But once Zeldovich had shown that Lambda might result from known physical processes, it suddenly had new credibility as a part of the sci/religious cannon. Some researchers sought to explain why the vacuum energy is so small, while others sought to understand how it might influence the evolution of the cosmos. Edward Tryon, then just beginning his academic career at Columbia University, tried something much more radical. Starting in the late 1960s, he explored the possibility that the vacuum energy could explain not just the expansion of the universe, in its guise as Lambda, but where it came from in the first place. In essence, he was suggesting that the rules of physics could absorb the old-time role of God as creator in addition to God as rule-maker. Even Einstein had never attempted anything so outrageous.
Tryon thought about the particles that constantly appear out of nowhere in the vacuum. Normally they disappear before they can have any permanent existence. But what if just once the process got out of hand? Maybe the entire universe emerged from a vacuum fluctuation that, instead of immediately collapsing back in on itself, expanded outward in a rush of spontaneously generated matter and radiation. If so, our universe is a fluke, a random twitch of physics. Einstein, who hated physical explanations that depended on a roll of the dice, would have been aghast. Physical Review Letters rejected a paper describing Tryon's wildly fanciful proposal, and many scientists took it as a joke. But in 1973, the prestigious journal Nature published Tryon's paper, “Is the Universe a Vacuum Fluctuation?” “I offer the modest proposal that our Universe is simply one of those things which happen from time to time,” he wrote.
As the 1970s progressed, physicists began to feel increasingly at home in cosmology, and vice versa. As this new kind scientific unity took hold—convergence between very large and very small—Tryon's ideas started not to look so silly after all. The impetus for this interdisciplinary dabbling once again came from a project started by Einstein. His cosmic religious faith told him that nature should be harmonious, whereas the physical laws described in textbooks sounded like cacophony. So starting in the early 1920s, Einstein attempted to show that gravity and electro-magnetism, two very different kinds of natural forces, are in fact two aspects of a single fundamental force that can be described by a single set of equations. This mystical project occupied and confounded him for the last three decades of his life.
Twenty years after Einstein's death, three physicists partially vindicated his effort. Using elaborate mathematical arguments, Sheldon Glashow, Abdus Salam, and Steven Weinberg exposed an underlying commonality between electromagnetism and another force, the weak nuclear force that governs radioactive decay. The physicists showed that these two forces should in fact appear identical at high temperatures. In the early universe, when everything was extremely hot, the two forces were one and the same. All of a sudden, particle physicists wanted to know more about the physical conditions immediately after the big bang. At the same time, cosmologists had a new tool for exploring how the universe evolved into its present state.
All of these mind-spinning ideas—vacuum energy, Lambda, unified forces, matter emerging from nowhere—came dancing together in the head of Alan Guth during the late 1970s. At the time, Guth was an eager physics grad student at the Massachusetts Institute of Technology,, working under Weinberg, who indoctrinated him into the latest “grand unified theories,” which attempted to create an even more general physics theory that combined all forces except for gravity into a unified whole. Guth was no astronomer; he stumbled into cosmology only in a roundabout way. He was actually trying to fix a vexing glitch in these unified physics models. Theory predicted that the universe should be filled with junk particles, called “magnetic monopoles,” that should have formed almost immediately after the big bang. But the doctrine of falsification through observation soon cut that theory down: experiment after experiment revealed not a trace of the expected particles.
Looking for a way out, Guth played around with different descriptions of the vacuum energy in the early universe, which depended on some highly esoteric guesses about how the laws of physics behave at temperatures and pressures that lie far beyond the reach of experiment. Why not? After all, he was a youthful researcher working in a speculative realm where there were no absolute answers. At the end of 1979 he found, to his delight, that there is indeed an alternative account of the first moments of existence that gets rid of the unwanted monopoles. Basically, he did it by resurrecting Lambda, but in a much more potent form than anything Einstein envisioned.
In Guth's revised scenario, the universe passed through a momentary phase just after the first moment of existence when a tremendous amount of energy is trapped in the vacuum. That energy acted like a supercharged Lambda, a repulsive force that caused the infant universe to expand at breakneck speed. This energetic phase would have lasted just a moment, but it was a very eventful moment. During the brief episode—about 10-35 second, so when Guth said brief, boy, he really meant it—the universe grew by a factor of 1030. The human mind cannot really fathom such minute and enormous numbers. If you expanded an atom by a factor of 1030, it would dwarf our galaxy. The visible universe, which before the growth process was far smaller even than an atom, emerged about the size of a very large beach ball. Perhaps inspired by the staggering jumps in the American consumer price index during the late 1970s, Guth called this period of runaway cosmic expansion “inflation.”
Guth had explored inflation as a way to sweep away the unwanted monopoles, by expanding the universe so much they were diluted to undetectability. But he soon recognized that inflation had much bigger implications for cosmology. It resurrected that strange model of empty, exponentially expanding space that Willem de Sitter had introduced as “solution B” in 1917. Inflation resurrected Lambda and again gave it a central role as shaper of the universe. It implied that we see only one tiny corner of a much vaster universe, which might be a trillion trillion times bigger than what is visible to us. And above all, as Guth quickly discovered, it handily solved several of the big problems looming over the big bang.
A year earlier, Guth had heard a lecture by Dicke in which he had discussed the flatness problem. Now Guth had a brainstorm. Return once more to the rubber-sheet analogy of the universe. In the first instant after the big bang, that sheet could have had any arbitrary shape. But if you kept stretching the sheet, it would begin to look flat no matter how it started out. Guth realized that the inflation process would do the same thing to the shape of space-time. No matter what the initial conditions of the big bang, inflation would produce a flat universe with a mass very close to the critical density. In a fit of giddy enthusiasm, he jotted down this idea in his notebook under the heading SPECTACULAR REALIZATION. This page now sits like a holy relic, on display behind glass at the Adler Planetarium in Chicago.
Soon after this insight, Guth had another flash of inspiration during lunch at the Stanford Linear Accelerator Center (invariably referred to as SLAC), where he was working at the time. “December 1979 was my lucky month,” Guth writes. “A few weeks after the invention of inflation, I stumbled upon another key piece of evidence to support it.” Some theorists were explaining one of the other key cosmological conundrums: the smoothness of the cosmic microwave background, that much debated horizon problem. Inflation could provide a cure here, too, Guth realized. The universe could have started out extremely irregular, with hot and cold spots in any random pattern. Once inflation set in, however, any one spot would grow so large that it would fill our entire visible universe. It would not matter if our particular spot were a cold one or a warm one. The important point is that the temperature would be everywhere the same, just as astronomers observe when they study the cosmic microwave background. Any primordial lumpiness in the big bang would likewise get smeared out in the course of the mad stretching caused by inflation. Interestingly, the old steady state model of cosmology offered the same basic solution to the horizon problem, only it assumed that the smoothing took place over billions of years of normal expa
nsion, not an inflationary frenzy lasting a tiny fraction of a second.
Too much smoothess just brings us back to the question of how galaxies could have formed. But further theoretical explorations revealed a plausible explanation for the origin of structure in the universe. The answer lies once again in the quantum cornucopia of the vacuum. Recall that empty space is constantly pulsing with energy and matter. Normally these fluctuations are invisibly small, comparable in scale to subatomic particles. During inflation, however, the vacuum blips would have been tremendously magnified and stretched along with everything else. Stephen Hawking calculated in 1982 that inflation would expand quantum fluctuations to about the right size and density needed to form galaxy clusters. He thus linked Guth's ideas to something concrete and observable. If he were correct, subatomic physics operating at the smallest scales we can study is directly responsible for the largest structures we can see—another piece of beautiful sci/religious harmony.
Inflation was an instant smash in the world of cosmology. Suddenly, everyone wanted to know more about the shaggy-haired upstart from SLAC who had cured the big bang of its ills. There was only one little problem: As originally formulated by Guth, inflation didn't work. He quickly ran into problems when he explored the details of how the extra energy is first trapped and then released from the vacuum.
In a remarkable instance of parallel thinking that recalled the independent discovery of expanding cosmologies by Friedmann and Lemaitre in the 1920s, Andrei Linde at the Lebedev Physical Institute in Moscow had developed a theory of inflation almost identical to Guth's during the late 1970s. He failed to see the merit in the idea, however, and in a fit of despair decided “there was no reason to publish such garbage.” Then in 1979, Alexei Starobinsky at the Landau Institute for Theoretical Physics, also in Moscow, hit on a more realistic version of the theory, and Linde got excited again. “For two years, it remained the main topic of conversation at all conferences on cosmology in the Soviet Union,” Linde explains. In the West, however, this work was unknown. By 1981 Linde had heard of Guth's difficulties and devised an improved version of inflation that circumvented the problems. Two physicists at the University of Pennsylvania, Andreas Albrecht and Paul Steinhardt, arrived independently at the same fix. The new version of inflation differed in many technical respects, but its outline was unchanged: It started with a brief early era during which Lambda was enormous, causing the universe to swell exponentially, and ended with a return to conditions that put the universe back on the conventional big bang track.
Within a few years, inflation entered the sci/religious mainstream, completing the marriage of cosmology and theoretical physics that began in the 1940s when Gamow, Alpher, Hermann, Hoyle, and others had used nuclear physics models as a tool to test their notions about the big bang. Guth and Linde were now blending ideas from quantum physics, unified field theory, and observational cosmology to push the authority of science even closer to the first moment of existence. “The big bang says nothing about the bang itself—it really starts just after the bang. Inflation attempts to fill in the gaps, providing a prehistory that gives a possible explanation for the initial conditions that the big bang just assumed,” Guth says. Once again, Lambda was helping to uncover the secrets of the Old One.
Unfortunately, inflation theory did not offer much for the observers to test in the 1980s. There were a few ways to see if Guth's version of Lambda was any more credible than Einstein's, but they were difficult and likely to turn out inconclusive. For instance, the simplest version of inflation predicts that the density of the universe should be almost exactly the critical value. But measuring the cosmic density was no mean feat, especially considering that most of it seems to exist in an invisible form. Inflation also predicted a distinctive distribution of lumpiness in the universe, the fluctuations that seeded the formation of galaxy clusters. Maps of cosmic structure were not up to the task. Guth and Linde needed inflation to score some obvious empirical successes before it could properly join the expanding universe and the primal fireball in the canon of sci/religion. For the moment, inflation had the status of a powerful but unverified prophesy, similar to Einstein's cosmological model of 1917.
Emboldened by half a century of remarkable cosmological advances, many theorists were willing to ignore the lack of evidence for inflation, because they saw the theory as the only way to explain why the universe looks the way it does without a distasteful detour back into old-time religion or an unholy alliance with the anthropic principle. Veteran cosmologists regularly call for caution until sci/religion has brought more evidence to bear on the issue. “Inflation is a new idea, very attractive, I don't know any alternative to it, but I don't know if I should believe it,” says Peebles. But other members of the congregation are eager to proceed toward revelation. “I do not think we'll be able to disprove the theory of inflation,” says Linde, who lauds it as the “only explanation” for the odd flatness and smoothners of the universe. Inflation shoved aside the nasty anthropic principle, which is no longer needed to explain the fine-tuning of the universe, and required no special appeals to God. Best of all, inflation gave new respectability to the kind of starry-eyed speculation pioneered by Edward Tryon when he suggested that our universe popped out of the nothingness of the quantum void. One of the most exciting and baffling aspects of inflation is that it gives a plausible explanation of where all the matter in the universe came from. During inflation, the expansion was driven by the tremendous amount of energy trapped in the vacuum. At the end of the inflationary era, an abrupt change occurs. The amount of energy trapped within the vacuum plummets, and this energy appears in tangible form as heat. So much heat is released at this point that particles literally boil out of the vacuum. During this episode, still within the first moment of the first second of existence, all the mass of the universe pops out of nowhere. Inflation is the reason the big bang went bang.
Any sensible person would ask how something could be created from nothing. Einstein's famous E=mc2, which states that matter can transform into energy and vice versa, provides a partial answer. But if the matter came from energy, the energy still had to come from somewhere. What powered the big bang? Guth's cunning reply is that the mutual gravitational attraction among all the particles in the universe gives rise to an enormous amount of negative, or potential, energy. A vase sitting on a high shelf has negative energy relative to the floor. If it falls off the shelf, the energy is liberated and the vase gains enough velocity to shatter itself when it hits the ground. Likewise, Guth notes, the universe abounds with negative gravitational energy that would be liberated if everything were to fall together. This energy is so great that it could exactly equal the mass of the universe.
Guth, Linde, and the many other inflation theorists argue that the negative energy created as particles expanded away from each other in the early universe exactly balanced the positive energy needed to create that matter. Matter in motion creates the negative energy that allows the creation of more matter. Voila! Something from nothing. As Guth likes to say, “The universe is the ultimate free lunch.” (Actually, Guth notes that a tiny universe already exists when inflation kicks in, so the lunch is not entirely free. He corrects himself and declares that “the universe is a very cheap lunch.”) It sounds like unbelievable sleight of hand, but other scientists take the idea as a serious solution to the problem of our origin. In the inflationary view, once inflation ended and all the particles popped into existence, the universe would look just like the hot big bang that cosmologists had envisioned since the time of Gamow's landmark 1948 paper. There would be no obvious sign of where the matter came from or why it was now so uniform.
For all that, inflation still leaves some of the most fundamental questions hanging. It doesn't explain why the strength of gravity, the electron structure of the carbon atom, and countless other details are exactly what they need them to be for people to exist. It doesn't explain why we inhabit a universe whose properties allowed inflation to occur in th
e first place.
“Why was it necessary for God to work to such precision?” Linde wondered. He has provided his own answer with an elaboration of the idea, called “chaotic inflation.” Linde envisions that the universe arose among the random field fluctuations in a preexisting universe. In one spot, the state of the quantum fields was just right to touch off an episode of inflation, and there our universe was born. But this was far from a onetime occurrence. As Linde pictures it, new regions of space are constantly budding off old ones, inflating, experiencing their own big bangs, and evolving into entire separate universes that could each have their own physical laws. A heroic kind of super-Lambda takes on the role of the Creator, keeping the process going endlessly.
“When I invented this theory, I was so excited I stopped everything,” he says. Chaotic inflation seemed to him an amazingly general and economical way to account for the nature of our cosmos. If there are, as he believes, an infinite number of universes, then we do not have to wonder about what happened before the big bang or what lies beyond the visible edge of the cosmos. Our universe is just a speck in an eternal, limitless “multiverse.” “Here the universe produces itself in all possible forms,” he says. This idea echoes the endless cycles of renewal and decline postulated by the pre-Christian Greeks and contains elements of the oscillating or “phoenix” universe that once appealed to both Einstein and Gamow. But the self-reproducing universe has a theological beauty that neither of these possessed. If Linde's interpretation is on target, then the anthropic principle starts to make more sense. Most of the universes may well be hostile to life; many will be short-lived; maybe every other one is sterile. But with an endless number to choose from, it is not surprising that at least one of them turned out to be just right for us. “Anything in physics that isn't forbidden is compulsory,” Guth says. “Once you accept eternal inflation, anything that can happen will happen.”
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