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 uncertainty—if 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 that’s 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. I’m 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 there’s a new component of energy in the universe which we didn’t know existed before. This new component is usually referred to as dark energy. As the name clearly suggests, we still don’t know exactly what this new component is. It’s a component of energy which in fact is very much like the repulsive gravity matter I talked about earlier—the 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 I’m trying to advertise that we’ve understood a lot, and we have, there are still many uncertainties. In particular, we still don’t know what most of the universe is made out of. There’s the dark energy, which seems to comprise in fact about 60 percent of the total mass/energy of the universe. We don’t know what it is. It could in fact be the energy of the vacuum itself, but we don’t know that for a fact. In addition, there’s what we call dark matter, which is another 30 percent, or maybe almost 40 percent, of the total matter in the universe. We don’t 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 properties—it’s attractive and it clusters, but we don’t know what it’s made of. The stuff we do know about—protons, neutrons, ordinary atoms and molecules—appear 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.
2
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 haven’t 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 we’re 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 we’ve 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, ’30s, and ’40s; 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 I’ve 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 don’t 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 we’re 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 years—a 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 I’m 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 Lemaître, and Richard Tolman were interested in the cyclic idea. I think it’s 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 that’s cyclic, it’s eternal, so you don’t have to explain the beginning.
During the attempts to try to bring cyclic ideas into modern cosmology, it was discovered in the 1920s and ’30s 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 we’re 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, it’s 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. It’s a standard assumption that people have made over the last fifty years, but it’s 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. Typically this is assumed to be due to an inflation field. You have to assume that in those patches of the universe that began at less than the critical density, a significant fraction of the energy is stored in inflation energy so that it can eventually overtake the universe and start the period of accelerated expansion. All of these are reasonable assumptions, but assumptions nevertheless. It’s important to take into account these assumptions and ingredients, because they’re helpful in comparing the consensus model to the challenger.
Assuming these conditions are met, the inflation energy overtakes the matter and radiation after a few instants. The inflationary epoch commences, and the expansion of the universe accelerates at a furious pace. The inflation does a number of miraculous things: It makes the universe homogeneous, it makes the universe flat, and it leaves behind certain inhomogeneities, which are supposed to be the seeds for the formation of galaxies. Now the universe is prepared to enter the next stage of evolution with the right conditions. According to the inflationary model, the inflation energy decays into a hot gas of matter and radiation. After a second or so, there form the first light nuclei. After a few tens of thousands of years, the slowly moving matter dominates the universe. It’s during these stages that the first atoms form, the universe becomes transparent, and the structure in the universe begins to form—the first stars and galaxies. Up to this point, the story is relatively simple.
But there is the recent discovery that we’ve entered a new stage in the evolution of the universe. After the stars and galaxies have formed, something strange has happened to cause the expansion of the universe to speed up again. During the 15 billion years when matter and radiation dominated the universe and structure was forming, the expansion of the universe was slowing down, because the matter and radiation within it is gravitationally self-attractive and resists the expansion of the universe. Until very recently, it had been presumed that matter would continue to be the dominant form of energy in the universe and this deceleration would continue forever.
But we’ve discovered instead, due to recent observations, that the expansion of the universe is speeding up. This means that most of the energy of the universe is neither matter nor radiation. Rather, another form of energy has overtaken the matter and radiation. For lack of a better term, this new energy form is called dark energy. Dark energy, unlike the matter and radiation we’re familiar with, is gravitationally self-repulsive. That’s why it causes the expansion to speed up rather than slow down. In Newton’s theory of gravity, all mass is gravitationally attractive, but Einstein’s theory allows the possibility of forms of energy that are gravitationally self-repulsive.
I don’t think either the physics or cosmology communities, or even the general public, have fully absorbed the full implications of this discovery. This is a revolution in the grand historic sense—in the Copernican sense. In fact, if you think about Copernicus—from whom we derive the word “revolution”—his importance was that he changed our notion of space and of our position in the universe. By showing that the Earth revolves around the sun, he triggered a chain of ideas that led us to the notion that we live in no particular place in the universe; there’s nothing special about where we are. Now we’ve discovered something very strange about the nature of time: that we may live in no special place, but we do live at a special time, a time of recent transition from deceleration to acceleration; from one in which matter and radiation dominate the universe to one in which they are rapidly becoming insignificant components; from one in which structure is forming in ever larger scales to one in which now, because of this accelerated expansion, structure formation stops. We are in the midst of the transition between these two stages of evolution. And just as Copernicus’ proposal that the Earth is no longer the center of the universe led to a chain of ideas that changed our whole outlook on the structure of the solar system and eventually to the structure of the universe, it shouldn’t be too surprising that perhaps this new discovery of cosmic acceleration could lead to a whole change in our view of cosmic history. That’s a big part of the motivation for thinking about our alternative proposal.
With these thoughts about the consensus model in mind, let me turn to the cyclic proposal. Since it’s cyclic, I’m allowed to begin the discussion of the cycle at any point I choose. To make the discussion parallel, I’ll begin at a point analogous to the Big Bang; I’ll call it the Bang. This is a point in the cycle where the universe reaches its highest temperature and density. In this scenario, though, unlike the Big Bang model, the temperature and density don’t diverge. There is a maximal, finite temperature. It’s a very high temperature, around 1020 degrees Kelvin—hot enough to evaporate atoms and nuclei into their fundamental constituents—but it’s not infinite. In fact, it’s well below the so-called Planck energy scale, where quantum gravity effects dominate. The theory begins with a bang and then proceeds directly to a phase dominated by radiation. In this scenario you do not have the inflation one has in the standard scenario. You still have to explain why the universe is flat, you still have to explain why the universe is homogeneous, and you still have to explain where the fluctuations came from that led to the formation of galaxies, but that’s not going to be explained by an early stage of inflation. It’s going to be explained by yet a different stage in the cyclic universe, which I’ll get to.
In this new model, you go directly to a radiation-dominated universe and form the usual nuclear abundances; then go directly to a matter-dominated universe in which the atoms and galaxies and larger-scale structure form; and then proceed to a phase of the universe dominated by dark energy. In the standard case, the dark energy comes as a surprise, since it’s something you have to add into the theory to make it consistent with what we observe. In the cyclic model, the dark energy moves to center stage as the key ingredient that is going to drive the universe, and in fact drives the universe, into the cyclic evolution. The first thing the dark energy does when it dominates the universe is what we observe today: It causes the expansion of the universe to begin t
o accelerate. Why is that important? Although this acceleration rate is 100 orders of magnitude smaller than the acceleration that one gets in inflation, if you give the universe enough time it actually accomplishes the same feat that inflation does. Over time, it thins out the distribution of matter and radiation in the universe, making the universe more and more homogeneous and isotropic—in fact, making it perfectly so—driving it into what is essentially a vacuum state.
Seth Lloyd said there were 1080 or 1090 bits inside the horizon, but if you were to look around the universe in a trillion years, you would find on average no bits inside your horizon, or less than one bit inside your horizon. In fact, when you count these bits, it’s important to realize that now that the universe is accelerating, our computer is actually losing bits from inside our horizon. This is something that we observe.
At the same time that the universe is made homogeneous and isotropic, it is also being made flat. If the universe had any warp or curvature to it, or if you think about the universe stretching over this long period of time, although it’s a slow process it makes the space extremely flat. If it continued forever, of course, that would be the end of the story. But in this scenario, just like inflation, the dark energy survives only for a finite period and triggers a series of events that eventually lead to a transformation of energy from gravity into new energy and radiation that will then start a new period of expansion of the universe. From a local observer’s point of view, it looks like the universe goes through exact cycles; that is to say, it looks like the universe empties out each round and a new matter and radiation is created, leading to a new period of expansion. In this sense it’s a cyclic universe. If you were a global observer and could see the entire universe, you’d discover that our three dimensions are forever infinite in this story. What’s happened is that at each stage when we create matter and radiation, it gets thinned out. It’s out there somewhere, but it’s getting thinned out. Locally, it looks like the universe is cyclic, but globally the universe has a steady evolution, a well-defined era in which, over time and throughout our three dimensions, entropy increases from cycle to cycle.
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