In the 1920s, Hubble measured the red shifts of galaxies, giving their recession velocities, and their distances, using standard candles such as Cepheids. He found that the further away a galaxy is, the faster it is moving away from us.Hubble's law: the recession velocity of a galaxy is directly proportional to its distance:v = H0d H0 is the Hubble constant. A graph of v against d is a straight line through the origin with gradient H0.
In SI units H0 is measured in s−1 (v in m s−1, d in m); astronomers often quote it in km s−1 Mpc−1. Current measurements give about 70 km s−1 Mpc−1, which is about 2.3 × 10−18 s−1.The value of H0 is uncertain because the distances to far galaxies are hard to measure accurately, and different methods give slightly different values.Combined with z ≈ v/c, Hubble's law lets a galaxy's distance be found from its red shift alone.Worked example: a galaxy has z = 0.050. Take H0 = 2.2 × 10−18 s−1.v = zc = 0.050 × 3.00 × 108 = 1.5 × 107 m s−1 d = v ÷ H0 = 1.5 × 107 ÷ 2.2 × 10−18 = 6.8 × 1024 m
In SI units H0 is measured in s−1 (v in m s−1, d in m); astronomers often quote it in km s−1 Mpc−1. Current measurements give about 70 km s−1 Mpc−1, which is about 2.3 × 10−18 s−1.The value of H0 is uncertain because the distances to far galaxies are hard to measure accurately, and different methods give slightly different values.Combined with z ≈ v/c, Hubble's law lets a galaxy's distance be found from its red shift alone.Worked example: a galaxy has z = 0.050. Take H0 = 2.2 × 10−18 s−1.