Measuring the expanding universe. Computer artwork demonstrating two ways that are used to measure the rate at which the universe is expanding. At left the 'standard candle' method is shown. Supernovae have a known brightness. The further you are from a supernova, the fainter it appears (as does a candle). Therefore by measuring how bright a supernova appears, its distance can be determined. At right the 'standard ruler' method is shown. Pairs of galaxies have a preference to be separated by 490 million light years. This separation distance appears to get smaller, the further away the galaxies are (as would a ruler). By measuring this change, the distance can be calculated.