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Book Review for
The Inflationary Universe: The Quest for a New Theory of Cosmic Origins
by Alan Guth
Addison-Wesley, 1997

Prepared by the staff of Jupiter Scientific

The Big Bang is the scientific version of genesis, the creation of the Universe. In the beginning, the world was extraordinarily hot and bathed in light. As time passed, the Universe expanded, meaning that the fabric of space was stretched in all directions. The possibility that space can be dynamic is a consequence of Einstein's General Theory of Relativity. The expansion of the Universe pulled material apart and caused the world to cool.

When the Universe was just a tiny fraction of a second old, it is believed that space underwent a tremendous stretching, probably by more than a factor of 1050. This event, know as inflation, was the idea of Dr. Alan Guth, currently a professor of physics at MIT. If true, inflation represents a dramatic moment in cosmic history. Light, radiation and matter were pulled apart and greatly diluted, while the cosmos cooled considerably. After this enormous expansion of space, the Universe reheated to a high temperature and then proceeded to expand steadily and slowly. The Universe has been expanding and cooling ever since.

Inflation is an attractive idea because it solves three important problems of non-inflationary cosmology: (1) the flatness problem, (2) the horizon problem, or the causality connectedness problem and (3) the GUT monopole problem. It was the latter that motivated Dr. Guth to invent inflation.

Grand Unified Theories, or GUTs, unify the strong nuclear force with the electroweak interactions. The strong force is responsible for indelibly tying together three quarks in a proton or a neutron and for causing protons and neutrons to stick to one another in a nucleus, which is the tiny heavy core of an atom. In short, the strong force is responsible for all nuclear binding.

Starting in the early 1970's, it was realized that electromagnetism and the weak subnuclear force are distorted manifestations of a single unity of forces called the electroweak interactions. A similar unification had been achieved for electromagnetism in the nineteenth century: The electric and magnetic forces are the consequences of a single theory. For example, a charge at rest produces an electric force but, when moving, it also creates a magnetic field.

But how can the weak force, which only operates inside a nucleus, be so different from the electromagnetic force, which acts at human-distance scales, if they are part of the same theory? The answer is due to breaking, or distortion. A sphere looks the same to you when it is rotated, but if it is squashed into the shape of a rugby ball then its symmetry is reduced: Its appearance remains the same when turned in one direction [the quarterback in American football throws a ball so that it spins about this direction] but not so in the other directions [a kicker typically sends a football flipping end over end]. The former direction corresponds to an unbroken symmetry, while the other directions correspond to broken symmetries. Amazingly, it turns out that particle forces are closely related to symmetries, so that when a theory is fully symmetric, all forces are the same. Due to dynamical effects, all but one symmetry in the electroweak theory are broken, much like a squashed sphere. The single unbroken direction is associated with electromagnetism, while the broken ones yield the weak subnuclear forces. In summary, electromagnetism and the weak interactions can be so different because of symmetry breaking.

Theorists during the last three decades of the twentieth century have hypothesized that perhaps the strong subnuclear force is united with the electroweak forces. Such a theory unifies three (the strong, weak and electromagnetic interactions) of the four known fundamental forces. Gravity, the fourth force, has defied unification (although string theory has the potential to achieve success) -- even Einstein failed in this task. If the world is governed by a Grand Unified Theory, then, again, the different behaviors of the strong, weak and electromagnetic forces are due to symmetry breaking.

During the early Universe, the temperature was so extraordinarily hot that distortions were eliminated, thereby causing all particle interactions to be similar. Unification was achieved. As space expanded and the world cooled, symmetry breaking set it and led to the different manifestations of the fundamental forces. If a GUT governs our world, then the breaking process also generated magnetic monopoles. Magnetic monopoles create magnetic forces in the same way that electric charges create electric forces. Magnetic monopoles have never been detected by scientists. [Magnets are dipoles because they always consist of two poles: a north one and a south one. If it were possible to cut a magnet so that only one pole remained then a magnetic monopole would be created. By the way, slicing a magnet in half leads to two magnets each with half the magnetic strength and each with both a north pole and a south pole.]

How does inflation solve the problem of the generation of magnetic monopoles during GUT symmetry breaking? It does not eliminate them. Rather, it greatly dilutes them. When space expands by an enormous factor, the monopoles are carried so far apart that they are very unlikely to be found in our region of the Universe. Believers in inflation and GUTs would say that monopoles exist but experimentalists have not detected them because they are too hard to find.

But if inflation causes monopoles to be dilutely distributed then does it not do the same for quarks (the constituents of protons and neutrons) and electrons? Why does the world currently have so much matter? The answer is as follows. At the end of inflation when the cosmos reheated, fluctuations in the field that drove the rapid expansion of space produced the elementary particles. Thus for a brief moment in cosmic history, the Universe was almost empty of ordinary matter. Then suddenly, quarks and electrons started popping up to fill the world.

Subsequently to the monopole work, Dr. Guth realized that inflation solves the flatness problem of cosmology. What is this puzzle that has perplexed cosmologists for decades?

According to Einstein's general theory of relativity, the topology of space assumes one of three forms. The Universe may have positive curvature, in which case it closes upon itself. A traveler venturing in a straight line in one direction eventually will return to the starting point from the opposite direction! The situation is similar to a person who voyages around the Earth; at the beginning, he or she heads to the east but eventually arrives home from the west. The second possibility is that the Universe has negative curvature. Such a space is infinite in extent. A part of a two-dimensional analog of a negatively curved space is the surface of a saddle. Finally, space might be flat. In this case, like second case, the Universe extends forever. The two-dimensional version of a flat space is the infinite plane.

The curvature of space depends on the amount of matter in the Universe. This follows from Einstein's theory, which states that mass causes the bending of space. If the world is densely filled with material, then the Universe has positive curvature. If the matter in the world is dilute, then space is negatively curved. For only one critical value of mass density is the Universe flat.

Astronomers have not yet been able to measure the mass of the Universe sufficiently well to determine the curvature of the cosmos. However, the mass density is within a factor of ten of its critical value. Thus space is roughly flat.

But this almost flat feature of space is surprising for the following reason. For the mass density to be within a factor of ten of its critical value nowadays, it needed to be fine-tuned to near its critical value with a precision of one part per million-billion at one second after the Big Bang. How could this have been achieved? Did Nature somehow twiddle a dial to an incredible accuracy? In a non- inflationary cosmology, a miracle needed to have happened.

Inflation solves the puzzle quite simply. Extensive expansion caused space to become almost flat. A two-dimensional analogy illuminates the situation. A six-inch balloon has significant curvature to the human eye. Indeed, a person can easily see that a balloon is round. Suppose the balloon expands to the size of the Earth. Then it is difficult to determine whether the Earth is flat or curved. Indeed, for millennia this issue was debated. A localized region of land looks flat. Analogously, perhaps the Universe on an enormous scale is curved, while the region that humans can observe is almost flat.

There is a second way of thinking about the flatness problem.

The mass density of the Universe determines its fate. If the world is filled with lots of material -- the case of positive curvature -- then the Universe has a finite lifetime. Eventually, the gravity of mutually attracting material will cause the expansion of space to cease, after which space will collapse. All matter will fall in upon itself in a Big Crunch, which is similar to the Big Bang but in reverse. If the mass density of the Universe is less than its critical value, so that space is negatively curved, then the Universe lives forever. It will continue to expand and cool, eventually becoming so cold that virtually nothing, including stars and life, can exist.

Having understood the relation between mass density and the ultimate fate of the cosmos, the flatness problem can be rephrased as follows. Unless the mass density is finely tuned to the critical value, then the world could not be as we humans observe it. If, at the earliest moments of creation, the density was more than its critical value, then the Universe would have quickly "died," undergoing a Big Crunch at a tiny, tiny fraction of a second. On the other hand, if, at the earliest moments of creation, the density were less than its critical value, then the Universe would be exceedingly cold and dilute nowadays. Stars never would have formed because material would have been too thinly distributed.

Inflation "saves" the world from "too early a death" and "too cold an old age" by automatically tuning the density close to its critical value. This was accomplished at the end of the process in which space expanded, when fluctuations produced all matter.

The third puzzle that inflation solves is the horizon problem.

No object can travel faster than the speed of light. This sets a limit on how one event can affect another. If a disturbance occurs in a distant region from us, it cannot be instantly felt here on Earth. It takes time for the effects of the disturbance to propagate to us since they cannot travel faster than the speed of light. For example, when a star explodes in a supernova elsewhere in the Universe, only years and years after the explosion does the bright object appear in the night sky on Earth. If the Sun were to explode at this very instance, humans would not know it until eight and a third minutes later, the time it takes light to reach our planet from the Sun.

When the Universe was about 300,000 years old, the first atoms formed: Through the attractive electric force, electrons were captured by protons and began to orbit them as neutral objects. Earlier, the Universe was too hot for these atoms to exist: electrons and protons swam in separate cosmic seas.

Each time an electron joined a proton in an atom, a spark of light was given off. This happened everywhere throughout the Universe, thereby creating a dramatic flash. Subsequently, when the Universe expanded, the light from the flash was stretched along with space. What does it mean for light to stretch? Light is a wave. As such, it has a wavelength, the distance between two successively wave crests. When space stretched, the distance between wave crests also increased.

The color of light is governed by its wavelength. When blue light was stretched, it became yellow light, when yellow was stretched, it became red light, and when red light was stretched, it became infrared radiation -- it was no longer ordinary light; it became a less energetic form of an electromagnetic wave. Eventually, all visible light became infrared radiation and the Universe turned completely black. As millions and millions of years passed, space continued to expand and the waves of infrared radiation were stretched until they became microwaves. Today, these microwaves propagate throughout the Universe and are known as the cosmic microwave background radiation. They are the "echo" of atom formation; not an echo of sound but an echo of "light." In 1965, scientists discovered this radiation and obtained a glimpse -- a kind of photograph -- of the Universe when it was just 300,000 years old.

The cosmic microwave background radiation has been observed to have very similar features even when examined at opposite places in the Universe. For example, the temperature of the radiation is the same to within one part per 100,000. Without an inflationary period, such a result is surprising because the radiation from one region of the Universe has not had enough time to influence the radiation from another region of the Universe. In a Big Bang without inflation, there would not be enough time for the radiation to thermalize to the same temperature -- thermal disturbances would have had to travel many times the speed of light. Theorists phrase this by saying distant parts of the Universe are not causally connected. It is difficult to understand the uniformity of the causally disconnected regions of the cosmic microwave background radiation.

How does a rapid expansion of space in the early Universe solve this problem? Before inflation, the visible Universe, that is that part of the Universe that astronomers can in principle observe, was a small region of the total Universe. In such a small region, it does not take long for it to achieve thermal equilibrium and thus a constant temperature throughout it. Inflation caused that small region to expand into our visible Universe. Hence the cosmic microwave background radiation is uniform in its properties because conditions were set that way during the very earliest moments of existence.

Click here to read a short metaphorical account of the inflationary event that is narrated in the Fifth Book of Creation of The Bible According to Einstein; The marble in the poem "A Marble on a Hill" symbolizes the field that drove the expansion of space. In this regard, see Figure 12.2 of The Inflationary Universe.

The Inflationary Universe by Alan Guth contains his personal account of the development of the theory of inflation. It also provides a historical discussion of cosmology going all the way back to the ancient Greeks. Intermingled with these are scientific explanations of what is discussed in this review. Of course, in a 300-page book, Dr. Guth is able to provide more details.

For us humans here on Earth,
the philosophical consequences of inflation are astounding.

A reader of his book probably should have had some physics training at the college level: The Inflationary Universe avoids explicit equations only by describing them with words instead. The book sometimes uses graphs portraying relations between technical quantities. In several places, it challenges the mind by presenting intricate logical lines of thinking. To appreciate these aspects of Guth's book requires a reader to be able to think like an amateur physicist.

Who should read this book? The undergraduate physics major is the ideal candidate. Half the book -- the part involving intricate reasoning -- is not accessible to the reader who does not have a background in science. An excellent glossary helps those who are unfamiliar with the terms and concepts of cosmology and particle physics.

The book is well written and clear, reminiscent in style to The First Three Minutes, which was authored by Nobel laureate Steven Weinberg and was a New York Times best seller.

If inflation is correct then it has great cosmological consequences: First, the visible Universe is only a small part of the entire Universe. Second, almost all matter and energy in the world emerged from the oscillations of a single quantity, know as a field. This field drove the inflationary expansion. Third, it is quite likely that there exist other universes, which are completely detached from ours.

Inflation is a great idea that solves three famous cosmological problems. Its attractiveness immediately created a great following among scientists. At present, there is limited direct experimental confirmation. The inflationary theory has predicted a certain feature called scale invariance in the fluctuations of the cosmic microwave background radiation that has been recently confirmed by observations with detectors in satellites. The detailed mechanism of what drives inflation is poorly understood. Proposals, such as the use of a field, often have problems. Nevertheless, most theorists believe that it is quite likely that the cosmos underwent a period of rapid expansion in its youth. For us humans here on Earth, the philosophical consequences of inflation are astounding.


The Inflationary Universe: The Quest for a New Theory of Cosmic Origins by Alan Guth, The First Three Minutes by Steven Weinberg and The Bible According to Einstein made be purchased through the internet at Amazon.com by clicking here for The Inflationary Universe, clicking here for The First Three Minutes or here for The Bible According to Einstein. Jupiter Scientific participates in Amazon.com's Associates Program.



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