Big Bang Theory
Before the twentieth century, astronomers could only assume that the universe had existed forever without change, or that it was created in its present condition by divine action at some arbitrary time. Evidence that the universe may be evolving did not begin to accumulate until the 1920s. Today, the idea that all matter in the universe was created from a gigantic explosion called the "big bang " is widely accepted by students of cosmology.
It was Albert Einstein's theory of relativity, published in 1915, that set the stage for the conceptual development of an expanding universe. Einstein had designed his theory to fit a static universe of constant dimensions. In 1919 a Dutch astronomer, Willem de Sitter, showed Einstein's theory could also describe an expanding universe. Mathematically, de Sitter's solution for Einstein's equation was sound, but observational evidence of expansion was lacking, and Einstein was skeptical.
Beginning in 1914, Vesto Melvin Slipher began to discover large red shifts in the spectra of the galaxies he was studying; these red-shifts indicated that the galaxies are continually moving apart at tremendous velocities. By 1929, American astronomer Edwin Powell Hubble had observed the same phenonmenon. This has been called the most important astronomical discovery of the century. Like de Sitter, Georges Henri Lemaître, a Belgian astronomer who worked with Hubble in 1924, developed a simple solution to Einstein's equations that described a universe in expansion. Slipher's and Hubble's stunning observations provided the evidence Lemaître was seeking for his theory. In 1933 Lemaître clearly described the expansion of the universe. Projecting back in time, he suggested that the universe had originated as a great "cosmic egg," expanding outward from a central point. He did not, however, consider whether an explosion actually took place to initiate this expansion.
George Gamow further investigated the origin of the universe in 1948. Since the universe is expanding outward, he reasoned, it should be possible to calculate backward in time to its beginning. If all the mass of the universe was compressed into a small volume 10 to 15 billion years ago, its density and temperature must have been phenomenal. A tremendous explosion would have caused the start of the expansion, left a "halo" of background radiation, and formed the atomic elements that are heavier than the abundant hydrogen and helium. Physicists Ralph A. Alpher (1921-) and Robert C. Herman (1914-) established a model to show how such heavier particles could form under these conditions.
Gamow's theory implied there was a specific beginning and end to the universe. But a number of other scientists, such as Fred Hoyle, Thomas Gold, and Hermann Bondi (1919-) felt that the theory of expansion required no beginning or end. Their model, called the steady state theory, suggested that matter was being continuously created throughout the universe. As galaxies drifted apart, matter would "condense" to form new ones in the void left behind. For nearly two decades, supporters of the competing theories seemed to be on equal footing.
In 1965 Robert H. Dicke (1916-) made calculations relative to the cooling-off period after the initial big bang explosion. His results indicated that Gamow's residual radiation should be detectable. During the intervening eons it would have cooled to about 5° Kelvin (five degrees above absolute zero). Unknown to him, radio engineers Arno Penzias (1933-) and Robert W. Wilson (1916-) had already detected such radiation at 3° K in 1964 while looking for sources of satellite communication interference. This was the most convincing evidence yet gathered in support of the big bang theory, and it sent the steady-state theory into decline.
No theory exists today that can account for the extreme conditions that existed at the moment of the big bang. The theory of relativity does not apply to objects as dense and small as the universe must have been prior to the big bang. Cosmologists can project only as far back as 0.01 seconds after the explosion, when the cosmos was a seething mass of protons and neutrons. (It is possible there were many exotic particles that later became important as dark matter.) Based on their theories, cosmologists suggest that during this time neutrinos were produced. Detectors built to search for neutrinos are only now beginning to produce unambiguous results.
It is believed protons and neutrons began to form atomic nuclei about 3 minutes and 46 seconds after the explosion, when the temperature was a mere 900,000,000° K. After 700,000 years hydrogen and helium formed. About one billion years after the big bang, stars and galaxies began to appear from the expanding mass. Countless stars condensed from swirling nebulae, evolved, and died, before our sun and its planets could form in a galaxy named the Milky Way.
Although the big bang theory accounts for most of the important characteristics of the universe, it still has weaknesses. One of the biggest of these involves the "homogeneity" of the universe. Until 1992, measurements of the background radiation produced by the big bang have shown that matter in the early universe was very evenly distributed. This seems to indicate that the universe evolved at a constant rate following the big bang. But if this is the case, the clumps of matter that we see (such as stars, galaxies, and clusters of galaxies) should not exist.
To remedy this inconsistency, Alan Guth (1947-) proposed the inflationary theory, which suggests that the expansion of the universe initially occurred much faster. This concept of accelerated expansion allows for the formation of the structures we see in the universe today.
In April 1992, NASA made an electrifying announcement: its Cosmic Background Explorer (COBE), looking 15 billion light-years into space (hence, 15 billion years into the past), detected minute temperature fluctuations in the cosmic background radiation. It is believed these ripples are evidence of gravitational disturbances in the early universe that could have resulted in matter to clumping together to form larger entities. This finding lent support to Guth 's theory of inflation and was a major stride toward eliminating one of the flaws with the big bang hypothesis.
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