Hubble's Law (1929): The Discovery That the Universe Is Expanding

Hubble’s law, published by Edwin Hubble in March 1929, says that galaxies move away from us at speeds proportional to their distance. Using 24 galaxies, Hubble found a near straight-line relation between distance and velocity. It became the first observational pillar of the expanding universe and of Big Bang cosmology.

The question before the paper

By the late 1920s astronomers knew two odd things about the spiral “nebulae”. Their spectra, measured mostly by Vesto Slipher, were usually shifted toward the red, which meant they were moving away from us. And thanks to Hubble’s own Cepheid measurements, they were clearly galaxies far outside the Milky Way. The obvious follow-up was the one this paper asks: do the speeds of these galaxies depend on how far away they are?

What the paper found

• A linear velocity-distance relation. The farther a galaxy, the faster it recedes.
• A slope of about 500 km/s per megaparsec. Hubble’s constant K came out at +465 ± 50 km/s per million parsecs using individual galaxies, and +513 ± 60 when he grouped them into 9 bins.
• A small, careful sample. Only 24 galaxies had distances he trusted, from Cepheid variables, the brightest stars in each galaxy and average galaxy brightness.
• A cautious interpretation. Hubble did not claim the universe was expanding. He discussed the result in terms of the “de Sitter effect”, a feature of one theoretical model of the cosmos.

How Hubble discovered the expanding universe

The paper combined two independent kinds of measurement. Velocities came from the Doppler shift of spectral lines, most of them already published by Slipher. Distances were Hubble’s own work at Mount Wilson Observatory, today part of Carnegie Science, where the 100-inch Hooker telescope could pick out individual stars in nearby galaxies. Cepheid variables, whose pulsation period reveals their true brightness, set the scale for the nearest galaxies; brighter, cruder indicators carried it farther.

Plotting velocity against distance gave a cloud of points that rose roughly along a straight line. In modern notation the law reads v = H₀ × d, where H₀ is the Hubble constant.

Why it mattered

A straight-line relation is exactly what uniform expansion of space produces: every galaxy sees every other galaxy receding, faster in proportion to distance. Within a few years the result, read through Einstein’s general relativity, turned the static universe into a dynamic one with a history. It is the observational foundation of cosmology as we know it, from the Big Bang to the far future traced in Deep Time.

Credit is shared. Georges Lemaître had derived the same relation from general relativity and estimated its slope in a 1927 paper that few people read. In October 2018 an electronic vote of International Astronomical Union members, 78% in favor, recommended calling it the Hubble-Lemaître law.

What we know now

Hubble’s slope was about seven times too large because his distance scale was badly calibrated. Modern values sit between about 67 and 73 km/s per megaparsec, and that spread is itself a problem. The early-universe prediction from the Planck satellite gives 67.4, while the SH0ES measurement with Cepheids and supernovae gives 73.04, a 5-sigma gap known as the Hubble tension. The 1998 discovery that the expansion is speeding up grew from the same measurement, pushed to much greater distances.

Who wrote it

Edwin Hubble was a staff astronomer at Mount Wilson. He wrote this short paper alone, building on Slipher’s velocities and on his own 1920s work showing that spiral nebulae are separate galaxies. The question of who “discovered” expansion is best answered with both names: Lemaître first, in theory and with numbers; Hubble, with the observational case that convinced the field.

Read the original

Hubble, E. (1929). “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae.” Proceedings of the National Academy of Sciences 15(3): 168. doi:10.1073/pnas.15.3.168. Free to read in full on PubMed Central. Semantic Scholar lists about 2,300 citations (October 2026).

Questions people ask

Who discovered the expanding universe?

Georges Lemaître predicted it from general relativity in 1927 and estimated the expansion rate from galaxy data. Edwin Hubble published the observational velocity-distance relation in 1929, which persuaded astronomers. The IAU now recommends the name Hubble-Lemaître law to credit both.

What is Hubble’s law in simple terms?

Hubble’s law says a galaxy’s recession speed equals the Hubble constant times its distance. A galaxy twice as far away moves away twice as fast. That pattern is what you expect when space itself is expanding evenly in every direction.

Did Hubble think the universe was expanding?

Not in this paper. Hubble reported the velocity-distance relation and linked it to the “de Sitter effect” of one cosmological model, without claiming expansion. Others, including Lemaître and Eddington, made the physical interpretation explicit.