Early models of the cosmos placed our species at the center. The Earth was the nexus of all existence, and we, the purpose for everything. It was a reasonable assessment, based on the evidence we had. From the perspective of a terrestrial observer, the Sun, Moon, planets, and stars appear to trace paths around us. Upon conducting the simplest inspection of the sky, anyone could see that we are special. We could rejoice: oh, how significant we must be! The entire universe is made for us, and the Earth acts as a pedestal, exhibiting our species at the center. This gratifying discovery was integrated into the language and disseminated by literature and education. It was rarely contested and remained predominant for some 1500 years.
In the mid-16th century, a different hypothesis emerged to explain the motions of the heavens1—that which featured the Sun, rather than the Earth, at the center of the universe. It was widely ignored, disregarded as an outlandish proposal. However, with the advent of the astronomical telescope, evidence began mounting to bolster the notion, challenging the established system. The tenets of geocentrism gradually eroded. Support dwindled as heliocentrism was further substantiated by improved instrumentation. By the nineteenth century, the Earth-centered conception of the universe was dispelled.
We were decommissioned from the leading role at cosmic center stage. The Sun superseded the Earth to hold the foremost position at the center of the universe. Nevertheless, this revision could still be conveyed to preserve the human-centered worldview. It was said that the distance from the Earth to the Sun was trivial when juxtaposed against the immense distances to the stars. By this reasoning, we could claim our displacement to be slight; we are nearly at the center of the universe. Heartened by a sense of significance, we found the cosmos, still, a haven for human pride.
The cosmic model of even just a century ago is hardly recognizable when compared to our modern equivalent. At that time, the Milky Way galaxy was not regarded as merely an occupant of the universe, but rather, the universe itself. The entire cosmos was a sole congregation of stars, surrounded by darkness and void. Some astronomers, though, disputed the prevailing notion as spiral clouds, called nebulae, were more frequently witnessed in the depths of space. They thought the strange mists to be galaxies like the Milky Way—“island universes.” Once again, humanity’s privileged position had come under threat; perhaps we are not central figures in a single-galaxy cosmos, but the inhabitants of a starry isle that is one among a multitude of others. The astronomical community at large, however, considered this absurd. It was unimaginable. Spiral nebulae could not be independent stellar assemblies; they are most certainly local—just solar systems in formation—nested within the Milky Way. Due to a lack of compelling evidence in support, this newfangled theory was dismissed. We could then return with contentment to what was familiar, what was comfortable. We could sleep soundly in the heart of the cosmos.
After a period of dormancy, the controversy surrounding the spiral nebulae resurged in the early 20th century. Heber Curtis was at the forefront of this ideological resurrection. An astronomer at the Lick Observatory in California, he was delegated the task of inquiring into the enigmatic clouds. Although at first he objected to the notion of exterior galaxies, Curtis was convinced otherwise as he accumulated new evidence. During his early research, Curtis imaged nebulae which had been previously examined and noticed an absence of rotational motion (a necessary condition to account for their spiral appearance). If the nebulae were to dwell within the Milky Way, such a rotary movement would be perceptible; if they were extremely large and distant, however, the movement would be too small to discern. Curtis dedicated much of his time during the 1910s to persuading the astronomical community. He lectured frequently, all the while developing and substantiating his arguments. Curtis became synonymous with the island-universe theory—the spokesman, advocating ours is a moderately-sized galaxy, 30,000 light-years wide, in which we are nestled just shy of the center; outside its periphery, there exist other assemblages of stars—a great cosmic citizenry of which the Milky Way is a part.
Curtis’s contentions were not widely accepted. He and his colleagues composed a faction of island-universe proponents, while the majority remained steadfast in opposition. One such contesting astronomer was Harlow Shapley; once a subscriber to the island-universe theory, he rejected the concept, drawing from his own observations. Shapley had measured the distance to all of the known globular clusters—dense collections of stars orbiting the galactic center. The Milky Way, which was thought to span 10,000-30,000 light-years, he deduced was much larger—300,000 light-years across. Shapley then reasoned that such exceptional dimensions of this galaxy preclude the existence of others equal in size. The Milky Way is of such vastness, it must be the primary component of the universe. The spiral nebulae are therefore nearby, settled within the galactic edge. Although the single-unit cosmos persisted, the conventional model did not remain entirely intact. By Shapley’s calculations, our solar system is not ensconced in the cosmic hub, as it had always been; rather, it resides 65,000 light-years distant - an astounding and unforeseen displacement from the center of the universe.
In 1920, these two astronomers, resolute in their respective theories, championing different universes, would come to clash. Curtis and Shapley engaged on the topic of the spiral nebulae at a meeting of the National Academy of Sciences. This historic confrontation was dubbed the “Great Debate.” However, it was really nothing of the sort. The interaction consisted of two successive lectures, given under the title “The Scale of the Universe.” It was just one feature during a three-day event and was hardly publicized. The greatness of the debate, however, evolved so dramatically over time that the encounter has since been reimagined as a fantastic brawl between two giants of astronomy.
Shapley was first at the podium. Rather than directly addressing the spiral nebulae, he relied on implication. If he convinced the audience that the Milky Way is vaster than previously thought, then the nebulae, accordingly, would be considered local members. He, additionally, underscored his discovery that the Sun does not hold a central position in the Milky Way, noting that we had been duped by the appearance of the sky.
Curtis began his lecture with a strike, fervently dismantling Shapley’s argument for the tremendous resizing of the Milky Way. Thereafter, Curtis directed his attention to the spiral nebulae, systematically presenting the array of evidence which he had amassed on the subject.
Ultimately, there was no clear-cut victor. Essentially, nothing changed. Everyone departed with mindsets no different than they had prior. The spiral nebulae remained shrouded in mystery.
Edwin Hubble, a talented and keen astronomer, stepped in soon after, determined to reveal the secret of the spiral nebulae and finally settle the long-fought battle. Hubble studied the Andromeda nebula at the Mount Wilson Observatory with a 100-inch telescope, the largest in the world at the time. He was on the search for novae—stars which rapidly and temporarily increase in luminosity. Novae had been spotted in Andromeda before and used to calculate its distance, but the limited knowledge of the stellar processes caused wildly varying results. Hubble thought that further sightings might advance the investigation of the nebulae and, perhaps, uncover their true nature.
Hubble imaged Andromeda and identified three objects which he suspected to be novae. On the resulting photographic plate, he denoted them: “N.” However, upon consulting earlier photos of the nebula, he noticed that one of the three so-called novae acted atypically. The star was not absent from the archived plates—as would be the case for the transient flare-up of a nova—but rather, the star brightened and dimmed regularly, from one plate to the next. It was, in fact, a variable star; Hubble soon recognized the variable to be a Cepheid. Classified as “standard candles”—astronomical objects with known luminosities—Cepheid variables are precise distance markers, reliable tools for cosmic measurements.2 Elated by this discovery, Hubble crossed out the mistaken “N” and wrote beneath: “VAR” (indicating variable), accompanied by an exclamation point.
At last, Hubble was able to perform a definitive calculation of distance. A profound conclusion followed: the Andromeda nebula is not within our galaxy. It is exceedingly beyond—900,000 light-years away3—far surpassing even the most generous estimates of the size of the Milky Way. Andromeda, as well as the other spiral nebulae, are not nebulae; they are galaxies, each one enormous and remote—vast collections of stars, marooned in the great cosmic ocean.
Hubble continued his observations, further reinforcing Andromeda’s extragalactic status. He transcribed his findings in a letter to Shapley. Upon its reading, the famed astronomer lamented: “Here is the letter that has destroyed my universe.”
Concerning the issue of the spiral nebulae, Curtis was correct. Shapley, however, was correct on other matters. Indeed the Milky Way extends much farther than previously considered4 and our Sun is situated at a great distance from the center. Shapley displaced humanity from where it had previously held sway. No longer are we in the core of the Milky Way, but in a quite unexceptional locale, thousands of light-years away.
The late 20th century saw the dawn of space-based studies of the universe. The Hubble Space Telescope was the first major observatory carried into orbit and is now a widely-celebrated champion of modern astronomy. When the new space telescope was named after the eminent astronomer Edwin Hubble, we had not the slightest inkling as to what the exploratory namesake would uncover and how fitting that posthumous honor would prove to be.
In 1995, the astronomer Dr. Robert Williams was serving as director of the Space Telescope Science Institute, the operations center for the Hubble Space Telescope. Accordingly, he was allocated 10% of the telescope’s observing time. Williams formulated a rather daring scheme: he would point the telescope at an empty region of the sky, lacking known celestial phenomena—devoid of anything considered at all interesting or remarkable—in hopes of seeing distant galaxies. However, he encountered criticism from his colleagues who thought that the telescope would be incapable of detecting the faint galaxies Williams anticipated and feared the fallout should this experiment be nonproductive. Nevertheless, Williams believed that the potential discoveries justified the gamble. He assembled a team and proceeded with his galactic quest.
For more than 100 hours, over ten days, the Hubble Space Telescope observed an unimpressive sliver of sky. The resulting composite image, the Hubble Deep Field, is among the most iconic snapshots of the universe.
R. Williams (STScI), the Hubble Deep Field Team and NASA
Hubble Deep Field
Aside from a few stars in the foreground of the Hubble Deep Field, every radiant smudge, every luminous blur, every point of light is a galaxy, each the aggregate glow of perhaps 100 thousand million stars. It was an unequivocal success, a triumph of astronomical prowess and bold leadership. The Hubble Space Telescope, peering into the depths of the cosmos, revealed a host of island universes—nearly 3,000 of them—residing in the darkness. Some are 4 billion times fainter than what is visible to the eye; some date back to when the universe was young, more than 10 billion years ago. In a single sector of the sky, one 24-millionth of the whole, once notable only for its emptiness and mediocrity, there exists a multiplicity of galaxies—a mesmerizing kaleidoscopic display in a patch of “nothing.”
Subsequent deep field images were compiled using the Hubble Space Telescope as the onboard technology improved. The Hubble Ultra Deep Field and the Hubble eXtreme Deep Field are deeper views into the universe than their predecessor, capturing vistas of space and time never before seen. Extrapolating from these images to fill the entire sky, we acquaint ourselves with an astonishing and grand cosmos, populated by at least 100 billion galaxies.
Since our most humble days as novice stargazers, the known universe has profoundly expanded. A cosmos once thought to be of modest proportions—consisting of the Sun, the Moon, a few planets, and a smattering of stars—we have discovered to be dimensions of unfathomable immensity and brimming with galaxies. While these cosmic revelations are stirring and magnificent, in their wake, we are confronted with different conditions—a newfound universe with which to reconcile ourselves.
Much has changed since the days when feeling important was just a matter of looking up. The breadth of the universe reduced us. As we came to understand the vastness, our notions of significance degraded and fell apart. We then found ourselves rummaging through the wreckage of what we thought made us special, searching for a scrap - something, anything which we could salvage. But it became apparent that the universe is not obliged to honor our expectations, nor our dearest wishes.
For now, we are tethered to this planet. The myriad galaxies, stars, and worlds are showcased beyond reach; human embrace lingers far-off in time. But the Earth is easily accessible. If we cannot generate some enormous and powerful impact in the universe, at least we can do so on our home planet. We can change the world. The idea seems inherently optimistic, but the prospect – admittedly - unlikely. We question how we might be the agents of sweeping phenomena. Even on this immediate planetary stage, we appear quite small players. Yet, the consequences of our daily activities indicate otherwise. Upheavals in the global environment now threaten our livelihood and put our long-term survival in doubt. We are changing the world. All of us. Every day. But we are changing it into something we never wanted it to be.
We are not at the center of the universe, but we are at the center of our own lives. We are responsible for our actions and interactions, for our progressions and retrogressions, and for what becomes of our world and ourselves. Our impacts may be local in space, but not in time. They propagate onward, through the coming generations and into the distant future. We affect people we will never know, who will live during times we will never experience, with our attentiveness to the problems we face today.
We humans are ineffably complex—intricate, nuanced, beautiful, and unpredictable. We have lives composed of triumphs and failures, discoveries and disasters, sacrifices and regret and sadness and loss and love. We are capable of looking across billions of light-years of space and back billions of years in time to investigate the deepest mysteries of the cosmos, and yet, we can live out entire lives beside other people and never truly know who they are—never know the wars they are secretly fighting within them. As it turns out, we are island universes too.
Somewhere in the cosmos, hidden within one of the more than 100 billion galaxies, thousands of light-years from the center, revolving an average star among hundreds of billions of others, there is a familiar world. That globe, tiny and obscure, is our refuge from the desolation, our shelter from the darkness, our home in the universe. On this world, experience of the human condition is commonplace, and the comfort we had sought from the universe, we can discover in each other. In the multiverse of humanity that exists here on the planet Earth, we can find solace that we are all, for the first time, puzzling out how to live, and trying to make sense of this vast and extraordinary universe in which we came to be.
1. The emergence of the heliocentric theory was, more precisely, a reemergence. Sun-centered models of the universe had been suggested before - most famously by the ancient Greek astronomer Aristarchus of Samos - but were rejected by the astronomical community, adhering to the geocentric theory.
2. About a decade prior, the astronomer Henrietta Leavitt discovered a link between the period of a Cepheid variable and its brightness; the period refers to the time elapsed while the star undergoes a single cycle in luminosity. Employing this period-luminosity relationship, astronomers are capable of deducing the true brightness of a Cepheid. Upon comparing the star’s true brightness and its observed brightness, the distance to that star can be calculated.
3. Improved measurements have yielded a distance of about 2.5 million light-years to Andromeda.
4. The Milky Way has since been downsized to 100,000 light-years across.