Hubble's Law

Hubble's law is the observed linear relationship between the recession velocity of distant galaxies and their distance from Earth: galaxies recede at speeds proportional to their distance, with the constant of proportionality known as the Hubble constant (H₀). First published by Edwin Hubble in 1929 using distance measurements to nearby galaxies combined with Vesto Slipher's redshift data, the law is the primary empirical evidence that the universe is expanding.

The law does not imply that Earth is at the center of the expansion. In a homogeneous expanding universe described by the FLRW metric, every observer sees every other observer receding according to the same linear relation — a consequence of the cosmological principle, not of our privileged position. The Hubble constant sets the age scale of the universe: a rough estimate of the time since the Big Bang is the inverse of H₀, though this simple calculation ignores the effects of matter, radiation, and dark energy on the expansion history.

Despite its name, Hubble's law is not a physical law in the strict sense but an empirical relation that holds approximately for galaxies in the local universe. At very large distances, the linear relationship breaks down due to the dynamical effects of dark energy and the changing matter density, requiring the more general Friedmann equations to describe the expansion accurately. The precise value of H₀ has been the subject of the 'Hubble tension' in modern cosmology, with early-universe measurements from the cosmic microwave background giving a lower value than late-universe measurements from supernovae and Cepheid variables — a discrepancy that, if real, may signal new physics beyond the standard cosmological model.