Hubble's Expanding Universe (1929): Galaxies Rushing Apart

In 1929 the American astronomer Edwin Hubble published a short paper showing that distant galaxies are moving away from the Milky Way at speeds roughly proportional to their distances. This velocity-distance relation, made with the 100-inch telescope at Mount Wilson Observatory in California, became the key evidence for the expanding universe. It transformed the cosmos from a static, unchanging backdrop into something with a history, and it led directly to the Big Bang theory.

Background: what were the nebulae?

At the start of the 20th century astronomers disagreed about the nature of faint, fuzzy “spiral nebulae”. Some thought they were clouds of gas inside our own galaxy; others believed they were separate “island universes” of stars far beyond it. In the so-called Great Debate of April 1920, Harlow Shapley and Heber Curtis argued the two positions before the National Academy of Sciences, without a decisive result.

Two developments laid the groundwork for an answer. In 1912 Henrietta Swan Leavitt at Harvard College Observatory found that Cepheid variable stars pulsate with periods that reveal their true brightness, so they could be used as “standard candles” to measure distance. Meanwhile, from 1912 Vesto Slipher at Lowell Observatory in Arizona measured the spectra of spiral nebulae and found that most showed a redshift: their light was shifted towards longer wavelengths, which suggested they were moving away at hundreds of kilometres per second.

Hubble at Mount Wilson

Edwin Hubble, born in Missouri in 1889, had studied law at Oxford as a Rhodes Scholar before turning to astronomy. In 1919 he joined Mount Wilson Observatory, home of the 100-inch Hooker Telescope, then the largest in the world. In late 1923 he identified a Cepheid variable in the Andromeda nebula and used it to show that Andromeda lay far beyond the Milky Way. His results, announced at the start of 1925, settled the Great Debate: the spiral nebulae were galaxies in their own right.

Hubble then set out to combine distances with velocities. Working with Milton Humason, a former mule driver and janitor at the observatory who had become a skilled observer, he estimated distances to two dozen or so galaxies using Cepheids, bright stars and overall brightness, and compared them with redshifts, most of them measured by Slipher.

The 1929 paper

Hubble’s paper, “A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae”, appeared in the Proceedings of the National Academy of Sciences in March 1929. It contained a scatter plot of velocity against distance. Despite considerable scatter, the trend was clear: the more distant the galaxy, the faster it appeared to be receding. Hubble described the relation as roughly linear.

The rate of increase, now called the Hubble constant, came out at about 500 kilometres per second per megaparsec (a megaparsec is about 3.26 million light-years). This was far too high, because Hubble’s distances were underestimated, partly because two different kinds of Cepheids had been confused. Later corrections brought the value down to the range of roughly 67 to 73 km/s/Mpc measured today. In 1931 Hubble and Humason extended the relation to much more distant galaxies, confirming it convincingly.

Modern cosmology interprets these redshifts not as galaxies flying through space like shrapnel but as space itself stretching, which lengthens the wavelength of light as it travels. The expansion has no centre: observers in any galaxy would see the others receding in the same way. Nearby galaxies can still move towards us where local gravity dominates; the Andromeda galaxy, for example, is approaching the Milky Way at roughly 110 kilometres per second.

Hubble also brought order to the galaxies themselves. In 1926 he introduced a classification of elliptical, spiral and barred spiral galaxies, often drawn as a “tuning fork” diagram, and in 1936 he summarised his work for a wider audience in the book The Realm of the Nebulae.

Lemaître and the theory of expansion

The theoretical framework already existed. Albert Einstein’s general theory of relativity, completed in 1915, described gravity as the curvature of space-time. In 1917 Einstein added a “cosmological constant” to make his equations describe a static universe. In 1922 the Russian mathematician Alexander Friedmann showed that the equations also allowed universes that expand or contract.

In 1927 Georges Lemaître, a Belgian Catholic priest and physicist, independently derived an expanding-universe solution and, using Slipher’s velocities and Hubble’s earlier distances, estimated an expansion rate of around 625 km/s/Mpc. His paper appeared in a little-read Belgian journal. When it was translated into English in 1931, the passage containing the estimate was left out; research published in 2011 showed that Lemaître made the translation himself and chose to omit it, since Hubble’s better data were by then available.

Arthur Eddington championed Lemaître’s work after 1929, and the combination of Hubble’s observations with relativistic theory persuaded most astronomers that the universe was expanding. Einstein visited Mount Wilson in 1931 and afterwards abandoned his cosmological constant.

Key people

• Edwin Hubble (1889–1953): the astronomer whose observations established the velocity-distance relation.
• Georges Lemaître (1894–1966): physicist who predicted the relation and proposed the “primeval atom” origin of the universe.
• Vesto Slipher (1875–1969): measured the first galaxy redshifts at Lowell Observatory.
• Milton Humason (1891–1972): Hubble’s observing partner, who measured many redshifts.
• Henrietta Swan Leavitt (1868–1921): discovered the Cepheid period-luminosity relation that made distance measurements possible.

Consequences: towards the Big Bang

An expanding universe implied that galaxies were once closer together. In 1931 Lemaître proposed that everything began in a single dense “primeval atom”. In the 1940s George Gamow and colleagues developed the idea of a hot, dense early universe, while Fred Hoyle and others proposed a rival steady-state theory, in which new matter is created as the universe expands. Hoyle coined the term “Big Bang” in a BBC radio talk in 1949.

The debate was largely settled by the discovery of the cosmic microwave background in 1964, the faint afterglow of the early universe predicted by Big Bang theory. In 1998 two teams found that the expansion is actually accelerating, a discovery that earned the 2011 Nobel Prize in Physics.

Legacy

Hubble’s work extended a long tradition of observational astronomy, from Galileo’s first telescopic observations to the measurement of nebulae with giant reflectors. He became one of the best-known scientists of his time, although astronomers were not then eligible for the Nobel Prize in Physics. The orbiting Hubble Space Telescope, launched in 1990, was named in his honour and helped measure the Hubble constant far more precisely.

In 2018 members of the International Astronomical Union voted to recommend renaming Hubble’s law the Hubble–Lemaître law, recognising Lemaître’s earlier contribution.

Timeline

• 1912: Leavitt publishes the Cepheid period-luminosity relation; Slipher measures his first galaxy velocity.
• 1917: Einstein adds the cosmological constant for a static universe.
• 1920, April: The Shapley–Curtis “Great Debate” on the nature of spiral nebulae.
• 1922: Friedmann publishes expanding-universe solutions.
• 1923, October: Hubble identifies a Cepheid in the Andromeda nebula.
• 1927: Lemaître predicts the velocity-distance relation.
• 1929, March: Hubble publishes the velocity-distance relation.
• 1931: Hubble and Humason extend the relation; Lemaître proposes the primeval atom.
• 2018: The IAU recommends the name Hubble–Lemaître law.

Why it matters

Hubble’s 1929 relation showed that the universe is not static but evolving, giving cosmology its central fact and making it possible to ask how old the universe is and how it began. Every modern estimate of the age of the universe, around 13.8 billion years, traces back to the expansion first measured at Mount Wilson.