The Big Bang theory is the prevailing cosmological model of the early development of the universe. The major premise of the Big Bang theory is that the universe was once in an extremely hot and dense state that expanded rapidly (a "Big Bang"). This rapid expansion caused the young universe to cool and resulted in its present continuously expanding state. According to recent measurements, scientific evidence, and observations, the original state happened around 13.7 billion years ago, which can be referred to as the time that the Big Bang occurred.
Georges Lemaître proposed what became known as the Big Bang theory of the origin of the Universe; he called it his "hypothesis of the primeval atom". The framework for the model relies on Albert Einstein's general relativity and on simplifying assumptions (such as homogeneity and isotropy of space). The governing equations had been formulated by Alexander Friedmann. In 1929, Edwin Hubble discovered that the distances to far away galaxies were generally proportional to their redshifts—an idea originally suggested by Lemaître in 1927. Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.
