Inflationary Theory

I

Introduction

Inflationary Theory, in cosmology, theory that proposes that the universe expanded very rapidly for a fraction of the first second of life after the big bang, the explosion at the beginning of the universe (see Big Bang Theory). The idea that the universe is expanding has long been one of the pillars of modern cosmology. The present rate of expansion seems to be fairly uniform, but inflationary theory proposes that the rate of expansion was much faster at the beginning of the universe.

The inflationary theory was developed in the 1970s to solve several mysteries still remaining in the universe as it was described by the big bang theory. In particular, it explains why the universe is expanding at approximately its current rate. It also explains why the universe appears so homogeneous, or uniform. Further, it explains why scientists have never detected magnetic monopoles (single north or south poles that are not connected to an opposite pole).

II

The Theory

The inflationary theory holds that very early in its life, the universe expanded much more rapidly than it now expands. This period of expansion started 1 × 10^-35 sec after the big bang and lasted only 1 × 10^-32 sec (1 × 10^-35 and 1 × 10^-32 are extremely small numbers—a decimal point followed by thirty-four zeroes and then a 1, or thirty one zeroes and then a 1, respectively). During that brief interval, the universe expanded to be 1 × 10⁵⁰ times its previous size (1 × 10⁵⁰ is a very large number—a 1 followed by 50 zeroes). The intense inflation then ended—still well within the first second of time. Then the universe assumed a rate of expansion much closer to that of the present.

In the inflationary theory, the observable universe began in a minuscule region of space. The universe was so compact that all parts of it could be in contact with each other. This allowed the universe to become very homogeneous. The inflationary period rapidly and evenly spread out this region, giving us the almost homogeneous universe we see today. This homogeneity of the universe may not seem apparent on a small scale, with planets, stars, galaxies, and clusters of galaxies adding matter in lumps. But on a larger scale, clusters of galaxies are spread out fairly evenly.

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The inflationary theory also explains why the universe’s density is so close to the limit that determines whether the universe will expand forever or will eventually begin to contract. The universe’s outward expansion is propelled by the initial force of the big bang. If the universe is dense enough, the force of gravity may eventually overcome the universe’s expansion and start pulling matter in the universe back together. Cosmologists are still trying to find out how dense the universe is. The estimations fall very close to the dividing value between a universe that expands forever (an open universe) and a universe that eventually contracts (a closed universe). The inflationary period in the theory would have expanded the universe just fast enough for it to reach the size that led to the estimated density of today’s universe.

The inflationary theory incorporates the existence of magnetic monopoles, which other physical and cosmological theories predict. But it also explains that the universe expanded so much in that fraction of a second that the relatively few magnetic monopoles were spread far apart. Cosmologists use the inflationary theory to predict that there may be only one monopole in the observable universe.

The best evidence for the inflationary universe—the homogeneity of the universe, the mass density of the universe, and the absence of magnetic monopoles—was also the justification for the theory. For this reason, some scientists have criticized the theory as not being testable. In the philosophy of science, an idea only becomes a theory when it can be tested for truth. Because of this, astrophysicists are divided on the question of whether the inflationary theory should really be considered a theory. The current version of the inflationary theory predicts that the density of the universe is very close to the value that divides the two possibilities for the future of the universe: expanding forever or eventually ending expansion and beginning to shrink. If scientists collect definitive evidence that the universe is “open,” or will expand forever, the inflationary theory would be proved false.

III

History

American cosmologist Alan Guth first formulated the idea of an inflationary universe at the Massachusetts Institute of Technology (MIT) in the 1970s. Guth developed the inflationary theory when he was applying the ideas of particle physics to questions about the expansion of the universe. In Guth's original version of the theory, the inflationary era did not end automatically. Several scientists helped to remove this flaw of the original theory in subsequent models. These scientists included Soviet-American cosmologist Andrei Linde, then at the Lebedev Institute in Moscow; British cosmologist Andreas Albrecht, now at Imperial College, London; and American cosmologist Paul Steinhardt, of the University of Pennsylvania. The equations they generated explained why the initial burst of rapid expansion ended, changing the expansion of the universe into today’s more placid era.

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