In the 1920s Edwin Hubble measured the red shifts of galaxies and estimated their distances. He found that the further away a galaxy is, the faster it is moving away from us.Hubble's law: the recession speed v of a galaxy is directly proportional to its distance d from us:v = H0d where H0 is the Hubble constant. A graph of v against d for galaxies is a straight line through the origin, and its gradient is H0. Real data show some scatter, because galaxies also have their own random motions and their distances are hard to measure.
The units of H0 depend on the units of v and d. Astronomers usually use km s−1 Mpc−1, and the current value is about 70 km s−1 Mpc−1. In SI units (v in m s−1 and d in m) this is about 2.3 × 10−18 s−1.Worked example: a galaxy is 150 Mpc away. Taking H0 = 70 km s−1 Mpc−1, find its recession speed.
v = H0d = 70 × 150 = 1.05 × 104 km s−1 = 1.05 × 107 m s−1Hubble's law is evidence that the universe is expanding. The galaxies are not flying apart through space from a central point: space itself is expanding, carrying the galaxies apart. Observers in any galaxy would see the same pattern, which is consistent with the cosmological principle.
The units of H0 depend on the units of v and d. Astronomers usually use km s−1 Mpc−1, and the current value is about 70 km s−1 Mpc−1. In SI units (v in m s−1 and d in m) this is about 2.3 × 10−18 s−1.Worked example: a galaxy is 150 Mpc away. Taking H0 = 70 km s−1 Mpc−1, find its recession speed.v = H0d = 70 × 150 = 1.05 × 104 km s−1 = 1.05 × 107 m s−1Hubble's law is evidence that the universe is expanding. The galaxies are not flying apart through space from a central point: space itself is expanding, carrying the galaxies apart. Observers in any galaxy would see the same pattern, which is consistent with the cosmological principle.