Hubble's law

Edwin Hubble published the relationship between galaxy recession velocity and distance in 1929, but Georges Lemaître had derived the same result from the same observational data two years earlier in 1927. The International Astronomical Union voted in 2018 to recommend renaming it the Hubble–Lemaître Law to acknowledge Lemaître's priority.

Measurements of the Hubble constant (H₀) currently fall in the range of approximately 67–73 km/s/Mpc depending on the method used. This discrepancy between the Planck satellite's CMB-based measurement (~67 km/s/Mpc) and supernova-based measurements (~73 km/s/Mpc) is known as the 'Hubble tension' and remains one of the biggest open problems in cosmology.

Hubble's Law demonstrates that galaxies are receding from us at velocities proportional to their distances, which means the universe is expanding. This was the first major observational evidence supporting the Big Bang model over a static, eternal universe, because an expanding universe implies everything was once compressed into a much hotter, denser state.

Two primary methods are used: the 'distance ladder' approach combining Cepheid variable stars and Type Ia supernovae in nearby galaxies, and the analysis of the cosmic microwave background by instruments like the Planck satellite. These two methods produce conflicting values — the 'Hubble tension' — which may require new physics such as early dark energy or additional relativistic species to resolve.