One of my favorite moments in Woody Allen's film Annie Hall is when Alvy Singer (Allen's alter ego) is shown having an existential crisis as a little boy. His mother summons a psychiatrist, one Dr. Flicker, to find out what's wrong.
"Why are you depressed, Alvy?" Dr. Flicker asks.
"The universe is expanding," Alvy says. "The universe is everything, and if it's expanding, some day it will break apart and that will be the end of everything."
"Why is that your business?" interrupts his mother. Turning to the psychiatrist, she announces, "He's stopped doing his homework!"
"What's the point?" Alvy says.
"What has the universe got to do with it!" his mother shouts. "You're here in Brooklyn! Brooklyn is not expanding!"
Dr. Flicker jumps in: "It won't be expanding for billions of years, Alvy, and we've got to enjoy ourselves while we're here, eh? Ha ha ha." (Cut to a view of the Singer house, which happens to be under the Coney Island roller coaster.)
I used to take Dr. Flicker's side in this matter. How silly to despond about the end of everything! After all, the cosmos was born only around 13 billion years ago, when the Big Bang happened, and parts of it will remain hospitable to our descendants for a good hundred billion years, even as the whole thing continues to spread out.
A half-dozen years ago, however, astronomers peering through their telescopes began to notice something rather alarming. The expansion of the universe, their observations indicated, was not proceeding at the stately, ever-slowing pace that Einstein's equations had predicted. Instead, it was speeding up. Some "dark energy" was evidently pushing against gravity, sending galaxies hurtling away from one another at a runaway rate. New measurements earlier this year confirmed this strange finding. Last July 22, the New York Times ran an ominous headline: "ASTRONOMERS REPORT EVIDENCE OF 'DARK ENERGY' SPLITTING THE UNIVERSE." David Letterman found this so disturbing that he mentioned it several consecutive nights in his Late Show monologue, wondering why the Times buried the story on Page A-13.
Until recently, the ultimate destiny of the universe looked a little more hopeful—or remote. Back around the middle of the last century, cosmologists figured out that there were two possible fates for the universe. Either it would continue to expand forever, getting very cold and very dark as the stars winked out one by one, the black holes evaporated, and all material structures disintegrated into an increasingly dilute sea of elementary particles: the Big Chill. Or it would eventually stop expanding and collapse back upon itself in a fiery, all-annihilating implosion: the Big Crunch.
Which of these two scenarios would come to pass depended on one crucial thing: how much stuff there was in the universe. So, at least, said Einstein's theory of general relativity. Stuff—matter and energy—creates gravity. And, as every undergraduate physics major will tell you, gravity sucks. It tends to draw things together. With enough stuff, and hence enough gravity, the expansion of the universe would eventually be arrested and reversed. With too little stuff, the gravity would merely slow the expansion, which would go on forever. So, to determine how the universe would ultimately expire, cosmologists thought that all they had to do was to weigh it. And preliminary estimates—taking account of the visible galaxies, the so-called "dark matter," and even the possible mass of the little neutrinos that swarm though it all—suggested that the universe had only enough weight to slow the expansion, not to turn it around.
Now, as cosmic fates go, the Big Chill might not seem a whole lot better than the Big Crunch. In the first, the temperature goes to absolute zero; in the second, it goes to infinity. Extinction by fire or by ice—what's to choose? Yet a few imaginative scientists, haunted, like Woody Allen, by visions of the end of the universe, came up with formulations of how our distant descendants might manage to go on enjoying life forever, despite these unpleasant conditions. In the Big Chill scenario, they could have an infinity of slower and slower experiences, with lots of sleep in between. In the Big Crunch scenario, they could have an infinity of faster and faster experiences in the run-up to the final implosion. Either way, the progress of civilization would be unlimited. No cause for existential gloom.
So, Letterman had reason to be upset by the dark energy news. It spells inescapable doom for intelligent life in the far, far future. No matter where you are located, the rest of the universe would eventually be receding from you at the speed of light, slipping forever beyond the horizon of knowability. Meanwhile, the shrinking region of space still accessible to you will fill up with a kind of insidious radiation that would eventually choke off information processing—and with it, the very possibility of thought. We seem to be headed not for a Big Crunch or a Big Chill but something far nastier: a Big Crackup. "All our knowledge, civilization and culture are destined to be forgotten," one prominent cosmologist has declared to the press. It looks as if little Alvy Singer was right after all. The universe is going to "break apart," and that will indeed mean the end of everything—even Brooklyn.
Hearing this news made me think of the inscription that someone once said should be on all churches: important if true. Applied to cosmology—the study of the universe as a whole—that is a big "if." Cosmic speculations that make it into the newspapers should often be taken with a pinch of salt. A few years ago, some astronomers from Johns Hopkins made headlines by announcing that the cosmos was turquoise; two months later they made headlines again by announcing that, no, it was actually beige. This may be a frivolous example, but even in graver matters—like the fate of the universe—cosmologists tend to reverse themselves every decade or so. As one of them once told me, cosmology is not really a science at all since you can't do experiments with the universe. It's more like a detective story. Even the term that is sometimes applied to theorizing about the end of the universe, "eschatology" (from the Greek word for "furthest") is borrowed from theology.
Before I was going to start worrying about the extinction of absolutely everything in some inconceivably distant epoch, I thought it would be a good idea to talk to a few leading cosmologists. Just how certain were they that the cosmos was undergoing a disastrous runaway expansion? Was intelligent life really doomed to perish as a result? How could they, as scientists, talk about the ultimate future of "civilization" and "consciousness" with a straight face?
It seemed natural to start with Freeman Dyson, an English-born physicist who has been at the Institute for Advanced Study in Princeton since the 1940s. Dyson is one of the founding fathers of cosmic eschatology, which he concedes is a "faintly disreputable" subject. He is also a fierce optimist about the far future, one who envisions "a universe growing without limit in richness and complexity, a universe of life surviving forever and making itself known to its neighbors across unimaginable gulfs of space and time." In 1979, he wrote a paper called "Time Without End," in which he used the laws of physics to show how humanity could flourish eternally in a slowly expanding universe, even as the stars died and the chill became absolute. The trick is to match your metabolism to the falling temperature, thinking your thoughts ever more slowly and hibernating for longer and longer periods while extraneous information is dumped into the void as waste heat. In this way, Dyson calculated, a complex society could go on perpetually with a finite energy reserve, one equivalent to a mere eight hours of sunlight.
The day I went to see Dyson, it was raining in Princeton. It took me a half-hour to walk from the train station to the Institute for Advanced Study, which sits by a pond in 500 acres of woods. The institute is a serene, otherworldly place. There are no students to distract the eminent scientists and scholars in residence from pursuing their intellectual fancies. Dyson's office is in the same building where Einstein spent the last decades of his career fruitlessly searching for a unified theory of physics. An elfin, courtly man with deep-set eyes and a hawklike nose, Dyson frequently lapsed into silence or emitted snuffles of amusement. I started by asking him whether the evidence that the universe was caught up in an accelerated expansion had blighted his hopes for the future of civilization.
"Not necessarily," he said. "It's a completely open question whether this acceleration will continue forever or whether it will peter out after a while. There are several theories of what kind of cosmic field might be causing it and no observations to determine which of them is right. If it's caused by the so-called 'dark energy' of empty space, then the expansion will keep speeding up forever, which is bad news as far as life is concerned. But if it's caused by some other kind of force field—which, out of ignorance, we label 'quintessence'—then the expansion might well slow down as we go into the future. Some quintessence theories even say that the universe will eventually stop expanding altogether and collapse. Of course, that, too, would be unfortunate for civilization since nothing would survive the Big Crunch."
Well, then, I said, let's stick with the optimistic scenario. Suppose the acceleration does turn out to be temporary and the future universe settles into a nice cruise-control expansion. What could our descendants possibly look like a trillion trillion trillion years from now, when the stars have disappeared and the universe is dark and freezing and so diffuse that it's practically empty? What will they be made of?
"The most plausible answer," Dyson said, "is that conscious life will take the form of interstellar dust clouds." He was alluding to the kind of inorganic life forms imagined by the late astronomer Sir Fred Hoyle in his 1957 science fiction novel, The Black Cloud. "An ever-expanding network of charged dust particles, communicating by electromagnetic forces, has all the complexity necessary for thinking an infinite number of novel thoughts."
How, I objected, can we really imagine such a wispy thing, spread out over billions of light-years of space, being conscious?
"Well," he said, "how do you imagine a couple of kilograms of protoplasm in someone's skull being conscious? We have no idea how that works either."
Practically next door to Dyson at the institute is the office of Ed Witten, a gangly, 50-ish fellow who is widely regarded as the smartest physicist of his generation, if not the living incarnation of Einstein. Witten is one of the prime movers behind superstring theory, which, if its hairy math is ever sorted out, may well furnish the Theory of Everything that physicists have long been after. He has an unnerving ability to shuffle complicated equations in his head without ever writing anything down, and he speaks in a hushed, soft voice. Earlier this year, Witten was quoted in the press calling the discovery of the runaway expansion of the universe "an extremely uncomfortable result." Why, I wondered, did he see it that way? Was it simply inconvenient for theoretical reasons? Or did he worry about its implications for the destiny of the cosmos? When I asked him, he agonized for a moment before responding, "Both."
Yet Witten, too, thought there was a good chance that the runaway expansion would be only temporary, as some of the quintessence theories predicted, rather than permanent, as the dark-energy hypothesis implied. "The quintessence theories are nicer, and I hope they're right," he told me. If the acceleration does indeed relax to zero, and the Big Crackup is averted, could civilization go on forever? Witten was unsure. One cause for concern was the possibility that protons will eventually decay, resulting in the dissolution of all matter within another, oh, 10^33 years or so. Freeman Dyson had scoffed at this when I talked with him, pointing out that no one had ever observed a proton decaying, but he insisted that intelligent beings could persist even if atoms fell to pieces, by re-embodying themselves in "plasma clouds"—swarms of electrons and positrons. I mentioned this to Witten. "Did Dyson really say that?" he exclaimed. "Good. Because I think protons probably do decay."