Big went the bang

The Big Bang.

In physical cosmology, the Big Bang is the scientific theory that the Universe emerged from an enormously dense and hot state about 13.7 billion years ago. The Big Bang theory developed from observations and theoretical considerations. Observationally, it was determined that most spiral nebulae were receding from Earth, but those who made the observation weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way. In 1927, the Belgian Catholic priest Georges Lemaître independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed, on the basis of the recession of spiral nebulae, that the Universe began with the "explosion" of a "primeval atom"—what was later called the Big Bang.

In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. He discovered that, relative to the Earth, the galaxies are receding in every direction at speeds directly proportional to their distance from the Earth. This fact is now known as Hubble's law (see Edwin Hubble: Mariner of the Nebulae by Edward Christianson). Given the cosmological principle whereby the Universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places, Hubble's law suggested that the Universe was expanding. http://en.wikipedia.org/wiki/Big_bang