Chapter 16
ALAN GUTH
"A Universe in Your Backyard"
Lee Smolin: The idea of inflation has probably been the most influential idea in cosmology in the last fifteen years, and it's Alan's idea. It's an idea that hasn't entirely convinced me, and I'm not alone in this, but it's had an enormous effect on everybody's thinking.
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ALAN GUTH is a physicist; Victor F. Weisskopf Professor of Physics at MIT; author of The Inflationary Universe: The Quest for a New Theory of Cosmic Origins, forthcoming, 1997.
Alan Guth: Cosmology has very much become an observational science; it's no longer people sitting back in armchairs inventing unfounded theories about what the universe might look like. Observations are being made all the time: observations of the distribution of galaxies in the universe, observations of the microwave background radiation and the nonuniformities in that radiation; estimates of the mass density of the universe; estimates of the age of the universe, based on a variety of different techniques.All that has an impact on the kinds of theories of the universe which are viable. In 1980, I developed the idea of the inflationary universe. It was a new theory of how the big bang might have begun. It's a theory consistent with the standard big- bang picture, which is one of the reasons it's become as well accepted as it has. It doesn't require people to throw out what was believed previously about cosmology. But it adds a lot. It adds a whole story about what happened during the first fraction of a second of the universe, a time period that had not been explored before. It answers a number of questions left open by the standard big-bang model. The inflationary universe is a theory about reality. I, and probably most physicists, regard reality as a genuine physical reality, a reality influenced by people only insofar as we can reach and move things and so on. Reality exists independent of people. The goal of the physicist is to understand that reality.
One of the most amazing features of the inflationary- universe model is that it allows the universe to evolve from something that's initially incredibly small. Something on the order of twenty pounds of matter is all it seems to take to start off a universe. This is very different from the standard cosmological model. Before inflation, the standard model required you to assume that all the matter that exists now was already there at the beginning, and the model just described how the universe expanded and how the matter cooled and evolved. Given the inflationary model, it becomes very tempting to ask whether, in principle, it's possible to create a universe in the laboratory — or a universe in your backyard — by man-made processes.
The first question to look at is what would happen if you had a small patch of inflationary universe in the midst of our universe, never mind how it might have gotten there. Let's pretend that it exists, and ask how it evolves. It turns out that if this patch is big enough, it will grow to become a new universe, but it does this in a very strange way. It doesn't — and this is very important for environmental purposes — displace our universe. Instead, the patch forms a wormhole and slips through it. From our universe, it always appears very small and looks more or less like an ordinary black hole. But on the inside, the new universe is expanding and can become arbitrarily large, creating new space as it grows. It can easily become large enough to encompass a universe like the one we see. In a very short length of time, a small fraction of a second, it completely pinches off from our universe and becomes a totally isolated new universe.
Inflationary cosmology is a new twist on the big-bang theory. It doesn't in any way do away with the big-bang theory. It's completely consistent with everything that's been talked about in terms of the big-bang model. What it does is change our conception of the history of the first small fraction of a second of the big bang. According to the new theory, the universe during this sliver of time underwent a period of inflation, a brief era of colossal expansion.
There are two key features that are different in inflationary cosmology from the standard big bang. One is that the inflationary model contains a mechanism by which essentially all the matter in the universe can be created during the brief period of inflation. In the standard big-bang model, by contrast, it was always necessary to assume that all the matter was there from the beginning, and there was no way to describe how it might be created. By the way, the inflationary production of matter is consistent with the principle of energy conservation, even though it can literally produce a universe from almost nothing. Energy is still conserved — this is all calculated in the context of standard classical general relativity. The unusual feature is that gravity plays a major role in the energy balance. It turns out that the energy of a gravitational field — any gravitational field — is negative. During inflation, as the universe gets bigger and bigger and more and more matter is created, the total energy of matter goes upward by an enormous amount. Meanwhile, however, the energy in gravity becomes more and more negative. The negative gravitational energy cancels the energy in matter, so the total energy of the system remains whatever it was when inflation started — presumably something very small. The universe could, in fact, even have zero total energy, with the negative energy of gravity precisely canceling the positive energy of matter. This capability for producing matter in the universe is one crucial difference between the inflationary model and the previous model.
The other big difference is the ability of the inflationary theory to explain several prominent features of our universe which remain unexplained in the standard big-bang model. Take, for example, the large-scale uniformity of the universe. When we look out to great distances, it appears that the universe is remarkably uniform. The best evidence for this comes from the oldest thing we can see — the cosmic microwave background radiation, a kind of afterglow of the big bang itself. When we look at this background radiation, we're seeing a snapshot of what the universe looked like when that radiation was released — something that happened only a few hundred thousand years after the big bang — and it's telling us that the universe was then incredibly uniform.
In the context of the standard big-bang model, that was always a mystery. The early universe was so large that there wasn't nearly enough time for light to travel across it in the time available. We can imagine, for example, observing the microwave radiation from two opposite directions in the sky, and then we can use the big-bang theory to trace each of the two microwave beams back to its source. When the radiation was released, the two sources were separated from each other by a distance about a hundred times larger than the total distance that light could have traveled up until that time. Since we believe that nothing can travel faster than light, it means that the point on one side of the universe had no way of being influenced by what was going on at the opposite point, but somehow they managed to be at the same temperature at the same time to the extraordinary precision of a few parts in a hundred thousand. The standard big-bang theory could account for this uniformity only by assuming, without explanation, that the universe started out incredibly uniform.
The inflationary model, on the other hand, posits a short period in the very early universe during which the universe expanded far, far faster than in the standard cosmology. This implies that the early universe was far smaller than people had previously thought. There was plenty of time for this microscopic proto-universe to come to a uniform temperature before inflation began, and then inflation magnified this very small region to become large enough to encompass the observed universe. The large-scale uniformity of the universe is therefore no longer a mystery, but can now be understood as the natural consequence of cosmic evolution. To account for the observed degree of large- scale uniformity, we must assume that the universe expanded during the inflationary era by at least a factor of a trillion trillion. It's quite likely that the expansion factor was much larger than this stupendous number, but we have no way of knowing how much the universe actually inflated.
I've recently been working on wormholes and on the question of whether it's in principle possible to create "a universe in your backyard." A few years ago I worked with Steven Blau and Eduardo Guendelman to figure out what would happen if there were a region of an inflating universe in the midst of our universe. We found that the question could be answered very cleanly and unambiguously, since the behavior is determined by general relativity. The only new ingredient for this problem is an idea from particle physics about a certain kind of matter called a "false vacuum," which is the driving force behind inflation. We discovered that a large enough region of false vacuum would create a new universe, which, as I described earlier, would rapidly disconnect from ours and become totally isolated.
The next question, which turns out to be much harder, is what does it take to produce this small region of false vacuum — to start everything going? Since the mass density of the false vacuum is approximately 1060 times larger than the density of an atomic nucleus, it would certainly not be easy. There's no technology in the present or the foreseeable future that would allow us to do this sort of thing. Nonetheless, one can talk about the physics of universe creation as a matter of principle, and I find it a very interesting question.
I'm going to imagine that somebody can make a false vacuum and learn to manipulate these extraordinary energy densities. But then there's still another problem. As you start to collect this material, its own gravitational force is so strong that it tends to collapse into a black hole. The formation of a black hole can be prevented only by starting the material expanding at a very high speed. We found that if the region is to expand fast enough to produce a new universe, it must begin from what in technical terms is called an initial singularity — also known as a white hole. A white hole is essentially the opposite of a black hole: while matter can fall into a black hole but cannot escape, matter is ejected from a white hole but cannot enter it.
The instant of cosmic creation in the big-bang theory is an example of a white hole, but certainly nobody has ever seen a white hole, and nobody knows how to make one in the laboratory. So if you ask whether a new universe can in principle be created in the laboratory, the answer, according to classical general relativity, is no, since such a creation requires a white hole. But classical general relativity is not the final word. The evidence is overwhelming that we live in a quantum universe — a universe that isn't governed by deterministic classical laws. We've found that quantum theory is absolutely essential for understanding molecules, atoms, and subatomic particles, and physicists firmly believe that quantum theory is also essential to an understanding of the true nature of gravity. Unfortunately, however, there are very complicated technical problems in trying to construct a quantum theory of gravity. The riddle of quantum gravity is perhaps solved by the superstring theory, but that theory is so poorly understood that it hasn't yet been used to answer any of the central questions that quantum gravity is expected to address.
While classical physics implies that a universe can't be created without a white hole, there's a possibility that quantum effects could make it easier. Edward Farhi, Jemal Guven, and I attempted to study the quantum question using an approximate formulation of quantum gravity that's much more tractable than superstring theory. We discovered two things. First, we found that one of the standard approximations to quantum gravity led to inconsistencies and had to be modified to obtain any answer at all. Second, we found that if we believed our modified rules of quantum gravity, then it is in principle possible to create a universe in the laboratory without starting from a white hole. The procedure isn't guaranteed to succeed, but in the context of quantum mechanics we were able to estimate a probability for success. Since our calculations relied on a modification of an approximation that was uncertain in the first place, we found it reassuring that Willy Fischler, Daniel Morgan, and Joseph Polchinski obtained the same results with a different method. The probability of success was found to depend crucially on the energy density of the false vacuum. If it's at a scale typical of what particle physicists call "grand unified theories," then the probability would be outlandishly small. On the other hand, it's conceivable that the energy level associated with the false vacuum might be a thousand times larger than those of the grand unified theories, and then the probability of successful universe production would be high.
Our calculations remain somewhat tentative, however, as the uncertainties of quantum gravity haven't been overcome. Since synthetic universe creation is well beyond the range of experiment, the only chance for discovering within our lifetimes whether it's possible would be the development of detailed theories of quantum gravity and the behavior of matter at extremely high energies. Those two challenges are linked, since the gravitational interactions of elementary particles become significant only at extraordinarily high energies.
An interesting aspect of the universe-creation work was the role of wormholes — elongated tubes of space that can in principle connect one universe to another, or a part of a universe to a distant part of the same universe. In the universe-creation scenario, the child universe is initially connected by a wormhole to its parent, although the wormhole pinches off in about 10-35 seconds. The same kinds of wormholes are also relevant to the question of whether the laws of physics allow the possibility of time travel.
The question of time travel hinges on the lifetime of the wormholes. For time travel to work, one needs to have a stable wormhole — a wormhole that can be built large and exist for a long time, so that you could travel through it. The scenario would begin with the construction of a wormhole linking our universe to itself, whenever it becomes technologically feasible. Then the aspiring time traveler would keep one entrance of the wormhole alongside her as she evolves normally into the future. She must keep the entrance moving at near the speed of light, but it can travel in a circle, so that it returns periodically. Years or millennia later, she or her descendants would be able to return to the time at which the wormhole was constructed by traveling through the wormhole.
The laws of physics, however, are not very cooperative with wormhole-transportation engineers. The rapid collapse of the wormhole in child-universe production is characteristic. In fact, if the wormhole is constructed from any "normal" material, it will collapse before anything can go through it. To hold the wormhole open requires a material with a negative energy density. There's room for hope, however, since relativistic quantum theories are known to allow the existence of regions of negative energy density. The size and duration of such regions are limited, however, so no one has yet designed a theoretically traversable wormhole. On the other hand, no one has proven it impossible.
People might wonder whether it makes any sense at all to be playing with theories that involve numbers such as 10-35 seconds. "How can you assign value or meaning to a number like that?" some may ask, since it's so far beyond direct experience. One of the amazing things about science, though, is the spectacular success we've had in extrapolating mathematical relationships. When the equations of electricity and magnetism were assembled by Maxwell in 1864, for example, they were based on tabletop experiments, with distances ranging from centimeters to meters. Today we successfully use these same equations to describe phenomena ranging from the size of atomic nuclei to the size of the visible universe. Obviously, however, one cannot claim that such extrapolations are always valid. When Newton's laws of motion are extrapolated to half the speed of light, they are found to be wrong! While large extrapolations aren't necessarily trustworthy, I would claim that they are always worth exploring. Special relativity was discovered, in fact, when Einstein attempted to extrapolate Newton's laws to near the speed of light. What would it look like to ride on a light wave? Einstein asked himself. Today physicists are similarly asking what it would look like to view the universe 10-35 seconds after its birth. It's speculative, but it's also intriguing, and we hope that it's productive.
I tend to take a rather hard-nosed point of view as to the underlying nature of the universe. The universe exists as a physical object, and physicists and other scientists are making a lot of progress in trying to understand the rules by which it works. It's important in science, and in life, to recognize that at any given time there will always be some questions you can't answer. You continue to try to answer them, but you shouldn't be surprised if you find you're incapable of answering them.Lee Smolin: The idea of inflation has probably been the most influential idea in cosmology in the last fifteen years, and it's Alan's idea. It's an idea that hasn't entirely convinced me, and I'm not alone in this, but it's had an enormous effect on everybody's thinking. The idea came from attempts to understand some very difficult problems about the universe as a whole; particularly why it's so symmetric, why it's not much, much more disordered than it is. If we believe in the standard theories of cosmology, and believe that the big bang was the first moment of time, then there was not enough time from that initial moment to the moment we see when we look at the cosmic microwave background radiation for the different parts of the universe to have interacted with each other and come to the same state. Every part of the universe we can see has the same temperature to a few parts in a hundred thousand. The idea of inflation was invented to explain that and other puzzles.
There are two ways to talk about inflation. You can say that at some very, very early moment the universe expands exponentially fast, growing by many, many powers of ten; or you can say that time slows down extraordinarily during this inflationary period. Both have the same effect; the period allows all the different parts of the universe we see to have been in communication.
Another very interesting thing about the idea is that it makes a prediction, which is that omega is precisely equal to one. Omega is a measure of the density of matter in the universe. There's a certain density of matter which will eventually cause the universe to collapse back — this collapse would be brought about by the matter's mutual gravitational pull. We can compute, since we know the speed at which the expansion is happening, how much matter there needs to be to stop the expansion. The ratio of the density of the matter that's actually here in the universe to this critical density is called omega. It is the critically important quantity in cosmology. The theory of inflation predicts that omega should be equal to one, which means that the universe is precisely balanced at the border between those that collapse and those that expand forever. A fraction more matter and it must collapse, a fraction less and it will at some point enter an eternity of rapid, runaway expansion. Therefore the inflationary theory is subject to experimental tests, observational tests. These tests are going to occur in the next ten or fifteen years.
Martin Rees: Inflation has been the stimulus for a great deal of the cosmological discussion about the ultra-early universe. There have been various fashions — old inflation, new inflation, chaotic inflation — and the actual details are still uncertain, in that we can't, for instance, calculate exactly when it happened in the universe, or how the fluctuations arose which evolved into galaxies, clusters, and superclusters, because the answers depend on uncertain physics. It's an exciting new possibility, however, that the small temperature fluctuations first detected by COBE, and now by about ten other experiments, may reveal, spread across the sky, the imprint of physical processes that occurred when the entire observed universe was squeezed smaller than a golfball. The exotic physics of this ultra-early era, when quantum-uncertainty effects were important on the cosmic scale, is now accessible to observations, and speculations can soon be at least constrained. But most cosmologists would bet fairly high odds that the idea of inflation, which was first very clearly stated in Alan Guth's paper, is going to be an element of any correct theory about the early universe. It was one of the developments that made it possible to talk seriously about not just the first second but the first 10-36 seconds of the universe.
It was fortunate that Alan Guth did his work at the same time that another idea came into fashion, which was the theory that we could understand why the universe contains matter and not antimatter in terms of some asymmetry, some favoritism for matter over antimatter in the early universe; it's no good having a scheme that can inflate the universe to enormous dimension if it's not possible to create matter to fill that large universe. Guth brought these two ideas together in his inflationary cosmology.
Excerpted from The Third Culture: Beyond the Scientific Revolution by John Brockman (Simon & Schuster, 1995) . Copyright © 1995 by John Brockman. All rights reserved.