What is the Big Bang?

The Big Bang was not an explosion in space but the rapid expansion of space itself from a hot, dense beginning around 13.8 billion years ago. Belgian priest and physicist Georges Lemaître proposed the idea in 1927, and in 1929 Edwin Hubble showed that distant galaxies are rushing away from us. The name was coined, somewhat mockingly, by rival astronomer Fred Hoyle on BBC radio in 1949. The strongest evidence came in 1965, when Arno Penzias and Robert Wilson detected the cosmic microwave background, the faint afterglow of the early universe, earning a Nobel Prize. About 380,000 years after the Big Bang, the universe cooled enough for atoms to form and light to travel freely.