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Eternity in an Hour: The Accelerating Universe

Copernicus taught us that we don’t live in a special place in space. Translated into time, that led to the very important Copernican principle that all points in time are the same. Now we’ve discovered that the universe is accelerating, and we do live in a special place in time. We’re right near the transition point between deceleration and acceleration, and not all times are the same. I think that is something that has to have profound meaning for science.

Paul Steinhardt, Princeton cosmologist

EINSTEIN’S GREATEST BLUNDER

After Einstein completed general relativity, he was satisfied but rather exhausted. The intensity of the project took a toll on his health. Nevertheless, he felt intellectually compelled to apply his master-work toward unraveling one of the deepest mysteries of science: the shape and form of the cosmos itself.

Today, the notion of galaxies as immense groupings of stars is so familiar that it’s hard to believe the concept is less than a century old. Before Hubble measured the distances to Andromeda and other spiral forms in the sky in 1924 and established them as “island universes” in their own right, many astronomers thought they were



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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos 3 Eternity in an Hour: The Accelerating Universe Copernicus taught us that we don’t live in a special place in space. Translated into time, that led to the very important Copernican principle that all points in time are the same. Now we’ve discovered that the universe is accelerating, and we do live in a special place in time. We’re right near the transition point between deceleration and acceleration, and not all times are the same. I think that is something that has to have profound meaning for science. Paul Steinhardt, Princeton cosmologist EINSTEIN’S GREATEST BLUNDER After Einstein completed general relativity, he was satisfied but rather exhausted. The intensity of the project took a toll on his health. Nevertheless, he felt intellectually compelled to apply his master-work toward unraveling one of the deepest mysteries of science: the shape and form of the cosmos itself. Today, the notion of galaxies as immense groupings of stars is so familiar that it’s hard to believe the concept is less than a century old. Before Hubble measured the distances to Andromeda and other spiral forms in the sky in 1924 and established them as “island universes” in their own right, many astronomers thought they were

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos simply nebulas (gas clouds) within the Milky Way itself. In other words, astronomers believed that the Milky Way constituted the entire universe and that all celestial bodies belonged to it. The cosmos, they thought, was a homogeneous sea of stars (and other formations) that had remained roughly the same since the beginning of time. Before Hubble’s discoveries, Einstein shared this early perception, believing that the overall distribution of material in space was essentially static. Therefore, when he applied general relativity toward the universe, he was astonished to discover that his result was highly unstable. Like an acrobat teetering on a wire, a slight push in any direction would send his model flying. A bit too much matter and his solution collapsed. A bit too little and it blew up. In either case, the universe seemed a fleeting creation, not a rock of the ages. Reluctantly, the German physicist felt compelled to supplement his elegant equation with an extra term, known as the cosmological constant or the Greek letter (lambda). This addition served to stabilize his model of the universe by counteracting gravitational attraction with a kind of antigravitational repulsion. It effectively offered a balancing pole to the teetering acrobat. Where the anti-gravity came from, Einstein couldn’t say. Finding it a bit crazy, he informed his friend Paul Ehrenfest that he had “committed something in the theory of gravitation that threatens to get me interned in a lunatic asylum.” The geometry Einstein had chosen for his model of the universe too was rather unusual. Instead of a stretched-out, speckled sheet, as we often imagine the canopy of the heavens to be, it resembled a polka-dot balloon. Rather than infinite, it was closed and finite. A beam of light heading in any direction would circumnavigate the entire universe and eventually return to its starting place. Einstein selected a bounded, rather than unlimited, cosmos purely for philosophical reasons. He ardently wanted general relativity to obey Mach’s principle—with the distant stars guiding

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos local inertia—but found that he couldn’t do so for an infinite collection of stars. A finite universe would fit that model much more easily. Naturally, though, the universe couldn’t end with a wall. It would be far more eloquent to imagine the cosmos as sufficiently curved that it connects up with itself—in other words, as what mathematicians call a “hypersphere.” A hypersphere is a higher-dimensional version of an ordinary sphere. Take a dot, spin it around a loop, and it becomes a circle. Twirl that circle about an axis and it becomes a sphere. Now choose an additional dimension, perpendicular to the ordinary three dimensions of space, and whirl that sphere around. It traces out a higher-dimensional object. Naturally, that last step of this extrapolation is hard to fathom. Yet there are creative ways of picturing higher dimensions and of determining the actual geometry of the universe. THE SHAPE OF THE MATTER Let’s say that you’ve never heard of the game of basketball. You come from a tiny island nation where the only two sports are synchronized and unsynchronized swimming. Suppose you enter a gym in the United States and see a basketball on the floor. Without picking it up, how do you know it’s spherical? The answer is trickier than you might think. Our vision carves out two-dimensional planes in three-dimensional space. Yet nuances of shade and color, the diminution of apparent size with distance, and varied perceptions from each eye offer us a sense of depth. These optical tools help us ascertain objects’ shapes and positions. Artists make use of such subtle cues to enliven their works, lending them an extra dimension. Such illusions leap out at us most vividly in 3D movies. How then can we really be certain that a basketball is spherical, not just a cleverly disguised orange pancake? A sure way of telling involves measuring angles on its surface. If you trace out a triangle

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos on a flat pancake and add up its angles, the sum is precisely 180 degrees. Do the same for the exterior of a ball, and you arrive at a figure greater than 180 degrees. The differences between flat and curved surfaces were first noted by the mathematicians Gauss, Lobachevsky, and Bolyai in the early 19th century, and involve the subject known as non-Euclidean geometry. Now imagine an ant crawling along the basketball—perhaps the same savvy insect that rode on top of Newton’s bucket. Constrained on the ball’s surface, it would be unaware that the ball has depth. It would believe that it lives in a two-dimensional world. However, two unmistakable facts would strongly suggest that the ball is a sphere. First, the ant could easily circumnavigate the surface and return to its original position. That would at least tell it that the ball is finite, not stretched out indefinitely. Second, it could measure out its own triangle and sum up the angles. A quick calculation would prove that the surface is curved. But suppose the ant was easily distracted and somehow never completed a full circle around the ball. If it could never physically enter the ball’s interior, how could it really be sure that it lived in a three-dimensional world? Maybe it would even discount the presence of a third dimension, since it couldn’t actually see it. Dismissing mysterious, unseen directions, perhaps the ant would conclude that it resided on a two-dimensional pancake with strange geometric properties. Becoming an expert in non-Euclidean geometry, it would consider the basketball’s inside merely a hypothetical construct, lacking a physical basis. Similarly, what if astronomical observations in space and time show that the four-dimensional space-time of general relativity is actually curved? Then we are led to ask: Curved into what? The logical answer is that space-time bends into the fifth dimension, which we may not be able to sample directly because we cannot step out of our world; which leads us to a fundamental question not so much technical as conceptual: Are extra dimensions merely hypo-

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos thetical constructs, convenient for mathematical discourse, or are they in some sense real? The nature of reality in science is a tricky game. For example, in quantum physics, particles are described by wave functions, containing all manner of information. However, at any given time not all these data can be accessed. According to Heisenberg’s uncertainty principle, by taking a measurement of one quantity of a particle (position, for instance), other quantities (such as momentum) tend to blur. Hence, these measured quantities, called observables, generally don’t constitute a complete picture of the particle. One might ask, then, which is the true physical reality—the shadowy realm of wave functions or the incomplete set of observables? Astronomers face a similar dilemma when they examine phenomena that cannot be directly observed. Consider, for example, the hundreds of planets discovered, during the past decade, to be orbiting distant stars. Most of them were detected through their gravitational tugs on their suns. Assuming (as in most of the cases) that the planets themselves are too dim to be seen, astronomers must infer their existence. They are presumed real because that’s the best explanation researchers have developed to account for their parent stars’ slight movements. In our own solar system, for many years Neptune’s existence was merely presumed. Well before its image was seen with a telescope, astronomers surmised its presence from perturbations in the orbits of the other planets. Did those observations alone make Neptune real, or did its light have to be detected first? Most physicists and astronomers today would say that something is real if they infer its existence through a logical explanation that preserves the established laws of nature. The subatomic particle called the neutrino is a good example of this philosophy. Theorist Wolfgang Pauli postulated its existence through applying the principles of the conservation of energy and momentum. Although his peers gently taunted him about his advocacy of a particle that had never been seen, Pauli stood his ground. Almost two decades later,

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos experimenters finally proved him right. The neutrino, though notoriously one of the most elusive particles in nature, was indeed real. Einstein went back and forth throughout his career on whether or not extra dimensions were real. This indecision related to his mixed feelings about the role of experimentation in physics. Philosophically, he had one foot firmly in each of two camps. He often argued that experimentation was needed to establish any proposition. That’s why he breathed easier once the Mercury precession and light-bending measurements seemed to confirm general relativity. On the other hand, he spent much of his later years trying to use his own intuition to surmise the deep mathematical principles underlying reality. At least to the outside world, these musings seemed to have little to do with what was experimentally known at the time. Einstein’s propositions that the universe is shaped like a hypersphere, and that a cosmological constant is needed to bolster it from either expansion or collapse, could not be tested for many years. Only in recent times have astronomers been able to map the likely shape of the cosmos and consider the likelihood of an antigravity term. Nevertheless, just by bringing up these issues Einstein ushered in a new age for cosmology. For the first time, science addressed the possibility that space itself has an overall shape. A sphere is not the only way a surface can be curved. Saddles, for example, are often curved one way on the bottom, to accommodate the horse, and another way on the top, to provide comfort to the rider. Similarly, three-dimensional spaces can curve several ways into a higher dimension while preserving constant curvature. Besides a hypersphere (known as closed or positively curved), spaces can be saddled-shaped hyperboloids (known as open or negatively curved). The third possibility is for the space to be completely flat (known as zero curvature). In 1922, Russian mathematician Alexander Friedmann explored each of these geometric possibilities for the universe. In the absence of a cosmological constant, he found that they corresponded to three

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos distinct cosmologies. He characterized these by a parameter, known as the scale factor, that measures the size of space. If space grows, for instance, the scale factor increases over time. This results in its content (galaxies in the present era) moving farther apart. The universe, as Friedmann envisioned, started out extremely small. Then, like a pumped-up balloon, it began to expand. If the universe’s overall geometry is closed, this expansion will eventually reverse itself—like air being let out of a balloon—and collapse it back down. This catastrophic demise is often called the Big Crunch. If, in contrast, the universe is open or flat, it will expand forever. The difference between the two models pertains to how quickly the scale factor grows; it grows faster for open than flat geometries. These three possibilities (closed, open, and flat) delineate what are known as the Friedmann cosmological models. Which geometry is feasible for a particular universe depends on its overall density. Denser universes follow a closed scenario, while sparser ones obey an open scenario. Universes of densities precisely equal to a critical value are flat. The ratio between the actual density and critical density is called the omega parameter. For omega greater than one, the cosmos is closed; less than one it is open; and equal to one it is flat. According to physicist George Gamow, Friedmann sent his results to Einstein, pointing out inaccuracies in Einstein’s static model. Einstein did not reply for quite some time. Finally, he responded with a “grumpy letter,” reluctantly agreeing with Friedmann’s conclusions. Although Friedmann published his results in a prestigious German journal, they were overlooked for several years—until Hubble’s remarkable findings brought them to prominence. EXPANDING PERSPECTIVES Hubble’s discovery, in 1929, of the expansion of the universe came at a fortuitous moment for cosmology. By that time, general relativity

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos was a well-established theory with a host of solutions. In addition to Einstein’s static universe and the dynamic Friedmann models, de Sitter had proposed a curious cosmological model that was completely empty but expanded anyway. Thus, scientists wishing to describe the cosmos could choose from possibilities galore: stable, dynamic, expanding, collapsing, full of matter, bereft of matter, and so on. Consequently, when Hubble revealed that all the galaxies in creation were fleeing from each other like a roomful of angry solipsists, theorists were well prepared. They dusted off expansion scenarios and put them to good use—relegating static models to the bottom drawers of musty filing cabinets. The final vestige of the Newtonian cosmos—the notion that space doesn’t evolve—crumpled under the weight of immutable facts. It did not take long for Einstein to realize he had erred in presuming that the cosmos was immutable. In January 1931, during a trip to the United States, he visited Mount Wilson Observatory in California to see for himself the instrument that had provided a window to cosmic truth. By that time Einstein was extraordinarily famous, so film crews accompanied him as he rode the elevator up to the 100-inch Hooker telescope and glanced through its eyepiece. Hubble was beaming with pride as he showed the German physicist the most powerful telescope on Earth and the spectral evidence he had gathered with it. Paying tribute to Hubble’s work, Einstein admitted that the cosmological constant had been a mistake. “Not for a moment,” said Einstein, “did I doubt that this formalism was merely a makeshift to give the general principle of relativity a preliminary closed form.” The purest form of the general relativistic equations, he declared, had been the correct one. The age of the expanding universe had begun. One of the articles unearthed at that time was remarkable for its prescience. Written in 1927, it came into prominence in 1931, when Eddington had it translated from French into English and published

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos in the Monthly Notices of the Royal Astronomical Society. The piece was remarkable for predicting not only a growing universe but also one whose growth is tempered or accelerated during various phases of its life. Moreover, it suggested a seeming connection between cosmological theories and the biblical notion of a moment of creation. Perhaps this was not surprising, given that the author of the paper, Georges Lemaitre, was an ordained priest as well as a scientist. Born in Belgium in 1894, Lemaitre studied math and physics while attending seminary, devouring all he could read about general relativity. After his ordination in 1923, he attended the University of Cambridge, where he took courses under Eddington. He completed his education at MIT, obtaining his Ph.D. in 1927. In his seminal research paper, Lemaitre devised a hybrid between the cosmological theories of Einstein and Friedmann. Adding a cosmological constant to Friedmann’s disparate geometries, Lemaitre found that Einstein’s equations produced a curious assortment of behaviors. The resulting solutions became known as the Friedmann-Lemaitre models. The solution Lemaitre found the most promising is sometimes called the “hesitation universe.” According to his theory, all of space and time began with a solitary burst of energy—a singular moment of genesis. Before that explosive instant, absolutely nothing existed. Afterward, the universe was a rapidly growing fireball, hurled outward by the blast. Fred Hoyle, a leading critic of this idea, later dubbed it the Big Bang. Lemaitre preferred to call the initial state the primordial atom. (It was sometimes also called the cosmic egg.) During its initial era, the cosmos was very dense. Consequently, the sticky force of its gravity was strong enough to slow the expansion. As the universe got bigger and bigger, its expansion became slower and slower. Eventually, its expansion was languid enough that galaxies could assemble from the hot matter. The galaxies in this model were distributed like a fluid with no center and no edge. As in

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos the Einstein universe, they resembled dots painted on the surface of a higher-dimensional “balloon.” Just as every point on a balloon is as central as every other point, no galaxy can rightly claim to be in the middle of the universe. According to Lemaitre, the lazy-growth period or “hesitation era” lasted for billions of years, allowing for the formation of all the galaxies we see in the sky. Then a new force began to dominate cosmic dynamics—the repulsive power of the cosmological constant term. We now call this extra push the “dark energy.” One of the advantages of Lemaitre’s proposal was its flexibility. By tinkering with the value of the cosmological constant, one could reduce or extend the hesitation era as much as one wanted—like tuning a radio dial to produce the best reception. Presumably the optimal time frame of Lemaitre’s model would be one that reproduced the known age of the universe and other observed astrophysical facts. Although Eddington helped bring Lemaitre’s paper to publication, he vehemently disagreed with its premise of a universal beginning. The British astronomer found distasteful the idea that time could have a starting gate, preferring to believe that the cosmos existed eternally. To remove the concept of genesis from the equations, Eddington pondered an infinitely long quiescent period, similar to Einstein’s static realm, in which the universe was like a solid lump of dough. This space-time dough would have persisted in the same state forever, except that somehow a disturbance (acting as a kind of cosmic yeast) caused it to rise. It expanded, under the influence of a cosmological constant, until it reached its present-day size—hence the colossal cosmos we observe today. Why would a sleeping cosmos of infinite duration suddenly wake up? This profound philosophical question dates at least as far back as St. Augustine of Hippo. In City of God, he argued that there was no contradiction between an immortal creator and a finite creation at a fixed instant in time. Eddington believed the awakening stemmed

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos from a chance occurrence that could have occurred at any moment. If someone were to bet in the lottery an infinite number of times, eventually they’d win and their life would be changed forever. The universe simply won the lottery. A third alternative, to both a singular explosion and a slow waking up, dates back (at least philosophically) to traditional Eastern notions of eternal cycles. The Hindus, Babylonians, dynastic Chinese, ancient Greeks, and many other cultures have advocated an ever-repeating universe in which the slate is periodically wiped clean. In the mid-1930s, Caltech physicist Richard Tolman explored a similar concept with his “oscillatory universe.” According to this model, instead of a universal beginning, the Big Bang was preceded by the “Big Crunch” of an earlier cycle. That crunch stemmed from the earlier era’s collapse, which was precipitated by a previous Big Bang, and so forth. Each era resembled a closed Friedmann model, glued by fate to its predecessors and successors. Tolman realized, however, that his model could not produce an endless succession of viable worlds. Rather than starting afresh, each era would preserve the entropy (amount of disorder) of the previous era. Like a movie theater that never sweeps up between screenings, the universe would accumulate more and more disorderly energy. Tolman calculated that this entropy increase would make each cycle longer and longer, with higher and higher temperatures, while less and less hospitable to the development of galaxies, stars, planets, and life. Ultimately, the cosmos would recycle itself into an indefinite array of lifeless stages. We might ask a philosophical question: If a universe arises that no living being is around to observe, does it truly exist? Note that the various cosmological theories of that period had markedly different suppositions. Both Lemaitre’s model and Eddington’s model made use of a cosmological constant term. Even though Einstein called this term his greatest blunder, it offered cosmologists greater freedom to “fine-tune” each universe model to

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos If, on the other hand, it was too long, there would be little sign of structure today. These delimiters still allowed for a number of options that, researchers hoped, more data would whittle down. Steinhardt joked that constructing the right inflationary model was like picking from a Chinese menu—selecting one item from column A, one from column B, and so on. The approach that led to the correct solution would have very particular properties and would match up precise models of the microscopic and macroscopic worlds. Linde, on the other hand, was convinced that inflation was a ubiquitous and natural phenomenon, akin to Darwinian evolution in biology. All one needed was the tabula rasa of empty space. On this blank pad the quill of quantum randomness would sketch fluctuations of various sizes. Through pure chance, at least one of these fluctuations would produce a scalar field able to spark the fuse of an inflationary blast. That region of space would expand exponentially, thereby dominating less explosive sectors. As it blew up, it would produce the familiar byproducts of inflation—flatness, correlations between remote domains, and so forth. Because of its reliance on sheer randomness, Linde dubbed his model “chaotic inflation.” Steinhardt, Linde, and their various collaborators spent much of the 1980s and early 1990s developing alternative models of inflation. Many others joined in on the quest. Like the makers of Coke and Pepsi, each research team produced various concoctions, hoping that one of them would pass the taste test of astronomical inquiry. Some of the models du jour developed by various groups included extended inflation (where a field interacts with gravity, causing it to eventually put the brakes on inflation), hyperextended inflation, power law inflation, natural inflation, hybrid inflation, eternal inflation, and so on. Each posited a distinct mechanism for inducing, then halting, a burst of ultrarapid expansion. An exponential proliferation of papers filled the science journals to the bursting point—leading to a curvature dilemma for the flimsy shelves in researchers’ offices. Physicists eagerly awaited experimental data that

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos would help distinguish these approaches—and confirm or disprove the theory itself. One critical source of information would be provided by new observations of the CMB’s profile. From the time of Penzias and Wilson until the end of the 1980s, the only features known about that radiation were its average temperature, spectral distribution, and overall isotropy. Experiments also accounted for a small discrepancy in the radiation’s temperature in opposite directions of the sky due to Doppler shifting caused by the Milky Way’s motion through space. Researchers, though, believed that a more precise examination would yield evidence for the seeds of structure formation in the universe. These seeds would be minute anisotropies due to slight differences in the early distribution of matter. Because inflationary theorists aspired to explain the process of structure formation in terms of stretched-out quantum fluctuations, they hoped such anisotropies would soon be found. Conversely, if no such bumps existed, advocates of any variation of the Big Bang model would have a hard time explaining how galaxies and other structures emerged. CRINKLES IN THE FABRIC On November 18, 1989, NASA launched the Cosmic Background Explorer (COBE) satellite, designed to take an unprecedented look at the primordial radiation bathing the cosmos. It carried several instruments, including a differential microwave radiometer, able to discern anisotropies in the background spectrum as tiny as a few parts per million. A team led by George Smoot of Lawrence Berkeley Laboratories (LBL) analyzed and interpreted the data. Physicists nervously awaited the experiment’s results. Would it confirm the Big Bang picture of a fiery beginning? Would it detect the minute raisins in the tapioca pudding of uniformity? Tension mounted as months passed by with no wrinkles to be

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos found. Smoot urged patience, realizing that it would take some time for the data collected by the satellite to reach levels of statistical significance. Even after some signs of anisotropy appeared, about a year after the launch, he was reluctant to publish any results until he was absolutely certain they were valid. During this wait, some science journalists stirred up a storm with dire warnings that the Big Bang theory was in jeopardy. A Sky and Telescope news item inquired, “The Big Bang: Dead or Alive?” A popular book by the physicist Eric J. Lerner, revising an outdated plasma cosmology, proclaimed in its title that The Big Bang Never Happened. Steady state theorists waited in the wings, eager to come to the rescue with alternative hypotheses. In an ironic twist, Narlikar and Burbidge each pointed to the smoothness of the microwave background as evidence against the Big Bang theory. Along with Hoyle they developed what came to be known as quasi-steady state cosmology. Unlike the original model, it predicted an isotropic radiation spectrum— albeit produced in “mini big bangs” rather than a single explosion. Few mainstream cosmologists, however, rallied to their cause. Finally, on April 23, 1992, Smoot enthralled scientists at a meeting of the American Physical Society with his long-awaited announcement of success. He had kept his results top-secret until the very end, checking and double-checking them to eliminate ambiguities. At one point he had even flown to Antarctica (where the cold night sky is especially clear) to take extra measurements. By the time he stood on the podium he was confident that his team had recorded the stunning visage of the early universe. The wrinkles that the COBE group found matched up beautifully with the concept that galaxies were seeded in the early universe. Corresponding to slightly hotter or colder regions of the background, the COBE picture identified primordial zones of greater or lesser density. The denser areas constituted the kernels of cosmic structure. Nevertheless, these results still weren’t precise enough to nail down key cosmological parameters and distinguish particular early-universe

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos models (such as various inflationary scenarios). Consequently, astronomers began planning in earnest for a more detailed probe. Meanwhile, other researchers at LBL and elsewhere pursued a wholly different way of investigating the early universe. Their investigations of distant supernovas would soon jolt the field of cosmology. CLOCKING THE SUPERNOVAS Saul Perlmutter, leader of the Supernova Cosmology Project, grew up in a family of respected academicians. His father, Daniel, was a professor of chemical engineering at the University of Pennsylvania, and his mother, Felice, was a professor of social administration at Temple University. Nurtured in a supportive, intellectual household, his interests turned to science at an early age. As a child, recalled Perlmutter, “I always enjoyed looking at the sky, but I was never one of those people who had their own backyard telescope. It was only because I started needing telescopes to answer the fundamental questions that I started learning much about astronomy.” In addition to his scientific talents, Perlmutter became adept at music—corroborating popular theories that the two abilities go hand in hand. He’s an avid violinist and enjoys playing in orchestras. Blending his talents with others—whether harmonizing in music or collaborating in science—has become an important part of his personal philosophy. “I was somebody who had fewer individual heroes and more collective heroes,” he stated. “The idea that people working together could understand the world and that no single one of them by themselves could understand the world, that really captured my imagination.” After receiving a Ph.D. from the University of California at Berkeley in 1986, he was appointed to a position at LBL. Along with an international team of astronomers, including Berkeley astronomers Carl Pennypacker and Gerson Goldhaber, he set out to measure the overall dynamics of the universe and the change in its expansion rate over time. This measurement would provide a way of delving into

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos the past and predicting the fate of the cosmos. To perform this task, they developed powerful techniques to measure the energy output of Type Ia supernovas in extremely remote galaxies. Type Ia supernovas, the catastrophic explosions of a certain kind of star, are valuable to astronomers because they can serve as “standard candles.” Standard candles are objects with well-known energy output. Imagine walking along a dark desert road and seeing a faint street lamp way off in the distance. If you know the intrinsic power of the lamp, you can deduce from its dimness how far away it is. Type Ia supernovas serve a similar purpose for astronomers eager to map the scale of the universe. By matching the apparent brightness of such stellar blasts to their actual luminosities, astronomers can reliably ascertain their distances. Light curves, indicating the progression of each burst, offer added information. Thus, they are solid celestial yardsticks, useful for measuring the remoteness of the galaxies in which they are situated. Once astronomers know the distances to the galaxies in a given region of space, they can readily determine the expansion rate of that region. Each galaxy’s spectral lines are shifted by the Doppler effect. By measuring this shift, they can assess the galaxies’ velocities. Finally, by combining this information with the distance data, they can calculate how fast each part of the universe is pulling away. In astronomy the farther out you look, the deeper into the past you see. Therefore, Perlmutter and his colleagues realized they had the perfect tool for determining how the cosmological expansion rate has altered over the eons. This tool could allow them to assess omega, the universe’s density parameter, and help them decide how much of its dynamics is driven by visible material versus dark matter. A second team, led by Brian Schmidt of Australian National University and Nicholas Suntzeff of Cerro Tololo Inter-American Observatory in Chile, enacted an independent program with a similar purpose. Throughout the 1990s the two groups jockeyed for valuable telescope time and competed in a race for publications.

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos One aspect of the cosmos that the researchers did not expect to challenge was its deceleration. The simplest Friedmann models portrayed a universe slowing down with time. The only difference lay in how quickly this braking would occur—with the closed model representing the extreme. Adding a cosmological constant would change the situation, allowing for the option of speeding up, but few physicists saw a point to doing that. After all, even Einstein had discarded the term. Supernova mapping is an arduous process, given that they are rare and unpredictable. It’s like knocking on doors all around the country hoping to find a family with quintuplets who had just won the lottery. If observers anywhere in the world spot a distant burst, researchers everywhere must leap into action. They may need to redirect a telescope to track the supernova’s light curve, with no time to spare. Then they can use that information to plot just one more point on their charts. As data trickle in, statistical significance builds over many years. In 1998 each supernova team felt it had enough evidence to render a verdict. In startling announcements the groups proclaimed that the universe is not currently decelerating at all but rather is speeding up. Thus, not only will the cosmos expand forever, its expansion is accelerating. Each remote galaxy is moving farther and faster away from the others, with no end in sight. DARK ENERGY By the end of the 20th century, scientists realized that many previous assumptions about the behavior of the cosmos were dead wrong. Along with the COBE data, the supernova results pointed to a flat universe. However, unlike the simplest flat Friedmann model, with omega equal to one, the parameter associated with the expansion was only about three-tenths. In other words, the universe had approximately 30 percent of the material density associated with flat

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos cosmologies. Something else must be hidden in the blackness of space. That extra component was named dark energy. The simplest way of incorporating dark energy into cosmology is to reinstate the cosmological constant term, also known by the Greek letter (lambda). Although that makes general relativity less elegant, it also makes it more accurate. A mathematical clarification is one thing, but a true physical explanation is another. Theorists scrambled to try to explain the origins of cosmological antigravity. It would be incorrect, though, to picture the universe as always speeding up in its expansion. Additional supernova measurements by Schmidt’s group and Perlmutter’s group have revealed that the universe began to accelerate relatively recently in its history—within a few billion years of the present day. Before then the universe was dense enough so that matter terms dominated the cosmological constant term. The attraction of gravity overpowered the repulsion of lambda, slowing the expansion. Therefore, space was decelerating before it began to accelerate. Only when the universe’s matter was dispersed enough did lambda begin to dominate and the universe start to speed up. As Steinhardt has pointed out, the outstanding coincidence that we live within a few billion years of the turnaround time of the universe seems to contradict the Copernican ideal that humankind occupies no special place or time. Thus, the new results cry out for a wholesale rethinking of our basic concept of the universe. As he has remarked: I think people are really missing the boat on this. This is truly a revolution of Copernican nature; this is not just another addition. What the cosmology community has done for the most part is say, “Oops, we’re missing an ingredient. Let’s add that ingredient. Everything fits beautifully. We have a wonderful model.” My reaction is: time to step back and reevaluate. The full extent of the implications hasn’t been worked out yet. If

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos you were around at the time of Copernicus you might have said, “He wants to make the Sun the center. You want to make the Earth the center. It doesn’t mean too much.” But then by the time you get to Kepler and Newton it means a lot. So it wasn’t just another detail. I can imagine this will be a very profound thing by the time it’s through. Steinhardt has proposed that the dark energy is a hitherto-unknown substance, called “quintessence.” Its name hearkens back to the ancient notion of four natural elements—earth, air, water, and fire. Quintessence would be the fifth. Instead of a steady cosmological constant, it would be a field that kicked in during a particular epoch of the universe, causing a far milder version of an inflationary burst. Using a variable field offers greater flexibility in modeling different cosmic phases. However, current observations have not been able to distinguish between variable and constant forms of dark energy. To resolve these and other vital issues, astronomers have pressed on with further testing. The supernova teams have continued their endeavors, accumulating a bevy of examples to enhance their data. The LBL group has proposed a space-based mission, called the Supernova Acceleration Probe, to improve their capability by 20-fold. Meanwhile, spectacular new results from the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, have uncovered a treasure trove of critical information about the young cosmos. PORTRAIT OF THE COSMOS AS A YOUNG EXPANSE The beginning of the 21st century has witnessed cosmology becoming an exacting enterprise, with ample tools to elucidate the state of the observable universe. It has also ushered in considerable confusion as to the future direction of the field. A snapshot of the early

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos universe captures this mixture of profound new knowledge and grave uncertainty. The stunning “baby picture” of the cosmos produced by WMAP represents one of the landmark scientific images of our times—akin to the double helix or the first photos of the Martian surface. When represented in color, like a weather map of hotter and colder sites, it is a fantastically intricate mosaic of multihued spots. Clearly the background radiation’s artist painted in pointillism. Paintings capture moods, and the WMAP portrait is no exception. It shows the cooled-down form of a once-scalding universe, releasing long-pent-up energy into the gaps between atoms. The atoms were slightly clumped together, in patterns that depended in part on the geometry of space. Their particular arrangements indicated that they were happily settled into a flat, expanding hyperplane—with omega exactly equal to one. Perhaps they were especially content because they recalled a far more explosive period earlier on that flattened their vistas. But now they could move away from each other at a gentler pace, awaiting the day their gravitational attraction would compel them to reunite into myriad celestial bodies. At a 2003 conference of the American Physical Society, physicist Michael Turner reveled in the high precision of the new data. He emphasized that, for the first time in the history of cosmology, researchers were able to perform exact-enough statistics to present their results with error bars (precise ranges of values). Turner also pointed out that the WMAP results ruled out the simplest inflationary models. He counseled, however, that there were other possibilities. “Fortunately, Andrei [Linde] had another 300 models left,” Turner joked. The combined power of the supernova and microwave background observations enables cosmologists to define a “concordance model” of the universe. Any theory that satisfies known results about the geometry, age, and content of the observable universe falls into this category. You would think that this would narrow things con-

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Brave New Universe: Illuminating the Darkest Secrets of the Cosmos siderably. However, it still leaves the door open to diverse explanations—an inflationary era being only one of many possibilities. Although we are now reasonably sure that the universe is flat, we still don’t know exactly what caused it to be flat. (To recall, we mean here that the ordinary three-dimensional part of a four-dimensional Friedmann model is flat.) Was it born that way, molded through inflation, or smoothed out through another mechanism? Data from WMAP and other sources have converged on an estimate of 13.7 billion years for the observable universe, since the time of the Big Bang. But what about eras that may have preceded that colossal burst of energy? Perhaps there was no Big Bang singularity at all, just a transition between different phases of the universe. And could the observable universe be part of a greater whole, conceivably in higher dimensions? What of the dark energy that constitutes some 73 percent of the substance of the cosmos? Could it be a sign of something missing in our concept of gravitation? Could fundamental constants, such as Newton’s gravitational constant or the speed of light, actually change over time? We will consider these disparate possibilities in chapters to come. Finally, let’s remember that, in addition to visible matter and dark energy, the cosmos appears to contain a third major component—dark matter. Readings from WMAP indicate that this hidden material represents 23 percent of the universe. Although theories abound, no one has yet developed a satisfactory explanation of what dark matter actually is. This enigma has grown even deeper with the recent discovery of an entire galaxy as inscrutable as the Cheshire cat.

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