Endless Universe: Beyond the Big Bang | Paul Steinhardt | Talks at Google

Thank you. It's a pleasure to be here. So I'm going to try to rock back and forth a little bit so that no one has a completely obstructed view seat. But this post in here is an interesting challenge. Well, the book we're going to be talking about today that we tried to write is really, from our point of view, a tale of two universes, or more precisely, a tale of two competing theories for explaining the real universe in which we live. One of these tales is what we might call the standard picture, the standard Big Bang picture, and nowadays it's more often called the Big Bang inflationary picture. It's the idea that has been in development for, oh, nearly a century. It's the idea that most of you have read about in school. It's the idea which most cosmologists adhere to, and we might call it the currently reigning champion. And we're also talking about a second story or a second tale about the universe, a challenger, a new idea that really is just born in the 21st century. And this is the idea of a cyclic universe or an endless universe. The idea, as we'll see, matches the standard Big Bang theory in, in terms of the evolution of the universe over the last 10 billion years or so, but gives a radically different view of what the universe was like before and where it's headed in the future. Now, the interesting situation at the moment is that we have this long standing champion and this new challenger. Both of them are able to. Sorry, both of them are able to explain all the wealth of data we've gathered in the universe over the last decades. Both of them are able to explain it in the same exquisite detail. Ah, okay. They both should be on. That's that one that's on, right? Are we better there? Okay, sorry. Okay, so what I was saying is that both these stories of the universe, both the conventional Big Bang inflationary picture and the new alternative we'll be talking about, were able to explain the entire wealth of data we have about the universe at present with the same exquisite detail. So essentially the same data can be explained by two different stories. So far, although, as we'll talk about in the future, there are things we can measure that might help distinguish them, but at the moment, we can't distinguish these two stories. So, in fact, we intentionally wrote this book at an interesting time in which you can capture science, real science, as it's happening before you know the answer. I don't claim to know which of these theories is right before you know the answer. When you have a Long standing champion, a new challenger, a new. And neither the theorists nor the experimentalists know which is right. Each of us are risking or taking a certain amount of risk and pursuing whichever idea we choose, not knowing which answer is going to be right. And this kind of high risk science is, you know, that's what science is really about. That's what makes it exciting. I don't have to explain to you this analogous things here at Google. It's taking this kind of risk which makes the subject exciting. And there's a lot at stake as we'll talk about. It's not simply a matter of understanding the cosmos and the history of the cosmos. Really the outcome of this debate very much will have a bearing on our view of how much we can learn about the universe, the fundamental laws of physics generally. What are the limits of science, given the fact that we can only observe a certain patch of the universe, how much can we really learn about the entire universe based on that limited information that's at stake as well? So I'm going to begin, I'm going to actually talk about both tails of the universe, both beginning with today's Big Bang picture. And then we'll talk about the alternative cyclic picture. So I've called it Today's Big Bang Theory to emphasize the fact that today's Big Bang theory is different than many of the. What is for many of you, the story of the Big Bang that you might have learned in school. So the story that you might have learned in school is a very simple story. There was a moment in time about 14 billion years ago when the universe sprang from nothingness into somethingness, into a fireball of space, time, matter and energy. And it's been expanding and cooling ever since. Simple story. And when it hears that the Big Bang theory explains everything we observe, it sounds like it's a simple story explaining a lot. But as I explained, the theory as it stands today is really more complicated than that. As we've learned more about the universe and more about the theory, we've had to add more and more bits to the story. So the story is actually not. Today's Big Bang theory is not nearly as simple as the theory you learned about in school. And that's important because that gives a motivation for thinking about alternative stories which might be simpler. So today's Big Bang theory still begins with an assumption. It is an assumption. It's an unproven assumption that somehow it's possible to spring from nothingness. That means no space, no time, of course, no matter and energy into somethingness and that once you've sprung that way, then, well, then things proceed more according to laws of physics we understand better. The space expands and the matter and radiation spread out so that things cool over time. But if that were the entire idea at the beginning of the universe, we'd have immediately a problem, which is when the universe would begin from such a violent, rapid event, space and time would likely be highly distorted and warped, and the distribution of matter would be very uneven. So if all you did was have a Big Bang and then just let it expand under the influence of the matter and the gravity inside it, what would happen is that when you looked at the universe today, you'd still see, when you looked at the universe in the large remnants of a very inhomogeneous, non uniform, curved, warped universe that it began with simply just stretched out. And that's not what we see. One thing we've learned for sure about the universe over the last few decades is that on large scales, it's remarkably uniform. You look in opposite directions in the sky, and the distribution of matter and radiation, the temperature of the universe, the density of the universe is the same to within one part and 10 to the fifth, so it's remarkably uniform. In contradiction to this idea of simply beginning with a violent Big Bang and letting it go. And for that reason, an amendment is immediately put in. A second assumption is immediately put in to fix the problem, which is the assumption of inflation, that after the Big Bang, after you emerge in this rather turbulent, disordered state, something happened that smoothed out the universe. And this is this inflation. And what inflation is, is a period of extraordinarily rapid expansion in which the universe doubles in size at a fixed rate of time. Every 10 to the minus 35th seconds, it doubles in size, accelerating, in other words, in its expansion. So that, let's say in 10 to the minus 30th seconds, it expands by a factor of two to the 100,000 or more. So that's enough expansion to take a region which is smaller than an atomic nucleus and expand it to, well, exponentially larger than the entire patch of universe. We're able to see today an extraordinary rate of expansion, which has the feature that by stretching out what is initially a curved and warped and non uniform distribution, tends to smooth it out, producing the kind of smoothness and uniformity that we see in the patch of the universe that we see today. So we would be born from part of the universe we see today, which is about 14 billion light years in radius. That's a patch of one larger patch of universe that has inflated in this way. And this inflation not only smooths the universe, not only removes any curvature of space and any warps in space that you have, but it has one other interesting feature which has to do with the way it ends. Well, assumption about the way it ends. So, first of all, in order to have this inflation, you have to assume there's a form of energy which causes the universe to inflate. It isn't matter or radiation. It has to be some other peculiar form of energy, which maybe you can ask me about afterwards, but I'm not going to say too much about. But the other thing you have to have is that this form of energy is unstable. It has to decay in the same way that a uranium nucleus decays so that we can end the inflation and turn this energy, this inflationary energy, into something which then produces the matter and radiation that composes the galaxies and the stars and ourselves. So it has to have a finite lifetime, last for only a short time, and then decay. And it decays through a quantum process. And a quantum process is inherently unpredictable, not perfectly predictable. It has some degree of randomness to it, so that when the inflationary energy decays, it doesn't decay everywhere at exactly the same time in the same way as if I had a row of uranium nuclei, they wouldn't decay exactly at the same time. There's some average decay time, and then there's some variance around it. And what that means is that when the universe turns, when this energy is converted to matter and radiation, you don't produce a perfectly smooth universe. You produce a kind of wrinkled structure to the universe, a wrinkled distribution of matter and radiation, which is what this little map is supposed to represent. This is supposed to represent sort of the entire surface of the furthest distances in the universe we can see today, mapped into an oval. And the colors are supposed to represent the variations in temperature and density in the universe, which are due to the quantum effect due to sort of a wrinkling effect due to this quantum uncertainty as to when inflation ends. And this is a key prediction of the inflationary prediction that you get a pattern with very special statistical properties. And what you're looking at here is actually not a theoretical prediction, but an experimental measurement. It's a measurement from what's called the Wilkinson Microwave Anisotropy Probe, or WMAP satellite. And it's actually an image taken by actually gathering light from the most distant regions of the universe and taking a photograph of what the universe looked like when it was a few hundred thousand Years old. And so far as we know, to the accuracy to which this has been measured, which is pretty fine, it matches beautifully with the inflationary predict. So that is what gives many cosmologists strong faith in the fact that this Big Bang inflationary picture is in excellent shape, some would even say proven on this basis. But you have to be careful because I gave you a story which gave you a result. You have to be sure that's the only story that could give that result. That's the issue that we're going to be fencing with today. Okay, so this uneven end to inflation leads to something good, which is this wrinkling, which is consistent with what we observe. And it is not only observed by the WMAP satellite, but it also forms the seeds. This slight wrinkling in the distribution of energy is what also forms the seeds for the formation of galaxies. So you can go from the theory to this picture, and from this picture and theory to a theory of what the distribution of galaxies should be, what should be their statistical properties. And it also agrees. That's a very important verification of this whole framework. Once you have this inflation, the idea is that then, although you've had to make some assumptions, you have to assume a Big Bang, you had to assume a special form of energy that decays in a certain way, you had to make a number of assumptions. But once you've done that, now you've set up the large scale structure of the universe, even including the wrinkles. And in principle at least, the idea was you should be able to explain everything else that happens in the universe from that point onwards, hopefully without making further assumptions. But it hasn't quite worked out that way. It's true that when inflation ends, you get a burst of matter and radiation, and then that radiation begins to condense to form atoms and molecules and structure and the like. We go through a phase of so called radiation domination and matter domination, and we can confirm those phases. We can actually measure things about and confirm that the theory works well over that period. But recently, so about a decade ago, we discovered something we didn't expect that wasn't predicted by the inflationary picture, and that is that a new form of energy has overtaken the universe, so called dark energy, an energy which is causing the expansion of the universe to speed up again. So inflation was actually a speed up in the expansion of the universe in which the universe was doubling at a constant rate. This has started again in this picture. You'd say it started again, except the doubling rate is different. Before it was once every 10 to the minus 35th seconds. Now it's about once every 10 billion years. Now, that's long compared to a human lifespan or even to the lifespan of a star. But it's a finite amount of time. And if you continue doubling every 10 billion years and you come back in a trillion years, okay, that's 100 doublings, you will find that you have essentially an empty universe. So in this Big Bang picture, we begin from nothingness. We have the universe evacuated by inflation. Then finally we have matter and radiation that we're familiar with formed. And then just about the time that we've come around, dark energy has taken over and the universe is going to be returned to a uninhabitable wasteland, vacuous universe. And the universe as we know it, the matter as we know it, only exists within a, what is a sort of cosmic scale. It's an instant of time sandwiched between two periods of vacuum, one dominated by inflation, one dominated by dark energy. Whatever you think of that picture, once you put all the, if you're willing to put all these ingredients together, you can explain all the data that we have. You may or may not like this picture, but it is consistent if you put in all the assumptions and ingredients with what we have, including the assumption that there exists some form of dark energy with just that was produced at inflation with just the right amount of concentration, that it would agree with the data. So you have to add yet another form of energy and another assumption to go along with it. And it seems to have no connection to the inflation. It seems to be added ad hoc. So if you talk about today's Big Bang theory, although on the one hand it agrees with the data, it agrees at a certain price. And that's the thing one should be concerned about when one's comparing different theories. First of all, it's definitely finely tuned. I didn't go into the details, but to get this inflation to give really the right statistical distribution of wrinkles that we see in the microwave background radiation in this WMAP satellite, the energy has to be have very specific properties. It has to have a very specific amount of energy, a strength of energy, a concentration of energy, and it has to decay in a very specific way, otherwise you get the wrong pattern. So it works. But the cost of fine tuning, as I already mentioned, when you add the dark energy part of the story, and it also works, but again, you have to add that in by hand at the cost of fine tuning. Secondly, as should have been apparent from the way I described it, it is Kind of a patchwork. Every time we've discovered something new about the universe, its smoothness, we had to add inflation, the recent acceleration, we had to add dark energy and other things I kind of talked about, like dark matter as well. Every time we've discovered something about the universe, we've actually had to amend the picture. So we began with a very simple idea in the 1920s. Just the universe emerging from nothingness into somethingness and expanding and cooling. But the subject, it's definitely gotten more complicated over time. And thirdly, the model doesn't really work exactly as advertised. And this is where we get to some really interesting issues. What I described to you is the way the model is usually described in most text and newspaper articles. Okay, so you have Big Bang. It smooths the universe out, everything ends inflation, and you end up with a nice smooth universe, except for those wrinkles. It's really not the way the model works that would suggest that first you smooth it. That's what that gray like square is supposed to represent. And then it decays and forms stars, and everywhere in the universe looks the same as it does here. That was the initial idea back in the 1980s, but that's actually not what happens in inflation. What really happens is something more subtle. This is how it really works. What happens is you have the. Inflation comes and dominates the universe. It makes some patch of it smooth. And then I said the inflationary energy begins to decay. Well, you might imagine that some large patch of sub patch, like I've drawn it here as a circle. So some large piece of that patch in fact decays and forms matter and radiation as we know it. And it may not be as regular as that circle, but some large fraction. But it's not 100% of the original patch due to the same quantum effects that regulate the rate at which the inflationary energy decays, that control whether it decays early or late through that same random process. It's always possible that there's some region left behind that hasn't ended inflation. Just like if I have a block of uranium. It's always possible that one of those uranium nuclei hasn't yet decayed, even after, well, after the average lifetime. Now you might say, what do I care? Almost all of it is decayed. Well, the reason why you have to care is because inflation amplifies these rare events. The region that's left behind keeps inflating while the region that ended inflation has stopped. So initially, most of it is completed inflation. Only a little bit has ended up. Afterwards, the situation is reversed. Okay, the pocket of ordinary matter universe is now, hasn't grown very much, while everything that was left behind with those rare quantum fluctuations has expanded by a huge exponential factor. It dominates the volume of the universe. So the universe is kind of turned inside out, instead of most of it having ended with only a few bits left behind. The real story is when you include the inflationary effect, is that almost none of it has ended and, and only pockets are left behind. We would live in one of those pockets. And of course, after time, a new pocket would form and a new pocket would form and the structure of the universe would look something like this. Not a nice uniform, not a simple uniform universe where each one of these pockets would be a place where inflation has ended and has sort of normal universe consisting of matter and radiation, you might think, and the rest of it would be still inflating. Now when people first realized this idea, they thought, hurrah, this is even better than we hoped. We don't just make one universe which is habitable, we make an infinite number of universes which is habitable. That's great news. Ah, but remember why this occurred. This occurred because inflation is amplifying rare quantum events, rare random events. And when you think about that a little bit, if you amplify something that's quantum, which is inherently random, and instead of making the universe very ordered, you're going to amplify randomness, you're going to amplify the disorder. So actually, if you look at these pockets, it turns out when you include the effects of this random quantum effects, they aren't all the same. Some of them will be habitable and look like ours, and some of them will not be. Some of them will be barren or have different physical, some of them will have the same physical laws as ours, and some of them will not. When I say some, I should have said an infinite, an infinite number of them will be like ours, an infinite number of them will not be like ours. And you might say, okay, well, which is more probable to be like us or to be like something else? Well, it seems that this distribution that we get is in a fundamental mathematical sense, not normalizable, which is to say we can't answer the question. If you have an infinite number of quantities of two things, you can't say which is more probable than the next. It's not a well behaved distribution. So in fact, although people talk about inflation, predicting this, that or the other thing, it predicts that for an infinite number of pockets, and it predicts something different for another infinity of pockets. So, for example, it predicts an infinite number of them which should be flat, and an infinite number of them should not be. We can't really say, can't really speak with confidence now, but what even inflation predicts, and this fact about inflation is it's really only known to those who work on the theory. It's not been so spread out. You don't see that in the general literature. But it's a very important point and it sort of conveys, and it's important to understand that because it's a motivation for thinking about alternatives. So I would say the Big Bang theory that we were headed for, we were hoping was a simple natural explanation of the universe we see. But it's not so simple these days as it's presently constructed. It involves this patchwork of ideas which are not tightly connected with one another. It's not yet natural. In spite of many attempts to try to see how you could get inflation with just the right properties to come out of fundamental physics, such as superstring theory or other ideas, we've not yet found a compelling example. It always seems that we have to do fine tuning as to whether it explains well, even that's under question now that we have this idea of what we call an eternally inflating universe in which you have these pockets being produced all the time, but they don't all have the same properties. So this is a somewhat critical look on the status of the Big Bang inflationary theory today. And it's important to emphasize that we were led down this path of thinking through a series of observations and logic driven by physical laws. So the moment we assumed that there was a beginning, that the Big Bang is the beginning, we were forced to introduce inflation to explain the uniformity. But inflation, because it decays by this quantum process, inherently produces this randomness on large scales. There's no way of escaping this logic as long as you have the idea that there was a definite beginning to the universe. So if you want to escape it and look for an alternative, you have to ask the question, what if that's not so? What if the Big Bang is not the beginning? And what if the things that set the large scale structure of the universe was not something violent and, and random, so random as inflation that occurs in the early universe, but something that occurred over a much longer time scale, say before the Big Bang or before whatever happened 14 billion years ago. And that, in a sense, was the idea that we were fencing with when we first began to think about this idea, chain of ideas that led to this idea of a cyclic universe. So if the Big Bang is not the beginning yet, you see the universe is expanding and cooling, what's the alternative? Well, you can think of various possibilities, but one of them is that the Big Bang was not the beginning. And that in fact the evolution of the universe may even be cyclic. It may even repeat Big Bangs, repeat periods of evolution over time, giving us plenty of time to set the large scale structure of the universe and the properties we observe. Now, we didn't come to that idea immediately. It came also through a sort of chain of ideas. Our first inspiration came from thinking about recent ideas in string theory. And that was kind of our motivation for thinking about this cosmological model and continues to be. And I'm going to express the, I'm going to express the cyclic model in that language. But before I do, I want to emphasize that even though that was our motivation and even though I'm going to use that language, the cosmology and string theory are not. One does not require the other. String theory does not require the cosmological picture that I'm going to present, nor does the cosmology require string theory. They can fit together very nicely. And it's good to know they can fit together nicely because it says there's a hope for both solving the fundamental laws of physics at the same time as cosmology, but they really are independent. So bear that in mind. And in particular, we're going to use ideas from string theory like the existence of extra dimensions and structures known as branes, which if you're not familiar with these ideas, or if you're familiar with these ideas and think that they're way far out and not worth thinking about, I'll just say that you can also re express the same cosmology in more normal in language which you might find more appealing, the same language which is usually used to describe inflation, the ideas of quantum fields in the usual number of dimensions, you can express it that way, although I think this picture is at least more, is at least more geometrically appealing and as I said, dovetails with many current ideas in fundamental physics. So our inspiration for thinking about this idea was actually both through sitting at a lecture, I guess it was about nine years ago now, opposite ends of the room, as I say. My co author, Neil Turek and I were sitting in the lecture at opposite ends of a room, hearing a lecture about these new ideas of what may be the structure, the geometrical structure of the universe. The idea known as M theory, a version of string theory in which our Three dimensional world. Our three dimensions is really a surface embedded in a space with extra dimensions. And for the purposes of illustration, I'm going to show one extra dimension, and it's separated by a small, almost infinitesimal distance from another such world with a certain gap in between. Now, it turns out this structure, this sort of geometrical structure of the universe, automatically allows string theory to have a spectrum of particles and forces which is in agreement with what we observe in nature so far. So this structure, I want to emphasize this geometrical structure was not inspired by anything to do with cosmology. It was inspired trying to find a version of string theory which matches up with facts we already know about elementary particles. So we would live in one of these worlds, our stars and galaxies, everything we see would live in one of these worlds. We can touch, feel, and see anything that lives in our brain, but anything that lives in this other world, this other so called membrane or brain world, lives across a gap in extra dimension which none of our particles or nothing we can make, can traverse. So far as we're concerned, it is in some sense a separate world, except that gravity exists everywhere, including between the two worlds. So if you make a clump of stuff in that world, it will attract us almost the same way as if it were in our world. Just like the stuff in our world gravitationally attracts the other world. So in fact you get mixtures of whatever's on our side a short gap away, there's stuff on the other side joined together. Now, sitting on opposite sides of the room. We both asked the question of ourselves, and then we both came up afterwards to ask the speaker, well, you showed a picture in which these two worlds are just sitting there. But why should we imagine they're just sitting there? Let's imagine they began empty, but then just smashed into one another. That would seem to us look like something like beginning of the universe which has nothing, becoming something, something like a kind of bang, not really the Big Bang as we normally read about in textbooks, because there really was something there before there was space and time there before there were these brains before they collided. Space and time exist before, but this is a collision now which takes us from empty universe to suddenly something universe with the energy not coming from one brain or the other, but actually the two colliding with one another, sort of a force that would draw them together and cause them to collide with one another. And we want to know if it's possible that there'd be such a force and there'd be such a motion and we're surprised to find that that's all quite possible within the theory. In fact, likely within the theory. And that was really the springboard for our thinking about an alternative universe. A new theory of the bang not being a beginning but being a collision suddenly opens up the issue, well, if that's the case, what happened before? Why did they collide? And can we re explain everything we know in a completely different story? Well, there was a chain of ideas like that and a chain of challenges like that which led to this alternative cyclic idea. So now, so basically, we began with the idea of this bang, and then we realized that once you produce this bang, these brains would either they. You can either think of them as passing through one another or bouncing off of one another. I'm sort of showing them bouncing off of one another here. They would then bounce off one another. They'd be both filled with matter and radiation. Let's just be concerned with our brain, which would be the one in the background here, be filled with matter and radiation. It would. Now what happened is that these branes, our three dimensions, would begin to expand and cool. They begin to condense to form stars and galaxies. And if you work out the equations, the equations that describe this expansion look exactly like the equations we use in the usual Big Bang theory. You can't really distinguish them. So the same physics would follow from the same start point of, in this case, not beginning from nothing but from a collision, and not beginning from infinite temperature, but beginning for whatever finite temperature you reach at this collision. And then along comes the dark energy. And here's where the story gets interesting. In this picture, there is automatically a source of dark energy there. It doesn't have to be added. Odd hoc, if there's going to be a force which is drawing these two brains together, it's like having a spring which is drawing them together. Associated with that force is both a kinetic energy and a potential energy. And it turns out the potential energy of such a spring looks a lot like, or can look a lot like dark energy. It can have the same effect as dark energy, causing the brains to. Then once it takes over, once the radiation and matter have been diluted enough through expansion, it takes over the energy of the universe and it causes the expansion to accelerate the universe to start doubling in size every 10 billion years or so. Now, actually, if you want to have a cyclic universe, one of the key challenges is the following. I need to make a universe that's going to make all the complex structure we see needs to evolve to make that complex structure we see. But yet I somehow have to get back to a simple beginning again. Well, I would claim that if you want to imagine such a universe, dark energy is the ideal form of energy for making such a universe. Because what the dark energy does is it takes the universe with all this structure and, and over time, it smooths it out and dilutes it, bringing the universe back to a very simple state where the brains are essentially empty. All the matter has been spread out and diluted to a point where it's completely insignificant. And it brings us back to an extremely simple beginning, to the universe of just two parallel empty branes which are then now free to collide again. In fact, because this dark energy is a kind of spring like potential energy, it's not going to last forever. It's also at the same time that it's causing the expansion this way. It's slowly beginning to draw the brains towards one another. It's associated with the spring like force. And eventually it will shut itself off. That is to say, it will stop accelerating the universe, because eventually the brains will pick up enough speed that the kinetic energy dominates the potential energy. And then the dark energy phase ends and we go into a kind of contraction phase. Not a contraction of the brains, the brains are always spreading in this picture, but a contraction of the dimension in between. So unlike the old idea of a cyclic universe that people talked about in the 20s and 30s, where it was our three dimensions which were kind of breathing in and out over time, an idea which we now know fails because it requires too much matter in the universe. And because this breathing mode doesn't keep at a regular pace, this model instead has the idea that there's kind of a mixture of expansion along the usual dimensions and regular contraction and expansion along the extra dimension. And this kind of mixture of sort of a cyclic behavior with a stretching behavior turns out to get around all the problems of all previous cyclic models. And during this contraction phase does one other amazing thing. So during this contraction phase, these brains started off flat and parallel and empty, made that way by the dark energy. But as they begin to move towards one another and they stop expanding, they instead quantum physics takes over and they begin to wrinkle. So in fact, if you look closely at that picture, they're not quite as smooth as they were before. They've actually begun to wrinkle. And that wrinkling is important because it means that when they hit one another and go through a kind of crunch, you don't hit everywhere at exactly the same time. So some regions collide and heat up before others, so that when you're through and they come apart again, the distribution of matter and energy is not precisely the same as it was before, but it's some kind of. And you get back to the bang, and it's not quite what it is before, but it's somewhat wrinkled. Sorry, it's somewhat wrinkled. And curiously enough, it's wrinkled in exactly the same way as the Big Bang inflationary picture predicts in exactly the same way as the WMAP picture measures, at least to the level we've been able to measure so far. So at least a leading order, the leading effect, looks indistinguishable. The physics is completely different. It's the physics of wrinkling brains colliding at different times. Compared to inflation, which is stretching the universe after the bang at very high energies, the physics is completely different, but the mathematical equations, curiously enough, are nearly identical, at least up to the level that we can measure. Any distinction so far. So that's a real tease. It really is telling us that the same data can have two different stories associated with it, Two radically different stories associated with it, one in which the universe begins with a Big bang, and one in which the universe is cyclic. So the story would look something like this. This is just a little cartoon to sort of review what we just said. We had the two brains collide in a bang. The time scale isn't right here, of course, but then they move apart, which would take just a few microseconds. They would then begin to expand along the usual three dimensions, forming the structure of the universe. And then for maybe a trillion years, it can keep up that expansion until finally the dark energy, the force, begins to draw the brains close together, and they collide together, and the process begins and just goes over and over again. Now, of course, the question we want to know is which model? Either is right, could be both of them are wrong. But if one of them is right, which one's right? How can we tell? And that's really what makes this not just fantasy, but makes it real science. So so far, we made a number of measurements about the universe which could have eliminated both models. For example, that WMAP picture I showed you, it could be that we found in that picture was inconsistent with either picture, and we'd have to go to something else entirely. But so far, all the measurements we've made so far are consistent with both. That's where things stand at the moment. Suppose we then say that one of the two is likely to be true, given that how can we distinguish one from the other? Well, there are several ways described in the book. But the one which is probably the easiest, in a sense, they're all difficult. The one that probably that people are going after right now has to do with the formation of gravitational waves. So in both of these pictures, when you have this wrinkling effect that produced either at the end of inflation or from this collision of brains, you have, in addition, wrinkles in space time itself formed. Wrinkles that, once the universe begins to evolve after the bang or after inflation, propagate as what we call gravitational waves, ripples in space time that travel throughout the universe. And as they pass you, they kind of stretch you in one direction and squeeze you in another and then do the reverse as they go past. And both of them predict a precise spectrum of such gravitational waves that would have been formed in the early universe. But those two spectra are completely different from one another. Different amplitudes, different dependences on wavelength. So that's the first thing that people are going for. And in particular, the best place to look for them, the easiest place to look for them is not directly, because gravitational waves are very weak and hard to detect directly, but through an indirect effect that they have on the microwave background radiation, the radiation that was produced a few hundred thousand years after the bang, after the first atoms were forming. So I showed you a picture that I called the WMAP image, which is an image of how the temperature and intensity of the radiation varies across the sky. To look for gravitational waves, you don't just want to measure the intensity. You actually want to measure the polarization of that light. So you want to imagine making a satellite which has polaroid lenses on it, which is what that's supposed to cartoon is supposed to represent. Because it turns out gravitational waves produce a very special, recognizable, mathematically distinct polarization signature in that map. So if we ask, where do we find a combination of intensity and polarization that match? You would get a kind of swirling like pattern. If you had gravitational waves in such a polarized map of the microwave background. And one of the great efforts underway, experiments are right now being conducted on mountains in Chile and a number of experiments at the South Pole. Balloons which are circumnavigating Antarctica, and satellites which are planned to go up in space in the next year. One of their prime goals is to look for this polarization pattern. Essentially, the way it works is this inflation is a very high energy, violent event. It produces much stronger gravitational waves than the cyclic picture. So much stronger, I mean, by a factor of 10 to the 50th, not a little bit stronger, I mean, by a huge factor. So if you were to see this swirling pattern characteristic of gravitational waves, it would tell you that whatever formed the initial structure of the universe had to occur when the temperature and density of the universe were high shortly after the bang, and that the inflationary theory is right. On the other hand, if you look and you don't see any sign of those gravitational waves, well, that would be most easily. That would be exactly what you'd expect from the cyclic picture, which says you can get the wrinkles without producing many gravitational waves. So it would favor the cyclic picture. Although I have to say that already theorists have gone around and figured out, well, how can I tinker with inflation to let the gravitational waves hide from our detectors? So they've thought of some ways of fine tuning things further to do that. So it's not so clear what happens if we don't find the gravitational waves. We'll go on to other tests. But if you do see the gravitational waves, that will kill at least one. This whole idea of there being something before the Big Bang and that that's what created the structure in the universe. So it's definitely in the realm of scientific tests. These tests are being performed now. So stepping back, what's at stake here is quite a bit, and we can understand it best if you sort of step back and sort of say, what's so different about these models? And the most obvious thing that's different about these models has to do with the nature of time. Has to do with a matter of time. If I were to go to a blackboard and sort of trace, sort of outline what each model looks like, even just what I'd run, even just the shape, geometrical shape of the model as drawn as a sort of flowchart tells you a lot about the different viewpoints about time and where that takes us in our thinking. So the Big Bang picture is definitely a linear picture or a ray like picture which has a definite beginning. And then time proceeds going forward in it. And it leads to some consequences which are profound. The first is that our vision is limited. By vision being limited. I mean, suppose you want to use the cosmos to learn something about the Big Bang itself. In the standard Big Bang inflationary picture, that's really impossible because what inflation has done when it's spreading out the distribution of matter and radiation in the universe is simply dilute anything that existed between them when the Big Bang occurred, and inflation and replace it with the inflationary energy and whatever it decays to. So everything we see, including the cosmic microwave background, is very insensitive to. In fact, it's sort of blocked by a curtain from anything having to do with the Big Bang. So we're limited in how much we can see of the history of the universe. And in a certain sense, the future is bleak. That is to say, where we're going in this picture. It's a one way trip of expansion. Our patch of the universe will become less and less habitable in a trillion years, will be essentially from that point onwards, an uninhabitable vacuum. And it leads to a theory in which our universe in a larger scale is in some sense fundamentally random. If you ask why the physical laws are as they are, why the properties as they are as they are, well, this picture I showed you of this eternal inflating universe with these pockets is basically telling us that the properties in our pocket are just chosen by chance and could have lived in a different pocket with different properties. There's nothing special about anything that we can measure in our universe that will tell us anything fundamental about the universe entire. So it has profound implications for how much we can ultimately learn from science. It leads to a landscape of possibility. That's another language that people are often using now these days, where each pocket sort of corresponds to a different valley in this landscape, which has different properties. And just studying everything about what's in our particular pocket, I don't know, it's not so interesting since it's just one pocket out of many. Some people view this picture, this radical new picture of the universe, as something very appealing and as a fundamental breakthrough in scientific thinking. Others, and I include myself, regard it as basically the end of science. That if you tell me that no matter what I measure in my local region, it's not telling me anything fundamental about the universe entirely. That's really a limit, a fundamental limit of science, because we're restricted by the laws of physics from seeing beyond what we can see. So it's important to note there's an alternative possibility. And the cyclic picture tends to draw the universe. The way you can get a cyclic universe is you have laws which are kept regulated. In fact, cycling acts like a regulator. If you have pockets that begin to deviate and begin to develop different properties, they become kind of like defects, they cease to cycle anymore. The cycling kind of forces you to a unique set of properties. In this picture, our vision, when we look at the microwave background, not only takes us to the Big Bang, it actually extends beyond the Big Bang. What we're seeing in the cyclic picture, in the interpretation of the cyclic picture when we look at this map is literally the big splat that occurred when these two brains hit. The pattern of splat. And the reason why it's not particularly even in this picture is because of events that occurred before the bang. So we can use this picture to determine what the conditions were like before the bang, even things like what rate these brains were moving when they collided into one another. And the future is in some sense hopeful, in the sense that this patch of space does not become a barren wasteland. We are just the latest generation in what may have been many or perhaps even endless cycles which new matter was formed, new bangs occurred, new matter was formed, new galaxies were formed, new planets and life. We're the most recent addition and the future. The universe continues to be a fertile land, if you like, a region where there's always the opportunity to produce new galaxies, stars, planets and life. And instead of getting an endless landscape, you get a universe, which is what many of us were aiming for back in the 1980s when the big Bang and inflationary picture was invented. A universe which really is the same everywhere, which has the same properties and the same physical laws, almost everywhere, which is consistent as far as we can see in the universe. So it's in some sense a simpler picture, although we can't say for sure what lies beyond. So there's really a lot at stake, as I tried to emphasize in this picture. I just tried to give you a sort of feeling for what some of the issues are at stake. The book goes into much more detail about both the ideas and the history that comes before them. And basically the issue comes down to do we live a universe in which bang goes to crunch, goes to bang goes to crunch, sort of endless universe, or do we live in a universe which had a definite beginning and it's approaching a wasteland? And the very interesting scientific enterprise that's underway now is one that will help us decide which of those two pictures is correct. I think it's a story which will have a profound influence on all of us, really. I don't think no matter what your philosophical or religious or personal point of view, the answer to which one of these is right has to have a profound effect on the outlook. And what the book tries to do is invite you to join in on this enterprise when we still don't know the answer. So you can kind of follow as experiments occur, you can appreciate why they're so important, why people are so eager to find the answers. So I think I'll stop there and let you ask questions. Yes. So why do we need to have a cycle here instead of just saying that when the brains collide, that's the beginning of the universe and they might just continue going on away from each other forever. So in fact that we began with that idea. So the cyclic universe came developed as a series of steps. We just began first of all with the idea of collision. Then we said, okay, maybe the brains could wrinkle and that could produce the distribution of fluctuations we see in the microwave background. Maybe we can get by with just one collision. But where we got stuck there was that things would only come out right if you prepared the brains in just the right way, in some way that they were flat, parallel and nearly empty. And to assume that at the outset is a little bit like assuming the universe is smooth at the outset. It's a very special assumption. So we thought, well, how do we know any mechanism that might smooth the universe? Well, we do know such a mechanism. Dark energy is such a mechanism. It's just that dark energy has only taken over the universe recently, whereas we want to smooth the universe out before the bang. But if you think about that thought a moment, you immediately jump to the idea, well, maybe, maybe, just maybe, it's possible that the same dark energy that's smoothing the universe out now actually was dominant before the bang. And so that's what led to this idea of a cyclic universe. So I don't say it's the only possibility. We did, people do, and one certainly can provide examples of sort of single collision universes. But the nice thing about the cyclic universe is it brings the dark energy we're seeing today, it connects it directly to the beginning of the universe. Whereas in all these other pictures it's kind of odd hope something that begins it and then there's dark energy later. But this sort of ties them together in a very natural way that occurs when you have this setup of these brains. So if the Platinus pre collision is due to the last universe's dark energy, presumably this applies to that previous universe as well. That's right. So like you start out with a rough universe where they aren't parallel and each time they bend, they get more and more parallel and smooth. Well, yeah, so the process. So, so we don't. These cycles could have been forever. They're self regulating in the sense that after you smooth the universe out, they develop wrinkles, they form structure, and then dark energy takes over and smooths things out to what they were before. So it could last even forever. Or you could. I think what you're imagining is I could begin with the universe many, many cycles ago, which was quite rough, much rougher than anything that we've seen in this cycle. And it would be smoothed out and become, and would settle into the structure we see today. So it has what we call an attractor behavior that even if it didn't begin in this cycling mode, you started off in some random state. As it begins colliding more and more, it sort of settles into or is attracted to the kind of structure we're seeing here. So as we described in the book, there's really two. You know, although the cyclic universe the first people thinking about is one which is eternally cycling, it's also perfectly possible to have a universe that begins in some more random way and then settles into cycling. And so then there'd be a. They'd still have to explain the beginning in that case. But the conditions today would be very insensitive to that beginning because they would have undergone so many cycles since then. Yes. A question about gravity. I was wondering, you're talking about gravity, but why is it that gravity can penetrate between the brains, between the brains when it seems like most other forces and energy can't? And is it somehow, because it has that property, is it somehow related to dark energy? That's. A good question. So gravity as we understand it, is associated with the elasticity of space time. So wherever there's space time on the brains or between the brains, you can always stretch and deform it. And that's the effect of gravity. Now, these brains are, we call, are very special structures. They have special physical properties which allow particles like electrons and photons and quarks to, to live on them, to exist along them. Whereas most points in space don't have the right physical properties to allow particles with the particular spin and what we call chirality properties that they have to exist between them to exist any point in space. They're very special, like end points along this extra dimension, which have very special mathematical properties which allow certain particles to exist that can exist in between, including the particles we know. So other forms of matter and other forces aren't directly associated with space time in the same way that gravity is. That's the reason for the distinction. The dark energy is not so much to do with directly with gravity as the fact that these two brains themselves, they do interact with one another. They can interact through virtual quantum processes. And those will provide a spring like force which will produce both a potential energy A dark energy and eventually a kinetic energy when it draws them together. So that's yet a third effect. Yes. I was just wondering how the scientific community has received your theory. Have you been hailed? Ostracized? I would say, thoughtfully. In other words, it is an unusual situation. Usually when a new theory comes along, sometimes you have a situation where a theory contradicts something we know. We know it has to be replaced, and then when you find something better, everyone hails it. So in the case of inflation, we had discovered that the Big Bang can't produce the smoothness or is unlikely to produce the smoothness that we see. Inflation was the first solution to be found. When it was found, it was pretty soon hailed as the leading idea. The current situation is a little bit different because at least the naive view of the Big bang inflationary theory seems to be in beautiful agreement with everything we see. I mentioned some of these things about the randomness. People aren't so sensitive about that. But the idea that we can reproduce this WMAP picture is something that people find very compelling. And so people are more loyal to, more drawn to, more. You know, they view the inflationary theory as the champion. Now, here's this challenger. Why do we need a challenger in this? Well, I think I've given you some reasons why we're looking ahead at a challenger, but I think people haven't yet caught up to those ideas. So I wouldn't say it's either been. Certainly not been hailed, I wouldn't say ostracized. I think people are kind of looking askance, you know, and they will not spend a lot of time on it until they feel there's a compelling reason to do so. And it's our job to improve the theory, find a compelling reason to do so, convince our experimental colleagues we've already done that, to go after testing them. And this test will answer it. I mean, if we don't, if we see those gravitational waves, then they'll have been perfectly right to ignore us. And if we don't see them, I think people will have to wake up and say, oh, well, now we have to begin to think more seriously about these alternatives. So I would say on the whole, it's been a pretty healthy reaction. It's the right thoughtful reaction that one should have. Yeah. In this theory, there is no. I mean, you don't mention anything about the beginning, right? Yes. Yeah. So either. Well, that's because. I don't know. I mean, so what we've shown is that it's possible to cycle for arbitrarily long periods of time. Now, could that be infinite in the past, so far as we know. I mean, we've asked ourselves, is there anything that forces us to have a beginning in this model? And the answer is no, not yet. So we've never discovered. You know, although people had various ideas about, you know, you might run into contradiction with perpetual motion machines or thermodynamics or something like that, there are things you could imagine you might run into trouble with. You look at those things you discover. So far as we know, there's nothing that prevents these cycles from going forever into the past. So that's interesting that you could actually have something that cycles forever. On the other hand, there's nothing about the model that requires that either. The model is really about explaining the universe we observe, which is a product of what happened just before the bang in the present. So you could imagine that well into the past there really was some beginning event, okay, Which I can't explain to you what it is. And then it would naturally settle into the cycling behavior because the cycling behavior has the effect that it creates lots and lots of space with the same physical properties. So it would be a very. It sort of draws the universe towards that. So that's another logical possibility. So I'm completely agnostic on the issue of whether there's a beginning or not. I just think it's interesting that you don't have to have one. But did you have a follow up to that or did you want to? The other question I have was regarding this. They heat and the galaxies they just created and it started expanding. Then they get closer. So what happens to the stars, galaxies who are going off, who are being stretched out there? Well, essentially what's happening is that they're being stretched. The space is growing so much during these periods of expansion that the stars and galaxies are moving out of each other's fields of view. So let's just take an even much shorter time scale. A trillion years from now, everything that's beyond Andromeda will have moved out of our field of view. Okay? And most of space won't have any stars and galaxies within its view. That's what's happening here over similar time scale is that most of space is emptied out. That's the part that cycle that will continue cycling in those rare regions where there are still clumps of stuff, probably nothing much will happen. It'll probably just be left behind and it won't neither grow nor. It'll probably just collapse into some kind of black hole or something. But most of space will be empty in the way that I've shown you. And that will always be a continuing process. There'll always be more empty cycling space than there will be anything else. Also, there's a possibility of another bang in some location that we don't see. It's part of our. Well, you could imagine that depends how you want to name things. We say another bang outside of our field of view. That's always a. So here's the thought that came to mind, and then I have to tell you the qualification to it. So you could imagine that the collision clear. There's a collision here and along this section of brain. And here's someplace far away outside our field of view, so called space, like separated from us. We did another such collision, but it occurs at a very different time. It's sort of out of sequence with what's happening with us. You could imagine something like this. Although in general relativity you're free to choose your surfaces of constant time when comparing two regions which are so separated, there's not a unique way of choosing your clocks in those two regions. So I can always am free to choose the clock so that in fact they're simultaneous. So in some fundamental sense, as long as general relativity is right, I can always choose coordinates where they are effectively simultaneous. But in any event, the fact that it's so separated means it also doesn't affect us. It would never have an effect on our region of the universe. Yes. In this theory, is it believed that there's only two membranes operating over just a single extra dimension? Or is it thought that there's an infinite number of these brains? It's a good question. So. Well, if we are borrowing this theory. So this assumes we're using this whole setup from M theory and this setup from M theory, there are R dimensions which I've represented by the brain. And there's this extra dimension which actually only has a finite extent. So whereas I can have infinite lines on our brain between the two brains, there's really only a line segment. And they are kind of endpoint brains in this extra dimension. So there's something peculiar about the extra dimension in that it only lasts over a finite distance. Although the picture shows something on the outside. Forget that. Only think about there being space in between. Now, what's important for our picture is what happens to the size of the extra dimension. So it's important what happens to the endpoints. It's not so important what happens to points in between because they don't affect the size of the Extra dimension. So, yes, I could have brains in between. In fact, you will have brains in between. They will play some interesting role in having to do with the details of the way elementary particles behave. But they don't, by their motion, affect the size of the extra dimension. So everything we discuss when we're talking about the cosmology, it's really that the endpoint brains that we're interested in, sometimes called the end of the world brains that we're interested in because their separation is what controls the size of the extra dimension, which goes from finite to zero, back to finite again. Yes. So if the collision of the planes causes changes in the structure of the brains. I'm not able to hear. Can you say it louder again? So if the collision of the planes causes changes in the structure. Structure of the brains, then does it mean. I mean, is there a transfer of energy of the spring, like energy between the branes, to energy within the brains? Yes. Yes. So actually, it's even more complicated than that, because if you just took an ordinary spring and had it go this way, and it was dissipating energy into heat, so this collision's like dissipating energy into heat. It would wind down. I mean, there's only a finite amount of energy available. Conservation of energy and thermodynamics would say it would necessarily wind down. So what's really happening here is something more subtle, something that can't be shown in the picture, which is the role of gravity in this. And the fact that the model really, though we call it cyclic, is, in a subtle way, not so cyclic. So I already mentioned that in a bit when I said we're cyclic in this direction, but we're always stretching in this direction. So. So that means it isn't exactly the same from cycle to cycle. If I had a global view of the universe, I could see, for example, there'd be more black holes produced every cycle, and they keep spreading out. I'd see a higher, you know, the number of black holes and the total entropy of the universe increasing from cycle to cycle. But. So what's the engine that drives it? Well, the engine that drives it is really gravity. What's happening during each of these. So what's really happening in this universe is that during the course of their drawing towards one another, we're converting gravitational potential energy as well as the spring energy into kinetic energy. So we're getting some energy not just from the spring, which would be, but also from gravity. At the end of the cycle, the spring goes back to where it was so we're back to where we were so far as the spring energy is concerned. But we haven't restored the gravitational potential energy. So the gravitational potential energy has decreased. And that's how we've gotten the energy we needed to make the matter and radiation that we see now. Next cycle, it happens again and again and again. Now, a key difference between the energy of a spring and gravitational potential energy is that the energy of a spring isn't a bottomless pit. There's only so much energy you can draw before it disappears. But gravitational potential energy, one of the interesting things about gravity is you can. It is a bottomless pit. You can keep drawing energy from gravity as long as you have a mechanism that allows you to do it. There's nothing that forbids. There is no gravitational potential energy becomes arbitrarily negative, which means you can always decrease. Decrease the gravitational potential energy and use that to make matter, if you can find a mechanism to do it. Here, the colliding brains are doing just that. You look at the equations. What they're automatically doing for you is, over the course of cycles, is drawing down potential energy, converting it to kinetic energy, and drawing the spring back to. That's how the spring can get back to where it was in the first place. So it's a kind of subtle story. We have these two brains, and there's a separation between them. I can define that distance between them, which could vary as you move along the brains. I could say, forget the extra dimension. I'll just define that to be a field which varies over space. So everything I do with the branes I can mimic with the behavior of this field. When the brains hit, that means the field goes to zero. When the brains move apart, that means the field grows. And I can do almost everything that we do in string theory. There are a few exceptions, but almost everything in string theory can be translated into such a field theoretic language. In fact, that's what string theorists do all the time. They're constantly moving from one mathematical language to the other, back and forth, because it turns out, thinking about them both at the same time is the most powerful way of figuring out what's going to happen. Now, the cosmology doesn't use every aspect of string theory and the cyclic cosmology. So, in fact, all the aspects it does use don't require that we ever go beyond four dimensions. We could do everything with fields that have to have certain complex interactions in that language which would mimic what's happening when the brains are hitting. But we've walked through that exercise, and it seems that there's nothing that stops us from translating from one picture to the other. So, in fact, now that people have realized that, that's gotten a lot more cosmologists involved, because it's kind of interesting. There's the string theorists, who are quite happy with the idea of string theory in extra dimensions. There's your observational cosmologists, who think that idea is rather highfalutin and abstract and should have nothing to do with the real world. And now that we can see that we can translate from either one picture to the other and back and forth again, it's gotten the cosmologists a lot more interested in this picture. They realize, oh, I can understand this using mechanisms which are not so different from the kind of mechanisms which ideas that we use for inflationary theory. And in the last year, we've actually seen a growth of various alternatives, various variations of this colliding, what I'd call the colliding brain picture translated into field language in a fewer dimensions. So it's. The way I like to think about it, is when you do that translation, the theory looks more complicated and messy than the geometry. Okay. But it uses more ordinary ingredients. So you can either use ordinary ingredients and they interact in a complicated way, or simpler ingredients like brains and extra dimensions, but less familiar ingredients which interact in a very. What looks to be a simpler way. Those are two different languages for describing the same thing. Think. Okay, well, thank you very much. Thanks for all of you for your attention.