Five Billion Years of Solitude (13 page)

In 1627, while using his new laws to calculate the future motions of Venus, Kepler surmised that the planet would occasionally cross the face of the Sun as viewed from Earth. He calculated that the next
transit of Venus would occur over the course of several hours on December 6, 1631, and that other than a near miss for a transit in late 1639, Venus would not cross the Sun’s face again until some time in 1761. Kepler hoped to witness the 1631 transit, but died in 1630. The transit of 1631 came and went, apparently unseen. In 1639, scarcely a month before the near miss Kepler had calculated, the young British astronomer Jeremiah Horrocks discovered an error in Kepler’s calculations—Venusian transits actually occurred in pairs separated by 8 years; the interval between each pair oscillated between 121 years and 105 years. Horrocks calculated that on the afternoon of December 4, 1639, the transit of Venus could be seen from his home in northern England. He and a friend, William Crabtree, rushed to plan their observations. On the fated day both men watched an event no human eyes had ever before seen, as the silhouette of Venus, one-thirtieth the apparent diameter of the Sun, glided across the blazing star. They were the only two souls on Earth to witness the 1639 transit. Horrocks’s correction of Kepler’s calculations set the timing for transits in future years. A pair would occur in 1761 and 1769, then in 1874 and 1882, then in the far-off years of 2004 and 2012, continuing on and on in what was thought to be an endless cycle.

Writing in the
Proceedings of the Royal Society
in 1716, the English astronomer Edmond Halley suggested how Venusian transits could provide an absolute Earthly reference point against which the rest of the universe could be measured. When viewed from different places on Earth, Halley wrote, the path of Venus across the Sun would shift slightly, also shifting the transit’s duration. By precisely timing the transit to distinguish the shift between two widely separated locations, it would be possible to triangulate the distance between the Earth and the Sun. From there, simple math would yield the Sun’s true size and each planet’s orbital distance, revealing the physical breadth of the solar system. In the years leading up to the next transit, that of 1761, states across Europe organized more than a hundred teams to travel to the world’s far corners to attempt Halley’s proposed measurements. It was
the first-ever flowering of international, state-sponsored science, and it was a spectacular failure. Astronomers hauled delicate equipment by ship, sled, and horseback into wild areas where the transit would be viewable, often only to find their cargo shattered and warped beyond repair at their destination. Wars, disease, and poor weather scuttled many attempts well before the transit actually occurred. The measurements that did trickle back from far-flung expeditions were too inaccurate and contradictory to be useful.

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.