The Big Bang Theory The Big Bang Theory is the currently accepted explanation of the beginning of the universe. The big bang theory proposes that the universe was once extremely compact, dense, and hot. Some original event, a cosmic explosion called the big bang, occurred about 10 billion to 20 billion years ago, and the universe has since been expanding and cooling.

The theory is based on the mathematical equations, known as the field equations, of the general theory of relativity set forth in 1915 by Albert Einstein. In 1922 Russian physicist Alexander Friedmann provided a set of solutions to the field equations. These solutions have served as the framework for much of the current theoretical work on the big bang theory. American astronomer Edwin Hubble provided some of the greatest supporting evidence for the theory with his 1929 discovery that the light of distant galaxies was universally shifted toward the red end of the spectrum (ie Red Shift). This proved that the galaxies were moving away from each other. He found that galaxies farther away were moving away faster, showing that the universe is expanding uniformly. However, the universe’s initial state was still unknown.

In the 1940s Russian-American physicist George Gamow worked out a theory that fit with Friedmann’s solutions in which the universe expanded from a hot, dense state. In 1950 British astronomer Fred Hoyle, in support of his own opposing steady-state theory, referred to Gamow’s theory as a mere “big bang,” but the name stuck. Indeed, a contest in the 1990s by Sky & Telescope magazine to find a better (perhaps more dignified) name did not produce one.

The Theory The big bang theory seeks to explain what happened at or soon after the beginning of the universe. Scientists can now model the universe back to 10-43 seconds after the big bang. For the time before that moment, the theory of gravity is no longer adequate. Scientists are searching for a theory that merges quantum mechanics and gravity, but have not found one yet. Many scientists have hope that string theory will tie together gravity and quantum mechanics and help scientists explore further back in time (ie Unified Field Theory).

Because scientists cannot look back in time beyond those early moments, the actual big bang is hidden from them. There is no way at present to detect the origin of the universe. Further, the big bang theory does not explain what existed before the big bang. It may be that time itself began at the big bang, so that it makes no sense to discuss what happened “before” the big bang.

According to the big bang theory, the universe expanded rapidly in its first microseconds. A single force existed at the beginning of the universe, and as the universe expanded and cooled, this force separated into those we know today: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.

The universe expanded from a primordial state of matter called ylem consisting of protons, neutrons, and electrons in a sea of radiation. They theorized the universe was

very hot at the time of the big bang since elements heavier than hydrogen can be formed only at a high temperature.

If the universe is open, it will keep expanding forever, even though the rate of expansion will gradually slow. If the universe is closed, the expansion of the universe will eventually stop and the universe will begin contracting until it collapses. Whether the universe is open or closed depends on the density, or concentration of mass, in the universe. If the universe is dense enough, it is closed.

Supporting Evidence The universe cooled as it expanded. The study of the distribution of deuterium, helium, and the other light elements is now a major field of research. The uniformity of the helium abundance around the universe supports the big bang theory and the abundance of deuterium can be used to estimate the density of matter in the universe.

The formation of atoms allowed many wavelengths of light to travel much farther than before. This change set free radiation that we can detect today. After billions of years of cooling, this cosmic background radiation was first detected and identified in 1965 by American astrophysicists Arno Penzias and Robert Wilson.

NASA’s Cosmic Background Explorer (COBE) spacecraft mapped the cosmic background radiation between 1989 and 1993. It verified that the distribution of the background radiation precisely matched the types predicted for the big bang theory. It also showed that the cosmic background radiation is not uniform, that it varies slightly. These variations are thought to be the seeds from which galaxies and other structures in the universe grew.

Refining the Theory Evidence indicates that the matter that scientists detect in the universe is only a small fraction of all the matter that actually exists. Cosmologists now think that much of the universe — perhaps 99 percent — is dark matter, or matter that has gravity but that we cannot see or otherwise detect. The fundamental particles known as neutrinos are a prime example of dark matter. The amount of dark matter that exists may be just enough to bring the universe to the boundary between open and closed.

The theories continue to match the observations, though there is no consensus on the type or types of dark matter that must be included. Supercomputers are important for making such models.

Astronomers continue to make new observations that are interpreted within the framework of the big bang theory. Although scientists have not found any major problems with the big bang theory, the theory is being constantly adjusted to match the observed universe.

Contributed By: Jay M. Pasachoff (revisions by MVG)

Big Bang

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