The Big Bang theory states that the universe began about 13.8 billion years ago from an extremely small, extremely hot and dense state, and has been expanding and cooling ever since.The Big Bang was not an explosion of matter into existing empty space. Space and time themselves began with it, and it is space itself that has been expanding since. So there is no centre of the universe where the Big Bang happened: it happened everywhere.The theory follows from Hubble's law. If galaxies are moving apart now, they must have been closer together in the past. Running the expansion backwards, all the matter and energy in the universe must once have been concentrated in a tiny, very hot region.There are two main pieces of evidence for the Big Bang:• Hubble's law: the red shift of distant galaxies shows that the universe is expanding.
• Cosmic microwave background radiation (CMB): microwave radiation arriving almost uniformly from every direction in space. It is radiation left over from the hot early universe, stretched to long wavelengths by the expansion.The theory also correctly predicts the amounts of light elements. About 25% of the ordinary mass of the universe is helium, most of which was made by fusion in the first few minutes after the Big Bang. Stars alone could not have made this much.The Big Bang theory replaced the rival steady state model, which proposed an unchanging, eternal universe and could not explain the CMB.
• Cosmic microwave background radiation (CMB): microwave radiation arriving almost uniformly from every direction in space. It is radiation left over from the hot early universe, stretched to long wavelengths by the expansion.The theory also correctly predicts the amounts of light elements. About 25% of the ordinary mass of the universe is helium, most of which was made by fusion in the first few minutes after the Big Bang. Stars alone could not have made this much.The Big Bang theory replaced the rival steady state model, which proposed an unchanging, eternal universe and could not explain the CMB.