Carroll found that process hidden inside one of the strangest and most exciting recent elaborations of the Big Bang theory. In 1984, MIT physicist Alan Guth suggested that the very young universe had gone through a brief period of runaway expansion, which he called “inflation,” and that this expansion had blown up one small corner of an earlier universe into everything we see. In the late 1980s Guth and other physicists, most notably Andrei Linde, now at Stanford, saw that inflation might happen over and over in a process of “eternal inflation.” As a result, pocket universes much like our own might be popping out of the uninflated background all the time. This multitude of universes was called, inevitably, the multiverse.
Carroll found in the multiverse concept a solution to both the direction and the origin of cosmic time. He had been musing over the arrow of time as far back as graduate school in the late 1980s, when he published papers on the feasibility of time travel using known physics. Eternal inflation suggested that it was not enough to think about time in our universe only; he realized he needed to consider it in a much bigger, multiverse context.
“We wondered if eternal inflation could work in both directions,” Carroll says. “That means there would be no need for a single Big Bang. Pocket universes would always sprout from the uninflated background. The trick needed to make eternal inflation work was to find a generic starting point: an easy-to-achieve condition that would occur infinitely many times and allow eternal inflation to flow in both directions.”
A full theory of eternal inflation came together in Carroll’s mind in 2004, while he was attending a five-month workshop on cosmology at the University of California at Santa Barbara’s famous Kavli Institute of Theoretical Physics with his student Jennifer Chen. “You go to a place like Kavli and you are away from the normal responsibilities of teaching,” Carroll says. “That gives you time to pull things together.” In those few months, Carroll and Chen worked out a vision of a profligate multiverse without beginnings, endings, or an arrow of time.
“All you need,” Carroll says, with a physicist’s penchant for understatement, “is to start with some empty space, a shard of dark energy, and some patience.” Dark energy—a hidden type of energy embedded in empty space, whose existence is strongly confirmed by recent observations—is crucial because quantum physics says that any energy field will always yield random fluctuations. In Carroll and Chen’s theory, fluctuations in the dark-energy background function as seeds that trigger new rounds of inflation, creating a crop of pocket universes from empty space.
“Some of these pocket universes will collapse into black holes and evaporate, taking themselves out of the picture,” Carroll says. “But others will expand forever. The ones that expand eventually thin out. They become the new empty space from which more inflation can start.” The whole process can happen again and again. Amazingly, the direction of time does not matter in the process. “That is the funny part. You can evolve the little inflating universes in either direction away from your generic starting point,” Carroll says. In the super-far past of our universe, long before the Big Bang, there could have been other Big Bangs for which the arrow of time ran in the opposite direction.
On the grandest scale, the multiverse is like a foam of interconnected pocket universes, completely symmetric with respect to time. Some universes move forward, but overall, an equal number move backward. With infinite space in infinite universes, there are no bounds on entropy. It can always increase; every universe is born with room (and entropy) to evolve. The Big Bang is just our Big Bang, and it is not unique. The question of before melts away because the multiverse has always existed and always will, evolving but—in a statistical sense—always the same.
After completing his multiverse paper with Chen, Carroll felt a twinge of dismay. “When you finish something like this, it’s bittersweet. The fun with hard problems can be in the chase,” he says. Luckily for him, the chase goes on. “Our paper really expresses a minority viewpoint,” he admits. He is now hard at work on follow-up papers fleshing out the details and bolstering his argument.
BIG IDEA 3: The Nows Have It
In 1999, while Steinhardt and Turok were convening in Cambridge and Carroll was meditating on the meaning of the multiverse, rebel physicist Julian Barbour published The End of Time—a manifesto suggesting that attempts to address what came before the Big Bang were based on a fundamental mistake. There is no need to find a solution to time’s beginning, Barbour insisted, because time does not actually exist.
Back in 1963, a magazine article had changed Barbour’s life. At the time he was just a young physics graduate student heading off for a relaxing trip to the mountains. “I was studying in Germany and had brought an article with me on holiday to the Bavarian Alps,” says Barbour, now 71. “It was about the great physicist Paul Dirac. He was speculating on the nature of time and space in the theory of relativity.” After finishing the article Barbour was left with a question he would never be able to relinquish: What, really, is time? He could not stop thinking about it. He turned around halfway up the mountain and never made it to the top.
Perhaps some universes move forward in time while an equal number move backward; the Big Bang is just our Big Bang.
“I knew that it would take years to understand my question,” Barbour recalls. “There was no way I could have a normal academic career, publishing paper after paper, and really get anywhere.” With bulldog determination he left academic physics and settled in rural England, supporting his family translating Russian scientific journals. Thirty-eight years later, still living in the same house, he has worked out enough answers to rise from obscurity and capture the attention of the world’s physics community.
In the 1970s Barbour began publishing his ideas in respected but slightly unconventional journals, like The British Journal for the Philosophy of Science and Proceedings of the Royal Society A. He continues to issue papers, most recently with his collaborator Edward Anderson (pdf) of the University of Cambridge. Barbour’s arguments are complex, but his core idea remains simplicity itself: There is no time (pdf). “If you try to get your hands on time, it’s always slipping through your fingers,” Barbour says with his disarming English charm. “My feeling is that people can’t get hold of time because it isn’t there at all.”