Einstein pointed out that his relativistic concepts could just as well be applied to the universe as to a star or a planet. To avoid the same cosmological instability that afflicted Isaac Newton’s formulation of gravity, Einstein amended his field equations, inserting an ad hoc term to keep his model universe in stasis. Although legitimate in the abstract, Einstein’s mathematical counterpoise—his so-called cosmological constant—had no obvious manifestation in the physical world. Yet its inclusion was compelled by astronomers’ general belief that the universe must be static.
Einstein solved the field equations for a cosmos encompassing a uniform distribution of matter at rest. (Whether the material is fluidlike, as Einstein had posited, or bunched in discrete units, such as stars and nebulae, is of no consequence, as long as it is evenly sown.) The resultant curvature of this model is spherical: the space–time continuum closes in on itself, forming a finite universe whose size depends on the amount of matter it contains. The transmission of light among widely separated objects is governed by the spherical geometry: light beams must travel along the surface of the sphere and cannot take the shortcut through the sphere’s interior. Indeed, a light beam might eventually circumnavigate the bounded space and arrive back at its place of origin. The shortest path between two points is no longer a straight line, à la Euclid, but an arc; hence, cosmic distances are construed differently than in a “flat” Euclidean world.
Perhaps the most significant philosophical and scientific ramification is the conceptual shift away from the infinite Newtonian universe and its attendant complexities. Einstein even roughed out the diameter of his cosmological model—one hundred million light-years—as well as its mean density of matter.
In 1917, Dutch astronomer Willem de Sitter offered an alternative cosmological solution to Einstein’s field equations, one that maintained the imperative of a static cosmos. However, the conceptual price for this mandatory stasis was high: de Sitter’s model was pure space, devoid of matter. Equally strange, if a hypothetical star or nebula were plunked into this vacuum, its light would appear redshifted to a distant (and likewise hypothetical) observer. Some astronomers noted the curious parallel of de Sitter’s theorized redshifts and Vesto Slipher’s growing list of redshifted spiral galaxies. Furthermore, de Sitter found that his predicted redshifts grew as the distance from the observer increased, a linkage not yet evident in Slipher’s sparse data set. In response to the obvious criticism that our universe is far from empty—witness the planets, stars, and galaxies—de Sitter pointed to the sheer abundance of space; given a sufficiently large volume, the overall density of matter might be low enough to approximate his mass-starved model.
In 1922, Russian physicist Alexander Friedmann introduced yet other solutions to Einstein’s field equations. While preserving Einstein’s notion of a closed, hyperspherical world, one of Friedmann’s models allowed an evolution of its overall curvature: the radius of the universe changes over time. Starting from a geometric point eons ago—“the time that has passed since Creation,” Friedmann termed it—the universe might expand for a while, then reverse and gradually draw back in on itself. Such a universe might be reborn, cycling through its life sequence without end. In a different scenario, the universal fabric has an underlying hyperboloidal form, like de Sitter’s model, only seeded with matter: a one-shot cosmos destined to expand forever.
Theoretical physicists offered only possible cosmologies; which, if any, of their proposals might apply to the real universe would have to be determined by astronomical observations. But astronomers were slow on the uptake. Though intrigued by the various cosmological implications of general relativity, most were confounded by its mathematical complexity. Mount Wilson’s director, George Ellery Hale, confessed to a colleague that “the complications of the theory of relativity are altogether too much for my comprehension. If I were a good mathematician I might have some hope of forming a feeble conception of the principle, but as it is I fear it will always remain beyond my grasp.” (Many modern-day researchers have the same reaction to cosmic string theory.)
The few astronomers who delved into the esoteric world of general relativity during the 1920s focused their attention on the static-universe models of Einstein and de Sitter. Alexander Friedmann’s published papers from 1922 and 1924 were ignored. So were the nonstatic cosmologies of the Belgian cleric-mathematician George Lemaître, who made an explicit argument for a linear increase of galaxian velocity with distance. The key advantage of the Friedmann–Lemaître cosmologies is that they permit the universe to contain matter (unlike de Sitter’s empty model), while providing a theoretical basis for the observed redshifts of galaxies. Yet their ideas languished, the scholarly current behind the static models too strong to deflect into alternative channels of cosmological thought.
A devout observationalist, Edwin Hubble was agnostic about the various cosmological theories. Whether Einstein’s, de Sitter’s, or some other world model prevailed was of marginal concern to him. All that mattered were their observational ramifications to his research program. That the majority of galaxies exhibit large redshifts had been established to Hubble’s satisfaction. And de Sitter’s model, he knew, predicted a correlation between redshift and distance in an ultra-low-density universe. Indeed, several observers had asserted a linear correspondence, using galactic diameter or faintness as a makeshift substitute for distance. But in Hubble’s mind, there could be no substitute for a calibrated measurement of a galaxy’s distance.
With the one-hundred-inch reflector, he could fix the distances of galaxies, derived from the brightness of their constituent stars. Then, swapping the camera for a spectrograph, he could obtain redshifts of extragalactic systems beyond the reach of Vesto Slipher’s twenty-four-inch refractor. Given the urgency to publish—much attention had converged of late on the issue of cosmological redshifts—Hubble knew there was insufficient time to do both: he decided to couple his own distance estimates with Slipher’s velocities. (Fully forty-one of the forty-six radial velocities of nearby galaxies had been acquired by Slipher.) As his own observations progressed, Hubble needed a proven partner to photograph the spectra of remote galaxies; even a single, additional redshift might prove decisive in swaying the collective opinion of astronomers. To the strategic enterprise, veteran Mount Wilson observer Milton Humason brought a grammar-school education, a fondness for gambling, a desk-drawer flask of “Panther Pacifier”—and a supreme gift for deep-space photography.
Coaxing his pack mule along the treacherous path up Mount Wilson in 1910, Milton Lasalle Humason could hardly have suspected that one day he would plumb the cosmic depths with the world’s largest telescope, much less coauthor a landmark paper in the Astrophysical Journal. Humason had worked in the Pasadena area for many years, and its rugged hillsides felt like home. After stints as a bellboy, handyman, and citrus farmer, Humason returned to the mountain in 1917 as a janitor. The following summer, he learned the rudiments of celestial photography from an undergraduate intern, and became so adept at it that Harlow Shapley recommended him for a staff position. By the time Edwin Hubble came calling in 1928, Humason was Mount Wilson’s foremost imaging expert and self-appointed guardian of the facility, instructing night assistants—and the occasional astronomer—on the proper use and care of the equipment.
Milton Humason, circa 1930.
Hubble detailed his proposed project, spelling out the division of labor: he would focus on galaxies’ distances and Humason on their velocities. Hubble envisioned a cosmological distance ladder extending into extragalactic space, each rung defined by a standard candle whose calibration rests upon that of the prior rung. Thus, Cepheids disclose the distances of the nearest galaxies, which collectively establish the average luminosity of a galaxy’s most radiant stars. These stellar beacons, in turn, become the standard candle to assess the distances of more remote galaxies, whose Cepheids are imperceptible to the camera. The final rung in Hubble’s stepwise scheme is set by a galaxy’s overall glow. At
this extreme range, stars are irresolvable, and merge into an amorphous whorl of light. In relatively short order, Hubble told Humason, he could gauge the distances of a sufficient number of Slipher’s galaxies to justify publication.
Although narrower in scope, Humason’s role was vital to Hubble’s plan. Humason would photograph spectra of several nearby galaxies, and validate his own measured radial velocities against Slipher’s. Assuming agreement between their respective data sets, Humason was to venture at once into the depths beyond the range of the Lowell refractor. He would target the galaxy NGC 7619, in Pegasus, a system whose dimness was indicative of great distance. This solitary data point, although beyond what Hubble termed the “realm of positive knowledge,” might strengthen the case for a linear correspondence between galactic distance and velocity.
Within a year, Hubble had extended his cosmological distance ladder more than six million light-years into space, encompassing twenty-four galaxies. (Subsequent recalibrations have septupled his initial range estimate.) He presented his results to the National Academy of Sciences on January 17, 1929, in a paper titled, “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae.” The measurements of galactic distance were his own, yet fully twenty of the two dozen radial velocities were Slipher’s (whose name appears nowhere in the paper). Compared to prior works, Hubble’s made a compelling case for the predicted linear escalation of a galaxy’s radial velocity with distance, an astronomical precept that would ultimately bear his name. In graphical form, the data points congregate loosely about a rising straight line drawn through their midst. Knowing his audience, Hubble intuited that his diagram called out for more data to establish the reality of the line.
As instructed, Humason had affirmed that his spectroscopic readings were in accord with Slipher’s. With that, he took a multinight, thirty-three-hour time-exposure of the spectrum of the remote galaxy NGC 7619, followed by one of forty-five-hour duration. The result: a recession velocity of twenty-four hundred miles per second, more than twice that of any previously recorded. Paired with Hubble’s own distance estimate of twenty-four million light-years, the data point for NGC 7619 indeed became the graphical anchor that secured the linear linkage between galactic velocity and distance. So critical was this measurement that Hubble featured Humason’s announcement as the lead-in article to his own. Two years later, in 1931, a promised follow-up paper appeared, in which Hubble and Humason add forty new radial velocities to their previous list and extend their survey of galaxies sixteenfold in distance. (Hubble makes amends for his prior snub by citing Vesto Slipher’s “great pioneer work.”)
Hubble’s velocity–distance graph of extragalactic nebulae, presented to the National Academy of Sciences on January 17, 1929. The solid and dashed lines represent alternative straight-line interpolations to the data points.
With the release of the 1931 paper, there was near unanimity among astronomers that the extragalactic velocity-distance relationship was real. Even longtime critic Harlow Shapley joined the ranks of the believers, although he claimed (rightly) that Hubble’s distances were skewed by mistaking compact, gaseous nebulae for a galaxy’s brightest stars. Albert Einstein conferred with Hubble during a visit to Mount Wilson in 1931 and declared his once-ascendant static-universe model null and void.
Ninety years earlier, upon Friedrich Bessel’s presentation of the first credible measurement of a star’s distance, astronomer John Herschel challenged the Royal Astronomical Society to overcome their doubts and accept the result. “Oculis subjecta fidelibus,” Herschel offered, echoing the lines of the Roman poet Horace about the trustworthy eye. “If all this does not carry conviction along with it, it seems difficult to say what ought to do so.” In the latter-day case of systematic cosmic motion, the astronomical community likewise decided that the requisite threshold of observational evidence had been reached. Yet Hubble himself remained noncommittal with regard to the physical interpretation of galactic redshifts. Indeed, he used the term “apparent radial velocity” to indicate that the observed spectral-line shifts might stem from a cause other than a headlong recession of galaxies; the confounding theories of general relativity and quantum mechanics had alerted him to the manifold ways in which nature upends logical expectations.
Hubble’s stark, pen-and-ink diagram, with its canted, empirically derived line, endowed James Keeler’s turn-of-the-century extragalactic spacescapes with profound meaning. The line and its associated mathematical formula, V = HR, later dubbed Hubble’s law, became twin avatars of what astronomers took to be an expanding universe. The universe is dynamic, they asserted, space itself billowing to vaster proportions, sweeping apart its luminous points of reference—the galaxies. In the time machine of one’s imagination, the cosmic clock can be driven backwards, the observed dispersal of galaxies reversed until atoms and photons meld into an infinitesimal, primeval amalgam. Thus, universal expansion compels a beginning: an ab initio, hyperdense fireball from which all cosmic energy and matter emerged—in modern-day parlance, the Big Bang.
From a 1936 article by Milton Humason, photographs and associated spectra for a sample of galaxies, arranged in order of distance. Arrows indicate the increasing redshift of spectral lines with galactic distance.
Edwin Hubble did not discover the expanding universe so much as lend the accumulated weight of his professional authority to the observed velocity–distance relation from which this phenomenological inference followed. Once Einstein and de Sitter signed on—and credited Edwin Hubble as its observational midwife—the expanding universe became the reigning paradigm among cosmological cognoscenti. That Hubble’s work paralleled, and in some cases duplicated, the efforts of others does not diminish the galvanizing effect his publications had upon the course of extragalactic astronomy. In the end, whether Hubble’s law or Slipher’s law or Lemaître’s law or some permutation of these, the iconic formula V = HR represents nature’s declaration of the way things are, heedless of any label affixed by inhabitants of one or another of its surfeit of planets.
The constant H, likewise named for Hubble, expresses the current rate of cosmic expansion and is computed from the array of observed galactic radial velocities and distances. A large value of H implies rapid expansion, thus, a relatively youthful universe, whereas a small value of H indicates a slower growing, Methuselan cosmos. Hubble’s initial estimates of his eponymous constant tended toward the former: for each million-parsec increment in distance—equivalent to 3.26 million light-years—a galaxy’s speed ratcheted up a staggering five hundred kilometers per second (eight hundred miles per second). Had the universe maintained this frenzied rate of growth from the start, it would have come into existence a mere two billion years ago. To scientists’ chagrin, contemporary estimates of Earth’s geological time line considerably exceeded the derived age of the universe: illogically, our own planet appeared to predate the very space it occupies. Subsequent revisions have cut the Hubble constant from its original five hundred to about seventy kilometers per second per million parsecs, raising the estimated age of the universe to some fourteen billion years, in full accord, not only with Earth’s age, but that of the galaxy’s oldest stars.
In recent decades, astronomers have detected the cosmic microwave background, the remnant radiation from an early, high-temperature phase of the universe. They have posed a compelling theory that the infant universe underwent a burst of hyperinflation, doubling its dimensions more than one hundred times over a tiny fraction of a second. Observers have confirmed the gravitational signature of dark matter—distributed material whose overall bulk far exceeds all the visible stars and gas clouds in galaxies. And in a remarkable development, astronomers have learned that the universal expansion is accelerating, as though space were being propelled to ever-larger scales by a mysterious antigravity. This so-called dark energy now stands at the forefront of cosmological research.
In a series of lectures at Yale University in 1935, published the following year as The Rea
lm of the Nebulae, Hubble declared that the “history of astronomy is a history of receding horizons.” On the one hand, these horizons might be spatial: a record-setting number of light-years into the void. However, with the advent of sensitive electronic cameras, advanced telescopes, and computer-based image processing, an optical horizon also beckons: the ultrafaint, resolution-limited threshold of detectability. One need not target only the farthest reaches of the observable universe, when exoplanets await discovery around nearby stars and black holes gorge themselves on matter in our own galaxy. By their nature, Hubble explained to the audience, frontier explorations of space lead to a state of uncertainty: “We are, by definition, in the very center of the observable region. We know our immediate neighborhood rather intimately. With increasing distance, our knowledge fades, and fades rapidly. Eventually,” Hubble concluded, with the assurance of one who has sailed over the horizon and returned, “we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial.”
Albert Einstein and Edwin Hubble at Mount Wilson’s one-hundred-inch telescope on the evening of January 29, 1931.
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