The Big Bang theory states that the universe began about 14 billion years ago from an extremely hot, dense state and has been expanding and cooling ever since. Space itself expanded; there was no centre and no pre-existing space for it to expand into.Evidence for the Big Bang:
• Hubble's law: galaxies are receding with speeds proportional to distance, so they were once much closer together.
• Cosmic microwave background radiation (CMB): microwave radiation reaching us almost equally from all directions, with a black-body spectrum at about 2.7 K. It is radiation from the hot early universe, stretched to microwave wavelengths by the expansion.
• Relative abundance of hydrogen and helium: the theory predicts that fusion in the first few minutes produced a universe about 75% hydrogen and 25% helium by mass, which matches observations.Age of the universe: if the universe has always expanded at the same rate, a galaxy at distance d moving at v took time t = d/v to get there. Since v = Hd:t ≈ 1/H Worked example: H = 65 km s-1 Mpc-1 = 65 × 103 / 3.08 × 1022 = 2.11 × 10-18 s-1
t = 1 / 2.11 × 10-18 = 4.74 × 1017 s
t = 4.74 × 1017 / 3.15 × 107 = 1.5 × 1010 yearsThis is only an estimate, because the rate of expansion has not been constant and the value of H is uncertain.
• Hubble's law: galaxies are receding with speeds proportional to distance, so they were once much closer together.
• Cosmic microwave background radiation (CMB): microwave radiation reaching us almost equally from all directions, with a black-body spectrum at about 2.7 K. It is radiation from the hot early universe, stretched to microwave wavelengths by the expansion.
• Relative abundance of hydrogen and helium: the theory predicts that fusion in the first few minutes produced a universe about 75% hydrogen and 25% helium by mass, which matches observations.Age of the universe: if the universe has always expanded at the same rate, a galaxy at distance d moving at v took time t = d/v to get there. Since v = Hd:
t = 1 / 2.11 × 10-18 = 4.74 × 1017 s
t = 4.74 × 1017 / 3.15 × 107 = 1.5 × 1010 yearsThis is only an estimate, because the rate of expansion has not been constant and the value of H is uncertain.