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A Golden Age of Cosmology
Alan Guth
Father of the inflationary theory of the Universe and Victor F. Weisskopf Professor of Physics at MIT; inaugural winner, Milner Foundation Fundamental Physics Prize; author, The Inflationary Universe: The Quest for a New Theory of Cosmic Origins
Itās often saidāand I believe this saying was started by the late David Schrammāthat today we are in a golden age of cosmology. Thatās really true. Cosmology at this present time is undergoing a transition from being a bunch of speculations to being a genuine branch of hard science, where theories can be developed and tested against precise observations. One of the most interesting areas of this is the prediction of the fluctuations, the nonuniformities, in the cosmic background radiation, an area Iāve been heavily involved in. We think of this radiation as being the afterglow of the heat of the Big Bang. One of the remarkable features of the radiation is that itās uniform in all directions, to an accuracy of about 1 part in 100,000, after you subtract the term thatās related to the motion of the Earth through the background radiation.
Iāve been heavily involved in a theory called the inflationary universe, which seems to be our best explanation for this uniformity. The uniformity is hard to understand. You might think initially that maybe the uniformity could be explained by the same principles of physics that cause a hot slice of pizza to get cold when you take it out of the oven; things tend to come to a uniform temperature. But once the equations of cosmology were worked out so that one could calculate how fast the universe was expanding at any given time, physicists were able to calculate how much time there was for this uniformity to set in.
They found that in order for the universe to have become uniform fast enough to account for the uniformity we see in the cosmic background radiation, information would have to have been transferred at approximately a hundred times the speed of light. But according to all our theories of physics, nothing can travel faster than light, so thereās no way this could have happened. So the classical version of the Big Bang theory had to simply start out by assuming that the universe was homogeneousācompletely uniformāfrom the very beginning.
The inflationary universe theory is an add-on to the standard Big Bang theory, and basically what it adds on is a description of what drove the universe into expansion in the first place. In the classic version of the Big Bang theory, that expansion was put in as part of the initial assumptions, so thereās no explanation for it whatsoever. The classical Big Bang theory was never really a theory of a bang; it was really a theory about the aftermath of a bang. Inflation provides a possible answer to the question of what made the universe bang, and now it looks like itās almost certainly the right answer.
Inflationary theory takes advantage of results from modern particle physics, which predicts that at very high energies there should exist peculiar kinds of substances which actually turn gravity on its head and produce repulsive gravitational forces. The inflationary explanation is the idea that the early universe contains at least a patch of this peculiar substance. It turns out that all you need is a patch; it can actually be more than a billion times smaller than a proton. But once such a patch exists, its own gravitational repulsion causes it to grow rapidly, becoming large enough to encompass the entire observed universe.
The inflationary theory gives a simple explanation for the uniformity of the observed universe, because in the inflationary model the universe starts out incredibly tiny. There was plenty of time for such a tiny region to reach a uniform temperature and uniform density, by the same mechanisms through which the air in a room reaches a uniform density throughout the room. And if you isolate a room and let it sit long enough, it will reach a uniform temperature as well. For the tiny universe with which the inflationary model begins, thereās enough time in the early history of the universe for these mechanisms to work, causing the universe to become almost perfectly uniform. Then inflation takes over and magnifies this tiny region to become large enough to encompass the entire universe, maintaining this uniformity as the expansion takes place.
For a while, when the theory was first developed, we were worried that we would get too much uniformity. One of the amazing features of the universe is how uniform it is, but itās still by no means completely uniform. We have galaxies and stars and clusters and all kinds of complicated structure in the universe that needs to be explained. If the universe started out completely uniform, it would just remain completely uniform, as there would be nothing to cause matter to collect here or there or any particular place.
I believe Stephen Hawking was the first person to suggest what we now think is the answer to this riddle. He pointed outāalthough his first calculations were inaccurateāthat quantum effects could come to our rescue. The real world is not described by classical physics, and even though this was very high-brow physics, we were in fact describing things completely classically, with deterministic equations. The real world, according to what we understand about physics, is described quantum mechanically, which means, deep down, that everything has to be described in terms of probabilities.
The āclassicalā world we perceive, in which every object has a definite position and moves in a deterministic way, is really just the average of the different possibilities the full quantum theory would predict. If you apply that notion here, itās at least qualitatively clear from the beginning that it gets us in the direction we want to go. It means that the uniform density, which our classical equations were predicting, would really be just the average of the quantum mechanical densities, which would have a range of values that could differ from one place to another. The quantum mechanical uncertainty would make the density of the early universe a little bit higher in some places, and in other places it would be a little bit lower.
So, at the end of inflation, we expect to have ripples on top of an almost uniform density of matter. Itās possible to actually calculate these ripples. I should confess that we donāt yet know enough about the particle physics to actually predict the amplitude of these ripples, the intensity of the ripples, but what we can calculate is the way in which the intensity depends on the wavelength of the ripples. That is, there are ripples of all sizes, and you can measure the intensity of ripples of different sizes. And you can discuss what we call the spectrumāwe use that word exactly the way itās used to describe sound waves. When we talk about the spectrum of a sound wave, weāre talking about how the intensity varies with the different wavelengths that make up that sound wave. We do exactly the same thing in the early universe, and talk about how the intensity of these ripples in the mass density of the early universe varied with the wavelengths of the different ripples weāre looking at. Today we can see those ripples in the cosmic background radiation.
The fact that we can see them at all is an absolutely fantastic success of modern technology. When we were first making these predictions, back in 1982, at that time astronomers had just barely been able to see the effect of the Earthās motion through the cosmic background radiation, which is an effect of about one part in a thousand. The ripples Iām talking about are only one part in 100,000ājust 1 percent of the intensity of the most subtle effect it had been possible to observe at the time we were first doing these calculations.
I never believed we would ever actually see these ripples. It just seemed too far-fetched that astronomers would get to be a hundred times better at measuring these things than they were at the time. But to my astonishment and delight, in 1992 these ripples were first detected by a satellite called COBE, the Cosmic Background Explorer, and now we have far better measurements than COBE, which had an angular resolution of about 7 degrees. This meant that you could see only the longest wavelength ripples. Now we have measurements that go down to a fraction of a degree, and weāre getting very precise measurements now of how the intensity varies with wavelength, with marvelous success.
About a year and a half ago, there was a spectacular set of announcements from experiments called BOOMERANG and MAXIMA, both balloon-based experiments, which gave very strong evidence that the universe is geometrically flat, which is just what inflation predicts. By flat I donāt mean two-dimensional; I just mean that the three-dimensional space of the universe is not curved, as it could have been according to general relativity. You can actually see the curvature of space in the way that the pattern of ripples has been affected by the evolution of the universe. A year and a half ago, however, there was an important discrepancy that people worried about, and no one was sure how big a deal to make out of it. The spectrum they were measuring was a graph that had, in principle, several peaks. These peaks had to do with successive oscillations of the density waves in the early universe and a phenomenon called resonance that makes some wavelengths more intense than others. The measurements showed the first peak beautifully, exactly where we expected it to be, with just the shape that was expected. But we couldnāt actually see the second peak.
In order to fit the data with the theories, people had to assume there were about ten times as many protons in the universe as we actually thought, because the extra protons would lead to a friction effect that could make the second peak disappear. Of course, every experiment has some uncertaintyāif an experiment is performed many times, the results will not be exactly the same each time. So we could imagine that the second peak was not seen purely because of bad luck. However, the probability that the peak could be so invisible, if the universe contained the density of protons thatās indicated by other measurements, was down to about the 1-percent level. So it was a very serious-looking discrepancy between what was observed and what was expected.
All this changed dramatically for the better about three or four months ago, with the next set of announcements with more precise measurements. Now the second peak is not only visible but it has exactly the height that was expected, and everything about the data now fits beautifully with the theoretical predictions. Too good, really. Iām sure it will get worse before it continues to get better, given the difficulties in making these kinds of measurements. But we have a beautiful picture now which seems to be confirming the inflationary theory of the early universe.
Our current picture of the universe has a new twist, however, which was discovered two or three years ago. To make things fit, to match the observations, which are now getting very clear, we have to assume that thereās a new component of energy in the universe which we didnāt know existed before. This new component is usually referred to as dark energy. As the name clearly suggests, we still donāt know exactly what this new component is. Itās a component of energy which in fact is very much like the repulsive gravity matter I talked about earlierāthe material that drives the inflation in the early universe. It appears that, in fact, today the universe is filled with a similar kind of matter. The antigravity effect is much weaker than the effect I was talking about in the early universe, but the universe today appears very definitely to be starting to accelerate again, under the influence of this so-called dark energy.
Although Iām trying to advertise that weāve understood a lot, and we have, there are still many uncertainties. In particular, we still donāt know what most of the universe is made out of. Thereās the dark energy, which seems to comprise in fact about 60 percent of the total mass/energy of the universe. We donāt know what it is. It could in fact be the energy of the vacuum itself, but we donāt know that for a fact. In addition, thereās what we call dark matter, which is another 30 percent, or maybe almost 40 percent, of the total matter in the universe. We donāt know what that is, either. The difference between the two is that the dark energy causes repulsive gravity and is smoothly distributed; the dark matter behaves like ordinary matter in terms of its gravitational propertiesāitās attractive and it clusters, but we donāt know what itās made of. The stuff we do know aboutāprotons, neutrons, ordinary atoms and moleculesāappear to comprise only about 5 percent of the mass of the universe.
The moral of the story is we have a great deal to learn. At the same time, the theories that we have developed so far seem to be working almost shockingly well.
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The Cyclic Universe
Paul Steinhardt
Theoretical physicist; Albert Einstein Professor of Science, Princeton University; coauthor (with Neil Turok), Endless Universe: Beyond the Big Bang
If you were to ask most cosmologists to give a summary of where we stand right now in the field, they would tell you that we live in a very special period in human history where, thanks to a whole host of advances in technology, we can suddenly view the very distant and very early universe in ways we havenāt been able to do ever before. For example, we can get a snapshot of what the universe looked like in its infancy, when the first atoms were forming. We can get a snapshot of what the universe looked like in its adolescence, when the first stars and galaxies were forming. And we are now getting a full detail, three-dimensional image of what the local universe looks like today. When you put together this different information, which weāre getting for the first time in human history, you obtain a very tight series of constraints on any model of cosmic evolution.
If you go back to the different theories of cosmic evolution in the early 1990s, the data weāve gathered in the last decade has eliminated all of them save one, a model that you might think of today as the consensus model. This model involves a combination of the Big Bang model as developed in the 1920s, ā30s, and ā40s; the inflationary theory, which Alan Guth proposed in the 1980s; and a recent amendment that I will discuss shortly. This consensus theory matches the observations we have of the universe today in exquisite detail. For this reason, many cosmologists conclude that we have finally determined the basic cosmic history of the universe.
But I have a rather different point of view, a view that has been stimulated by two events. The first is the recent amendment to which I referred earlier. I want to argue that the recent amendment is not simply an amendment but a real shock to our whole notion of time and cosmic history. And secondly, in the last year Iāve been involved in the development of an alternative theory that turns the cosmic history topsy-turvy: All the events that created the important features of our universe occur in a different order, by different physics, at different times, over different time scales. And yet this model seems capable of reproducing all the successful predictions of the consensus picture with the same exquisite detail.
The key difference between this picture and the consensus picture comes down to the nature of time. The standard model, or consensus model, assumes that time has a beginning that we normally refer to as the Big Bang. According to that model, for reasons we donāt quite understand, the universe sprang from nothingness into somethingness, full of matter and energy, and has been expanding and cooling for the past 15 billion years. In the alternative model, the universe is endless. Time is endless, in the sense that it goes on forever in the past and forever in the future, and in some sense space is endless. Indeed, our three spatial dimensions remain infinite throughout the evolution of the universe.
More specifically, this model proposes a universe in which the evolution of the universe is cyclic. That is to say, the universe goes through periods of evolution from hot to cold, from dense to under-dense, from hot radiation to the structure we see today, and eventually to an empty universe. Then, a sequence of events occurs that cause the cycle to begin again. The empty universe is reinjected with energy, creating a new period of expansion and cooling. This process repeats periodically forever. What weāre witnessing now is simply the latest cycle.
The notion of a cyclic universe is not new. People have considered this idea as far back as recorded history. The ancient Hindus, for example, had a very elaborate and detailed cosmology based on a cyclic universe. They predicted the duration of each cycle to be 8.64 billion yearsāa prediction with three-digit accuracy. This is very impressive, especially since they had no quantum mechanics and no string theory! It disagrees with the number Iām going suggest, which is trillions of years rather than billions.
The cyclic notion has also been a recurrent theme in Western thought. Edgar Allan Poe and Friedrich Nietzsche, for example, each had cyclic models of the universe, and in the early days of relativistic cosmology Albert Einstein, Alexander Friedmann, Georges LemaĆ®tre, and Richard Tolman were interested in the cyclic idea. I think itās clear why so many have found the cyclic idea to be appealing: If you have a universe with a beginning, you have the challenge of explaining why it began and the conditions under which it began. If you have a universe thatās cyclic, itās eternal, so you donāt have to explain the beginning.
During the attempts to try to bring cyclic ideas into modern cosmology, it was discovered in the 1920s and ā30s that there are various technical problems. The idea at that time was a cycle in which our three-dimensional universe goes through periods of expansion beginning from the Big Bang and then reversal to contraction and a Big Crunch. The universe bounces, and expansion begins again. One problem is that every time the universe contracts to a crunch, the density and temperature of the universe rises to an infinite value, and it is not clear if the usual laws of physics can be applied.
Second, every cycle of expansion and contraction creates entropy through natural thermodynamic processes, which adds to the entropy from earlier cycles. So at the beginning of a new cycle, there is higher entropy density than the cycle before. It turns out that the duration of a cycle is sensitive to the entropy density. If the entropy increases, the duration of the cycle increases as well. So, going forward in time, each cycle becomes longer than the one before. The problem is that, extrapolating back in time, the cycles become shorter until, after a finite time, they shrink to zero duration. The problem of avoiding a beginning has not been solved; it has simply been pushed back a finite number of cycles. If weāre going to reintroduce the idea of a truly cyclic universe, these two problems must be overcome. The cyclic model I will describe uses new ideas to do just that.
To appreciate why an alternative model is worth pursuing, itās important to get a more detailed impression of what the consensus picture is like. Certainly some aspects are appealing. But what I want to argue is that, overall, the consensus model is not so simple. In particular, recent observations have forced us to amend the consensus model and make it more complicated. So, let me begin with an overview of the consensus model.
The consensus theory begins with the Big Bang: The universe has a beginning. Itās a standard assumption that people have made over the last fifty years, but itās not something we can prove at present from any fundamental laws of physics. Furthermore, you have to assume that the universe began with an energy density less than the critical value. Otherwise, the universe would stop expanding and recollapse before the next stage of evolution, the inflationary epoch. In addition, to reach this inflationary stage, there must be some sort of energy to drive the inflation. Typicall...
