The big bang is one of several theoretical beginning moments of the universe. The big bang theory states that the expanding universe originated 10-20 billion years ago in a single explosive event in which the entire universe suddenly exploded out of a single, minuscule, infinitely dense and hot particle, reaching a pea-sized supercondensed state with a temperature of 10 billion million million degrees Celsius in 1 million-million-million-million-million-millionth (10-36) of a second after the big bang. Some of the fundamental contributions of the expanding universe models come from the German-born American physicist Albert Einstein, who in 1915 proposed the general theory of relativity, which described how matter and energy warp space-time to produce gravity. When Einstein applied his theory to the universe in 1917, he discovered that gravity would cause the universe to be unstable and collapse, so he proposed adding a cosmological constant as a "fudge factor" to his equations. The cosmological constant added a repulsive force to the general theory, and this force
Milky Way Galaxy. Astronomers and cosmologists must estimate the amount of matter, dark matter, and energy in the galaxies and the interstellar medium to test different models for the evolution of the universe (Sander van Sinttruye, 2008, used under license from Shutterstock, Inc.)
counterbalanced gravity, enabling the universe to continue expanding in his equations. Dutch mathematician, physicist, and astronomer William de Sitter (1872-1934) further applied Einstein's theory of general relativity to predict that the universe is expanding. In 1927 Belgian priest and astronomer Georges Lemaitre (1894-1966) proposed that the universe originated in a giant explosion of a primeval atom, an event now called the big bang. In 1929 American astronomer Edwin Hubble (1889-1953) measured the movement of distant galaxies and discovered that galaxies are moving away from each other, expanding the universe as if the universe is being propelled from a big bang. This idea of expansion from an explosion negated the need for Einstein's cosmo-logical constant, which he retracted, referring to it as his biggest blunder. This retraction, however, would later come back to haunt cosmologists.
Also in the 1920s, the Russian physicist and cos-mologist from Odessa, George Gamow (1904-68), worked with a group of scientists and suggested that elements heavier than hydrogen, specifically helium and lithium, could be produced in thermonuclear reactions during the big bang. Later, in 1957, English astronomer Fred Hoyle (1915-2001) and his colleagues, American astrophysicist William Fowler (1911-55) and American physicists Geoff Bur-bidge (1925- ) and British-born Margaret Burbidge (1919- ), showed how hydrogen and helium could be processed in stars to produce heavier elements necessary to life, such as carbon, oxygen, and iron.
The inflationary theory is a modification of the big bang theory, and suggests that the universe underwent a period of rapid expansion immediately after the big bang. This theory, proposed in 1980 by American physicist Alan Guth (1947- ), attempts to explain the present distribution of galaxies, as well as the 3 K cosmic background radiation discovered by American physicists and Nobel laureates Arno Penzias (1933- ) and Robert Woodrow Wilson (1936- ) in 1965. This uniformly distributed radiation is thought to be a relic left over from the initial explosion of the big bang. For many years after the discovery of the cosmic background radiation, astronomers searched for answers to the amount of mass in the universe and to determine how fast the universe was expanding and to what degree the gravitational attraction of bodies in the universe was
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