by Lee Billings
Of all the astronomers who sought to study 1761’s Venusian transit, none were unluckier than Guillaume Le Gentil of France. Le Gentil left his home in Paris a year before the transit, bound for a French colony in India. After he left, war broke out between France and Britain, and his ship was blown far off course by a storm. When he finally arrived in Indian waters a few days before the transit, he was barred from coming ashore by British troops, who had seized the French colony. Le Gentil was forced to observe the 1761 transit offshore, where heaving seas made precise measurements impossible. He stoically remained in Asia to await the next transit. After eight patient, painstaking years, by 1769 Le Gentil had constructed a small observatory in India to record the event. All was ready and the weather was fair on the eve of the appointed day, June 4, 1769. A thin haze ominously accumulated overnight, then boiled off in the morning sun. Moments before Venus was to begin its passage across the Sun, a thick bank of clouds rolled in. They dissipated only later that afternoon, shortly after the transit’s conclusion. Le Gentil was briefly reduced to a gibbering mass of twitching nerves, but after a time regained his senses and began his long journey home. His homeward voyage was derailed first by dysentery, then by a hurricane that nearly sunk his ship. Arriving empty-handed back in Paris in 1771, eleven and a half years after he had left, Le Gentil found his former life in tatters: he had been declared legally dead and his estate had been dissolved.
Some were more fortunate than Le Gentil in 1769. From a hilltop in Tahiti, Captain James Cook successfully charted the transit of Venus for the Royal Navy and the Royal Society before going on to map and claim islands for the Crown on his voyages throughout the South Pacific. On his farm in Philadelphia, an astronomer named David Rittenhouse documented the transit for the American Philosophical Society, bringing the burgeoning colonial scientific community onto the world stage for the first time. As an astronomer, Rittenhouse was arguably already of somewhat delicate temperament, and was so overcome by the transit’s first moments that he fainted, leaving an otherwise inexplicable lacuna in his official records. Combining these and other measurements from expeditions scattered across the globe, astronomers pegged the Earth-Sun distance, the Astronomical Unit, at 150 million kilometers, or 93 million miles. At last, astronomers had a firm foundation for calibrating the size of the solar system, and with it, the universe. The Copernican Revolution could proceed.
Now knowing that the Earth drew out an approximately 186-million-mile baseline as it moved in its orbit around the Sun, astronomers revisited the ancient parallax measurements of Aristarchus and began measuring the distances to the stars. Over months and years, a handful of nearby stars revealed their proximity by moving against the more distant “fixed” stars, just as a low-flying bird would whiz through your field of view against the more stately motion of a passenger jet flying overhead much farther away in the sky. By the middle of the nineteenth century, astronomers were regularly measuring stellar parallaxes, establishing that most stars in the sky were, at minimum, tens of light-years away. Our own solar system seemed caught in a cycle of perpetual demotion, occupying an ever-shrinking region within a universe that grew with each new improvement in measurement.
In the first decades of the twentieth century, American astronomers built off the basis of stellar parallax to enact the next great Copernican demotions, establishing the field of modern cosmology. First, the spatial distribution of the Milky Way’s star clusters revealed that our solar system was not, as many had believed, at the center of the galaxy, but rather on its outskirts. Then, the American astronomer Edwin Hubble found that our galaxy was but one of many, and discovered that nearly all other galaxies in the sky were racing away from one another at incredible speeds. The universe was literally expanding, following a course that would soon be elucidated in the relativistic theories of Albert Einstein. Once again, at the largest scales that could then be measured, the cosmos was proving far larger and stranger than most anyone had dared to previously suppose, with our existence nowhere near the center.
Meanwhile, far back down the scale, in the realm of stars and their planets, the Copernican Revolution had stalled. Astronomers mapping nearby stars had gradually discovered that our Sun was not a typical star at all—most of its neighboring stars were smaller and dimmer, red and orange dwarfs. Perhaps the solar system was atypical as well, since no solid evidence for exoplanets had been obtained. Many astronomers began to believe our Sun might harbor one of only a very small number of planetary systems in the entire galaxy, though by the middle of the twentieth century mounting indirect evidence suggested planets were probably common around stars.
Still, the Space Age’s chilling revelations about Venus, Mars, and the solar system’s other apparently lifeless planets gave Earth a small fraction of its previous Platonic luster. Then came the exoplanet boom. To many modern planet hunters, finding another biosphere beyond the solar system became a quest for a comforting capstone to place atop the principle of mediocrity, forming the pinnacle of the Copernican Revolution. At last, our planet and all upon it would reach its final demotion—just another average world in a cosmos teeming with life.
And yet, leaving aside the vexing unsolved mysteries of life’s origins and the unknown quantity of Earth-like planets, the frontiers of cosmology have recently unearthed new difficulties with the Copernican Principle’s notions of our mediocrity. The majority of the observable universe looks to be empty space, offering at best one-in-a-million odds that, set down randomly within it, you would find yourself in a galaxy. Given that the universe is gradually expanding, these odds can only get worse as time marches on. Mysterious halos, filaments, and clouds of “dark matter,” seemingly immune to all forces in the universe save for gravity, are what hold galaxies and galactic clusters together. A galaxy’s interior is mostly void, filled with, on average, one proton per cubic centimeter. If a galaxy’s stars were the size of sand grains, the average distance between them would be on the order of a few miles. Only the slimmest fraction of the interstellar material within a galaxy is at any moment condensed into something so sophisticated and advanced as a hydrogen atom. To simply be any piece of ordinary matter—a molecule, a wisp of gas, a rock, a star, a planet, or a person—appears to be an impressive and statistically unlikely accomplishment.
The apparently privileged place of matter within such vast emptiness is compounded by the universe’s ongoing evolution, which seems set on a course toward ever-greater desolation. Surveys of supernovae detonating at the fringes of the observable universe have revealed that the space between galaxies is not only expanding, but also accelerating in its expansion, propelled by a mysterious force cosmologists know only as “dark energy.” Unless somehow the cosmos ceases its accelerating expansion, the universe of the very distant future will be far lonelier and emptier than it is now: other than a handful of galaxies gravitationally interacting with the Milky Way, known as the “Local Group,” all the other galaxies we presently see in our skies will by that late date have been swept beyond the horizons of our visible universe. The Local Group’s galaxies will also eventually become dark some hundred trillion years from now, as all their stars burn out one by one. Next, decillions upon decillions of years in the future, protons—the cornerstones of atomic structure—should all decay in dying bursts of radiation (a “decillion” is one followed by thirty-three zeroes, and a very, very long time indeed). As this process occurs, the last remnants of burnt-out stars and frozen planets will dissolve into oblivion. The universe will become incomprehensibly dark, diffuse, and cold, and in the minuscule sector that used to harbor our Local Group, the only remaining macroscale structures will be a few supermassive black holes, slowly evaporating due to quantum-mechanical effects. When at last the last black holes shrink and vanish in puffs of quantum foam, there will be little left but faint wafts of photons, electrons, and neutrinos streaming endlessly through the infinitely expanding emptiness.
Perhaps it is simply a failure of imagination to see
no hope for life in such a bleak, dismal future. Or, maybe, the predicted evolution of our universe is a portent against Copernican mediocrity, a sign that this bright age of bountiful galaxies, shining stars, and living planets, unfolding only a cosmic moment after the dawn of all things, is in fact rather special.
Just as the universe’s future challenges Copernican expectations, so too does its past. The essential idea behind the Big Bang, the leading scientific explanation for the universe’s past history, is that the cosmos developed from a singular, improbably dense point that somehow explosively expanded about 13.8 billion years ago. Not very Copernican. More problematically, the Big Bang itself is challenged by the universe’s structure. Beyond the granular distinctions of atoms, planets, stars, galaxies, and galactic clusters, on the largest scales astronomers can measure the cosmos appears preternaturally smooth. This large-scale smoothness is in keeping with Copernican predictions, but vexing, since even the slightest difference in expansion rates between separate regions of the early universe should have resulted in substantial deviations in their present structure—lumps, wrinkles, and the like. But regions of space now at opposite sides of the observable universe appear structurally identical, almost flawlessly smooth despite being so distant from each other that they are causally disconnected. Light itself has yet to travel between them, not to mention any information or energy or heat that could bring those far-removed sectors of the universe into equilibrium.
The leading cosmological explanation for this conundrum is an add-on to the Big Bang called “inflation,” which posits that fractions of a second after our universe’s birth, when everything was squeezed into a hot, dense region perhaps the size of a proton, an intense, mysterious blast of repulsive anti-gravity suddenly “inflated” the space to perhaps the size of a large grapefruit. This may sound minor, but it represents a leap in scale of some ten trillion trillion. Any major irregularities would have been erased by this accelerated expansion, like the creases that disappear from the rubber surface of an inflating balloon. According to inflationary models, the minor imperfections that remained came from vastly magnified quantum fluctuations, and formed slight pockets of density in the early universe from which galaxies and galactic clusters condensed.
The problem with inflation is that once it begins, it cannot be easily stopped. Some researchers have even speculated that dark energy may be a bizarre echo or shadow of primordial inflation, somehow returning after billions of years of dormancy. Though primordial inflation may rapidly decay and cease in a local region of space (such as our entire observable universe), because it so greatly boosts the rate of expansion, it should thrust a vastly larger bubble of space out far beyond the horizon of our visible universe. Indeed, a universe expanded far beyond our observable universe’s horizon is a standard outcome of primordial inflation. Deep within that exponentially larger, perhaps infinite volume, more inflationary Big Bangs could then occur again and again even if they were extremely improbable. Each time, yet another branching expansion without end would emerge. Inflation, once started, seems set to proceed eternally, generating an infinite, fractal ensemble of parallel bubble universes, each related to but causally distinct from the others. Most would be destined never to intersect and meet, as ongoing inflation in the spaces between them moved them apart faster than their boundaries expanded, like fleeting bubbles in a white-rushing river. Within different bubbles, the laws of physics that froze out from the fiery chaos of inflationary expansion could be utterly different than those that reign within our own local region of the universe.
In some small fraction, the laws of physics would be identical to or scarcely distinguishable from our own, and those regions would be more likely to generate galaxies, stars, planets, and living creatures. In the remainder of the regions, natural laws would be so alien that life as we know it would be impossible. The theory of an inflationary “multiverse” is consequently often used in modern cosmology to explain otherwise mysterious fundamental properties of our universe that seem fine-tuned to allow life to arise and persist. In some stillborn universes with physical laws that precluded life’s emergence, there would be no stars. In others there would be no atoms. Some would expand or contract so quickly they shuffled out of existence in an instant; others would contain exactly equal portions of matter and antimatter that would mutually annihilate in a blaze of energy, leaving behind nothing but vacuum and seething radiation fields. In the majority of universes we can conceive of, the existence of observers seems inconceivable. There would be no living creatures within them to gaze out at their surroundings and wonder how it all began. In this telling, the universe we see around us is of course fit for life, for otherwise we would not be here.
No one has yet devised a foolproof way to test most of these ideas—how, exactly, can you detect other universes that by definition are forever inaccessible to us? But if true, an inflationary multiverse holds muddled consequences for Copernican ideas. On the one hand, it would mean that our entire observable universe was only the most minuscule fraction of a much larger cosmos inflated from our Big Bang 13.8 billion years ago. This vastly larger cosmos would itself be only a single member of an infinite ensemble of other universes. Infinity being, well, infinite, it would follow that the multiverse would host infinitudes of living beings on a limitless number of other worlds. On the other hand, the infinitude of bubble universes incapable of supporting life would appear to be very much larger than the infinitude that could. Against the principle of mediocrity, an inflationary multiverse suggests our local universe is a small part of an atypical bubble embedded in a much larger region of inflation, a member of a rather exclusive subset of universes that can harbor life. Whether the physical laws we observe are “average” within this subset, no one can say. A planet, a star, or a galaxy may be only as special and valuable as the cosmos that gave birth to it.
In contemplating eternal inflation, modern cosmology has, in effect, returned to some of the tenets first formulated by the Greek atomists some 2,500 years ago; Democritus would certainly laugh that it took so long. In the far future, as our own universe decays into a dark, cold senescence, life’s last holdouts may find some solace in believing that somewhere, far, far away, over the cosmic horizon, the ceaseless process of creation continues, giving birth to new lives, new worlds, and new universes. Hope springs eternal.
After the Gold Rush
From time to time when Laughlin was deep in thought at his office, he would absentmindedly reach across his desk for a small child’s toy he had purchased in the 1990s, back when he was a postdoc at UC Berkeley. The toy looked much like a hangman’s scaffold. Instead of a noose, the scaffold held a thin steel pendulum, loosely suspended above a steel square by a tiny embedded magnet. He would place magnets of various strengths and shapes strategically upon the square and give the pendulum a gentle bump; it would swing to and fro for long periods, kicking between magnetic fields with sufficient force to overcome the frictional loss of momentum from moving through the air. Its motions followed a chaotic random walk, never exactly repeating any given path. Laughlin savored the toy for how its complex behavior could unfold solely from the simple initial conditions of each magnet’s position and the strength and trajectory of an initiatory nudge. It reminded him of his struggles to predict the typical outcomes that emerged from the chaotic gravitational interactions of black holes, stars, and planets, and his efforts to squeeze faint signals from backgrounds of meaningless noise.
One night near the end of June in 2006, after coming home late from work, Laughlin sat down at his kitchen table and realized he had brought his work home with him—an idea was effervescing in his brain. Earlier that day he had been pondering the uncertain orbit of Proxima Centauri, a dim red dwarf tenuously bound by gravity to Alpha Centauri’s binary system of two Sun-like stars, Alpha Centauri A and B. Whether it was only a lone star passing in the galactic night or an estranged family member of the Alpha Centauri system, Laughlin was not sure. What mattered was tha
t the trio comprised the closest known stars to our solar system. As he had thought that day about the stars’ celestial motions, the question of whether they had any accompanying planets intermittently tickled the back of his mind. By that night, the tickle had become an irresistible itch. Laughlin scratched it with notes scrawled on scrap paper and calculations keyed into his laptop.
For decades, consensus had held that binary star systems were poor targets for planet searches, because it was thought that gravitational interactions between the two stars would either prevent planet formation or fling planets, once formed, on escape trajectories out of the system. But ever since the exoplanet boom, increasing numbers of binary-star planets had been discovered—the consensus had been wrong. Due to their close proximity to Earth, Alpha Centauri A and B offered plentiful photons for an RV planet search. Alpha Centauri B, a dusky orange star slightly smaller than our Sun, was particularly quiet and stable—an excellent candidate to scour for potentially habitable planets. Earlier searches had already ruled out the presence of any gas-giant planets within a few AUs of each star, but the presence of smaller worlds was still possible. Laughlin thought they might be just within reach.
