God In The Equation
Page 22
Lee Smolin at Pennsylvania State University, another kindred spirit, views this multiverse as analogous to the natural variations that allow biological evolution to occur. Most mutations are harmful, but every once in a while a beneficial one occurs. When that happens, natural selection quickly ensures that the organism with the slight innate advantage will come to dominate the population. In an analogous way, Smolin pictures a precession of universes, each having slightly different physical rules, that sprout whenever a star collapses into a black hole. Most of these are sterile places, but a universe whose laws allow the formation of stars soon begets other universes full of stars, planets, and perhaps life. These cosmic cradles quickly fill with living organisms. What we call “the cosmos” is one of these celestial oases.
The existence of a boundary in time—a moment of creation—always irked Einstein. Linde and Smolin cleverly dodge the temporal bullet by adding near-endless cycles of creation that push the moment of origin almost unthinkably far away. The process could have initiated with a quantum fluctuation that conjured up a true something-from-nothing universe, but still that fluctuation had to arise under the guidance of some preexisting set of natural laws. “So where do the laws of physics come from?” Guth asks. “Science has no answer at this time.” The sci/religious faithful find themselves at an impasse reminiscent of that of Saint Augustine, who believed that God created everything, even time itself. What then did God do before the existence of time? “Some people say that before He made Heaven and Earth, God prepared Gehenna for those who have the hardihood to inquire into such high matters,” Augustine wrote evasively in his Confessions. Hawking accepted Augustine's implied challenge and helped develop a complicated theory, the “no-boundary proposal,” in which time can wrap on itself so that it can be finite and unbounded, equivalent to the finite but unbounded space of Einstein's 1917 cosmology. “The universe would be completely self-contained and not affected by anything outside itself. It would neither be created nor destroyed. It would just BE,” he proudly writes.
Even as cosmologists plunged bravely toward Augustine's hell, their theories still left an awkward gap between the divine era of Lambda-driven creation and the more mundane world of today. All of the grandiose interpretations of inflation focus on events that occurred billions of years ago or in alternate universes cut off from our own. According to the usual interpretation, inflation ended when the cosmos was just 10-30 second old. From then on, the energy drained from the vacuum and the big bang went about its business calmly, without any influence from Lambda. Almost everyone believed that inflation was a phenomenon of the distant past. “The universe is not expanding in an inflationary way today,” Hawking asserts in A Brief History of Time. Likewise, Guth doubted that Lambda played an important role in the modern universe. He reiterated that the leading unified theories of particle physics predict a value of Lambda that is 120 orders of magnitude larger than the limits set by observations of the expansion of the universe. Clearly, something is wrong with the theories: either the expected vacuum energy does not exist or some process cancels it out. Either way, the most natural assumption is that the value of Lambda is zero. It seemed incredible, to Guth and to everyone else, that the universe would find a way to eliminate 99.999999999999999999999999999999999999999999999999 9999999999999999999999999999999999999999999999999999999999999999999 percent of the vacuum energy but leave exactly enough to influence the expansion of the universe without messing up the everyday physics of our lives.
Now that the idea of Lambda was once more a part of mainstream cosmology, however, some other theorists continued to play with it. Perhaps some residual Lambda survived after inflation ended. A dab of energy might yet lurk within the vacuum. In the search for the sci/religious divine, Einstein's old cosmological constant still made for an awfully convenient fudge factor to bring harmony to the universe. The age problem still vexed astronomers: the oldest stars appeared to be older than the estimated age of the universe. As Georges Lemaitre had discovered decades earlier, Lambda could clear up that problem handily. More fundamentally, efforts to measure the density of the universe kept coming up short. If some of the cosmic mass existed in the form of vacuum energy, that added component could increase the tally and reach the magic “critical” density that produces a flat universe. But Lambda was still viewed as something of an embarrassment. It had fooled Einstein, allowing him to believe in a static cosmos. What researcher could hope to succeed where the greatest mind in physics had failed?
This was where cosmology remained stuck for nearly two decades. Theory had gone about as far as it could go. In the back-and-forth process that propels science onward, the next move belonged to the observers. They made the most of the opportunity.
8. THE ANGEL OF DARK ENERGY
PAUL PERLMUTTER darts around his modest office at Lawrence Berkeley Laboratory, a cluster of drab-looking buildings nestled in the Berkeley hills above the University of California campus. Surrounded by institutional yellow walls and gray steel bookshelves, he riffles through a stack of journal articles and computer printouts, searching for the right one. With his edgy movements, shaggy hair, and self-deprecating, Woody Allenish gestures, he could be mistaken for a computer programmer—and he does in fact spend a lot of time at the keyboard. Then he picks out the key article, “Measurements of Omega and Lambda from 42 High-Redshift Supernovae,” and it becomes clear that the papers and computers and the entire California landscape are only a minuscule, superficial part of who he is. Perlmutter is a leader in the movement that revised the big bang and reinvigorated cosmology with a fresh dose of mysticism. He is one of the new high priests of sci/religion.
Perlmutter works in the tradition of Edwin Hubble and his self-proclaimed observational approach to cosmology. Like his illustrious predecessor, Perlmutter is driven by a low-key but irrepressible desire to see all the cosmos has to offer. “It goes back to childhood. I've always been interested in the most fundamental questions. I wanted to know the fundamental rules that make things look the way they do. This time around, I wanted to do something experimental—I wanted to see something about the world around me,” he says. Much has changed since the 1920s. Silicon light detectors have replaced glass photographic plates, and rumpled T-shirts have taken the place of jackets and ties during observing sessions, but the drill remains the same. You peer deep into the heavens, scan the countless specks of light, and search for the deeper levels of reality that nobody else has seen before. Once again this effort has given credibility to some of the most startling aspects of Einsteinian prophecy.
Along with a large group of collaborators, Perlmutter set out to determine how far he could go in penetrating what Hubble called “the dim boundary—the utmost limits of our telescopes.” Another team, under the direction of the soft-spoken Brian Schmidt, embarked on a similar quest. Both Perlmutter and Schmidt were in their twenties when they began. They were the children of the sci/religious revolution; they had grown up never knowing any doubt that cosmology could provide an all-encompassing view of the cosmos. But from inside the Temple of Einstein they watched as theorists compiled a list of neotheological questions. Is inflation the correct description of the big bang? Is space full of invisible energy? Where did our universe come from? Perlmutter and Schmidt wanted to drag those questions back into the observational realm.
Both men had the same basic goal. They wanted to uncover two of the most sought numbers in cosmology, the rate at which the universe is expanding and the deceleration parameter—how the pace of expansion is changing over time. These two values contain some of the most basic information about the nature of the universe we live in. Together they indicate when the universe began and how much matter it contains, pulling the galaxies together and slowing down their outward movement. And there was another, tantalizing possibility. If Lambda exists—if Einstein's 1917 conjecture was correct—then that should show up, too.
Perlmutter initially sought cosmic truth through studies of subatomic particles. By
1983 he was fed up with complicated physics experiments that would take years to provide any meaningful answers and set out in search of a different way to study the world. “It looked like astrophysics was going to let me get at fundamental problems,” he recalls. He ultimately struck up a collaboration with his Berkeley colleague Carl Pennypacker to measure the cosmic expansion rate. The duo expanded into a team as they picked up graduate students and colleagues to help with the effort. Eventually it mutated into the Supernova Cosmology Project, with Perlmutter in charge of an ever-changing team lineup.
Schmidt initially had a more concrete goal. He wanted to understand the mechanics of a supernova. When a star obliterates itself this way, for a few weeks it shines with more than a billion times the luminosity of the sun. In the process, as Fred Hoyle discovered in the 1950s, it spews a cloud of heavy elements that seed the universe with the heavy elements that make up planets and people. “I like them as physical objects. What are they doing inside? Why are they so bright?” Schmidt wondered. But like Perlmutter, he recognized that supernovas are also powerful tools for exploring cosmology's great spiritual questions. In 1994 Schmidt joined the hunt for enlightenment with some guidance from his mentor, the supernova guru Robert Kirshner of Harvard University. Schmidt called his effort the High-Z Supernova Search. (Z is the term cosmologists use to denote redshift, so high-Z refers to extremely distant objects whose light appears greatly reddened by the expansion of the universe.) In addition to a late start, Schmidt had to compensate for a geographical disability. In 1995 he took a position at Mount Stromlo and Sliding Springs Observatories in New South Wales, Australia (now the Research School of Astronomy and Astrophysics), which placed him a dozen time zones away from many of his colleagues.
Others had set down this path before. Hubble dreamed of mapping and analyzing the farthest cosmic depths using the two-hundred-inch Hale telescope, the “Big Eye” that opened on Mount Palomar in 1948. He began with a burst of late-life optimism. In 1951 he laid down a grand agenda: “We shall turn to the great problems of the universe with new confidence. Observational results can be stated positively, with limits of uncertainties evaluated accurately. Then theory after theory can be eliminated. . . And possibly, just possibly, we may be able to identify, in the shortened array, the specific type that must include the universe we inhabit.” His plan was to continue, on a vastly larger scope, the research that had led him to discover the apparent expansion of the universe in 1929. He would search for Cepheid variable stars in distant galaxies, this time knowing that such stars come in two varieties. He would measure how quickly they pulsed in order to determine their true luminosity and, hence, their distance. Then he would combine those measurements with Humason's redshift data to see how quickly the universe was running down. He even held out a vague hope of peering all the way to the “observational horizon” of the universe. Two years later Hubble died of a cerebral thrombosis, his program barely begun.
Allan Sandage and Hubble's other successors carried on this enormous undertaking. Even the Hale telescope could produce detailed images of Cepheids only in a handful of fairly nearby galaxies. Beyond that it was hopeless; individual stars were just too dim. Hubble had used cunning tricks to extend his reach. He looked at the brightest stars in galaxies, assuming they must all be similarly luminous; farther out, he looked at the brightest galaxies in clusters of galaxies. In this way, he attempted to add rung after rung to his cosmic distance ladder. But the uncertainties in each step of extrapolation were so great that Sandage and his rivals were still bitterly debating the correct rate of cosmic expansion four decades later. Humason, meanwhile, found that the background glow of the atmosphere obscured the redshifts of the faintest galaxies. “Well, there is apparently no horizon, at least as far as the two-hundred-inch goes,” he said with a sigh. Einstein and his disciples could describe the whole universe in their equations, but the observers were struggling to test the truthfulness of those mystical visions.
Back in 1938, Walter Baade at Mount Wilson had suggested another way to take the measure of the universe. Frustrated by the same limitations that had held back Hubble, Baade thought about tracking supernovas rather than Cepheid variable stars. Supernovas are so much more luminous that they can be seen clearly regardless of how far away they are. If all supernovas were essentially the same, they could be used as “standard candles” to reckon the correct distances to galaxies billions of light-years away. But as Baade and others soon learned, the universe does not yield its secrets so easily. Some exploding stars are at least five times as luminous as others. Without understanding the nature of those variations, a naive cosmologist might arrive at distance measurements that were off by a factor of two—far too crude for the delicate business of mapping the exact physical parameters of the expanding universe.
In 1941 the German astronomer Rudolph Minkowski recognized that supernovas fall into two broad categories: Type I, which do not appear to contain hydrogen; and Type II, which do. During the 1950s, Hoyle worked out the basic theory of supernovas as thermonuclear detonations, titanic relatives of the hydrogen bomb. Their diversity made it clear that there is more than one way for supernova explosions to occur, however, and distinguishing among them was not easy. Starting in the 1960s, astronomers began to recognize that the two types of supernovas are completely different kinds of objects. Type II supernovas are the death throes of massive stars that have exhausted all the nuclear fuel in their cores. With no energy blazing out from the center, the star collapses, creating so much heat and pressure that all of the star's outer layers undergo nuclear burning all at once. But at least some Type I supernovas occur through a different process. When a middleweight star like the sun grows old, it ends up as a stellar remnant called a white dwarf. Usually the story ends there. But if the white dwarf has a companion star, it can grab material from its partner and keep growing more massive. Eventually it hits a critical point at which gravity can no longer support all that bulk, and the star caves in on itself—then producing a nuclear blast similar to the one from a Type II supernova.
By the late 1970s, astronomers knew enough about the different types of supernovas that a number of eager cosmologists thought it was time to revisit the possibility of using the exploding stars to gauge distances and put the increasingly exotic cosmological theories to the test. The idea took off in 1985 when several researchers, including Kirshner, Sandage, and Gustav Tammann at the University of Basel uncovered a subdivision within the Type I supernovas. One specific kind of supernova, referred to as Type 1a, always seems to blow up the same way. These objects alone—not all the other, similar-looking Type I stars—are exploding white dwarfs. Fortunately, these explosions have a distinctive look that makes them easy to identify, and as luck would have it, they are also the most luminous variety. Type 1a supernovas are so potent, they are easily visible all the way across the universe, perfect for charting the future course of our cosmos.
Perlmutter got excited. “It became clear that Type 1a supernovas were the really useful ones, the ones that could provide a standard candle in a way that we had never thought we had before,” he says. At the time, he was still an aspiring particle physicist rattling around at Lawrence Berkeley Lab looking for a juicy research project. Nothing really captivated him until he heard the holy song of the supernovas and saw the light of sci/religion. Here was a chance to tackle the biggest physics problem of all, the origin and fate of the cosmos. In essence, Perlmutter was rediscovering the motivations behind Einstein's 1917 cosmology: to find the one set of laws that explains the universe. He learned to express this argument forcefully over the years in order to convince Lawrence Berkeley Lab that its sponsor, the Department of Energy, really should be spending its dollars on a cosmology program.
In 1988 Perlmutter and Pennypacker split off from the rest of their research group and started hunting for supernovas among nearby galaxies, with “nearby” meaning that they were no more than a few hundred million light-years distant. And so the Superno
va Cosmology Project was born. The first of many daunting tasks Perlmutter faced was simply finding the supernovas. Historically, astronomers have considered these stellar detonations extremely rare events. On the local scale, that is certainly true. The last visible supernova in our galaxy was the bright star recorded by Johannes Kepler in 1604, five years before Galileo turned his first telescope skyward. In any one galaxy a Type 1a supernova lights up roughly once every three hundred years. On the large scale, however, the numbers were on Perlmutter's side. There are so many galaxies in the universe—about one hundred billion of them, according to recent estimates from the Hubble Space Telescope—that a supernova visible to today's largest telescopes appears every few seconds. Based on statistics alone, Perlmutter would have plenty of events to study. The difficulty lay in locating a single faint blip of light from among the sky's countless specks and then gathering enough of its feeble glimmering to reconstruct the tale of the recently deceased star.