Edwin Hubble publishes the linear relationship between galaxy distance and recession velocity (redshift), establishing convincing observational evidence that the universe is expanding. The finding, based on his own program of distance measurement via Cepheids and on redshifts measured by Vesto Slipher, transformed cosmology from a static to a dynamic model, mathematically grounded Big Bang theory, and gave rise to the concept of the Hubble constant (H₀) as the fundamental measure of the rate of cosmic expansion.