Big Bang Theory

Big bang theory describes the origin of the knowable universe and the development of the laws of physics and chemistry fifteen billion years ago.

During the 1940s Russian-born American cosmologist and nuclear physicist George Gamow (1904–1968) developed the modern version of the big bang model based upon earlier concepts advanced by Russian physicist Alexander (Aleksandr Aleksandrovich) Friedmann (also spelled as Fridman, 1888–1925) and Belgian astrophysicist and cosmologist Abbé Georges Lemaître (1894–1966). Big bang based models replaced static models of the universe that described a homogeneous universe that was the same in all directions (when averaged over a large span of space) and at all times. Big bang and static cosmological models competed with each other for scientific and philosophical favor. Although many astrophysicists rejected the steady state model because it would violate the law of mass-energy conservation, the model had many eloquent and capable defenders. Moreover, the steady state model was interpreted by many to be more compatible with many philosophical, social, and religious concepts centered on the concept of an unchanging universe. The discovery of quasars and a permeating cosmic background radiation eventually tilted the cosmological argument in favor of big bang theory models.

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 was evolving did not begin to accumulate until the 1920s. The theory 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 German-American physicist Albert Einstein's (1879–1955) 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.

In 1929, American astronomer Edwin Powell Hubble made what has been called the most significant astronomical discovery of the century. He observed 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. Vesto Melvin Slipher, who took photographs of the red-shift of many of the same galaxies, also drew similar conclusions.

Like de Sitter, Lemaître, who worked with Hubble in 1924, developed out a simple solution to Einstein's equations that described a universe in expansion. Hubble's stunning observation 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. Because 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 ten to fifteen 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 and Robert C. Herman 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. However, a number of other scientists, including Fred Hoyle, Thomas Gold, and Hermann Bondi 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 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 five K (five kelvins above absolute zero). Unknown to him, radio engineers Arno Penzias and Robert W. Wilson already detected such radiation at three 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.
It is argued that the laws of physics and chemistry—manifested in the properties of the fundamental forces of gravity, the strong force, electromagnetism, and the weak force (electromagnetism and the weak force are now known to be different manifestations of a more fundamental electroweak force)—formed in the first few fractions of a second of the big bang.

Protons and neutrons began to form atomic nuclei about three minutes and forty six seconds after the explosion, when the temperature was a mere 900,000,000 K. After seven hundred thousand 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 would condense from swirling nebulae, evolve and die, before our Sun and its planets could form in the Milky Way galaxy.

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 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 fifteen billion light-years into space (hence, fifteen 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 lends support to Guth's theory of inflation.