Hubble's law

In the 1920s, astronomers measuring distant galaxies noticed something strange: almost every galaxy's light was shifted toward the red, as if it were rushing away from us, and the farther the galaxy, the faster it seemed to flee. Hubble's law is the simple pattern Edwin Hubble drew from this: a galaxy's recession speed is proportional to its distance. Double the distance, double the speed; ten times the distance, ten times the speed.

Written as a formula it is just v = H0 d, where v is how fast the galaxy recedes, d is its distance, and H0 (the Hubble constant) is the constant of proportionality that fixes the rate. The numbers are large: a galaxy 100 million light-years away recedes at very roughly 2,000 kilometres per second. This tidy straight-line relation is exactly what the uniform expansion of space predicts — if all distances grow by the same percentage each second, then more distant points must separate proportionally faster. Hubble's law is therefore not evidence that we sit at a special centre; an observer in any galaxy would measure the same law, because everyone sees the same expansion.

Hubble's law was the first hard observational evidence that the universe is expanding, and run backward it points to a hot, dense beginning. It is also the everyday workhorse of cosmology: measure a distant galaxy's redshift, and Hubble's law converts it into a distance, which is how we map the universe out to billions of light-years. One caution: the law is a smooth average. Nearby galaxies also have their own random motions and gravitational pulls (for instance, Andromeda is actually approaching us), so the clean v = H0 d relation only emerges once galaxies are far enough that expansion dominates their motion.