Many people envision it as a rapidly expanding fireball -- the way the "Big Bang" is described in virtually all the college textbooks and popular books published in the United States.
But this common conception may be wrong. At least it is being challenged by the latest versions of a theory called inflationary cosmology, which has successfully predicted a number of recent observations regarding the structure of the present-day universe.
According to inflationary theory, the universe begins not like an expanding ball of fire but more like a huge growing fractal made up of many inflating balls of space-time that produce new balls that, in turn, produce more new balls, ad infinitum.
"The theory is very simple, but we have had a lot of psychological barriers to overcome," said Andrei Linde, a physics professor at Stanford and one of the authors of inflationary cosmology. He summarized the latest thinking on the subject in a session titled "Keys to the Cosmos: The Unification of Particle Physics and Cosmology -- History and Prophecy" at the annual meeting of the American Association for the Advancement of Science on Monday, Jan. 25.
The standard "big bang" model describes the origin of the universe in terms of a hot, energetic explosion that took place about 15 billion years ago. The theory has been extremely successful in explaining many aspects of the visible universe. It can account for astronomers' discovery that the universe is expanding. It also explains the discovery in the 1960s that a faint and remarkably uniform microwave signal, called the cosmic background radiation, emanates from everywhere in the heavens. This signal has been interpreted as fossil radiation that dates back to a period when the universe was about 300,000 years old, the point when the primordial mixture of subatomic particles and radiation cooled to the point that light could travel freely.
But there are a number of important questions that the standard big bang model has failed to answer, Linde said. Where did the big bang come from, and what preceded it? Why does the visible universe, which is about 11 billion light years across, appear to be flat rather than curved? Why is the matter in the universe distributed extremely evenly at a very large scale, yet gathered into large clumps called galaxies at a smaller scale?
Inflationary theory gives answers to these questions. The underlying idea is elegantly simple. It proposes that the primordial universe underwent a period of rapid, exponential expansion. But the magnitude of this expansion is difficult to grasp. During a period shorter than an eye blink, Linde calculates, a microscopic speck of space would have expanded explosively until it was much larger than the visible universe.
Two of the benefits of this theory are immediately apparent. At a very large scale, matter is spread out with remarkable uniformity, departing from perfect homogeneity by less than one part in 10,000. If the visible universe started from a single, tiny volume, this extreme uniformity makes perfect sense.
Inflationary theory also predicts that the visible universe should be flat, rather than curved, as suggested by Einstein's theory of general relativity. That is because the inflating universe acts similarly to an expanding balloon. If you pick a small area on the surface of a balloon and then blow it up, the area becomes flatter and flatter. Recent astronomical observations suggest that the universe is as flat as inflationary theory predicts.
"It seems that inflation is doing very well, so far," Linde said. "In the last 20 years no other theory has been proposed that can explain the present state of the universe as well."
Originally, inflation was thought of as an intermediate stage in the evolution of the universe that could solve a number of problems. One of these problems involves magnetic monopoles, theoretical super-particles possessing only one magnetic pole. According to the standard big bang model, monopoles should have been produced in abundance, but none has ever been found. Inflation solves this problem because exponential expansion would make them exceedingly rare.
First inflationary theory introduced
The first version of inflationary theory was produced by Alexei A. Starobinsky of the L. D. Landau Institute of Theoretical Physics in Moscow in 1979. Although it created a sensation among Russian astrophysicists, it was quite complicated and did not say much about how inflation could actually start, Linde said.
In 1972, Linde and his colleague David Kirzhnits at the P. N. Lebedev Physics Institute in Moscow suggested that the early universe went through a series of phase transitions. As the universe expanded and cooled, it condensed into different forms, much like water vapor becomes liquid water that freezes into ice. In 1981, Alan H. Guth at Massachusetts Institute of Technology built on this idea by suggesting that the universe might have gone through an unstable, super-cooled state during which the universe would undergo exponential expansion. Super-cooling is common during phase transitions. For example, undisturbed water can be cooled below 32 degrees Fahrenheit. But the slightest disturbance causes it to freeze rapidly.
Despite its popularity the original inflation theory had a fatal flaw, Linde said. It portrayed the period of inflation as taking place in what physicists call a false vacuum. This is a state without any particles, but with a lot of potential energy. "The problem with this idea is that this completely symmetric and nice state is so empty that you do not have any preferable coordinate system," he said. That means there is no way to determine whether the universe is expanding or not and, if you cannot make that determination, then the expansion is not real; instead it is a "false expansion."
After exploring his idea for a year, Guth concluded that it could not work. But Linde found a way to rescue it in 1982 with what he dubbed "new inflationary theory." He did so by showing that inflation can take place in a false vacuum state that has begun to deteriorate. A few months later the same idea was proposed by Andreas Albrecht and Paul Steinhardt at the University of Pennsylvania. "If you have just a little bit of change, then you can have this preferable system that tells you when it is expanding," he said.
They proposed that the energy in this near-false-vacuum state would be contained in a scalar field, often called the inflaton field. There is no exact comparison to such a field in nature today. But an electrostatic field, like that generated by the static build-up in clothes that causes them to cling, is a close analogy. A uniform electrostatic field is virtually undetectable: It only generates electrical and magnetic fields when it is inhomogeneous or changes over time. The inflaton field has the same basic characteristics but differs in one important way: It carries its own energy.
The theorists argued that, when the inflaton field began falling, the primordial universe could undergo real, exponential inflation rather than false inflation. An imaginary observer equipped with a gravity meter would begin recording a slight weakening in the force of gravity and, if she were able to mark two different positions in nearby space, she would see them begin flying apart. As the scalar field decreases, it undergoes a phenomenon called quantum fluctuations. They are predicted by quantum electrodynamics, the laws that explain the behavior of subatomic particles. Initially, these oscillations would be sub-microscopic in scale. But as space inflates they become larger and larger, until they become the size of galaxies. Because these fluctuations correspond to variations in energy density, when the period of inflation ends, larger amounts of matter would be produced in areas where the field is high than in regions where it is low. Thus, they can explain the formation of galaxies, Linde said.
The mechanism is also consistent with the discovery of slight variations in the strength of the cosmic background radiation discovered in 1992. They are also interpreted as the product of quantum fluctuations in the glowing soup of matter and energy. They are much smaller because they occurred before the universe finished its period of exponential expansion.
Chaotic inflation eliminates a number of old assumptions
Like old inflation, new inflation retained the assumption that the universe began both hot and in thermal equilibrium, that is, at the same temperature everywhere. Then inflation took place and all the original particles were swept away in the extraordinary growth spurt. At the end of the inflationary period, particles were recreated and then reheated by the fluctuating scalar field. But, if an inflationary period occurred, Linde realized that it also swept away virtually all information about the conditions that preceded it. The evidence scientists had interpreted as indicating hot conditions at the birth of the universe, like the cosmic background radiation, must refer to the post-inflation period instead of the period before it.
"What evidence is there that the universe was originally hot? What evidence is there that it was in thermal equilibrium? None at all," Linde objected. So the cosmologist went back to the drawing board and came up with a third version of inflationary theory called chaotic inflation. In this approach, the big bang remains but becomes an aftereffect of cosmic inflation. He found that he could jettison a number of the other arbitrary assumptions on which past cosmological theories have been based. In fact, he found that all he needs to create a universe like our own is a patch of primordial universe with a large scalar field that is moving toward its minimum value. "If the scalar field falls down very slowly, it is nearly indistinguishable from a false vacuum and the universe will inflate," he said.
The cosmologist recounted the response he got from fellow scientists when he first suggested that the universe might not have been initially hot. "They said, 'No, we know it must be hot!'" Similarly, when he suggested it didn't need to be in a condition of thermal equilibrium, colleagues responded, "That is unnatural. It is not as beautiful as the assumption of initial equilibrium!"
For Linde, however, what is beautiful about chaotic inflation is its ability to explain how the universe may have begun using a minimum number of arbitrary assumptions: "You can start with any ugly part of the universe in a non-equilibrium state." Some regions do not inflate. But that just means they become insignificant. The parts that can undergo inflation, on the other hand, become huge and most of the volume of the universe comes from them. Chaotic inflation "creates order out of chaos, not by destroying previous chaos, but by exploding those parts that are capable of becoming non-chaotic," he said.
An eternally self-reproducing universe
Linde calls his latest variation on the inflation theme the eternally self-reproducing universe. He began by asking himself if the inflaton field would always go down. He concluded that, in very rare instances, quantum fluctuations would cause the field to jump up in some parts of the universe. These places would be extremely rare. When the inflaton field increases, however, some of these sites would begin inflating madly. In almost no time, they grow into very large regions with high scalar fields. Then, within these inflated regions, the process repeats itself. Quantum fluctuations strike again, causing the field strength to jump in a few localities, some of which undergo a second round of inflation. And so on, ad infinitum.
"So you have those parts of the universe where the field is going down," Linde said. "That is the part of the universe where we live. The energy density is already down. But there are some areas of the universe where the scalar field jumps up, against the normal laws of physics. In this way the universe reproduces itself."
While these expanding regions of the universe are inaccessible to us, they are still important, he said. "From the point of view of the general geometry of space, our part of the universe has been created, but other parts of the universe are still being created. If life in our part of the universe were to disappear, then it will appear again someplace else. So the universe as a whole becomes immortal."
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