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Reaching Into Space: Edwin Hubble’s Continuum of Discovery

Illustrated by Stéphanie Leprohon

In most fields of study, experts do not simply exclaim eureka! and cause a paradigm shift overnight. Scientists, for instance, as Isaac Newton once said only see “by standing upon the shoulders of giants” (Letter to Robert Hooke). We can only build upon what has been done as nothing; not even energy or matter is ever just created. It is from this standpoint that the work of Edwin Hubble should be understood. Often regarded as one of the 20th century’s most outstanding astronomers, Hubble’s work is significant because of his synthesis of various theories regarding celestial observations that were historically available to him. Had Hubble worked at the Mount Wilson Observatory even 20 years earlier––in particular, before the work of Harvard Observatory astronomer Henrietta Leavitt and that of other astronomers––he would have had little to work with. That said, through his evaluation of nebular distances, his focus on strengthening the link between light and distance, as well his use of his astronomical and mathematical background to enhance cosmology, Hubble did effectively change the way we understand the universe by proving its expansion from a point of origin and where we stand in it.

Born Edwin Powell Hubble on November 20, 1889 in Marshfield, Missouri, Hubble grew up a golden boy in turn-of-the-century America. He excelled at most academic and athletic activities, often winning track and field competitions and even nearly qualifying for heavyweight boxing. As the events of his life show, he was simply a good thinker. After attending the University of Chicago and Oxford to study law, Hubble returned to America not wanting to practice law (Goldsmith 75), and decided to pursue a career in astronomy. Fellow astronomer and Hubble’s friend, Nicholas Mayall, acknowledged early on how Hubble “had an innate feeling for what was significant and what was merely secondary” (76) as would be demonstrated in the nature of his work at Mount Wilson Observatory near Los Angeles, California  in 1919 where G.E. Hale had set up state-of-the-art telescopes. Historically, this time period falls right at the dawn of our understanding of modern physics, after Einstein’s publication of the Theory of Special Relativity in 1905 and the Theory of General Relativity in 1915. This colossal scientific insight led many astrophysicists and astronomers to understand space not just as space, but as spacetime (Gott 162), and thus the physical observations of distant objects were not necessarily as they seemed, but part of the curved nature of spacetime (95). This allowed analysis of where and when objects are and greatly influenced the history of physics. It explained Hubble’s eventual conclusions that the universe is expanding and suggested the birth of spacetime at the Big Bang.

When Hubble entered the astronomical work force, the genuine belief was that the Milky Way was the entire static universe measuring a rough 30 000 light-years (LaViolette 254). Still, the proof of a larger and growing galaxy (and not universe) existed before 1919, an aspect Hubble did not neglect.

Once at Mount Wilson, Hubble began working with the institute’s new toy: the 100 inch Hooker Telescope installed by G.E. Hale. It was the world’s largest at the time (Kerrod 162). Hubble later credited the device, claiming, “[t]he break through was an achievement of great [technology]” (Hubble 4). With such technology, it became possible to investigate what pivotal Harvard Observatory astronomer Henrietta Leavitt (1868-1921) had explored nearly twenty years earlier as noted in her 1908 article “1777 Variables in the Magellanic Clouds”. At that point, Leavitt and the rest of the astronomical community had argued these two ‘clouds’ were nebulae; after Hubble, they were established as neighbouring dwarf galaxies.

Before Hubble had even achieved his PhD, Leavitt had been far ahead in astronomical breakthroughs that determined the crucial role of Cepheid variable stars, or “stars that pulsate in brightness with a well-defined cyclic period” (LaViolette 254). Her job at the Harvard Observatory meant she examined tens of hundreds of photographic plates of stars and nebulae within the Large and Small Magellanic Clouds. In what was considered a great leap for astronomy, as astronomy historian Donald Goldsmith states, “Leavitt […] found that the longer the period, the greater the luminosity […] [T]his was as if Leavitt had discovered that a person’s wealth can be determined by how often the person takes a vacation” (Goldsmith 78). In other words, Leavitt found that if two Cepheid variables have the same luminosity, they therefore have equivalent energy output (78). To justify, Leavitt kept a catalogue of nearly 1800 Cepheid variables found in her observations of the two Magellanic Clouds. Importantly, these variables play the role of templates of star distances; that is, their luminosity is so well determined, they can therefore imply their own distance. With Einstein’s new physics, this would prove to be a little trickier, but the principle still prevailed. In other words, any star can be compared to any Cepheid variable and after determining its luminosity, distance can be found.

Goldsmith assures us, “Hubble knew the importance of Leavitt’s work” (78) and arguably it was this correlation between luminosity and distance set him off on a streak of discoveries throughout the twenties. Regarding spiral galaxies, namely the Andromeda galaxy, otherwise known as M31, Hubble was set on finding galaxies’ “structure and contents, to discover their common features, and to devise general methods of estimating distances” (Hubble 8) with emphasis on the former objective. Hubble’s method relied heavily on Leavitt’s work when he compared the known luminosities and distances of Cepheid variables in the Magellanic Clouds to the luminosity of stars a little closer to home. From here he could switch and swap the values for either distance or luminosity to any observable star— mathematic ‘plug n’ play’. Saying the consequences were important is an understatement. The first announced distance Hubble made was that of the Andromeda Galaxy. Using Leavitt’s data on the Cepheid variables she had found in the Magellanic Clouds as his basic framework, Hubble came up with distance values for Andromeda that were “astronomically immense” (Goldsmith 79). His grand calculation came to 900 000 light years, well outside the supposed galactic boundary of the Milky Way. This data meant “the perceived universe suddenly increased thirtyfold” (LaViolette 254) and ultimately led Hubble and others in the scientific community to think perhaps there was much more beyond the Milky Way, meaning possibly that Andromeda and the Megellanic Clouds weren’t nebulae, but rather their own galaxies. Affirming Leavitt’s work, Hubble then quickly built upon his own breakthroughs by applying more of Leavitt’s and his hunches to further investigate the implications of more galaxies.

By 1924, Hubble had established that the Milky Way was indeed much bigger than previously imagined, and presumably not alone. Five years later, with the help of Mount Wilson janitor turned astronomer Milton Humason, Hubble found and calculated the distances of stars up to 25 million light-years away (LaViolette 255). However, once Leavitt (who was dead by 1921) and Hubble had firmly established the relationship between luminosity and distance, Hubble still noticed something relevant about Cepheid variables and nebulous stars in terms of the light and colour they emit, and began to focus on spectrum analysis. This is where another celebrated American astronomer’s work came into play. Starting much earlier in 1908 (when Hubble was only 19), Vesto Slipher (1875-1969) was studying “rational velocities of […] spiral nebulae” (256) at the Lowell Observatory in Arizona. After several years’ work analyzing the spectral blueshifts of Andromeda, he had successfully concluded that this nebula (now recognized as the galaxy that it is) was fast approaching us at a velocity of 300 kilometers per second (257). The question is: how could he have possibly deduced this? The answer lies in the fact that Slipher found Andromeda to be ‘blueshifting’; in the spectrum of light, the waves with the highest frequency are whites and blues, but as the wave length elongates and its peaks become less steep, it reddens in colour. In other words, if the light source is approaching, the frequencies get higher, and therefore the colour shifts to blue or white, but as the object moves farther away, the light waves elongate, meaning the colour shifts to red. This works almost exactly as sound does in the Doppler Effect (Goldsmith 91). As early as 1917, Slipher had analyzed the spectra of another 12 nebulae in which almost all of them were redshifting, therefore moving farther and farther away. Slipher had set the pace by “estimating the speeds at which galaxies were moving by examining the spectrum of their light” (Kerrod 163).

Hubble’s work ran with Slipher’s findings; calculating a star’s redshift over a period of time would evidently tell everything there was to know about its velocity through space. At this point, Hubble could then determine how far each star was judging by its redshift; therefore he could judge its distance and its velocity. Plotting his findings in a velocity/distance graph, Hubble published in 1929 what some consider one of astronomy’s most important graphs (Goldsmith 84). This graph, without Hubble’s intention, shows that there is a direct correlation between the relation of distance (in Parsecs) and velocity (in km/s). In other words, the farther the object, the faster it is travelling away. Hubble then deduced that the idea of an expanding universe was not out of the question seeing as it was “derived from the […] interpretations of red-shifts” (Hubble 1). This served as the evidence for a theory Hubble did not necessarily set out trying to validate. In all, he successfully made the connection between light, distance and velocity thus elaborating a theory that was special on its own.      

In addition to Hubble’s daunting observations and mathematical method, his work remains extremely relevant today because of its deep connection to cosmology. Until the era of the Copernican model, our anthropocentric assumption of Earth’s place in the universe made it seem that everything centers on it. Hubble’s realization of the correlation between distance and velocity made it appear to some as though “all galaxies appear to be receding from us” (Goldsmith 84, emphasis is Goldsmith’s). The time in which Hubble published his discovery was a time much more philosophically weary of our place in the universe. So taking into account our “fairly representative” (Hubble 11) view of the universe, Hubble explained the reasons that led to a “single conclusion” (Goldsmith 84) that the universe was expanding in all directions. This does not mean the nebulae, galaxies and stars are getting farther apart but rather the dark ocean of space itself is getting bigger, therefore pushing bodies away from one another (Charap 29) like chocolate chips in an inflating cooking muffin. Before Hubble, however, it must be acknowledged that Belgian astronomer Georges Lemaître (1894-1966), in 1927 published his calculations on a universal rate of expansion. Hubble, in 1929, demonstrated that this rate of expansion could be approximated to 500 km/s/mega parsec (Huchra “The Hubble Constant”) known as Hubble’s constant. This estimate was slightly more accurate than that of Lemaître’s. Today it is more accurate and significantly slower with a value of 67 km/s/mpc (Huchra “The Hubble Constant”). In any case, the constant bridges distance to velocity and thereby explains how the more distant the object, the more it accelerates.

One of the many important implications that can be drawn from Hubble’s law of expansion, making it one of “the most crucial parameters in the quantitative study of cosmology” (Charap 30), is that using Hubble’s constant, we can calculate the age of our universe and “trace back the expansion to a time when the whole universe was infinitesimally small” (30). Following a Newtonian reasoning, if we were to see a soccer ball rolling down a field, we would logically deduce that it was kicked. Hubble was seeing the universe rolling down this allegorical field and deduced that it too must have been kicked. This leads to the Big Bang Theory, a derivative model from Hubble’s work.   In the twilight years of his life, Hubble approximated the birth of the universe at 7 billion years (LaViolette 264). After his death in 1953, the answer was still contested and by the 1960s, the age “ranged from about 10 to 20 billion years, depending on the particular value adopted for the Hubble constant” (264). Such an estimate is comfortable today, when the age has been ‘settled’ roughly around 14 billion years. These estimates are direct results of Hubble’s cosmological inquiry expressed in a manner that was never done before, though the idea of Big Bang was indeed present in the work of European astronomers such as Lemaître, Carl Wirtz and Alexander Friedmann (256). While these men had all theorized their cosmological conjectures before Hubble’s 1929 publication (259), Hubble was the only one to draw the redshift/velocity relation thus granting him more acclaim and clamour. From his method, it is argued that this is why Hubble is so fondly remembered. He laid down the foundation for what we today often call our creation, but the interpretations never stop here.

Today’s modern physics attempts to go even further than Hubble had imagined. In recent years, applying Einstein’s idea of mass/energy’s ability to warp spacetime, and Hubble’s law of expansion, astrophysicists proposed the Big Crunch Theory in which the universe will gather so much mass that its own gravity will pull all “contents of the entire universe” (38) into a black hole singularity, disappearing into the unknown. Even more recently, with the popular support of Stephen Hawking and Alan Guth, the idea of the multiverse became evermore intriguing. This model is complexly based in String Theory and the grand idea that our still vastly unknown universe is only one of an “infinite number of closed big bang models laid out in time” (Gott 163). The possibilities are quite endless.

Nothing is ever just created, but is elaborated on. Edwin Hubble is remembered for first proving the universe is expanding, but the truth lies in the fact that he was able to do this by synthesis of several factors and through the possibility of affirming this by observation. His study of distances, his establishing of relations between light, distance and velocity, and his overall development of quantitative cosmology enabled him to make one massive conclusion. However, this conclusion is still up for debate as more theories derive and build upon it. The method of scientific building is, like spacetime, a continuum. Through Hubble’s understanding of the expanding universe, our knowledge has expanded as well; as he built upon the work of geniuses, we too build on his tremendous findings fostered in wanting to know more. Today, all information about the depths of space pass through the Hubble name first. The Hubble Space Telescope (launched in 1990) has opened a window to worlds far away in location and long ago in time, stimulating our wonder of the universe. Hubble himself stresses our knowledge is “yet complete; nevertheless, we should not replace a known, familiar principle by an ad hoc explanation unless we are forced to that step by actual observations” (Hubble 22). With fantastic success, Hubble has taken us to that next step and from there, we will go much farther.

Works Cited

Charap, John M. Explaining the Universe: A New Age of Physics. Princeton: Princeton U P, 2002. Print.

Goldsmith, Donald. The Astronomers. New York: St. Martin’s Press, 1991. Print.

Gott, J. Richard. Time Travel in Einstein’s Universe: The Physical Possibilities of Travel Through Time. Boston: Houghton Mifflin Company, 2001. Print.

Hubble, Edwin. The Observational Approach to Cosmology. Oxford: Oxford U. P. 1937, Print.

Huchra, John. “The Hubble Constant.” Harvard-Smithsonian Center for Astrophysics. June 2008. Web. 30 Sept. 2015.

Kerrod, Robin. Hubble: The Mirror on the Universe. Richmond Hill: Firefly Books, 2003. Print.

LaViolette, Paul A. Beyond the Big Bang: Ancient Myth and the Science of Continuous Creation. Rochester: Park Street Press, 1995. Print.

Leavitt, Henrietta S. “1777 Variables in the Magellanic Clouds.” Annals of Harvard College Observatory. 60.4 (1908): 87-108. SAO/NASA Astrophysics Data System (ADS). Web. 5 October 2015.

Newton, Isaac. “Letter to Robert Hooke”. 15 Feb. 1676. MS. London.