Big Bang Theory

The Big Bang Theory is the prevailing theory of the origin of the universe, and it is based on astronomical observations. According to this theory, about 15 billion years ago all the matter and energy in the visible universe was concentrated in a small, hot, dense region, which flew apart in a gigantic explosion.

Before the twentieth century, most scientists believed the universe was static in the sense that it was neither growing nor shrinking as a whole, although individual stars and planets were moving. In 1915 Albert Einstein proposed the general theory of relativity, which is a theory of gravity that has superseded Isaac Newton's theory of gravity for very massive objects. Since general relativity was invented, its equations have been used to describe the possible ways in which the universe might change as time goes on. Einstein, like others before him, thought the universe was static, but the equations of general relativity do not allow for such a thing; according to the equations, the universe has to grow or shrink. In 1917, in order to allow for a static universe, Einstein changed the equations of general relativity by adding a term called "the cosmological constant."

In the 1920s, cosmologists examined Einstein's original equations without the cosmological constant and found solutions corresponding to an expanding universe. Among those cosmologists was the Belgian Georges Lemaitre, who proposed that the universe began in a hot, dense state, and has been expanding ever since. This proposal came before there was any substantial evidence of an expanding universe.

Nearly all stars in the visible universe are in large clusters called galaxies. The Milky Way galaxy, the galaxy containing the sun and about 100 billion other stars, is one of about 50 billion galaxies that exist in the visible universe. In 1929 the astronomer Edwin Hubble, after making observations with a powerful telescope, discovered that distant galaxies are moving away from the earth and the Milky Way (and from one another). The farther these galaxies are from the earth, the faster they are moving, their speed being approximately proportional to their distance. Galaxies at the same distance from the earth appear to be moving away from us at the same speed, no matter in what direction in the sky the astronomers look. These observations do not mean that the earth is at the center of the universe; astronomers believe that if they made observations from any part of the visible universe that they would find the same general result.

If the galaxies are moving away from each other, then in the past they were closer to one another than they are now. Furthermore, it can be calculated fromthe present speeds and distances of the galaxies, that about 15 billion years in the past, all the matter and energy in the visible universe must have been in the same place. That is when the Big Bang happened. Scientists do not know what the universe was like "before" the Big Bang or even whether the concept of earlier time makes sense. The galaxies were formed out of the original matter and energy perhaps a billion years or more after the Big Bang.

An artist's impression of galaxies being formed in the aftermath of the Big Bang. The spiral clouds of gas have already started condensing into the shapes of future galaxies. (Photo Researchers Inc.)

Fred Hoyle, an astronomer and cosmologist who had a rival "steady state" theory of the universe, coinedthe name "Big Bang" in order to make fun of the theory in which the universe began in an explosion. The name stuck. Today, nearly all scientists prefer the Big Bang Theory because it can account for more observed properties of the universe than the steady state theory can. In particular, the observed microwave background radiation that appears everywhere in the sky is a remnant of the Big Bang. This radiation cannot be accounted for in a natural way by the steady state theory.

According to present theory, the galaxies are not flying apart into empty space, but space itself is growing larger. Another way of putting this is to say that the universe itself is expanding. Although the universe is expanding, one should not think that everything in the universe is expanding with it. Individual galaxies are not expanding, because their stars are prevented from flying apart by their mutual gravitational attractive forces. Likewise, other forces of nature keep the stars, the sun, Earth, and objects on Earth—down to atoms and nuclei—from expanding along with the universe.

What will be the ultimate fate of the universe? Will the expansion go on forever or will gravity slow and then reverse the expansion into a collapse? According to general relativity, whether or not the universe will continue to expand or eventually collapse depends on the amount of matter and energy in the universe. If this matter and energy together are greater than a certain critical amount, their mutual gravitational attraction will reverse the expansion, and the universe will end with what astronomers call the "Big Crunch." If the sum of the matter and energy is below the critical amount, then, although gravity will slow the expansion, the universe will continue to expand forever. At the present time, most observations seem to favor a universe that will expand forever, but the uncertainties are large.

Astronomical observations made in the late 1990s, which are still preliminary, indicate that the expansion of the universe is not slowing down, as required by the attractive gravitational force of general relativity, but is speeding up. One way to account for this speeding up is to put back the cosmological constant into the equations of general relativity. If the cosmological constant has a certain value, general relativity allows for the speeding up that astronomers think they are seeing. When Einstein first learned that the universe was expanding, he abandoned the cosmological constant, calling it his greatest mistake. If he were alive today, what would he think about the possibility that his constant might be needed after all, but for an entirely different reason? In any case, astronomers continue to make better and better observations with their telescopes and are hoping to obtain more definite answers about the universe during the first decades of the twenty-first century. However, based on the recent history of discoveries in astronomy, it is probable that more surprises are in store.

Matter and Energy; Particle Accelerators.

Guth, A. (1997). The Inflationary Universe. Reading, MA: Addison-Wesley.

Rees, M. (1997) Before the Beginning. Reading, MA: Addison-Wesley.

Weinberg, S. (1977). The First Three Minutes. New York: Basic Books.