The Big Bang

In physical cosmology, the Big Bang is the scientific "theory" that the universe emerged from a tremendously dense and hot state about 13.7 billion years ago.

Observational evidence for the Big Bang includes the analysis of the spectrum of light from galaxies which reveal a shift towards longer wavelengths proportional to each galaxy's distance in a relationship described by Hubble's law. If this observation that galaxies are moving away from us is not at any special position in the universe (the Copernican principle), then this suggests that space itself is expanding. Extrapolation of this expansion back in time yields an initial state in the distant past in which all matter and energy was at an immense temperature and density. This initial state is the key premise of Big Bang theory.

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Georges Lemaître

Monsignor Georges Henri Joseph Éduard Lemaître (July 17, 1894 – June 20, 1966) was a Belgian Roman Catholic priest, honorary prelate, professor of physics and astronomer.

Fr. (later Msgr.) Lemaître proposed what became known as the Big Bang theory of the origin of the Universe, although he called it his 'hypothesis of the primeval atom'.

He was a pioneer in applying Einstein's theory of general relativity to cosmology: suggesting a pre-cursor of Hubble's law in 1927, and then publishing his primeval atom theory in the pages of Nature in 1931. At the time Einstein believed in an eternal universe and had previously expressed his skepticism about Lemaître's original 1927 paper. A similar solution to Einstein's equations, suggesting a changing radius to the size of the universe, had been proposed in 1922 by Alexander Alexandrovich Friedman, as Einstein informed Lemaître when he approached him with the theory at the 1927 Solvay Conference (Friedman had also been criticized by Einstein), but it is Lemaître, with his proposed mechanism, that made the theory famous for several reasons according to historians.

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