The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos
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The mechanism that inflation provides that drives the universe toward flatness will in almost all cases overshoot, not giving us a universe that is just nearly flat today but a universe that’s almost exactly flat today. This can be avoided, and people have at times tried to design versions of inflation that avoided it, but those versions of inflation never looked very plausible. You have to arrange for inflation to end at just the right point, where it’s almost made the universe flat but not quite. It requires a lot of delicate fine-tuning, but in the days when it looked like the universe was open, some people tried to design such models. But they always looked contrived and never really caught on.
The generic inflationary model drives the universe to be completely flat, which means that one of the predictions is that today the mass density of the universe should be at the critical value that makes the universe geometrically flat. Until three or four years ago, no astronomers believed that. They told us that if you looked at just the visible matter, you would see only about 1 percent of what you needed to make the universe flat. But they also said they could offer more than that: There’s also dark matter. Dark matter is matter that’s inferred to exist because of the gravitational effect it has on visible matter. It’s seen, for example, in the rotation curves of galaxies. When astronomers first measured how fast galaxies rotate, they found they were spinning so fast that if the only matter present was what you saw, galaxies would just fly apart.
To understand the stability of galaxies, it was necessary to assume that there was a large amount of dark matter in the galaxy—about five or ten times the amount of visible matter—which was needed just to hold the galaxy together. This problem repeats itself when one talks about the motion of galaxies within clusters of galaxies. The motion of galaxies in clusters is much more random and chaotic than the spiral galaxy, but the same issues arise. You can ask how much mass is needed to hold those clusters of galaxies together, and the answer is that you still need significantly more matter than what you assumed was in the galaxies. Adding all of that together, astronomers came up only with about a third of the critical density. They were pretty well able to guarantee that there wasn’t any more than that out there; that was all they could detect. That was bad for the inflationary model, but many of us still had faith that inflation had to be right and that sooner or later the astronomers would come up with something.
And they did, although what they came up with was something very different from the kind of matter we were talking about previously. Starting in 1998, astronomers have been gathering evidence for the remarkable fact that the universe today appears to be accelerating, not slowing down. As I said at the beginning of this talk, the theory of general relativity allows for that. What’s needed is a material with a negative pressure. We are now therefore convinced that our universe must be permeated with a material with negative pressure which is causing the acceleration we’re now seeing. We don’t know what this material is, but we’re referring to it as dark energy. Even without knowing what it is, general relativity by itself allows us to calculate how much mass has to be out there to cause the observed acceleration, and it turns out to be almost exactly equal to two-thirds of the critical density. This is exactly what was missing from the previous calculations! So, if we assume that this dark energy is real, we now have complete agreement between what the astronomers are telling us about the mass density of the universe and what inflation predicts.
The other important prediction that comes out of inflation is becoming even more persuasive than the issue of flatness: namely, the issue of density perturbations. Inflation has what in some ways is a wonderful characteristic: that by stretching everything out—and Paul’s model takes advantage of the same effect—you can smooth out any nonuniformities that were present prior to this expansion. Inflation does not depend sensitively on what you assume existed before inflation; everything there just gets washed away by the enormous expansion. For a while, in the early days of developing the inflationary model, we were all very worried that this would lead to a universe that would be absolutely, completely smooth.
After a while, several physicists began to explore the idea that quantum fluctuations could save us. The universe is fundamentally a quantum mechanical system, so perhaps quantum theory was necessary not just to understand atoms but also to understand galaxies. It’s a rather remarkable idea that an aspect of fundamental physics like quantum theory could have such a broad sweep. The point is that a classical version of inflationary theory would predict a completely uniform density of matter at the end of inflation. According to quantum mechanics, however, everything is probabilistic. There are quantum fluctuations everywhere, which means that in some places the mass density would be slightly higher than average and in other places it would be slightly lower than average. That’s exactly the sort of thing you want, to explain the structure of the universe. You can even go ahead and calculate the spectrum of these nonuniformities, which is something that Paul and I both worked on in the early days and had great fun with. The answer that we both came up with was that, in fact, quantum mechanics produces just the right spectrum of nonuniformities.
We really can’t predict the overall amplitude—that is, the intensity—of these ripples unless we know more about the fundamental theory. At the present time, we have to take the overall factor that multiplies the predicted intensity of these ripples from observation. But we can predict the spectrum—that is, the complicated pattern of ripples can be viewed as ripples of many different wavelengths lying on top of each other, and we can calculate how the intensity of the ripples varies with their wavelengths. We knew how to do this back in 1982, but recently it has actually become possible for astronomers to see these nonuniformities imprinted on the cosmic background radiation. These were first observed back in 1992 by the COBE satellite, but back then they could only see very broad features, since the angular resolution of the satellite was only about 7 degrees. Now they’ve gotten down to angular resolutions of about 1/10 of a degree. These observations of the cosmic background radiation can be used to produce plots of the spectrum of nonuniformities, which are becoming more and more detailed.
The most recent data set was made by an experiment called the Cosmic Background Imager, which released a new set of data in May that is rather spectacular. This graph of the spectrum is rather complicated, because these fluctuations are produced during the inflationary era but then oscillate as the early universe evolves. Thus, what you see is a picture that includes the original spectrum plus all of the oscillations that depend on various properties of the universe. A remarkable thing is that these curves now show five separate peaks, and all five of the peaks show good agreement between theory and observation. You can see that the peaks are in about the right place and have about the right heights, without any ambiguity, and the leading peak is rather well-mapped-out. It’s a rather remarkable fit between actual measurements made by astronomers and a theory based on wild ideas about quantum fluctuations at 10-35 seconds. The data are so far in beautiful agreement with the theory.
At the present time, this inflationary theory, which a few years ago was in significant conflict with observation, now works perfectly with our measurements of the mass density and the fluctuations. The evidence for a theory that’s either the one I’m talking about or something very close to it is very, very strong.
I’d just like to close by saying that although I’ve been using “theory” in the singular to talk about inflation, I shouldn’t, really. It’s important to remember that inflation is really a class of theories. If inflation is right, it’s by no means the end of our study of the origin of the universe, but still it’s really closer to the beginning. There are many different versions of inflation, and in fact the cyclic model that Paul described could be considered one version. It’s a rather novel version, since it puts the inflation at a completely different era of the history of the universe, but inflation is still doing many of the same things. There are many versions of in
flation that are much closer to the kinds of theories we were developing in the ’80s and ’90s, so saying that inflation is right is by no means the end of the story. There’s still a lot of flexibility here, and a lot to be learned. And what needs to be learned will involve both the study of cosmology and the study of the underlying particle physics, which is essential to these models.
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Andrei Linde
Theoretical physicist, Stanford University; father of eternal chaotic inflation; inaugural winner, Milner Foundation Fundamental Physics Prize
I should probably start by explaining what happened during the last thirty years in cosmology. This story will begin with old news: the creation of inflationary theory. Then we will talk about the relatively recent developments, when inflation became a part of the theory of an inflationary multiverse and the string theory landscape. Then—what we expect in the future.
Let me start by saying that many, many years ago—and I mean like almost a century ago—Einstein came up with something called the cosmological principle, which says that our universe must be homogeneous and uniform. And for many years people used this principle. In fact, it was formulated even much earlier, by Newton. The universe is still represented this way in current books on astrophysics, where you can find different versions of the cosmological principle.
For a while, this was the only way of answering the question of why the universe is everywhere the same—in fact, why it is the universe. So we did not think about the multiverse, we just wanted to explain why the world is so homogeneous around us, why it is so big, why parallel lines do not intersect. Which is, in fact, part of the same question: If the universe was tiny, like a small globe, and you drew parallel lines perpendicular to the equator of the globe, they would intersect at the south and the north poles. Why has nobody ever seen parallel lines intersecting?
These kinds of questions, for many years, could seem a bit silly. For example, one may wonder what happened before the universe even emerged. The textbook of general relativity that we used in Russia said that it was meaningless to ask this question, because the solutions of the Einstein equations cannot be continued through the singularity, so why bother? And yet people bothered. They are still trying to answer these kinds of questions. But for many people such questions looked metaphysical, not to be taken seriously.
When inflationary theory was invented, people started taking these questions seriously. Alan Guth asked these questions and proposed the theory of cosmic inflation, a framework in which a consistent answer to these questions could be found. The problem was, as Guth immediately recognized, that his own answer to these questions was incomplete. And then, after more than a year of work, I proposed a new version of inflationary theory, which helped to find a way to answer many of these questions. At first it sounded like science fiction, but once we found possible answers to the questions which previously were considered metaphysical, we couldn’t just forget about it. This was the first reason for us to believe the idea of cosmic inflation. So let me explain this idea.
Standard Big Bang theory says that everything begins with a big bang, a huge explosion. Terrorists started the universe. But when you calculate how much high-tech explosives these guys would have to have at their disposal to start the universe formation, they would need 1080 tons of high-tech explosives, compressed to a ball smaller than 1 centimeter, and ignite all of its parts exactly at the same time with a precision better than 1 in 10,000.
Another problem was that in the standard Big Bang scenario, the universe could only expand slower as time went on. But then why did the universe start to expand? Who gave it the first push? It looks totally incredible, like a miracle. However, people sometimes believe that the greater the miracle, the better: Obviously, God could create 1080 tons of explosive from nothing, then ignite it and make it grow, all for our benefit. Can we make an attempt to come up with an alternative explanation?
According to inflationary theory, one may avoid many of these problems if the universe began in some special state, almost a vacuum-like state. The simplest version of such a state involves something called a scalar field. Remember electric and magnetic fields? Well, a scalar field is even simpler: It doesn’t point to any direction. If it’s uniform and doesn’t change in time, it is invisible like a vacuum, but it may have lots of energy packed in it. When the universe expands, the scalar field remains almost constant, and its energy density remains almost constant.
This is the key point, so let us talk about it. Think about the universe as a big box containing many atoms. When the universe expands two times, its volume grows eight times, and therefore the density of atoms decreases eight times. However, when the universe is filled with a constant scalar field, its energy density remains constant while the universe expands. Therefore when the size of the universe grows two times, the total energy of matter in the universe grows eight times. If the universe continues to grow, its total energy, and its total mass, rapidly becomes enormously large, so one could easily get all of these 1080 tons of matter starting from almost nothing.
That was the basic idea of inflation. At the first glance, it could seem totally wrong, because of energy conservation. One cannot get energy from nothing. We always have the same energy with which we started. Once, I was invited to give an opening talk at the Nobel symposium in Sweden on the concept of energy, and I wondered, Why did they invite me there? What am I going to tell these people who study solar energy, oil, wind? What can I tell them? And then I told them, “If you want to get lots of energy, you can start from practically nothing, and you can get all the energy in the universe.”
Not everyone knows that when the universe expands, the total energy of matter does change. The total energy of matter plus gravity does not change, and it amounts to exactly zero. So the energy conservation for the universe is always satisfied, but it is trivial: Zero equals zero. But we are not interested in the energy of the universe as a whole; we are interested in the energy of matter.
If we can have a regime where we have some kind of instability, where the initial zero energy can split into a very big positive energy of matter and a very big negative energy of gravity, the total sum remains zero. But the total energy of matter can become as large as we want. This is one of the main ideas of inflation.
We have found how to start this instability, and how to stop it, because if it doesn’t stop, then it goes on forever, and then it’s not the universe where we can we live. Alan Guth’s idea was how to start inflation, but he didn’t know how to stop it in a graceful way. My idea was how to start it, continue it, and eventually stop it without damaging the universe. And when we learned how to do it, we understood that yes, we can start from practically nothing, or even literally nothing, as suggested by Alex Vilenkin, and account for everything that we see now. At that time, it was quite a revolutionary development: We finally could understand many properties of our universe. We no longer needed to postulate the cosmological principle; we finally knew the real physical reason why the world we see around us is uniform.
But then, soon after inventing new inflation, I realized something else. If you take a universe which initially was tiny, tiny, but still contained different parts with different properties, then our part of the universe might have exploded exponentially, and we no longer see other parts of the universe, which become far away from us because the distance between different parts of the universe increased exponentially during inflation. And those who live in other parts of the universe will be unable see us, because we will be far away. We look around, we do not see other parts of the universe with different properties, and so we think, “This is our universe, it is everywhere the same, other parts do not exist.” And those who live in other parts of the universe will also think, “All of the universe is the way we see it.”
For example, I could start in a red part of the universe, like in the Soviet Union, and you can start in a blue universe,
and then after inflation, after each part becomes exponentially large, each of us would look around and say, just like Einstein and Newton did, “This is the universe, this is the whole thing, it is single-colored.” And then some of us will try to explain why the universe must be red, and others will try to explain why the universe must be blue, all over the world. But now we know that from the point of view of inflation, it’s quite possible that our universe is divided into many regions with different properties. Instead of the cosmological principle—which asserted that the whole world is the same everywhere and all of us must live in parts of the universe with similar properties—we are coming to a more cosmopolitan perspective: We live in a huge inflationary multiverse. Some of us can live in its red parts, some can live in blue parts, and there is nothing wrong about this picture as long as each of its parts is enormously large because of inflation.
Thus inflationary theory explained why our part of the universe looks so uniform: Everything that surrounds us was created by the exponential expansion of a tiny part of the universe. But we cannot see what happens at a distance much greater than the speed of light multiplied by the age of the universe. Inflationary theory tells that our part of the universe, the part we can see, is much, much smaller than the whole universe. Inflation of other parts of the universe may produce enormous regions with different properties. This was the first realization, which paved the way towards the theory of the inflationary multiverse. And the second realization was that even if we start with the same universe everywhere—a red universe, say—quantum fluctuations produced during inflation could make it multicolored.
I’m talking about different colors here only to help us visualize what happens during inflation and after it. Let me explain what I actually mean by that. Think about water. It can be liquid water, solid, or vapor. The chemical composition is the same, H2O. But fish can live only in liquid water. Liquid, solid, or vapor are different phases of water. The same may happen with different realizations of the laws of physics in the universe. We usually assume, for simplicity, that all parts of the universe obey the same fundamental laws of physics. Nevertheless, different parts of the universe may dramatically differ from each other, just as icebergs differ from the water surrounding them. But instead of saying that water can be in different phases in application of physical laws, we say that different parts of the universe may be in different vacuum states, and in each of them the same fundamental law of physics may be realized differently. For example, in some parts of the universe we have weak, strong, and electromagnetic interactions, and in other parts these interactions do not differ from each other.