Big Bang
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
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Big Bang
