Most people associate the discovery that faraway galaxies are receding from us — and thus, that the universe is expanding — with Edwin Hubble, thanks to his landmark 1929 paper. It was one of the most fundamental discoveries in the history of science.

But Hubble did not discover the expansion. In the 1910s, a Lowell Observatory astronomer named Vesto Slipher found that spiral nebulae, as galaxies were called, are “fleeing” from us at what were then unheard-of speeds. This was the first observational evidence of the expanding universe. Hubble’s paper established a linear relationship between Slipher’s nebulae velocities and the distances Hubble had measured — a relation that later became known as Hubble’s law — but failed to cite Slipher’s own publications containing his indispensable measurements or even to mention his name. Yet Slipher had done half the work!

It was only after Hubble had secured worldwide recognition and lasting fame that he acknowledged his use of Slipher’s data. But by then the damage had been done: Slipher had been eclipsed, and has never been properly commemorated for his critical contribution.

Today, astronomers continue in the footsteps of both Hubble and Slipher, measuring the velocities of receding galaxies and their distances. These crucial observations tell us how quickly the universe is expanding. Yet, the famous mathematical parameter that defines the relationship between Slipher’s velocities and Hubble’s distances bears a single name: the Hubble constant.

Modern science has made many strides in setting the historical record straight. In that tradition, we suggest the International Astronomical Union (IAU) formally rename the Hubble constant to the Hubble-Slipher constant, in long overdue recognition of Slipher’s historic achievement.

The challenge of the spirals

The son of an Indiana farmer, Vesto Melvin Slipher joined Lowell Observatory in Flagstaff, Arizona, in 1901, under the leadership of the flamboyant Percival Lowell. Slipher’s charge was spectroscopy, and he made good use of Lowell’s 24-inch Alvan Clark refracting telescope with its brand-new Brashear spectrograph. Within a few years he began reaping important results, such as confirming the rotation periods of Mars, Jupiter, and Saturn.

Slipher became a virtuoso in the new art of acquiring and interpreting spectra — the data obtained when light from astronomical objects is spread out into its constituent wavelengths, revealing details about composition and motion. And there were spectroscopic surprises everywhere he looked. Among his subsequent discoveries were methane and ammonia in the giant planets’ atmospheres; interstellar gas throughout the Milky Way; and the first known reflection nebula, in the Pleiades, which led to the discovery of interstellar dust.

But his next challenge tested his skills more profoundly. On Lowell’s instructions, he started to photograph spectra of the faintest of all objects: spiral nebulae.

These nebulae were the subject of an ongoing debate among astronomers in the early 20th century. Were they baby solar systems forming within our own Milky Way? Or were they instead remote “island universes,” each like the entire Milky Way, floating loose in the cosmic void?
Spectroscopes, attached to large light-gathering telescopes, held the best promise of providing an answer. Importantly, they could determine an object’s radial velocity — its motion toward or away from the observer — through subtle shifts in the object’s spectral features compared to a stationary reference. Displacement toward the red end of the spectrum (a redshift) means the object is receding from us; a blueshift signifies that it is approaching.

Slipher knew that long exposures would be required to produce detailed, high-quality spectra of the vexingly faint spirals. Switching out the slow spectrograph camera lens for a faster one enabled him to cut exposure times more than 30-fold. Even so, obtaining a single spectrum often required dozens of hours.

The obvious target for a first effort was the biggest and brightest spiral nebula of all, majestic M31 in the constellation Andromeda. An exposure over several nights in December 1912 produced a stunning surprise: M31’s spectral lines were shifted toward the blue end of the spectrum. The Andromeda Nebula was approaching our solar system at some 186 miles (300 kilometers) per second, the highest celestial velocity ever measured at the time.

An expanding universe

Encouraged by Lowell to continue the effort, Slipher turned his instrument in April 1913 to the Sombrero Nebula (M104), a dramatic edge-on spiral in the constellation Virgo. This time the lines were shifted immensely toward the red end of the spectrum, suggesting an extraordinary recession speed of 684 miles (1,100 km) per second.

Slipher continued his survey and in August 1914 traveled to Northwestern University in Evanston, Illinois, to present his findings at the annual meeting of the American Astronomical Society. By then, he had gauged the velocities of 15 spiral nebulae: Three were approaching Earth, while the rest were zooming away. The historic manuscript from which Slipher read is not well known and has never been published. It now resides in the Lowell Observatory Archives.

The conclusions he pitched to his audience were powerfully worded: “The striking preponderance of the positive sign [meaning recession] indicates a general fleeing from us or the Milky Way.” Moreover, most of the spiral nebulae were receding. Allowing that the data were not yet definitive, he nonetheless felt that “they do strongly indicate that the spirals are leaving the Milky Way, which fits in with their non-galactic distribution.” In other words, Slipher’s findings gave strong evidence for the controversial island universe hypothesis.

It was an astonishing finding. Although no one appreciated it at the time — including Slipher — he in effect had staked his claim for the observational discovery of the expansion of the universe. Over time, this realization would fundamentally transform our idea of the cosmos and our place in it.

When Slipher finished delivering his news of the great recession, his fellow astronomers rose to their feet and gave him a resounding ovation — an unprecedented spectacle at an astronomical meeting. Interestingly, Hubble was in the audience, attending the meeting just before he began his Ph.D. that fall at the University of Chicago’s Yerkes Observatory in Williams Bay, Wisconsin. It’s possible that witnessing this momentous reaction inspired Hubble to choose spiral nebulae as his dissertation topic.

Over the next three years, Slipher measured more velocities. By 1917 he had a total of 25, all but four of which were receding. Buoyed by this trend, he conservatively opined in a 1917 paper in the Proceedings of the American Philosophical Society that the island universe theory, “seems to me, gains favor in the present observations.” By 1922, he had topped off his survey with a further 17 spiral nebulae, all of which were receding — the fastest at an unprecedented 1,120 miles (1,800 km) per second.

Hubble steps in

In 1924, Hubble nailed down the proof that spiral nebulae were separate galaxies using Cepheid variable stars. These “standard candles” allow the accurate measurement of distance through an established relationship between their period and intrinsic luminosity. Measuring a star’s period allows an astronomer to deduce its inherent brightness; any dimming can therefore be attributed to distance.

By 1928, he began focusing on the nature of Slipher’s discovered cosmic exodus, searching for any pattern in the redshifts of galaxies as they rushed headlong through space. To do this, he teamed up with Milton Humason, a colleague at Mount Wilson. Hubble would continue to measure the galaxies’ distances (his specialty), while Humason would obtain the velocities.

Within a year, Hubble had prepared his first publication on his findings, that 1929 landmark paper titled “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae.” In it, he paired the distances of 24 galaxies with their velocities. The pattern, captured in a famous graph, jumped off the page: The velocity of galaxies steadily increased in a linear fashion as one looked farther into space. At double the distance from Earth, a galaxy’s speed doubled as well. When the distance triples, the velocity triples, and so on. By the late 1930s, astronomers were coming to refer to the slope of Hubble’s graph — the rate at which this recession increases with distance — as Hubble’s constant, and later simply the Hubble constant, or H0.

But in the lead-up to Hubble’s 1929 paper, his partner Humason was only getting started on his velocity measurements. He was primarily focused on getting redshifts of previously unmeasured targets that had been too faint for Slipher to determine with his smaller telescope. Nearly all the redshifts that Hubble used in calculating the rate of recession were Slipher’s measurements. In other words, half of the data that went into formulating the original Hubble constant came from Slipher. Yet anyone perusing Hubble’s paper would not have known this. Hubble used Slipher’s measurements without direct citation or acknowledgment — a serious breach of scientific protocol.

Hubble later made partial amends: In his next big paper on the redshift law, published in 1931, he inserted a sentence praising “the great pioneer work of V. M. Slipher at the Lowell Observatory.” And two years later, the Royal Astronomical Society (RAS) in England presented Slipher with its highest award, the Gold Medal, seven years before Hubble would earn the honor. During the presentation, RAS President Frederick Stratton announced that “if cosmogonists today have to deal with a universe that is expanding in fact as well as in fancy, at a rate which offers them special difficulties, a great part of the initial blame must be borne by our medalist.”

Although Hubble again praised Slipher’s work in his 1936 book The Realm of the Nebulae, ultimately, it was too late. Hubble’s law and the Hubble constant became entrenched among astronomers, while Slipher’s contribution was nearly forgotten. By nature, Slipher was never a showman and preferred publishing in his observatory’s Bulletin rather than well-known journals. He was simply too humble and reserved to demand his share of the glory. Hubble, by contrast — so handsome, so manly, so erudite — was a force of nature, far more accomplished in garnering publicity and protecting his legacy.

Slipher eclipsed

Hubble’s initial failure to cite Slipher was a major slight, but his jealous hegemony over the velocity-distance relationship extended beyond Slipher. In a 1930 letter, Hubble warned the Dutch cosmologist Willem de Sitter, who had innocently commented in a review article that several other astronomers had previously looked at the relationship, that he considered it “a Mount Wilson contribution and I am deeply concerned in its recognition as such.”

Colleagues also long complained that Hubble engaged in “selective referencing,” such as when he failed to mention Belgian cosmologist Georges Lemaître’s work in The Realm of the Nebulae or to specifically cite Harvard University’s Harlow Shapley in the same book for his early look at the velocity-distance relation. And when Hubble in 1941 again failed to cite Slipher — this time over Slipher’s priority in determining that a spiral nebula’s arms trail as it rotates — Slipher was compelled to pen an irritated note to Science in 1944 to correct the record.

In fairness, Slipher shares some blame for the lack of appreciation for his contribution. In 1915 and 1917 publications reporting his accumulating galaxy redshifts, instead of describing the galaxies as “fleeing,” “receding,” or “leaving” the Milky Way, as he earlier did in his 1914 talk, he substituted the more diffuse term “scattering.” Although astronomers understood its meaning (and Hubble subsequently used it), the word was unnecessarily conservative.

A deeper problem was that Slipher’s results appeared in second-tier journals rather than premier outlets like The Astrophysical Journal. Worse, he generously allowed his final list of 42 redshifts to be published in 1923 and 1925, with attribution, but under other authors’ names!
Yet Slipher’s data were unique, accurate, and determinative. His findings were communicated to the scientific community and his work spoke for itself, under whatever auspice it appeared. His lack of visibility is no reason to deny him priority for his discovery.

The Hubble-Lemaître law

Astronomers have made progress in undoing Hubble’s hegemony — though not yet to Slipher’s benefit. In 2018, the IAU, by vote after its 30th General Assembly in Vienna, adopted a resolution to rename the Hubble law to the Hubble-Lemaître law, to honor Lemaître for his 1927 dynamic solution to Einstein’s general relativity equations. His result, published two years before Hubble’s, predicted that the universe is expanding in such a way that galaxy redshifts are proportional to their distances. Lemaître even computed what came to be known as the Hubble constant, based on Hubble’s galaxy magnitudes and Slipher’s redshifts. (Although like Hubble, Lemaître also failed to cite Slipher.

The decision to rename the Hubble law without including Slipher was met with pushback. In January 2019, cosmologist Emilio Elizalde of the University of Barcelona published a meticulous historical review in the journal Symmetry: “Reasons in favor of a Hubble-Lemaître-Slipher law.” And in Astronomy magazine’s February 2020 issue, Lowell Observatory’s Director Jeff Hall and Historian Kevin Schindler also advocated for the addition of Slipher’s name to the law.

Why was Slipher denied yet again? The explanation offered at the IAU meeting was that he and others “did not use their data nor invent new theory to discover the universal expansion.” But as we have shown, Slipher in fact did use his redshift data to conclude, as early as 1914, that the spiral nebulae were “receding,” or “scattering,” from the Milky Way — which amounts to the observational discovery of universal expansion.
The IAU requirement that a law be derived from theory runs counter to the conventional view that a scientific law is simply a relation between observables. Hubble recognized this distinction in his George Darwin lecture to the RAS in 1953: “I propose to discuss the law of red-shifts — the correlation between the distances of the nebulae and the displacements in their spectra.”

But even conceding that Slipher did not “invent new theory” to explain the expansion, neither did Hubble. For all his life, Hubble, unlike Slipher, was wary that redshifts truly represented recession and worried this interpretation might be overturned.

For a Hubble-Slipher constant

For all these reasons, we too support renaming the Hubble-Lemaître law the Hubble-Lemaître-Slipher law. But we acknowledge such a reconsideration would likely be difficult.

So instead, given how the newly titled Hubble-Lemaître law set a precedent in honoring the overlooked, we propose that the Hubble constant, H0 — the proportionality between the recession velocities and distances of the galaxies — be renamed the Hubble-Slipher constant. This proposal is similar to that of Irish physicist Cormac O’Raifeartaigh, who suggested that Hubble’s famous “discovery graph” of 1929, whose slope is simply H0, could also be known as the Hubble-Slipher graph.

In the end, our proposal is best supported by Hubble’s own final accounting of events. The relationship between Slipher and Hubble largely remained collegial and respectful over the years, and in 1953, the year of his death, Hubble finally made full amends. During a RAS lecture, Hubble noted that his discovery “emerged from a combination of radial velocities measured by Slipher at Flagstaff with distances derived at Mount Wilson. … Slipher worked almost alone, and 10 years later … had contributed 42 out of the 46 nebular velocities then available.” In a letter to Slipher that same year, he cited Slipher’s first steps “as by far the most important of all” in “the combination of your velocities and my distances.

What better argument for accepting a Hubble-Slipher constant than the case made by Hubble himself?