In the early 1920s, American astronomer Edwin Hubble provided proof of the true size of the universe. Working at the Mount Wilson Observatory near Pasadena, California, Hubble used the newly constructed 100-in (2.5-m) Hooker Telescope, at that time the largest in the world, to settle the greatest argument then raging in astronomy. His observations would lead to the startling revelation that the universe is not only far, far larger than previously thought, but is also expanding.
Settling the Great Debate
At the time, the question of whether spiral nebulae were galaxies beyond the Milky Way or a special kind of nebula was the subject of a “Great Debate.” In 1920, a meeting was held at the Smithsonian Museum in Washington, D.C., in an attempt to settle the question. Speaking for the “small universe” was Princeton astronomer Harlow Shapley, who contended that the Milky Way comprised the entire universe. Shapley cited as evidence reports that the spiral nebulae were rotating, reasoning that this must make them relatively small because otherwise the outer regions would be spinning at speeds faster than the speed of light (these reports were later shown to be wrong). Opposed to Shapley was Heber D. Curtis, who supported the idea that each nebula was far beyond the Milky Way. Curtis cited as evidence the discovery by Vesto Slipher that light from most “spiral nebula” galaxies was shifted to the red part of the electromagnetic spectrum, indicating that they were moving away from Earth at enormous speeds—speeds far too high for them to be contained within the Milky Way.
Hubble set out to see whether there was a relationship between the distances of spiral nebulae and their velocity. His strategy was to search for Cepheid variable stars—stars whose luminosity changes predictably—within nebulae and to measure their distances from Earth. This provided Hubble with his first big discovery in the winter of 1923.
Beginning with photographic plates of the closest and clearest nebulae, Hubble spotted a Cepheid variable on one of the first plates he reviewed. The distances he calculated for even relatively nearby nebulae were so vast that it effectively killed the Great Debate immediately: NGC 6822 was 700,000 light-years away, while M33 and M31 were 850,000 light-years away. It was instantly clear that the universe extended beyond the Milky Way. Just as Curtis had maintained, the spiral nebulae were “island universes,” or “extragalactic nebulae” as Hubble termed them. Over time, the term “spiral nebulae” fell into disuse and they are now simply called galaxies.
“Observations indicate that the universe is expanding at an ever-increasing rate. It will expand forever, getting emptier and darker.” Stephen Hawking
In the realm of the nebulae
Hubble pressed ahead with his program of measuring the distances to galaxies beyond the Milky Way. Farther out, however, it became impossible to pick out individual Cepheid variables in such faint and fuzzy distant galaxies. He was compelled to fall back on indirect methods, such as the so called “standard ruler” assumption: reasoning that all galaxies of a similar type are the same size allowed him to estimate the distance to a galaxy by measuring its apparent size and comparing it to the expected “true” size. Thanks to Slipher’s measurements, Hubble already knew that light from most spiral nebulae was redshifted. In addition, the fainter spirals had higher values of redshift, showing that they were moving faster through space. Hubble realized that, if there were indeed a relationship between a galaxy’s distance from Earth and its recession velocity, these redshifts would serve as a cosmic yardstick, enabling the distances of the very farthest and faintest galaxies to be calculated, and a ballpark figure to be put on the size of the universe as a whole. Meanwhile, Milton Humason, the assistant astronomer at Mount Wilson, checked Slipher’s redshifts and collected new spectra from distant galaxies. It was hard, punishing work, and he and Hubble spent many bitterly cold nights in the observer’s cage at the top of the tube telescope on Wilson Mountain in California.
Hubble’s landmark paper, “A relationship between distance and radial velocity among extra-galactic nebulae,” was published in a journal called Proceedings of the National Academy of Science in 1929. It contained a straight-line graph that plotted 46 galaxies from near to far against their redshifts. Although there was a considerable scatter, Hubble managed to fit a straight line through the majority. The graph shows that, with the exception of the nearest galaxies, Andromeda and Triangulum, which are encroaching on the Milky Way, all other galaxies are receding. What is more, the farther away they are, the faster their movement.
Toward an interpretation
If, from Earth’s perspective within the universe, all galaxies are seen flying away, then the potential explanations are that (a) Earth lies at the center of the universe; or (b) the universe itself originated from a single point and is expanding as a whole.
Objectivity—a kind of foundational law in science—requires that there is no reason to assume that Earth occupies a unique position. Instead, the light from distant nebulae showed that the universe was not static. Many astronomers quickly reached the conclusion that this was due to the expansion of the universe, although Hubble never stated this explicitly. In reality, Vesto Slipher had indicated the trend in 1919, four years before Hubble made his observations, and Georges Lemaître had proposed the expansion of the universe from a “primeval atom” in 1927. However, Hubble’s result provided a simple link between his redshift-measured velocities and distance, and with it, the convincing proof that the scientific community needed. “Hubble’s law,” stating that the redshift of galaxies is proportional to their distance from Earth, was accepted almost unanimously.
“Equipped with his five senses, man explores the universe around him and calls the adventure Science.” Edwin Hubble
The revelation that the universe might be expanding made news all over the world—not least for the fact that it directly contradicted a theory of Albert Einstein’s. Einstein saw that gravity could eventually cause the universe to collapse under its own weight, so he used a value he called the cosmological constant—a kind of negative pressure—to prevent this from occurring in the field equations of general relativity. He abandoned the idea in the wake of Hubble’s discovery.
Einstein and others assumed that the observed velocities were Doppler effects caused by the galaxies’ speed of recession, but there were some dissenting voices. Swiss astronomer Fritz Zwicky suggested that the redshift might be due to “tired light” reaching Earth—caused by the interaction of photons with the intervening matter. Hubble himself found it hard to believe that the velocities indicated by the redshifts were actually real, and was happy to use them solely as distance indicators. In fact, the velocities of galaxies observed by Hubble are due to the expansion of spacetime itself.
Hubble showed how fast spacetime is expanding by plotting a straight-line graph—which he grandly called the “K-factor.” The gradient is described mathematically by a value now known as the Hubble Constant (H0). This important number determines not only the size of the observable universe but also its age. The Hubble Constant allowed astronomers to work backward and calculate the moment in time of the Big Bang itself, when the radius of the universe was zero.
The initial calculation of H0 was 300 miles (500 km) per second per megaparsec (one megaparsec is approximately 3.26 million light-years). This presented a problem, since it gave a figure of 2 billion years for the age of the universe, less than half the accepted age of Earth. The discrepancy was found to have been caused by systematic errors in Hubble’s distance measurements. Many were out by a factor of seven due to his method of taking the brightest star in any galaxy—or even the luminosity of the galaxy itself—and assuming it to be a Cepheid variable star. Luckily for Hubble, the inaccuracies were fairly consistent throughout the dataset, allowing him to plot the trend in spite of them.
Hubble Key Project
Calculating the rate of expansion of the cosmos drove the decision to develop the Hubble Space Telescope from its inception in the 1970s to its 1990 launch. NASA made one of the telescope’s “Key Projects” determining the Hubble Constant to within 10 percent. As a result, the instrument spent years measuring Cepheid light curves. The final result, delivered in 2001, gave an age for the universe of 13.7 billion years. This figure was fine-tuned to 13.799 billion years (with an error of 21 million years either way) by data from the Planck Space Observatory in 2015. The most dramatic revision to Hubble’s law, however, came in 1998 when astronomers discovered that the universe’s expansion is accelerating due to a mysterious and unknown agent known as dark energy, which has led to a renewal of interest in Einstein’s socalled blunder, the cosmological constant.
Born in 1889 in Missouri, Edwin Hubble was a gifted athlete as a youth, leading the University of Chicago’s basketball team. After graduating with a science degree, he studied law at Oxford University. Returning from England dressed in a cape and behaving like an aristocrat, he was described by Harlow Shapley as “absurdly vain and pompous.”
Despite his flair for self-publicity, Hubble was a cautious scientist. He described himself as an observer, and reserved judgment until he had sufficient evidence. He reacted furiously if anyone encroached on his field of research. It is to Hubble’s discredit, therefore, that he did not acknowledge that 41 of the 46 redshifts used to formulate his famous law were measured not by him but by Vesto Slipher. Hubble spent his last years campaigning for a Nobel Prize to be awarded in astronomy. He died in 1953.