So you have this set-up with these two parallel worlds, just literally geometrically parallel worlds, separated by a small gap. We did not dream up this picture. This picture emerges from the most sophisticated mathematical models we have of the fundamental particles and forces. When we try to describe reality—quarks, electrons, photons, and all these things—we are led to this picture of the two parallel worlds separated by a gap, and our starting point was to assume that this picture is correct.
These membranes are sometimes called end-of-the-world branes. Basically because they’re more like mirrors; they’re reflectors. There’s nothing outside them. They’re literally the end of the world. If you traveled across the gap between the two membranes, you would hit one of them and bounce back from it. There’s nothing beyond it. So all you have are these two parallel branes with the gap. But these two membranes can move. So imagine we start from today’s universe. We’re sitting here, today, and we’re living on one of these membranes. There’s this other membrane, very near to us. We can’t see it because light only travels along our membrane, but the distance away from us is much tinier than the size of an atomic nucleus. It’s hardly any distance from us at all. We also know that, in the universe today, there’s something called dark energy. Dark energy is the energy of empty space. Within the cyclic theory, the energy associated with the force of attraction between these two membranes is responsible, in part, for the dark energy.
Imagine that you’ve got these two membranes, and they attract each other. When you pull them apart you have to put energy into the system. That’s the dark energy. And the dark energy itself causes these two membranes to attract. Right now the universe is full of dark energy; we know that from observations. According to our model, the dark energy is actually not stable, and it won’t last forever. If you think of a ball rolling up a hill, the stored energy grows as the ball gets higher. Likewise, the dark energy grows as the gap between membranes widens. At some point, the ball turns around and falls back downhill. Likewise, after a period of dark-energy domination, the two branes start to move toward each other, and then they collide, and that’s the Bang. It’s the decay of the dark energy we see today which leads to the next Big Bang, in the cyclic model.
Dark energy was only observationally confirmed in 1999 and it was a huge surprise for the inflationary picture. There’s no rhyme or reason for its existence in that picture: Dark energy plays no role in the early universe, according to inflationary theory. Whereas in the cyclic model, dark energy is vital, because it’s the decay of dark energy that leads to the next Big Bang.
This picture of cyclic brane collisions resolves one of the longest-standing puzzles in cyclic models. The idea of a cyclic model isn’t new: Friedmann and others pictured a cyclic model back in the 1930s. They envisaged a finite universe that collapsed and bounced over and over again. But Richard Tolman soon pointed out that, actually, it wouldn’t remove the problem of having to have a beginning. The reason those cyclic models didn’t work is that every bounce makes more radiation and that means the universe has more stuff in it. According to Einstein’s equations, this makes the universe bigger after each bounce, so that every cycle lasts longer than the one before it. But, tracing back to the past, the duration of each bounce gets shorter and shorter and the duration of the cycles shrinks to zero, meaning that the universe still had to begin a finite time ago. An eternal cyclic model was impossible, in the old framework. What is new about our model is that by employing dark energy, and by having an infinite universe that dilutes away the radiation and matter after every bang, you actually can have an eternal cyclic universe, which could last forever.
7
Why Does the Universe Look the Way It Does?
Sean Carroll
Theoretical physicist, Caltech; author, The Particle at the End of the Universe
This seems, on the one hand, a very obvious question. On the other hand, it is an interestingly strange question, because we have no basis for comparison. The universe is not something that belongs to a set of many universes. We haven’t seen different kinds of universes so we can say, “Oh, this is an unusual universe,” or “This is a very typical universe.” Nevertheless, we do have ideas about what we think the universe should look like if it were “natural,” as we say in physics. Over and over again, it doesn’t look natural. We think this is a clue to something going on that we don’t understand.
One very classic example that people care a lot about these days is the acceleration of the universe and dark energy. In 1998, astronomers looked out at supernovae that were very distant objects in the universe, and they were trying to figure out how much stuff there was in the universe, because if you have more and more stuff—if you have more matter and energy—the universe would be expanding, but ever more slowly as the stuff pulled together. What they found by looking at these distant bright objects of type 1A supernovae was that not only is the universe expanding but it’s accelerating. It’s moving apart faster and faster. Our best explanation for this is something called dark energy—the idea that in every cubic centimeter of space, every little region of space, if you empty it out so there are no atoms, no dark matter, no radiation, no visible matter, there is still energy there. There is energy inherent in empty spaces. We can measure how much energy you need in empty space to fit this data, this fact that the universe is accelerating. This vacuum energy pushes on the universe. It provides an impulse. It keeps the universe accelerating. We get an answer, and the answer is 10-8 ergs per cubic centimeter, if that’s very meaningful.
But then we can also estimate how big it should be. We can ask, “What should the vacuum energy have been?” We can do a back-of-the-envelope calculation, just using what we know about quantum field theory, the fact that there are virtual particles popping in and out of existence. We can say there should be a certain amount of vacuum energy. The answer is, there should be 10112 ergs per cubic centimeter. In other words, 10120 times as much as the theoretical prediction compared to the observational reality. That’s an example where we say the universe isn’t natural. There’s a parameter of the universe, there’s a fact about the universe in which we live—how much energy there is in empty space—which doesn’t match what you would expect, what you would naïvely guess.
This is something a lot of attention has been paid to in the last ten years, or even before that—trying to understand the apparently finely tuned nature of the laws of physics. People talk about the anthropic principle and whether or not you could explain this by saying that if the vacuum energy were bigger we wouldn’t be here to talk about it. Maybe there’s a selection effect that says you can only live in a universe with finely tuned parameters like this. But there’s another kind of fine tuning, another kind of unnaturalness, which is the state of the universe, the particular configuration we find the universe in—both now and at earlier times. That’s where we get into entropy and the arrow of time.
This is actually the question I’m most interested in right now. It’s a fact about the universe in which we observe that there are all sorts of configurations in which the particles in the universe could be. We have a pretty quantitative understanding of ways you could rearrange the ingredients of the universe to make it look different. According to what we were taught in the 19th century about statistical mechanics by Boltzmann and Maxwell and Gibbs and giants like that, what you would expect in a natural configuration is for something to be high entropy—for something to be very, very disordered. Entropy is telling us the number of ways you could rearrange the constituents of something so that it looks the same. In air filling the room, there are a lot of ways you could rearrange the air so that you wouldn’t notice. If all the air in the room were squeezed into one tiny corner, there are only a few ways you could rearrange it. If air is squeezed into a corner, it’s low entropy. If it fills the room, it’s high entropy. It’s very natural that physical systems go from low entropy, if they are low entropy, to being high entropy. There are just a lot more w
ays to be high entropy.
If you didn’t know any better, if you asked what the universe should be like, what configuration it should be in, you would say it should be in a high-entropy configuration. There are a lot more ways to be high entropy; there are a lot more ways to be disorderly and chaotic than there are to be orderly and uniform and well arranged. However, the real world is quite orderly. The entropy is much, much lower than it could be. The reason for this is that the early universe, near the Big Bang, 14 billion years ago, had incredibly low entropy compared to what it could have been. This is an absolute mystery in cosmology. This is something that modern cosmologists don’t know the answer to: why our observable universe started out in a state of such pristine regularity and order, such low entropy. We know that if it does, it makes sense. We can tell a story that starts in the low-entropy early universe, trace it through the present day and into the future. It’s not going to go back to being low-entropy. It’s going to be compliant entropy. It’s going to stay there forever. Our best model of the universe right now is one that began 14 billion years ago in a state of low entropy but will go on forever into the future in a state of high entropy.
Why do we find ourselves so close to the aftermath of this very strange event, this Big Bang, that has such low entropy? The answer is, we just don’t know. The anthropic principle is just not enough to explain this. We really need to think deeply about what could have happened both at the Big Bang and even before the Big Bang. My favorite guess at the answer is that the reason the universe started out at such a low entropy is the same reason that an egg starts out at low entropy. The classic example of entropy is that you can take an egg and make an omelet. You cannot take an omelet and turn it into an egg. That’s because the entropy increases when you mix up the egg to make it into an omelet. Why did the egg start with such a low entropy in the first place? The answer is that it’s not alone in the universe. The universe consists of more than just an egg. The egg came from a chicken. It was created by something that had a very low entropy that was part of a bigger system. The point is that our universe is part of a bigger system. Then you can start to try to understand why it had such a low entropy to begin with. I actually think that the fact that we can observe the early universe having such a low entropy is the best evidence we currently have that we live in a multiverse, that the universe we observe is not all there is, that we are actually embedded in some much larger structure.
We are in a very unusual situation in the history of science where physics has become slightly a victim of its own success. We have theories that fit the data, which is a terrible thing to have when you’re a theoretical physicist. You want to be the one who invents those theories, but you don’t want to live in a world where those theories have already been invented because then it becomes harder to improve upon them when they just fit the data. What you want are anomalies given to us by the data that we don’t know how to explain.
Right now, we have two incredibly successful models—in fact three if you want to count gravity. We have for gravity Einstein’s general theory of relativity, which we have had since 1915. It provides a wonderful explanation of how gravity works from the solar system to the very, very early universe—one second after the Big Bang. In particle physics, we have the standard model of particle physics, based on quantum field theory, and it predicts a certain set of particles. It was assembled over the course of the ’60s and ’70s, and then through the ’80s and the ’90s all we did was confirm that it was right. We got more and more evidence that it fit all of the data. The standard model is absolutely consistent with the observations that we had. Finally, in cosmology we have the standard Big Bang model—the idea that we start in a hot dense state near the Big Bang. We expand and cool over the course of 14 billion years. We have a theory for the initial conditions, where there were slight deviations in density from place to place and these slight deviations grow into galaxy and stars and clusters of galaxies.
The three ingredients—the standard model of particle physics, general relatively for gravity, and the standard model of Big Bang cosmology—together fit essentially all the data we have. It makes it very difficult to move beyond that, but it’s crucial that we move beyond that, because these ideas are mutually inconsistent with each other. We know they can’t be the final answer. We have these large outstanding questions. How do you reconcile quantum field theory—and quantum mechanics more generally, which is the basis of the standard model—with general relativity, which is the way we describe gravity? These two theories are just speaking completely different languages, and that makes it very difficult to know how to marry them together. In cosmology, we have the Big Bang, which is a source of complete mystery. How did the universe begin? Why were the initial conditions like they were? That’s something we need to figure out. We also have hints of things that don’t quite fit into the model. We have dark matter, which cannot be accommodated in the standard model of particle physics, and we have dark energy making the universe accelerate, which is not something that we can do. We can basically put a fudge factor into the equations that fit the data, but again we don’t have an understanding of why it is like that, where that comes from.
What we want to do is move beyond these models that fit the data, and are phenomenological and basically about fitting the data, and move to a deeper understanding. What are the fundamental ingredients out of which gravity and particle physics arise? What are the things that could have happened at the Big Bang? There are a bunch of ideas out there on the market.
For fundamental physics, we have string theory as the dominant paradigm. We don’t know that string theory is right. It could be wrong, but for many years now people have been working on string theory, suggesting that we replace the idea of tiny little particles making up the universe by tiny little loops of string. That single idea taken to its logical conclusion predicts a whole bunch of wonderful things, which unfortunately we can’t observe. This is just a problem with our ability to do experiments compared to the regime in which string theory might become important. We can’t make a string by itself. We can’t observe the stringiness of ordinary particles, because the energies are just too high. In string theory, you predict that there should be extra dimensions of space. There should not only be the three dimensions of space that we know and love—up, down, forward, backward—but there should be extra dimensions, and those dimensions are somehow invisible. They could all be curled up in really tiny balls and we just can’t see them. In fact, we will never see them plausibly, depending on how small they are. Or some of them could be big, and we are stuck on some subset. We can’t get to the extra dimensions, and this is the idea that we live on a brane. One of the questions that string theory puts front and center is, if the theory itself—string theory—likes to predict that there are extra dimensions, then where are they? Not only where are they, but why are they not visible? What happened in the universe to make these dimensions invisible? There are a bunch of ideas.
I recently wrote a paper with Lisa Randall and Matt Johnson about how we could have started in a universe that had more dimensions and then undergone a transition to a big space where some of the dimensions were curled up. This is a provocative idea that also feeds into cosmology. The point is that you can’t just sit down and try to reconcile gravity—the laws of general relativity—with quantum mechanics without also talking about cosmology and why the universe started in the state that it was in. The way we have to go is to look at what happened before the Big Bang. Right now, the best model we have for what happened at what we now call the Big Bang, which is the favorite one among cosmologists, is inflation. The idea is that there was a temporary period of superfast acceleration that took a tiny little patch of the universe and smoothed it out, filled it with energy, and then that energy heated up into ordinary particles and dark matter, and that’s what we see as the Big Bang today.
But inflation has a lot of questions that it doesn’t answer. The most obvious question is, Why did
inflation ever start? You say, “Well, there’s a tiny little patch. It was dominated by some form of energy.” How unlikely can that be? Roger Penrose and other people have emphasized that it’s really, really unlikely that can be. Inflation doesn’t provide a natural explanation for why the early universe looks like it does unless you can give me an answer for why inflation ever started in the first place. That’s not a question we know the answer to right now. That’s why we need to go back before inflation, into before the Big Bang, into a different part of the universe, to understand why inflation happened versus something else. There you get into branes and the cyclic universe.
I really don’t like any of the models that are on the market right now. We really need to think harder about what the universe should look like. If we didn’t have some prejudice for what the universe did look like from doing experiments, we should try to understand what we would expect just from first principles as to what the universe should look like, and then see how that comes close to, or is far away from, looking like the actual universe. It’s only when we take seriously what our theories would like the universe to look like, and then try to match them with the universe that we see, that we can take advantage of these clues the experiments are giving us to try to reconcile the ideas of quantum mechanics, gravity, string theory, and cosmology.
One of the interesting things about the string theory situation, where we’re victims of our own success, where we have models that fit the data very well but we’re trying to move beyond them, is that the criteria for success has changed a little bit. It’s not that one theory or another makes a prediction that you can go out and test tomorrow. We all want to test our ideas eventually, but it becomes a more long-term goal when it’s hard to find data that doesn’t already agree with the existing theories. We know that the existing theories aren’t right and we need to move beyond them.
The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos Page 10
