laws of nature in different patches are different. You don't even need the deep underlying laws to be different. You just need some of the parameters to be different. to be different. You just need some of the parameters to be different. The numbers that describe things like how much vacuum energy is in that part of the universe, or what are the values of some background fields, we say gravitational fields or other fields in those parts of the universe. And if those things are different in different places, they will have a different expansion history. The universe at that region will expand based on what's in the universe and what are the interaction strengths of the stuff in that part of the universe. with a little bit more pedestrian descriptions of how the laws may change from one place to another,

or just even the content of the universe in each of those places. And so there's a simple idea of the multiverse, which is that there really is just one universe. The part of the universe that we exist in have a certain set of parameters controlling how life looks. And then if you go far enough away, farther than light could have traveled in the age of the universe, there are places in the universe where the laws are just different. The parameters are different. The expansion history is completely different.

Maybe those regions, no stars or galaxies could have formed. Nothing lives there. Or maybe in those parts of the universe, detailed properties like the mass of the Higgs boson, which controls the mass of lots of other particles. It controls what hydrogen is. It controls how chemistry works. All of those things could be different in different places if you go far enough out. It appears in our observable universe, the laws are pretty static. We have laws, we have initial conditions, and we have a description of how the universe works and how experiments work on Earth. They seem to be constant in time, and it doesn't really matter where our galaxy is

as the experiments are done, as our galaxy is actually moving quite fast relative to the background. So in that sense, the laws are very stable here, but you could imagine much farther distances in which the laws are different. And stars can or cannot form. Chemistry does or does not work. A different type of chemistry works. All kinds of goofy things could be happening in very different places, very different creatures. Who knows? Sure. So I like the way you phrased it, that very different things could be happening. And of course, you know, all of us in late night conversations in our college dorm rooms could think of that thought, that we have a parochial view. It's just in the nature of the finite speed of light and the finite time that the observable universe has been around

that we live in our patch. And who knows what happens in these other patches? We can't, by definition, we can't observe those parts. But there's a principled reason for believing in the multiverse, or maybe many principled reasons beyond this kind of like college dorm speculation. So can you tell me about that? Like, for instance, there are mysteries in physics that led physicists to come up with the idea of a multiverse. Maybe tell us about what some of those mysteries are that would bring us to this wild. You can tell I'm a little skeptical here. Of course. It feels a little like wild philosophizing, but I also know that physics is very coherent and you have reasons for this. It's not wild philosophizing.

First off, we have something called the standard model of particle physics. And the standard model is based on quantum field theory. Quantum field theory works extraordinarily well in describing the interactions of particles in complex systems, in simple systems, at high energies, internal properties of particles like magnetic moments of electrons and the strong force and confinement of nuclear matter. It has just an amazing breadth of description of how all matter works. And that theory involves the particles that we've seen. We identify them as fields. We identify the electron and every electron, in fact, as an excitation of the electron field in the universe. And therefore,

the standard model with a list of fields describes all possible fundamental particles that ever could be created. And so far, everyone we have seen, we've seen all those particles, and we haven't seen any others, directly at least. It also has a list of parameters which tell you how strong is the electrical force between the electron and the proton, and various other numbers which predict the probability of scattering various particles. And a number of those parameters seem very reasonable in the sense of you put those numbers in and you calculate quantum corrections or situations where the background is a little bit different and nothing funny happens with those parameters. Save two. There are two numbers in the standard model that are weird.

One of those numbers is the mass of the Higgs boson itself. The mass of the Higgs boson is actually the mass scale at which all other fundamental particles masses come from. So if you ask what the mass of the electron is, it's proportional to the mass of the Higgs boson. What's the mass of the top quark, the heaviest quark? It's proportional to the mass of the Higgs boson. The W, weak force boson, is proportional to the mass of the Higgs boson. So the Higgs boson controls one physical mass scale among all of the particles in the standard model. And that physical mass scale is something you simply put in by hand. You've measured it. We've measured the mass of the Higgs. We've measured the mass of the other

particles and their interactions with the Higgs. And that allows us to fix experimentally what that number is. Now, if you take a quantum field theory, it has a Higgs in it and all these other particles, and you estimate what the mass of the Higgs should be, what would be a typical mass of the Higgs in a quantum field theory with these particles in it, you get infinity, which is nonsense. And then you say, well, that's okay, because the reason I get infinity has to do with the fact that I'm assuming there are no new particles to arbitrarily high energy. And the way that quantum field theory works and quantum

theories work at all is that you have to incorporate what are, let's call them quantum fluctuations, in the calculations of any physical parameters. So you have a few parameters you put into your theory, but if you want to ask about a physical state, a physical measurement that you make in that quantum theory, it includes all quantum fluctuations that live in the theory. Now, in the case of the Higgs, you discover that the quantum fluctuations would be infinite if the standard model was correct up to infinite energies. We don't think the standard model is correct up to infinite energies.

In fact, we already know to incorporate gravity into the standard model brings in a new energy scale. It's called the Planck mass. It's called the Planck mass. But there could be plenty of other particles or new symmetries or other things for which when you get to that energy, you find, oh, there are no more contributions to the Higgs mass. It's finite. Everything's OK. You're going to get infinities if you assume that your model is correct to arbitrarily

high energy. So you cut these models off. You say, OK, there are going to be new particles at some mass. Above that, let's assume there are no contributions to the Higgs. And below that, there will be these quantum fluctuations that contribute to the Higgs. And what you find is wherever you put that cutoff, however high energy you say the standard model is correct to, there are contributions to the Higgs mass all the way up to that energy, which means you get contributions to the Higgs mass all the way up to that energy, which means you get contributions to the Higgs mass to these arbitrarily high energies or masses. And so the

Higgs mass should be the mass that's associated with the new unknown physics. There should be some new physics up there at some high energy, and the Higgs mass should actually be roughly that scale. And that means when you discover the Higgs and you measure its mass, you should also discover a whole bunch of other garbage, which is associated with the new physics, which is beyond the standard model. I see. Yeah. So instead of saying there's some unknown physics so far above the Higgs, and the Higgs mass actually gets contributions of that energy, you turn it around. You say, oh no, if I measure the Higgs, it's going to come with all this new stuff.

Because the Higgs mass is not infinite, it's finite. So let me, can I just ask, I think I got you. This is a new idea to me. It's very interesting. If I hear you right, you're saying the Higgs is going to set the borderland between the known and the unknown. And because it's this edge case, sort of, it is on the borderland. That means when we discover it, I'm going to see on one side, the stuff that we already kind of understand, but because it's a borderland particle, there's going to be stuff on the other edge of the border. And that's what we're going to be thrilled to discover and measure its property too, because that'll be new physics. Exactly. Exactly. And that led to decades of research to think about what would be on the other side of the border. What is the new theory that makes

the mass of the Higgs finite, not infinite? When you include all the quantum fluctuations, it should be now a reasonable theory. So it should come with a bunch of stuff. And people have posited symmetries, something called supersymmetry, or making the Higgs a composite particle. We've seen the proton is made of quarks. Maybe the Higgs is made of other stuff. And at high energies, actually, there's no Higgs. There's stuff inside, and we're seeing other fundamental particles. So it could be even a borderline of its own existence. And at high energies, there's a completely different description. These were the sort of quantum field theories that people introduced and explored

that we all wondered could be what we're seeing the hint of, that what seemingly looks like nonsense with the Higgs mass is only a statement that there's a new theory living just above it in energy and in mass, and we can even make suggestions of what that theory is, suggestions that cure this issue with the mass of the Higgs. And that was the hope people had when the LHC turned on, Large Hadron Collider, turned on, and they would discover the Higgs, and they'd discover some of these other new, we'll call them degrees of freedom.

These other particles, different masses with relationships to the Higgs. Or even that the Higgs itself is not fundamental and we see properties of its internal structure. And that's why in 2000, I think 2003, I started thinking, but it could be that that's not the case. 2003, I started thinking, but it could be that that's not the case, that the Higgs mass, while it should come with a whole bunch of stuff, you could at least theoretically imagine that the mass of the Higgs is accidentally small, that it would be at the borderline, but all of those new particles that have been added to make it finite, when those quantum fluctuations contribute to the mass of the Higgs, just by some horrible accident, those contributions cancel to such a large degree that the mass of the Higgs is anomalously small

compared to where the actual border to new physics is. So it's sort of like in my picture in my head where I've got this region of the known and then this borderland region and then the region of the new stuff. Right. In fact, when I go into the region of the new stuff, it's the Sahara Desert for as far as the eye can see. Right. Except that there's something way the heck off the edge of the map. Right. And in physics, because the Higgs is so sensitive to quantum fluctuations,

we would call that a fine-tuned situation, one in which there really has to be an accident because the other world that lives way out there, the new physics, the new particles, the new laws that govern how the Higgs behaves doesn't know anything in some weird sense about the low energy, low-lying masses. So there's no reason that it would pick an energy scale for the Higgs, which is arbitrarily light compared to all of the dynamics associated with that new physics. So an example of this is that the strong interactions, the interactions of quarks,

become strong in a particular energy scale at 200 mega electron volts. That's when the quark interactions become so strong, we cannot actually pull the quarks apart. And then you ask, what is the mass of the proton? It's made of quarks. It's about five times that. What's the mass of the neutron? It's about five times that. What are the mass of the various mesons made of quarks and anti-quarks? It's between a few times that and a few, you know, there's a very light one for symmetry reasons, but it's about half that energy. They're all about that energy. All the particles that you get when you go from the land of quarks to the land of the nuclei that we see in atoms, they all live at

that energy scale. So all the new physics is that energy scale and we would have assumed the same thing that whatever controls all of the dynamics associated with creating a higgs boson whatever it comes from the higgs is going to be at that energy scale it might be the lightest particle of all the mess yep yep and instead the higgs is way down here compared to, I mean, we just, as you said, we see a Sahara desert. We don't see, maybe there's something way off there. Sometimes we get hints of it, but you know, it's often a mirage. We don't see anything. So let me get you on this. This is fantastic. You're saying that in the story with the strong force, that was sort of exemplary. That's given us a lot of intuition for how things are supposed to behave, that when we have the energy scale for the strong force, we see this zoo of particles

all kind of in the same zoo. Exactly. Right? I mean, they're all in more or less the same scale. Okay, give or take a factor of five here or 10 there or two there. But is it like the Higgs is just this loner? It's like the Higgs is the only thing in its own neighborhood. There's no zoo. So far, there's no zoo. Oh my God. So it's people worried about it. I mean, it really was the 70s, mid to late 70s, when Ken Wilson pointed out that the pure standard model with just a Higgs is horribly fine-tuned. Okay.

That there's an instability just in the calculation or the contributions to the Higgs is horribly fine-tuned. Okay. That there's an instability just in the calculation or the contributions to the Higgs mass. And that instability is infinite unless you just say there's some energy where there's new physics. That kicked off an exploration of ideas. One was that there was no Higgs at all, that the behavior of the Higgs was just some new, strong, is that there was no Higgs at all, that the behavior of the Higgs was just some new strong confining group that did all the things the Higgs is supposed to do, give masses to particles.

It was hard to build such models. People came up with supersymmetry as a symmetry that could protect the Higgs as long as you had the partners, the rest of the zoo, to stabilize the Higgs mass. And that all happened within a few years, sort of 1978 to 1981. Those were the sort of initial ideas for these sorts of theories. And then later, wilder theories appeared. The early 2000s was an idea that maybe there is no more energy scale above the Higgs. Maybe even the scale of quantum gravity, which would be the Planck scale, for some manipulative reason, is not 17 orders of magnitude heavier than where we think the Higgs is.

It's actually right on top of it. But to do that, you need to do something slightly crazy, which is you posit the existence of extra dimensions of space. And extra dimensions of space do something very funny to gravity. It allows it to be extremely strong at a much lower energy than the Planck scale, but it dilutes in such a way in the extra dimensions that we would estimate it as being much weaker and not seem strong until a much higher energy. But that, in some sense, numerically solved the problem,

but physics-wise didn't really solve the problem. Yeah. Because we don't know what the theory of quantum gravity is. We don't know that the Higgs could have come from such a theory and why it would, and we have no predictions. So it was less satisfying, but experimentally, it was a bizarre possibility that was not ruled out. And so people could look for that possibility in totally different ways. But all of those type of theories, that whole class, were theories that suggested there's new stuff at the Higgs mass or

just above it. And so far, we haven't seen anything like that. So this is great because this gives a real intellectual motivation that I was expecting there would be, but I never really understood what it was. I've heard this phrase fine-tuning forever or for my whole scientific life, but I was never quite sure what it meant. And so, at least in this respect, the Higgs, as good as it is and as valuable as it's been to physics, it has created a lot of headaches, it seems. Yes. Because it's turning out to have properties, this fine tuning. You're telling me that the fine tuning, whatever we call it, enigma, somehow the multiverse is going to help us address that?

Is that the idea? Okay. Let's hear how. So what I would say is that fine tuning doesn't tell you the theory is wrong. It just tells you that it smells bad. And you think, you know, I have a theory, it does such and such at that. And you fine tune some of the parameters, you know, by one part in 10 to the 34, which is what you would need if the new physics is at the Planck scale. One part in 10 to the 34, which is what you would need if the new physics is at the Planck scale. One part in 10 to the 31 for cancellation

among all the different parameters, just so that the Higgs ends up at a mass which is so completely different than the scale of the physics that generated the Higgs in the first place. So that we'd say, well, that stinks. It could be true, but maybe we should use that hint to explore what else is going on. And that's where all these other theories came to say, oh, no, no, there could be new particles right around the mass of the Higgs.

It cancels infinities. The Higgs is composite, whatever, even extra dimensions. There's no high energy scales. But the other possibility is it is fine-tuned, and perhaps there's a totally different type of explanation for why it is fine-tuned. And now this is where you think, well, maybe we're thinking parochially, which is that we describe the laws of nature, but we do it in a very rigid way. We say locally in space-time, the part of the universe we've seen, this part has these static, unchanging, uniform across the space, laws of nature. That's all there is. And the Higgs mass is just super weird,

and there must be some fine tuning. But the other possibility is, some people call it a multiverse, mundanely, the parameters could just be different in different patches of the universe. And the parameters that control the Higgs mass could be different in different places. And in fact, because the Higgs interacts directly or indirectly with essentially every part of the standard model, then any parameter change in any part of the universe would change the mass of the Higgs, the properties of the Higgs. Now, you could imagine that the Higgs mass is naturally extremely heavy, and it's near all of its family, the particles, the excitations, the dynamics that created the Higgs in the first place, that for typical values of the parameters in the full theory of

the universe and of nature, that the Higgs mass is always roughly at that energy scale, a much higher energy scale than the one we've seen experimentally. But since the laws of the universe are somewhat different in different locations, then the Higgs mass itself will be different in different locations. And if the universe is vast enough, there should be locations where the mass of the Higgs is sort of anomalous. There's some accidental cancellation between the different quantum fluctuation contributions to the mass of the Higgs, which means accidentally there could be parts of the universe where the Higgs is exponentially smaller than it should be, the mass of the Higgs. That would require an exponentially large number of universes to allow that to happen randomly, but who knows how big the universe is? Who knows how the parameters vary? So this is an in principle possibility, but then you have to ask, well, why do we live

in the aberrant universe with the really crummy cancellation that makes it very hard to discover anything? And the answer would be that the Higgs or any of the underlying physics have a lot of control over whether we exist or not. And instead of calling us us and making it anthropic or personal, we can just talk about it as structure. Here's a very simple analogy. We live on Earth. We don't live in empty space. Why don't we live in empty space? Well, of course, we are part of the Earth. We are born out of the material that made the Earth.

It made us as well. Why are we near the sun? The Earth was made near the sun. Why are we near the sun? Because biological beings perhaps aren't living in places where there are no significant sources of energy. There's no fine-tuning argument to explain why we live on a planet Earth rather than the 10 to the 60 times bigger volume, which is the rest of the empty universe. So we don't talk about that as a problem, of course.

But what we can imagine is that the universe itself has different patches with different laws. And what you find with the Higgs mass is it's possible that for a huge range of Higgs masses, what we would call chemistry doesn't exist, which means molecules don't bind, which means structure cannot form in those places. Nobody has explored the possible laws of nature from all possible underlying laws of nature. In other words, if I give you the standard model, you wouldn't come back to me and tell me about the existence of a giraffe. You can't predict a very chaotic nonlinear process to tell me what could exist there. But at least at sort of the baseline level, you do need something which is non-trivial to happen. And you could imagine that the Higgs mass better be very low compared to the Planck scale, the quantum gravity scale, in order for structure to form in reasonable ways without creating black holes, for example. So you can imagine

rules in which there are special values of the Higgs mass, which could be extraordinarily fine tuned, but in those universes or those parts of the universe, truly non-trivial things happen, like the formation of stars and planets and anything on planets. So if I can just try to, I don't want to oversimplify what you said, but I think it sounds like what you're saying is that if you happen to live in a hospitable patch, meaning that fine-tuning just randomly happened in your patch, you, you meaning you molecules, you atoms, have a shot at developing the kinds of structure that can lead to sentient introspective life. And then can get puzzled about, gee, how come we're so lucky that we live in a place where this can happen? And it's kind of because that's what happened. The other parts are stillborn. They can't ask the question. They don't have any life. They don't have any consciousness. Exactly. We're observers, but we're part of the system. And so we have to be in a place where the laws of nature are such that in this region, we would be created.

So we have what we'll call an observational bias. Yes. People see this in astronomy all the time. You look out at the stars and count stars and say, oh, this is how many stars I see. And this is how many stars of this type versus how many stars of that type. But type A stars are very bright and type B stars are very dim. And it's hard to see type B stars. And you may come to the conclusion that there are 20 times as many type A's than type B's. But actually type B's are extraordinarily difficult to see. And so we have an observational

bias. Actually, there are a million times more type B's than type A's. I have no idea how many stars there are based on simple observations. And so I better get much more clever about how I observe. But all of that is in a region where I can observe in principle. The multiverse is a place we can't observe in principle. But there still could be an observational bias based on what the parameters of the standard model are and what forms in different patches of the multiverse. So far, we've been focusing on this fine-tuning kind of motivation for the idea of the multiverse. But I'm wondering, does something like, I remember hearing the phrase eternal inflation

or bubble universes popping out through an inflationary cosmology sort of scenario. Is that another kind of argument for a multiverse? Yeah. And in some sense, it's the easiest way to spit out a multiverse, assuming that it makes sense. So that the two parameters in the standard model that invoke ideas of a multiverse are the mass of the Higgs, but also the cosmological constant. And the cosmological constant is something that we have seen significant evidence for

in our universe. It causes the accelerated expansion of our universe. People call it dark energy. But cosmological constant is also, in effect, something in the standard model. It's also something that garners quantum fluctuations. And so the value of the cosmological constant here in this universe could have been any value, really. And it, too, is fine-tuned compared to at least the highest energy scales that we can imagine in physics, which in this case, again, is the Planck scale.

And that fine tuning is something like one part in 10 to the 123. So you have to cancel something in the new physics down to 123 digits in order for our universe to have a nice, small cosmological constant. So I think I remember you saying earlier, there are two numbers that are weird. So have we now come to that point that one is the mass of the Higgs and one is the cosmological constant? Correct. Those are the two. It's so interesting.

I mean, you have to feel kind of blessed to be alive to think about this. Maybe literally blessed, because if you want to get theological, I happen to not be religious. But if a person is religious, it's kind of tempting to go there, right? That these are two miraculous things that had to happen. These numbers had to be fine-tuned for us. Okay, I don't know. I get those sorts of questions when I was doing Q&As. And another explanation for fine-tuning is not that there's a statistical sample large enough to incorporate things on the tail of the distribution, but the things on the tail of the distribution are important for life and structure.

You could also say, no, there is a supreme being that has set the number the way they are because the supreme being really wanted life to exist and yes you could think of it in that way i don't have a model for the multiverse that i'm in love with but i certainly also don't have a model for an all-being creator setting these things up either so i don't personally find them compelling in the sense that my day job is to try to figure out what the heck is going on. And neither of those really have a lot of teeth. Not saying that the multiverse is not true, but if you want the multiverse to solve the problems of our universe, it does a sort of mediocre job of it.

All right. But back to the many bubble universes. Sure. What does this have to do again with the cosmological constant? So the cosmological constant itself is the one energy density in the universe that does not dilute. That's why it's called a constant. And what that means is when the universe expands, its expansion, which is driven by what's in the universe, changes the relative amounts of matter, of radiation, and of this constant. The constant is not diluting, but everything else is. And that means at some point in the history of the universe, the cosmological constant wins, and the expansion of the universe is driven by just the cosmological constant.

And a cosmological constant expands the universe much faster than other types of matter. In fact, most types of matter, while they allow for a certain rate of expansion, they also tend to slow the expansion. You can think of it in some ways as matter is attracted to itself or radiation is attracted to itself gravitationally. It's not a perfect analogy, but it's trying to slow that expansion. A cosmological constant, just the way it works in general relativity, it gives you the opposite sign. It tends to speed up expansion. And it speeds up

expansion proportional to itself in a way. So we would describe it as exponential expansion. And so what happens is when the cosmological constant eventually becomes the dominant energy of the universe, it starts to expand the universe exponentially fast. And if that happens too early in the history of the universe, there is no time for anything in the universe to form. Galaxies don't form or stars don't form or any sort of clumping of matter whatsoever would not have a chance to form because the cosmological constant expansion would blow everything apart. Nothing gravitational would form in the early universe.

So all the structure that lives in this universe that seems to be the important source of non-trivial things, interesting things, stars, planets, life, blah, blah, blah. If the cosmological constant was too big, there is no time in which those things could be created. Now, you can fiddle with other parameters to make sure those things get created at earlier times. But if you fix all other parameters

and say that the cosmological constant maybe is different in different parts of the universe, then those patches of the universe would expand very differently. And the ones with larger cosmological constants would essentially have nothing in them. And so there it's an even more trivial thing that we live where the stuff is, so we have to live in a place where exponential expansion didn't take over before stuff was formed. So let's just, as a slight side note, let's look at our universe and our cosmological

constant, the one that we have experimental evidence for. You can say that if I wait another 14 billion years, or let's say 140 billion years, or a trillion years, the exponential expansion would take over. The cosmological constant would be the dominant energy density in the universe. And you could ask, would our galaxy be ripped apart or their cluster be ripped apart? And the answer is no, in fact. That actually things that are gravitationally bound, like our galaxy, compete well, do better than the cosmological constant.

So locally, the gravity is still more important in the galaxy by the local matter than the cosmological scale of the cosmological constant. But things that are not gravitationally tied to each other, they are going to be ripped apart or pulled apart by exponential expansion. So all you need is that structure forms before the cosmological constant takes over. And weirdly, in our universe, the cosmological constant is roughly just big enough, keeping all other parameters fixed, such that matter just had a chance to form. So we could have seen zero cosmological constant or never detected it, and we'd say, oh, there's some magical reason why the cosmological constant is zero. We could have had a cosmological constant which was, say, a hundred times or a thousand times bigger,

in which case galaxies wouldn't have formed, there wouldn't have been time for the gravitational bound states that create our worlds to form. And therefore, there would be no interesting structure in the universe and no life. So the cosmological constant landed as small as it had to be, but as big as it could be. And when that was measured in 98, it was a sort of wake-up call that, oh, maybe there are parameters that are associated with the fact that we're biased by our observable universe parameters and not by what it would be a natural outcome of a deep high energy or underlying laws of nature. And so then you would imagine that,

okay, maybe it's the same accident. This patch has a tiny cosmological constant due to a bizarre amount of cancellation, but not bizarre if there are 10 to the 500 universes or 10 to the 500 patches where the cosmological constant is different. Most of those patches would have nothing in it, no structure. There could be even rarer parts or patches of the universe with a much tinier cosmological constant, but we're in the most populous type of patch of the universe where the cosmological constant is as big as it could be without destroying us or without causing us not to form.

causing us not to form. So the fine-tuning slash landscape of possible universes, the multiverse, all of that is a statistical argument. We don't know what the priors are. We don't know what parameters we're supposed to vary. But if you keep everything fixed and you just vary the cosmological constant, you get something interesting, which is that the cosmological constant is not far from the value that you would see in a typical universe in which life exists, or galaxies exist. Wow. It really is very philosophically mind-blowing thinking about all this, that our existence, I mean, it makes this anthropic principle, which goes back, I don't know, is it from the 1960s or something? But it seems more important all the time. I think that the anthropic principle often was more tautological.

Yeah. But here, it really is attempting to remove observational bias, which is to say that we have to take into account that the measurements we're making are the ones we have access to. And the fact that we're here means that we have access to a certain type of measurement or a certain range of parameters. We would not have been able to measure a cosmological constant, which is hundreds of orders of magnitude bigger. It would not have been possible because there would be no structure in existence. You could postulate that there's a different form of life that lives at a different time scale, you know, made of different particles that live long enough, but they don't have to live too long. That could be true. And then you could ask questions,

why aren't we that life? Or if this whole anthropic argument is true, does that tell us that the first real structure comes down to the things made of atoms? It's very hard to parse what can be asked scientifically at that point and what you're really asking about the initial conditions of the universe. And what famously with physics, you get two things. You get dynamics, which are the laws of nature, the dynamical laws, which are differential equations, and then you get initial condition. And the laws of nature are something you can test again and again and again. But the initial conditions are whatever you get. And if there's a strong bias in the initial conditions towards certain things by the way we're doing the measurements, then we're going to get things

that are not fundamental. They're in a sense historical. They depend on the history of this part of the universe. And that could be aberrant or anomalous with respect to a larger scope of what the universe could be. These ideas are so fascinating. Can I ask you to just look into your crystal ball, and we'll end with that. What would notions of the multiverse, cosmological constant, Higgs, all these ideas, but especially the multiverse, where's it going to lead us in physics? There's a lot of things we didn't cover. For example, how quantum mechanics plays with a multiverse and the multiverse that lives in quantum mechanics itself.

There are deeper explorations to do, of course, but what the questioning that came up in the birth of the use of the multiverse to explain parameters are in some sense warning signs that there are certain things that we may never get the answer to, and there are plausibility arguments for not ever coming up with an explanation for the mass of the Higgs or the cosmological constant. But they certainly don't mean that there isn't a different or even better explanation than the multiverse. It is just reminding physicists the scary part of doing physics, which is that it is a high-risk, high-reward game.

And you could be going down a path which you think might be fruitful for many decades and discover that it's a dead end. And you may, and discover that it's a dead end. And you may not even know why it's a dead end. So these are concepts in physics which could have an impact on individuals deciding whether or not solving the cosmological constant problem, why is it so small, is compelling or not, but it hasn't landed us on a solution. And just like the universe has certain limits,

there's an observable universe and there's a certain finite amount that can be discovered in the universe just because of the potentially finite lifetime, if it is. Similarly, all humans have finite life and there may be a finite capacity to discover all of the laws of nature. I personally have not been completely sold on the multiverse as explanations for these things, but I think they are plausible. But it doesn't stop me from thinking about those problems in different ways. It just raises the bar on the quality of the potential solutions I'll be looking for. Well, thank you. This really has been a fascinating conversation. David Kaplan, thanks for joining us today. Sure. My pleasure.

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