Hubble's Law
The cosmological observation that galaxies recede from Earth at velocities directly proportional to their proper distance.
Governing formula
v = H₀ d, H₀ ≈ 70 (km/s) / Mpc
SI unit
km/s/Mpc (Hubble Constant)
In depth
Discovered by Edwin Hubble in 1929. Recessional motion is caused by the expanding metric of space between galaxies rather than mechanical motion through space. The inverse of Hubble's constant H₀ provides an estimate of the age of the universe.
Examples in the real world
A galaxy located 100 megaparsecs away receding from Earth at a velocity of ~7,000 kilometers per second.