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The big bang theory describes a hot explosion of energy and matter at the time the universe came into existence. This theory explains why the universe is expanding. Recent versions of the theory also explain why the universe seems so uniform in all directions and at all places.







The work of Edwin Hubble, which showed that the universe is expanding, led cosmologists to begin tracking the history of the universe. The dominant idea is that the universe would have been hotter and denser billions of years ago. In the 1940s Russian American physicist George Gamow and his students, American physicists Ralph Alpher and Robert Herman, developed the idea of a hot explosion of matter and energy at the time of the origin of the universe. (This theory of an explosion at the beginning of the universe was given the originally derisive name “big bang” by British astronomer Fred Hoyle in 1950.) Current calculations place the age of the universe at about 13.7 billion years. Gamow and his students realized that some of the chemical elements in the universe today were forged in the hot early stage of the universe’s existence. They also hypothesized that some radiation that remains from the big bang explosion may still be circulating in the universe, though this idea was forgotten for some time.




Current methods of particle physics allow the universe to be traced back to a tiny fraction of a second—1 × 10-43 seconds—after the big bang explosion initiated the expansion of the universe. To understand the behavior of the universe before that point cosmologists would need a theory that merges quantum mechanics and general relativity. Scientists do not actually study the big bang itself, but infer its existence from the universe’s expansion.




In the 1950s American astronomer William Fowler and British astronomers Fred Hoyle, Geoffrey Burbidge, and Margaret Burbidge worked out a series of calculations that showed that the lightest of the chemical elements (those of lowest atomic weight) were formed in the early universe shortly after the big bang. These light elements include ordinary hydrogen, hydrogen’s isotope deuterium, and helium. Heavier elements, according to those calculations, were formed later. Scientists now know that the elements heavier than helium and lighter than iron were formed in nuclear processes in stars, and the heaviest elements (those heavier than iron) were formed in supernova explosions.

 

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