Inflationary Theory Explains the Early Universe

Date 1980

Alan H. Guth made an important contribution to scientists’ understanding of the universe when he proposed a new theory of cosmology that says that the expansion of the universe, which is currently slow and linear, was rapid and exponential for a very brief period near the beginning of time.

Locale Stanford, California

Key Figures

• Alan H. Guth (b. 1947), physicist who developed the first theory of inflation
• A. D. Linde (b. 1926), physicist who improved Guth’s theories by making the “bubble universe” of inflation large enough to agree with present observations
• Paul J. Steinhardt (b. 1952), physicist who contributed to the improvements necessary to make inflation a viable theory
• Andreas Albrecht (b. 1927), physicist who worked with Steinhardt on the developing inflationary theory

Summary of Event

According to the big bang theory, the present universe emerged out of a region of space smaller than a proton 13.7 billion years ago. Originally, it was too hot for material particles to form. As the universe cooled, however, elementary particles were able to crystallize out of the high-energy sea; further cooling allowed these particles to form atoms, then molecules, then clouds, then stars and planets, and finally living beings.

There are at least three serious problems with the standard model of the big bang theory: the horizon problem, the smoothness problem, and the flatness problem. The horizon problem exists because the standard model of the big bang theory cannot explain why the universe is so uniform over such great distances. If the different parts of the universe are so separated that they cannot communicate with one another, even at the speed of light, then why do they look exactly the same? Just as animals on widely separated continents evolve differently, so should different parts of the universe evolve differently.

The smoothness problem is the opposite of the horizon problem. In an otherwise uniform universe, how were galaxies formed? These deviations from perfect uniformity must have arisen from tiny irregularities in the very early universe. The problem lies in the fact that ten billion years later, even tiny deviations from perfect smoothness in the early universe will result in enormous nonuniformity. Galaxies represent relatively minor disturbances in the uniformity of the universe.

The flatness problem relates to the energy density (or, equivalently, the mass density) of the universe. If the energy density is only slightly higher than a particular “critical value,” the big bang will reverse eventually and be followed by a big contraction. If the density is slightly lower than this critical value, the current expansion will continue forever. If the density is exactly equal to the critical value, corresponding to a “flat” universe, the expansion will continue, but at a gradually slowing rate. Measured values of the present energy density yield results that are very close to the critical value. The flatness problem exists because any initial deviation from perfect flatness in the early universe is magnified tremendously by the subsequent expansion of the universe. To account for the current measured flatness of the universe, the early universe must have been flat to within one part in a thousand trillion (10¹⁵). This is an unbelievable constraint on the allowed values for the initial flatness.

The horizon, smoothness, and flatness problems are dealt with in the standard model of the big bang by simply assuming the specific initial conditions necessary to account for the present observed features of the universe. The fact that these conditions are assumed without any theoretical basis makes them arbitrary, which is considered to be a weakness in a physical theory. A good physical theory explains things; it does not assume them.

Alan H. Guth proposed the “inflationary model” of the big bang theory in 1980 in an attempt to explain the horizon, smoothness, and flatness problems. Guth suggested that the very early universe, which he called a “bubble,” underwent an initial, very rapid exponential expansion that was much faster than the linear expansion of the standard universe. This bubble inflated to become the present observable universe, which is thus embedded in a much larger unobservable universe. This exponential expansion increased the size of the universe by a factor of 10⁵⁰, from smaller than a proton to larger than a softball. Inflation began at 10^-35 seconds after the big bang and ended at 10^-33 seconds.

According to the inflationary model, the observable universe grew out of a region of space—the inflationary bubble—that was much smaller than was formerly believed. The material in this small primordial bubble was thus very densely packed and able to interact mutually for a longer period, homogenizing itself so that when it began to separate, all of it would evolve in the same way, thus solving the horizon problem. When the inflation occurred, the universe expanded dramatically, maintaining its newly established homogeneity, thus solving the smoothness problem. The flatness of the universe increased with the size of the universe, just as the flatness of a square drawn on a balloon increases as the balloon is blown up. By viewing the entire universe as being much larger, inflation explains successfully why the visible portion is so flat.

Furthermore, the inflationary model is able to explain the approximate distribution and size of the galactic clusters that populate the universe. In the original primordial bubble, the homogeneity would have been limited by the laws of quantum mechanics, which state that there will be small fluctuations even in a perfectly uniform region of space. These small fluctuations were magnified dramatically by inflation until they became the large structures that are seen as galaxies.

Guth’s original formulation had a few problems. The most serious of these concerned the length of the inflationary epoch, which was too short to produce a universe of adequate size. In 1984, A. D. Linde, Paul J. Steinhardt, and Andreas Albrecht solved this problem and improved the inflationary theory, which is now widely accepted as the most likely explanation for the observed features of the present universe.

Significance

The inflation theory has helped to explain the actual origin of the universe itself, one of the deepest mysteries in science, which has repercussions for philosophy and religion. In a remarkable application of quantum theory, Guth has calculated that the tiny fluctuations present even in a vacuum—an empty region of space—might be adequate to initiate the process of inflation. According to quantum theory, which is very well established, a vacuum is not completely inactive. There must be a small energy field present that is fluctuating about zero. There is a probability that one of these fluctuations could erupt and produce a new universe. The laws of physics permit this because a universe such as Earth’s has almost no net energy in it. The positive energy associated with all matter (Einstein’s E = mc²) is balanced by the negative energy associated with the gravitational force. If the universe is indeed flat, which both measurements and inflationary theory seem to suggest, then it has no net energy, indicating that it could have erupted from an empty vacuum—from nothing—without violating the law of the conservation of energy.

Bibliography

Carrigan, Richard A., and W. Peter Trower, eds. Particle Physics in the Cosmos. New York: W. H. Freeman, 1989. Collection of articles reprinted from Scientific American includes discussion of the important role inflationary theories play in theories of cosmology. Of particular interest are “The Inflationary Universe,” by Alan Guth and Paul J. Steinhardt; “The Structure of the Early Universe,” by John Barrow and Joseph Silk; and “The Large Scale Structure of the Universe,” by Joseph Silk, Alexander Szalay, and Yakov Zel’dovich. Some chapters include postscripts updating the material.

Gribbin, John. The Omega Point: The Search for the Missing Mass and the Ultimate Fate of the Universe. New York: Bantam Books, 1988. A popular science writer with professional training in cosmology presents one of the most accessible explanations of the universe available to lay readers. Includes bibliography and index.

Harrison, Edward R. Cosmology: The Science of the Universe. 2d ed. New York: Cambridge University Press, 2000. Comprehensive volume includes discussion of the big bang and continuous creation cosmologies. A good overview for a wide audience.

‗‗‗‗‗‗‗. Masks of the Universe: Changing Ideas on the Nature of the Cosmos. 2d ed. New York: Cambridge University Press, 2003. Fascinating book, aimed at lay readers, discusses the various notions held about our galaxy and the universe from the dawn of time up to the late twentieth century. Shows how culture and science influence each other.

Pagels, Heinz R. Perfect Symmetry: The Search for the Beginning of Time. New York: Simon & Schuster, 1985. Volume by a top scientist in the field of cosmology contains much historical material as well as a discussion of the current status of cosmological theories. Includes several sections that discuss various aspects of the inflationary theory.

Silk, Joseph. The Big Bang. 3d ed. New York: W. H. Freeman, 2000. Excellent resource presents a complete discussion of the big bang, including the theory’s historical development. Provides an excellent introduction to inflationary theories, with an unusual diagram showing the relative size of the inflationary universe and the visible universe.