Definition
Hubble's law is the empirical relation v = H₀ × d: galaxies recede from us with a velocity proportional to their distance, with the proportionality constant H₀ (the Hubble constant) capturing the present-day rate of cosmic expansion.
Edwin Hubble published it in 1929, building on velocity measurements by Vesto Slipher and distance measurements made with Cepheid variable stars. Belgian priest and physicist Georges Lemaître had derived essentially the same relation theoretically two years earlier, in 1927.
Why it matters
How it works
Hubble combined two independent observations. Slipher had shown in the 1910s that the spectra of distant "nebulae" were systematically redshifted, implying recession. Hubble used Cepheid variables — pulsating stars whose intrinsic luminosity is determined by their period (the Leavitt period–luminosity relation) — to measure distances to 24 of those nebulae. Plotting recession velocity against distance gave a roughly linear trend with positive slope.
Today the law is usually written v = H₀ d, where H₀ is measured in km/s per megaparsec. Hubble's original value was about 500 km/s/Mpc — much too large because of distance-scale errors that took half a century to correct. Modern measurements cluster between 67 and 74 km/s/Mpc, depending on method.
Crucially, the recession is not galaxies flying through space. In the general-relativistic picture (the FLRW metric), galaxies sit nearly stationary in comoving coordinates, while the metric itself stretches with cosmic time. The redshift of distant light is then the cosmological redshift — light is stretched along with space — rather than a Doppler effect of motion through space. The two coincide for small distances; they diverge at high redshift.