How Did Our Universe Begin? | Paul Steinhardt

Paul, the question, did the universe have a beginning, is one of the great questions that human beings can ask. And in our era, we now have the agreement that there indeed was a beginning, there was a Big Bang. Now, when naive folk ask, okay, what was before then, the typical answer you get is that if you're creating space and time at this instant, it doesn't make sense to ask what was before then. And it's like, what was north of the North Pole? There is no answer to that. But you have an answer to that. Well, we're exploring a radical possibility, yes. We're exploring the idea that whatever happened 14 billion years ago, what we call the Big Bang, may have been involved in the creation of a lot of hot matter radiation, but was not really at the beginning, that space and time existed beforehand. So you might first ask, well, how do we, you know, how do we manage to challenge these ideas that everyone seems to hold as conventional? Well, the answer is that we're not really sure, scientifically, that there was a beginning to space and time. What we know is that the universe is expanding today. We know the laws of nature that are causing this expansion, and we can use those laws. Those are namely Einstein's theory of gravity, his theory of general relativity. If you extrapolate those equations back in time and try to predict what the conditions were at earlier times, you come to a moment 14 billion years ago when the temperature and density become infinite. Now when you use a set of mathematical equations to extrapolate and you get an infinite answer, that's something that mathematicians know is a breakdown in your equations, like they call it a singularity. In fact, another term that we use for the Big Bang is the cosmic singularity, meaning it's really a breakdown in our knowledge of the right physics. We know that Einstein's equations break down at that point, and it has to be replaced by a better theory. Now a common assumption is that when you finally come up with this better theory, it will still predict a beginning, and it will even explain to you how that beginning took place. We don't yet have that theory in place. In fact, we've been exploring the idea that when you actually replace Einstein's theory with an improved theory of gravity, that you'll find that the Big Bang was not the beginning, that there existed space and time beforehand, and in fact, the key events that created the large-scale structure of the universe and even all the galaxies and stars, all those events actually occurred before the Bang, and we only exist here because of those events. Let's find out what those are. Okay. The cyclic idea we've been exploring is derived from an alternative or a theory of gravity or an improved theory of gravity, which physicists call string theory. It's an attempt to unify all the fundamental forces of nature into a single unified theory that includes a quantum theory of gravity and a quantum theory of all the fundamental forces. By quantum, I mean it has to be not only consistent with Einstein's theory of relativity, but it also has to be consistent with the laws of quantum physics that we learned about a century ago. In this theory, one possibility for the structure of the universe is that our three-dimensional world is really a surface embedded in a space with an extra dimension that's invisible to us, and this extra dimension is in fact bounded a short distance away by another similar sort of three-dimensional world that we can't see or touch or feel, but even though it's a short distance away. It's a short distance in the fourth spatial dimension. That's right. The fourth spatial dimension would be microscopic. It might be a billionth or a trillionth the size of a nucleus, but the fact that we cannot reach in that fourth dimension means, in some senses, it's as if it's infinitely far away. However, there's space between us, and wherever there's space, according to the laws of physics, according to the laws of string theory, and even according to the laws of Einstein, there must be gravity. So gravity exists in the space between, and furthermore, if you make a lump of stuff in this other world, you can actually feel this gravity on our side, just like if you make a lump of stuff on our side, you can feel its gravity on the other side. So they can interact with one another, and what we're proposing is an even more dramatic kind of interaction, where they simply don't remain apart from one another, talking across this gap, but in fact, occasionally, what will happen is they will actually move towards one another and collide, and every time that happens, there'll be a huge creation of matter and radiation in these two worlds, they'll bounce apart, filled with matter and radiation, and they'll begin to stretch, causing an expansion and cooling. So it's very much like the Big Bang, that people think of as being the beginning of space and time, but it's not the beginning of space and time. These surfaces, we call them, they're called membranes sometimes, or brains for short, they exist before the collision. Not these kinds of brains. Not those kinds of brains. You use these kinds of brains to discover those kinds of brains. Exactly. We hope to, yes. So these brains, these membranes, exist beforehand, it's a collision we're talking about, they exist beforehand, there's a before a collision, there's a during, and there's an after, and so the Big Bang in this picture is not the beginning of space and time, it's a big collision. And this big collision injects into, maybe both sides, tremendous energy, and as we know from Einstein's famous E equals MC squared, that energy then will propagate and cool and become matter over time. Exactly. So everything that we're made of today is a remnant from this big collision that occurred 14 billion years ago. Now do we have any sense of how long it is between these collisions? Can they happen randomly, like a disintegration of a uranium atom, we don't know when it's going to happen, or is there some finite period of time that we can estimate? Well we don't have a firm theory that tells us exactly how long it would take between collisions, but what we imagine is that, well obviously it has to take more than 14 billion years, and we know at a minimum it must last several tens of billion years between collisions, so a nice round number would be that a minimum time between collisions would be roughly a trillion years or so. And particularly the idea we have in mind is not that these would occur at random intervals, but they would occur at regular intervals, this would be a cyclic universe that would go through regular periods of evolution, beginning with a bang, and these two brains, these two worlds flying apart, going through periods of expansion and cooling and creation of galaxies, stars and planets and life, going then through a period where a new form of energy takes over the universe, an energy associated with the force which is drawing these two brains together, a kind of invisible or dark energy that causes them to expand at an even faster rate and become empty, essentially, or cause the matter within them to spread out and dilute for them to become empty, returning them to a sort of pristine condition again, and then this force draws them together again, leading to another collision. And this might occur, as I said, maybe once every trillion years or so. When you look at these cycles, I guess you can see them infinite in either direction, or I guess you could have a finite past and an infinite future, or indeed an infinite past and possibly a finite future, or infinite on both. Is there any way of making progress in being able to see, is this whole cyclical universe that we live in, this grand universe, can you sense whether there really will be a beginning and an end? That's a very interesting question, especially since people have been thinking about this idea of a cyclic universe, well, throughout human history, but certainly recently, scientists over the last hundred years have tried various attempts at trying to make cyclic universes, including Einstein himself. And they always ran into troubles, to roadblocks, things that would force the cycles to last only a finite period of time in the past or in the future. But what we've discovered in this particular version of a cyclic universe, these colliding membranes, that so far as we can tell, we can get around all the known roadblocks, and we haven't discovered any new ones. So that in principle, these cycles could have gone forever into the past and could go forever into the future. It's not required, so I don't exclude the possibility that the universe had a beginning and then settled into cycling, but it's not required. So we could go either way with it. Does the cyclical model solve some of the traditional problems in cosmology, the flatness of the universe, the homogeneity that everything looks the same, everything being the same in all directions, these classical problems that inflation theory is originally supposed to solve? That's a good question, because that's an absolutely critical standard, which any model nowadays must satisfy to be considered even in the running. So one of the things we had to do if we were going to try to reorganize the history of the universe is answer exactly that question. How are we going to smooth the universe out without inflation, or how are we going to make the inhomogeneities we see in the universe that inflation produces at the end of inflation, how are we going to do that? So the way that happens in this picture is not through events that occur after the bang, the way inflation occurs. Inflation is an event that occurs shortly after the bang, because in inflationary theory there's nothing before the bang, so it's the only time it could occur. So what happens in these cycles is that after you've created matter and radiation and they've cooled to form stars, galaxies, planets and the like, there next comes a period where a new form of energy takes over the universe and controls the expansion. And this is an energy which is associated with the force which is about to draw the two brains together. There's a spring-like force which is about to cause them to slam into one another, and associated with that is an energy. When we draw a spring apart, it has an energy that it contains. In this case, this energy acts like a kind of dark energy, as we cosmologists call it, an energy which causes the brains to speed up in their expansion, so that as we look around us and this expansion speeds up, all the stars and galaxies we see today will eventually fall out of our view, and the universe will become an emptier, more vacuous place. And these brains will become flatter, smoother, more parallel, more uniform. So this is a smoothing influence, and furthermore, this contraction also has some magical properties which even do some additional smoothing, actually it's even the more critical smoothing that takes place. So that when the two brains hit, except for one effect which I'll mention in a second, they would hit almost exactly simultaneously everywhere, but not quite so. If it were exactly so, that would be actually bad news. It would smooth the universe out, but it would make it too smooth. And the universe isn't perfectly smooth, that's why we have stars and galaxies. So it's important that we do a little bit of unsmoothing. And what does that is quantum physics. So as these two brains go together, they are being stretched by the effects of dark energy, but quantum physics does its thing, which is always trying to wrinkle the brains. So when they hit, they hit at slightly different times at slightly different places, so that means they heat up and cool at slightly different times at slightly different places, and when they come apart again, that means the temperature on the brains isn't perfectly uniform, it's slightly wrinkled. And the bizarre thing, the really sort of hook that got us into this whole story was that when you calculate the pattern of temperature and density that you get from such a collision, it's actually just like you get from inflation, even though there's no inflation here, and just like we see when we look at the universe, when we look at the cosmic microwave background radiation, and we look at the distribution of galaxies. So what we discovered is that actually this kind of collision effect produces a new and alternative way, a new alternative way for explaining all that structure. All the same data. All the same data can be described in this way. So let me see if I get this right. If I am in our three-dimensional universe at the Big Bang, or just before the Big Bang in your model, or in some future time when everything has been so spread out that there is virtually no structure left, maybe an isolated photon here, and they're enormously large, and that's it. It looks like it'll stay that way forever. And then suddenly, boom, out of seemingly nothing, you have this injection of unfathomable amount of energy to create the Big Bang. Right. And unfortunately, you'd probably be evaporated unless you invented a way out of it. You might have gotten some hint that something was coming if you were doing some careful experiments. Before the bang comes, you would find something's happening to the laws of physics. It turns out the laws of physics are sensitive to what the distance is between these brains. So as they speed towards one another, you'd find the laws of physics that you were used to in the textbooks were changing. In fact, they were picking up, the change was picking up speed as the two brains were approaching. So you have a notion that something peculiar was going on based on your experiments in the laboratory. And then at the very last instant, you discover what the peculiar thing was, a sudden injection of energy that evaporated everything and created all the matter and radiation for a new stage of evolution of the universe. So what would look like the creation of something out of nothing really would be a nothing within our three-dimensional framework, but would be a something in a four-dimension spatial framework. Exactly. So it's cosmology taking full advantage of an idea we already had from fundamental physics that there might be another dimension. Here, that extra dimension is playing a crucial role in constantly re-infusing the universe with new matter and radiation.