Hubble Announces the Expanding Universe

On March 15, 1929, Edwin Hubble published his groundbreaking paper 'A relation between distance and radial velocity among extra-galactic nebulae' in the Proceedings of the National Academy of Sciences. Using the 100-inch Hooker telescope at Mount Wilson Observatory, Hubble demonstrated that galaxies are moving away from Earth at velocities proportional to their distance—more distant galaxies receding faster than nearby ones. This relationship, which became known as Hubble's Law, provided the first observational evidence of an expanding universe, fundamentally transformed our understanding of cosmic structure, and laid crucial groundwork for the Big Bang theory that would later explain the universe's origins.