Big Bang theory The most widely accepted theory of the origin and evolution of the Universe. According to the Big Bang theory, the Universe originated from an initial state of high temperature and density and has been expanding ever since. The theory of general relativity predicts the existence of a singularity at the very beginning, where the temperature and density were infinite. Most cosmologists interpret this singularity as meaning that general relativity breaks down at the Planck era under the extreme physical conditions of the very early Universe, and that the very beginning must be addressed using a theory of quantum cosmology. With our present knowledge of high-energy particle physics, we can run the clock back through the lepton era and hadron era to about a millionth of a second after the Big Bang, when the temperature was 10¹³ K. Using more speculative theory, cosmologists have tried to push the model to within 10^-35 s of the singularity, when the temperature was 10²⁸ K.

 The Big Bang theory accounts for the expansion of the Universe; the existence of the cosmic microwave background; and the abundances of light nuclei such as helium, helium-3, deuterium, and lithium-7, which are predicted to have been formed about 1 second after the Big Bang when the temperature was 10¹⁰ K. The cosmic microwave background provides the most direct evidence that the Universe went through a hot, dense phase. In the Big Bang theory, the microwave background is accounted for by the fact that, for the first million years or so (i.e. before the decoupling of matter and radiation), the Universe was filled with plasma that was opaque to radiation and therefore in thermal equilibrium with it. This phase is usually called the primordial fireball. When the Universe expanded and cooled to about 3000 K it became transparent to radiation, which we now observe, much cooled and diluted, as thermal microwave radiation. The discovery of the microwave background in 1965 resolved a long-standing battle between the Big Bang and its then rival, the steady-state theory, which cannot explain the black-body form of the microwave background. Ironically, the term Big Bang was initially intended to be derogatory and was coined by F.Hoyle, one of the strongest advocates of the steady state.

big bang chronology

Big Bang In cosmology, theory advanced to explain the origin of the Universe, developed from the ideas of Georges Lemaître and advanced in the 1940s by George Gamow. According to ‘Big Bang’ theory, a giant explosion 10 to 20 thousand million years ago began the expansion of the Universe, which still continues. Everything in the Universe once constituted an exceedingly hot and compressed gas with a temperature exceeding 10,000 million degrees. When the Universe was only a few minutes old, its temperature would have been 1000 million degrees. As it cooled, nuclear reactions took place that led to material emerging from the fireball consisting of about 75% hydrogen and 25% helium by mass, the composition of the Universe as we observe it today. There were local fluctuations in the density or expansion rate. Slightly denser regions of gas, the expansion rates of which lagged behind the mean value, collapsed to form galaxies when the Universe was perhaps a tenth of its present age. The cosmic microwave background radiation detected in 1965 is considered to be the residual radiation of the ‘Big Bang’ explosion. See also oscillating Universe theory; steady-state theory

Big Bang Theory

The Big Bang Theory is based on the observation that all the stars and galaxies of the universe are in motion and not stationary. The American astronomer Edwin Hubble (1889–1953) discovered in 1929 that the light of all visible stars was redshifted. Hence the movement of the myriad of galaxies is not random but everything is moving further away. If all galaxies are now racing away from one another then at one point all matter must have been clustered together in an infinitely dense space and its present motion might best be explained by an original explosion of matter. Hence the term Big Bang. The 1965 discovery by Arno Penzias (b. 1933) and Robert Wilson (b. 1936) of the background radiation produced by the intense heat of this "explosion" served to further confirm the theory. The Big Bang Theory brought to an end the idea of a static universe and made respectable again discussions of the beginning and possible creation of the universe.

See also Big Crunch Theory; Cosmology, Physical Aspects; Creation; Inflationary Universe Theory

mark worthing

big bang theory The current explanation for the origin of the universe, in which it expands and evolves from an initial very high-temperature condition about 15–20 billion years ago. The expansion time is given from the reciprocal of the Hubble constant (the rate at which galaxies are receding). All all-pervasive background radiation of 3K is considered to be residual from the big bang and is the strongest supporting evidence for the theory.

big bang (also Big Bang) • n. Astron. the explosion of dense matter that, according to current cosmological theories, marked the origin of the universe.