It was the day after Christmas in 2004, a bright winter's day in Berkeley, California. I was outside a café at the corner of Shattuck and Cedar, waiting for Saul Perlmutter, an astrophysicist at the University of California. The campus is nestled at the base of wooded hills that rise steeply from the city's edge. About 1,000 feet up in the hills is the Lawrence Berkeley National Laboratory (LBNL). In the 1990s, the UC campus and LBNL housed several members of two teams of astronomers that simultaneously but independently discovered something that caused ripples of astonishment, even alarm. Our universe, it seems, is being blown apart.
Perlmutter was the leader of one of those teams. His enthusiastic, wide-eyed gaze, enhanced by enormous glasses, along with a forehead made larger by a receding hairline, reminded me of Woody Allen. But what he had found was no laughing matter. In fact, Perlmutter admitted that their discovery had thrown cosmology into crisis. The studies of distant supernovae by the two teams had shown that the expansion of the universe, first observed by Edwin Hubble in 1929, was accelerating - not, as many had predicted, slowing down. It was as if some mysterious energy were creating a repulsive force to counter gravity. Unsure as to its exact nature, cosmologists call it dark energy. More important, it seems to constitute nearly three-quarters of the total matter and energy in the universe.
Dark energy is the latest and most daunting puzzle to confront cosmologists, adding to another mystery that has haunted them for decades: dark matter. Nearly 90 percent of the mass of galaxies seems to be made of matter that is unknown and unseen. We know it must be there, for without its gravitational pull the galaxies would have disintegrated. Perlmutter pointed out that cosmologists in particular, and physicists in general, are now faced with the stark reality that roughly 96 percent of the universe cannot be explained with the theories at hand. All our efforts to understand the material world have illuminated only a tiny fraction of the cosmos.
And there are other mysteries. What is the origin of mass? What happened to the antimatter that should have been produced along with matter after the big bang? After almost a century of spectacular success at explaining our world using the twin pillars of modern physics - quantum mechanics and Einstein's general theory of relativity - physicists have reached a plateau of sorts. As Perlmutter put it, he and others are now looking to climb a steep stairway toward a new understanding of the universe, with only a foggy idea of what awaits them at the top.
Part of this seemingly superhuman effort will involve reconciling quantum mechanics with general relativity into a theory of quantum gravity. In situations where the two domains collide - where overwhelming gravity meets microscopic volumes, such as in black holes or in a big bang - the theories don't work well together. In fact, they fail miserably. One of the most ambitious attempts to bring them together is string theory, an edifice of incredible mathematical complexity. Its most ardent proponents hope that it will lead us not just to quantum gravity but to a theory of everything, allowing us to describe every aspect of the universe with a few elegant equations. But the discovery of dark energy and recent developments in string theory itself have conspired to confound. On yet another winter's day in the Bay Area, more than two years after meeting Perlmutter, I got a taste of just how grave things had gotten in physics.
It was a late February afternoon in 2007. A conference room on the ballroom level of the San Francisco Hilton was filled to capacity for this session at the annual meeting of the American Association for the Advancement of Science (AAAS). Three physicists were arguing about dark energy and how it relates to some of the most serious questions one can ask: Why is our universe the way it is? Is it fine-tuned for the existence of life? Dark energy, it turns out, is not merely mysterious; it seems to be at about the right value for the formation of stars and galaxies. “The great mystery is not why there is dark energy. The great mystery is why there is so little of it,” Leonard Susskind, Felix Bloch Professor of Theoretical Physics at Stanford and co-inventor of string theory, told the audience at the Hilton. He continued in a poetic vein: “The fact that we are just on the knife edge of existence, [that] if dark energy were very much bigger we wouldn't be here, that's the mystery.”
The hope until recently had been that string theory would explain this, that dark energy's value would fall out naturally as a solution to the theory's equations - as would the answers to other puzzling questions. Why does the proton weigh almost two thousand times more than the electron? Why is gravity so much weaker than the electromagnetic force? Essentially, why do the fundamental constants of nature have the values they do? The question of dark energy is emblematic of such concerns. Nothing in the laws of physics can explain why many aspects of our universe are what they are. They seem to be extraordinarily fine-tuned to produce a universe capable of supporting life - a fact that bothers physicists no end.
But string theory's hoped-for denouement is nowhere in sight. Indeed, some physicists are slowly abandoning the notion that everything about the universe can be reduced to a handful of equations. In San Francisco, Susskind rose to address this issue. His talk was titled “Why the Rats Are Fleeing the Ship.” However, abandoning reductionism hasn't meant abandoning string theory. Quite the contrary. For Susskind and many others, it has meant embracing the theory in all its mathematical glory, despite its mind-boggling consequences. One of the most outlandish implications of string theory, as it stands today, is the existence of a multiverse. The idea is that our universe is just one of a possible 10 to the five hundredth power universes, if not more. And in this extraordinary scenario lies an answer to the conundrum of why dark energy and other fundamental constants have the values they do. In a multiverse, all values of dark energy and fundamental constants are possible; in fact, the laws of physics can differ from universe to universe. To explain our universe, physicists don't have to resort to tweaking and fine-tuning. If a multiverse exists, then there is a fi- nite probability, however small, that our universe randomly emerged with the properties it has. The laws governing it give rise to stars and galaxies - and, indeed, planets and intelligent life, including physicists asking the question: Why is the universe the way it is?
This is the so-called anthropic principle, which, loosely stated, says that our universe is what it is because we are here to say so, and if it were any different we wouldn't exist to inquire. The idea is viewed by many as a cop-out, for then physicists don't have to work so hard to explain all things from first principles. Another speaker, cosmologist Andrei Linde, Susskind's colleague at Stanford, recalled his efforts to talk about the anthropic principle to physicists at Fermilab, outside Chicago, nearly twenty years ago. Linde had been warned that eggs were thrown at people who talked about such things, so he began by discussing something else entirely and switched topics midway, on the assumption that the Fermilabbers wouldn't “have enough time to go to Safeway and buy eggs.”
Given string theory's support for a multiverse, the anthropic principle is gaining traction. But string theory itself is so far from being experimentally verified that many physicists find it difficult, if not impossible, to take its implications seriously. The third parti...