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By definition, the universe seems like it should be the totality of everything that exists. Yet a variety of arguments emerging from cosmology, particle physics and quantum mechanics hint that there could also be unobservable universes beyond our own that follow different laws of nature. While the existence of a multiverse is speculative, for many physicists it represents a plausible explanation for some of the biggest mysteries in science. In this episode, Steven Strogatz explores the idea of a multiverse with the theoretical physicist David Kaplan and learns what it might mean about our own existence.

Listen on Apple Podcasts, Spotify, Google Podcasts, Stitcher, TuneIn or your favorite podcasting app, or you can stream it from Quanta.

Steve Strogatz (00:03): Im Steve Strogatz and this is The Joy of Why, a podcast from Quanta Magazine that takes you into some of the biggest unanswered questions in math and science today. In this episode, were going to ask: Do we live in a multiverse?

(00:16) 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 wont 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 were not alone, and what we can see with the help of telescopes and infrared cameras isnt 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.

(01:08) Its a mind-blowing concept, the idea that we could live in a multiverse. And its 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.

David Kaplan (01:38): My pleasure.

Strogatz (01:39): Well, this is great. Im very curious to hear your take on the multiverse. Before we get to the multiverse, lets talk about the good old-fashioned universe. 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?

Kaplan (01:57): 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 theres 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 couldnt 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 couldnt pass through the universe.

(02:53) In other words, in the early universe, if it was smaller and hotter and denser, light didnt 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. Thats 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 like a finite time to get to us, we just mathematically cannot see beyond a certain point.

Strogatz (03:57): 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.

Kaplan (04:04): When it comes to the multiverse or the initial start of the universe, all of those things theyre 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 whats going on is when you get to densities of order its called the Planck energy The description of gravity in terms of a geometry or a geometric theory of the universe breaks down. It doesnt mean that there isnt time and space of some other kind at earlier times. Its just that general relativity no longer applies in that time.

(04:53) And so, could it be that time goes back infinitely far? A different label of what we call time? Absolutely. We dont know. We dont 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.

(05:14) So people pop in and say, Well, time and space get created sort of instantaneously out of nothing. But thats not a mathematically well-defined description. Its a sort of hopeful compactification of space and time. And if you really have nothing, its hard to make predictions about when something appears because theres nothing there.

(05:37) 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 its possible that the laws of nature in different patches are different. You dont even need the deep underlying laws 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 whats in the universe, and what are the interaction strengths of the stuff in that part of the universe. And so you can really have very different universes 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.

(07:03) And so theres a simple idea of the multiverse, which is that there really is just one universe. The part of the universe that we exist in has 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 [in] 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.

(07:57) 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 doesnt 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?

Strogatz (08:47): Sure. So I liked 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. Its 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 cant, by definition, we cant observe those parts. But theres 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, this wild You can tell Im a little skeptical here.

Kaplan (09:36): Of course.

Strogatz: It feels a little like wild philosophizing. But I also know that physics is very coherent and you have reasons for this. Its not wild philosophizing.

Kaplan (09:44): 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 weve 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, every one we have seen weve seen all those particles, and we havent seen any others, directly at least.

(10:55) 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.

(11:26) 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, its proportional to the mass of the Higgs boson. Whats the mass of the top quark, the heaviest quark? Its proportional to the mass of the Higgs boson. The W weak force boson is proportional to the mass of the Higgs boson.

(12:02) 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. Youve measured it, weve measured the mass of the Higgs, weve measured the mass of the other particles and their interactions with the Higgs. And that allows us to fix experimentally what that number is.

(12:30) Now, if you take a quantum field theory that 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, thats OK. Because the reason I get infinity has to do with the fact that Im 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 lets call them quantum fluctuations in the calculations of any physical parameters. So you have a few parameters you put into your theory.

(13:18) 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.

(13:40) We dont 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 its 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 there are no more contributions to the Higgs mass. Its finite, everythings OK. Youre 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, lets 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 cut off 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 to these arbitrarily high energies or masses.

(14:43) And so the Higgs mass should be the mass thats 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.

Strogatz (15:10): I see, yeah.

Kaplan: So instead of saying theres 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, its going to come with all this new stuff. Because the Higgs mass is not infinite, its finite.

Strogatz (15:27): So let me can I just ask I think I got you. This is a new idea to me, very interesting. If I hear you right, youre saying the Higgs is going to set the borderland between the known and the unknown.

Kaplan (15:38): Yes.

Strogatz: And because its an edge case, sort of, it is on the borderland

Kaplan (15:43): Indeed.

Strogatz: that means when we discover it, Im going to see on one side the stuff that we already kind of understand. But because its a borderland particle, theres going to be stuff on the other edge of the border. And thats what were going to be thrilled to discover, and measure its property too, because thatll be new physics.

Kaplan (16:01): 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. Weve seen the proton is made of quarks; maybe the Higgs is made of other stuff. And at high energies, actually, theres no Higgs, theres stuff inside, and were seeing other fundamental particles. So it can be even a borderline of its own existence. And at high energies, theres a completely different description.

(16:52) These were the sort of quantum field theories that people introduced and explored, that we all wondered could be what were seeing the hint of. That what seemingly looks like nonsense with the Higgs mass is only a statement that theres 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.

(17:20) And that was the hope people had when the LHC turned on Large Hadron Collider turned on. And they would discover the Higgs and theyd discover some of these other new well 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.

(17:47) And thats why in (I think) 2003, I started thinking, But it could be that thats 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.

Strogatz (18:35): So its sort of like in my picture in my head, where Ive got this region of the known, and then this borderland region, and then the region of the new stuff.

Kaplan (18:44): Right.

Strogatz: In fact, when I go into the region of the new stuff, its the Sahara Desert for as far as the eye can see

Kaplan (18:49): Right.

Strogatz: except that theres something way the heck off the edge of the map.

Kaplan (18:53): 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 and new laws that govern how the Higgs behaves, doesnt know anything, in some weird sense, about the low energy, low-lying masses. So theres 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.

(19:37) 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. Thats 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? Its made of quarks. Its about five times that. Whats the mass of the neutron? Its about five times that. What are the masses of the various mesons made of quarks and antiquarks? Its between a few times that and a few Theres a very light one, for symmetry reasons, but its about half that energy. Theyre 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.

(20:30) So all the new physics is that energy scale. And we would assume 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. And instead, the Higgs is way down here compared to I mean, as you said, we see a Sahara Desert, we dont see Maybe theres something way off there! Sometimes we get hints of it. But you know, its often a mirage. I dont see anything.

Strogatz (21:04): So let me get you on this. This is fantastic. Youre saying that in the story with the strong force, that was sort of exemplary. Thats 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.

Kaplan (21:24): Exactly.

Strogatz: Right? I mean, theyre all in more or less the same scale, OK, give or take a factor of five here or 10 or two there. But is it like the Higgs is just this loner? Its like the Higgs is the only thing in its own neighborhood? Theres no zoo?

Kaplan (21:38): So far, theres no zoo.

Strogatz: Oh, my god.

Kaplan (21:40): So its people were 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 the Higgs is horribly fine-tuned, that theres an instability just in the calculation or the contributions to the Higgs mass. And that instability is infinite unless you just say theres some energy where theres new physics.

(22:06) 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 confining group that did all the things that 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.

(22:46) And then later, wilder theories appeared. In 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, its 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 much higher energy. But that, in some sense, numerically solved the problem. But physics-wise, didnt really solve the problem because we dont know what the theory of quantum gravity is. We dont know that the Higgs could have come from such a theory and why it would, and we have no predictions. So it was a 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.

(24:05) But all of those type of theories, that whole class, were theories that suggested theres new stuff at the Higgs mass or just above it. And so far, we havent seen anything like that.

Strogatz (24:16): 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. Ive 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 you know, valuable as its been to physics, it has created a lot of headaches, it seems. Because its turning out to have properties, this fine tuning Youre 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?

Kaplan (24:53): Yes.

Strogatz: OK. Lets hear how.

Kaplan (24:55): So what I would say is that fine-tuning doesnt tell you the theory is wrong, it just tells you that it smells bad. You think, you know, I have a theory it does such and such. And you fine tune some of the parameters, you know, by one part in 1034, which is what you would need if the new physics is at the Planck scale, one part in 1034 for cancellation among all the different parameters just so, 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.

(25:30) So that, wed say well, that stinks. It could be true, but maybe we should use that hint to explore what else is going on. And thats 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, theres no high energy scales. But the other possibility is, it is fine-tuned and perhaps theres a totally different type of explanation for why it is fine-tuned. And now this is where you think, Well, maybe were thinking parochially, which is that we describe the laws of nature but we do it in a very rigid way. We say locally in spacetime, the part of the universe weve seen, this part has these static, unchanging, uniform across-the-space laws of nature, thats all there is. And the Higgs mass is just super weird, and there must be some fine-tuning.

(26:26) 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.

(26:57) Now, you could imagine that the Higgs mass is naturally extremely heavy, and its 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 and 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 weve 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. Theres some accidental cancellation between the different quantum fluctuation contributions to the mass of the Higgs.

(27:53) 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 in principle a 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?

(28:29) 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. Heres a very simple analogy. We live on Earth; we dont live in empty space. Why dont we live in empty space? 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, arent living in places where there are no significant sources of energy. Theres no fine-tuning argument to explain why we live on a planet Earth, rather than the 1060 times bigger volume, which is the rest of the empty universe. So we dont talk about that as a problem, of course.

(29:37) 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 its possible that for a huge range of Higgs masses, what we would call chemistry doesnt exist, which means molecules dont bind, which means structure cannot form in those places. Nobody has explored the possible laws of nature from all possible underlying laws of nature.

(30:08) In other words, if I gave you the Standard Model, you wouldnt come back to me and tell me about the existence of a giraffe. You cant predict a very chaotic nonlinear process to tell me what could exist there. But at least its sort of the baseline level. You do need something which is nontrivial 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 nontrivial things happen, like the formation of stars and planets and anything on planets.

Strogatz (31:06): So if I can just try to I dont want to oversimplify what you said. But I think it sounds like what youre 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

Kaplan (31:30): Exactly.

Strogatz: and then can get puzzled about, Gee, how come were so lucky that we live in a place where this can happen? And its kind of because thats what happened, the other parts are stillborn. They cant ask the question. They dont have any life. They dont have any consciousness.

Kaplan (31:44): Exactly. Were observers, but were 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 a, what well call an observational bias.

Strogatz: Yes.

Kaplan (32:00): People see this in astronomy all the time. You look out at the stars, you can 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 its hard to see type B stars. And you may come to the conclusion that there are 20 times as many type As than type Bs, but actually type Bs are extraordinarily difficult to see. And so we have an observational bias. Actually, there are a million times more type Bs than type As. 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.

(32:40) But all of that is in a region where I can observe in principle. The multiverse is a place we cant 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.

Strogatz (32:59): So far, weve been focusing on this fine-tuning kind of motivation for the idea of the multiverse. But Im 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?

Kaplan (33:19): Yeah, and in some sense, its 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. Its also something that garners quantum fluctuations.

(33:20) 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 10123. So you have to cancel something in the new physics down to 123 digits in order for a universe to have a nice small cosmological constant.

Strogatz (34:34): 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?

Kaplan (34:43): Correct. Those are the two.

Strogatz (34:45): Its so interesting. This is so, 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, its 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. OK, I dont know.

Kaplan (35:09): I get those sorts of questions when I was doing Q&As. And another explanation for fine-tuning is not that theres 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 dont have a model for the multiverse that Im in love with. But I certainly also dont have a model for an All-Seeing Creator setting these things up either. So I dont 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.

Strogatz (36:00): OK.

Kaplan: Im not saying that the multiverse is not true. But if you want the multiverse to solve the problems of our universe, it does this sort of mediocre job of it.

Strogatz (36:11): All right, but back to the many bubble universes.

Kaplan: Sure.

Strogatz (36:14): What does this have to do again with the cosmological constant?

Kaplan (36:17): So the cosmological constant itself is the one energy density in the universe that does not dilute. Thats why its called a constant. And what that means is when the universe expands, its expansion, which is driven by whats 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.

(36:57) 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. Its not a perfect analogy, but its trying to slow that expansion.

(37:26) 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 dont form or stars dont 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 nontrivial 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.

(38:35) 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.

(38:58) And so there its an even more trivial thing that we live where the stuff is. So we have to live in a place where exponential expansion didnt take over before stuff was formed.

(39:11) So lets just, as a slight side note, lets 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 lets 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 the cluster be ripped apart?

(39:40) 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.

(40:11) 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 wed say, oh, theres some magical reason why the cosmological constant is zero. We could have had a cosmological constant, which was, say 100 times or 1,000 times bigger, in which case, galaxies wouldnt have formed. There wouldnt 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.

(41:05) And when that was measured in in 98, it was a sort of wake-up call that, oh, maybe there are parameters that are associated with the fact that were biased by our observable universe parameters, and not by what would be a natural outcome of a deep high energy or underlying laws of nature. And so then you would imagine that, OK, maybe its the same accident, this patch has a tiny cosmological constant due to a bizarre amount of cancellation, but [thats] not bizarre if there are 10500 universes, or 10500 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 were 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.

(42:12) So the fine-tuning/landscape of possible universes, the multiverse, all of that is a statistical argument. We dont know what the priors are, we dont know what parameters were 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.

Strogatz (42:44): 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 dont know, is it from the 1960s, or something? But it seems more important all the time.

Kaplan (42:58): I think that the anthropic principle often was more tautological. But here, it really is attempting to remove observational bias, which is to say that, you know, we have to take into account that the measurements were making are the ones we have access to. And the fact that were 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, you know, hundreds of orders of magnitude bigger. It would not have been possible because there would be no structure in existence.

(43:37) You could postulate that theres a different form of life that lives at a different timescale, you know, made of different particles that live long enough, but they dont have to live too long. That could be true. And then you could ask questions: Why arent 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?

(44:02) Its very hard to parse what can be asked scientifically at that point, and what youre really asking about the initial conditions of the universe. And 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 conditions. 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 theres a strong bias in the initial conditions toward certain things by the way were doing the measurements, then were going to get things that are not fundamental. Theyre, 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.

Strogatz (44:56): These ideas are so fascinating. Can I ask you to just look into your crystal ball, and well end with that. What would notions of the multiverse, cosmological constant, Higgs, all these ideas but especially the multiverse wheres it gonna lead us in physics?

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Are There Reasons to Believe in a Multiverse? - Quanta Magazine