For Paul Steinhardt and Neil Turok, the Big Bang ended on a summer day in 1999 in Cambridge, England. Sitting together at a conference they had organized, called “A School on Connecting Fundamental Physics and Cosmology,” the two physicists suddenly hit on the same idea. Maybe science was finally ready to tackle the mystery of what made the Big Bang go bang. And if so, then maybe science could also address one of the deepest questions of all: What came before the Big Bang?
Steinhardt and Turok — working closely with a few like-minded colleagues — have now developed these insights into a thorough alternative to the prevailing, Genesis-like view of cosmology. According to the Big Bang theory, the whole universe emerged during a single moment some 13.7 billion years ago. In the competing theory, our universe generates and regenerates itself in an endless cycle of creation. The latest version of the cyclic model even matches key pieces of observational evidence supporting the older view.
This is the most detailed challenge yet to the 40-year-old orthodoxy of the Big Bang. Some researchers go further and envision a type of infinite time that plays out not just in this universe but in a multiverse — a multitude of universes, each with its own laws of physics and its own life story. Still others seek to revise the very idea of time, rendering the concept of a “beginning” meaningless.
All of these cosmology heretics agree on one thing: The Big Bang no longer defines the limit of how far the human mind can explore.
Big Idea 1: The Incredible Bulk
The latest elaboration of Steinhardt and Turok’s cyclic cosmology, spearheaded by Evgeny Buchbinder of Perimeter Institute for Theoretical Physics in Waterloo, Ontario, was published last December. Yet the impulse behind this work far predates modern theories of the universe. In the fourth century A.D., St. Augustine pondered what the Lord was doing before the first day of Genesis (wryly repeating the exasperated retort that “He was preparing Hell for those who pry too deep”). The question became a scientific one in 1929, when Edwin Hubble determined that the universe was expanding. Extrapolated backward, Hubble’s observation suggested the cosmos was flying apart from an explosive origin, the fabled Big Bang.
In the standard interpretation of the Big Bang, which took shape in the 1960s, the formative event was not an explosion that occurred at some point in space and time — it was an explosion of space and time. In this view, time did not exist beforehand. Even for many researchers in the field, this was a bitter pill to swallow. It is hard to imagine time just starting: How does a universe decide when it is time to pop into existence?
For years, every attempt to understand what happened in that formative moment quickly hit a dead end. In the standard Big Bang model, the universe began in a state of near-infinite density and temperature. At such extremes the known laws of physics break down. To push all the way back to the beginning of time, physicists needed a new theory, one that blended general relativity with quantum mechanics.
The prospects for making sense of the Big Bang began to improve in the 1990s as physicists refined their ideas in string theory, a promising approach for reconciling the relativity and quantum views. Nobody knows yet whether string theory matches up with the real world — the Large Hadron Collider, a particle smasher coming on line later this year, may provide some clues — but it has already inspired stunning ideas about how the universe is constructed. Most notably, current versions of string theory posit seven hidden dimensions of space in addition to the three we experience.
Strange and wonderful things can happen in those extra dimensions: That is what inspired Steinhardt (of Princeton University) and Turok (of Cambridge University) to set up their fateful conference in 1999. “We organized the conference because we both felt that the standard Big Bang model was failing to explain things,” Turok says. “We wanted to bring people together to talk about what string theory could do for cosmology.”
The key concept turned out to be a “brane,” a three-dimensional world embedded in a higher-dimensional space (the term, in the language of string theory, is just short for membrane). “People had just started talking about branes when we set up the conference,” Steinhardt recalls. “Together Neil and I went to a talk where the speaker was describing them as static objects. Afterward we both asked the same question: What happens if the branes can move? What happens if they collide?”
A remarkable picture began to take shape in the two physicists’ minds. A sheet of paper blowing in the wind is a kind of two-dimensional membrane tumbling through our three-dimensional world. For Steinhardt and Turok, our entire universe is just one sheet, or 3-D brane, moving through a four-dimensional background called “the bulk.” Our brane is not the only one; there are others moving through the bulk as well. Just as two sheets of paper could be blown together in a storm, different 3-D branes could collide within the bulk.
The equations of string theory indicated that each 3-D brane would exert powerful forces on others nearby in the bulk. Vast quantities of energy lie bound up in those forces. A collision between two branes could unleash those energies. From the inside, the result would look like a tremendous explosion. Even more intriguing, the theoretical characteristics of that explosion closely matched the observed properties of the Big Bang — including the cosmic microwave background, the afterglow of the universe’s fiercely hot early days. “That was amazing for us because it meant colliding branes could explain one of the key pieces of evidence people use to support the Big Bang,” Steinhardt says.
Three years later came a second epiphany: Steinhardt and Turok found their story did not end after the collision. “We weren’t looking for cycles,” Steinhardt says, “but the model naturally produces them.” After a collision, energy gives rise to matter in the brane worlds. The matter then evolves into the kind of universe we know: galaxies, stars, planets, the works. Space within the branes expands, and at first the distance between the branes (in the bulk) grows too. When the brane worlds expand so much that their space is nearly empty, however, attractive forces between the branes draw the world-sheets together again. A new collision occurs, and a new cycle of creation begins. In this model, each round of existence — each cycle from one collision to the next — stretches about a trillion years. By that reckoning, our universe is still in its infancy, being only 0.1 percent of the way through the current cycle.
The cyclic universe directly solves the problem of before. With an infinity of Big Bangs, time stretches into forever in both directions. “The Big Bang was not the beginning of space and time,” Steinhardt says. “There was a before, and before matters because it leaves an imprint on what happens in the next cycle.”
Not everyone is pleased by this departure from the usual cosmological thinking. Some researchers consider Steinhardt and Turok’s ideas misguided or even dangerous. “I had one well-respected scientist tell me we should stop because we were undermining public confidence in the Big Bang,” Turok says. But part of the appeal of the cyclic universe is that it is not just a beautiful idea — it is a testable one.
The standard model of the early universe predicts that space is full of gravitational waves, ripples in space-time left over from the first instants after the Big Bang. These waves look very different in the cyclic model, and those differences could be measured — as soon as physicists develop an effective gravity-wave detector. “It may take 20 years before we have the technology,” Turok says, “but in principle it can be done. Given the importance of the question, I’d say it’s worth the wait.”
Big Idea 2: Time’s Arrow
While the concept of a cyclic universe provides a way to explore the Big Bang’s past, some scientists believe that Steinhardt and Turok have skirted the deeper issue of origins. “The real problem is not the beginning of time but the arrow of time,” says Sean Carroll, a theoretical physicist at Caltech. “Looking for a universe that repeats itself is exactly what you do not want. Cycles still give us a time that flows with a definite direction, and the direction of time is the very thing we need to explain.”
In 2004 Carroll and a graduate student of his, Jennifer Chen, came up with a much different answer (pdf) to the problem of before. In his view, time’s arrow and time’s beginning cannot be treated separately: There is no way to address what came before the Big Bang until we understand why the before precedes the after. Like Steinhardt and Turok, Carroll thinks that finding the answer requires rethinking the full extent of the universe, but Carroll is not satisfied with adding more dimensions. He also wants to add more universes — a whole lot more of them — to show that, in the big picture, time does not flow so much as advance symmetrically backward and forward.
Barbour argues that time is an illusion, with each moment — each “Now” — existing in its own right, complete and whole.
The one-way progression of time, always into the future, is one of the greatest enigmas in physics. The equations governing individual objects do not care about time’s direction. Imagine a movie of two billiard balls colliding; there is no way to say if the movie is being run forward or backward. But if you gather a zillion atoms together in something like a balloon, past and future look very different. Pop the balloon and the air molecules inside quickly fill the entire space; they never race backward to reinflate the balloon.
In any such large group of objects, the system trends toward equilibrium. Physicists use the term entropy to describe how far a system is from equilibrium. The closer it is, the higher its entropy; full equilibrium is, by definition, the maximum value. So the path from low entropy (all the molecules in one corner of the room, unstable) to maximum entropy (the molecules evenly distributed in the room, stable) defines the arrow of time. The route to equilibrium separates before from after. Once you hit equilibrium the arrow of time no longer matters, because change is no longer possible.
“Our universe has been evolving for 13 billion years,” Carroll says, “so it clearly did not start in equilibrium.” Rather, all the matter, energy, space, and even time in the universe must have started in a state of extraordinarily low entropy. That is the only way we could begin with a Big Bang and end up with the wonderfully diverse cosmos of today. Understand how that happened, Carroll argues, and you will understand the bigger process that brought our universe into being.
To demonstrate just how strange our universe is, Carroll considers all the other ways it might have been constructed. Thinking about the range of possibilities, he wonders: “Why did the initial setup of the universe allow cosmic time to have a direction? There are an infinite number of ways the initial universe could have been set up. An overwhelming majority of them have high entropy.” These high-entropy universes would be boring and inert; evolution and change would not be possible. Such a universe could not produce galaxies and stars, and it certainly could not support life.
It is almost as if our universe were fine-tuned to start out far from equilibrium so it could possess an arrow of time. But to a physicist, invoking fine-tuning is akin to saying “a miracle occurred.” For Carroll, the challenge was finding a process that would explain the universe’s low entropy naturally, without any appeal to incredible coincidence or (worse) to a miracle.
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. “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.”
Isaac Newton thought of time as a river flowing at the same rate everywhere. Albert Einstein unified space and time into a single entity, but he still held on to the concept of time as a measure of change. In Barbour’s view there is no invisible river of time. Instead, he thinks that change merely creates an illusion of time, with each individual moment existing in its own right, complete and whole. He calls these moments “Nows.”
“As we live, we seem to move through a succession of Nows. The question is, what are they?” Barbour asks. His answer: Each Now is an arrangement of everything in the universe. “We have the strong impression that things have definite positions relative to each other. I aim to abstract away everything we cannot see, directly or indirectly, and simply keep this idea of many different things coexisting at once. There are simply the Nows, nothing more and nothing less.”
Barbour’s Nows can be imagined as pages of a novel ripped from the book’s spine and tossed randomly onto the floor. Each page is a separate entity. Arranging the pages in some special order and moving through them step by step makes it seem that a story is unfolding. Even so, no matter how we arrange the sheets, each page is complete and independent. For Barbour, reality is just the physics of these Nows taken together as a whole.
“What really intrigues me is that the totality of all possible Nows has a very special structure,” he says. “You can think of it as a landscape or country. Each point in this country is a Now, and I call the country Platonia,” in reference to Plato’s conception of a deeper reality, “because it is timeless and created by perfect mathematical rules. Platonia is the true arena of the universe.”
In Platonia all possible configurations of the universe, every possible location of every atom, exist simultaneously. There is no past moment that flows into a future moment; the question of what came before the Big Bang never arises because Barbour’s cosmology has no time. The Big Bang is not an event in the distant past; it is just one special place in Platonia.
Our illusion of the past comes because each Now in Platonia contains objects that appear as “records,” in Barbour’s language. “The only evidence you have of last week is your memory — but memory comes from a stable structure of neurons in your brain now. The only evidence we have of the earth’s past are rocks and fossils — but these are just stable structures in the form of an arrangement of minerals we examine in the present. All we have are these records, and we only have them in this Now,” Barbour says. In his theory, some Nows are linked to others in Platonia’s landscape even though they all exist simultaneously. Those links create the appearance of a sequence from past to future, but there is no actual flow of time from one Now to another.
“Think of the integers,” Barbour says. “Every integer exists simultaneously. But some of the integers are linked in structure, like the set of all primes or the numbers you get from the Fibonacci series.” Yet the number 3 does not occur in the past of the number 5 any more than the Big Bang exists in the past of the year 2008.
These ideas might sound like the stuff of late-night dorm-room conversations, but Barbour has spent four decades hammering them out in the hard language of mathematical physics (pdf). He has blended Platonia with the equations of quantum mechanics to devise a mathematical description of a “changeless” physics. With Irish collaborator Niall Ó Murchadha of the National University of Ireland in Cork, Barbour is continuing to reformulate a time-free version of Einstein’s theory.
So What Really Happened?
For each of the alternatives to the Big Bang, it is easier to demonstrate the appeal of the idea than to prove that it is correct. Steinhardt and Turok’s cyclic cosmology can account for critical pieces of evidence usually cited to support the Big Bang, but the experiments that could put it over the top are decades away. Carroll’s model of the multiverse depends on a speculative interpretation of inflationary cosmology, which is itself only loosely verified.
Barbour stands at the farthest extreme. He has no way to test his concept of Platonia. The power of his ideas rests heavily on the beauty of their formulation and on their capacity to unify physics. “What we are working out now is simple and coherent,” Barbour says, “and because of that I believe it is showing us something fundamental.”
The payoff that Barbour offers is not just a mathematical solution but a philosophical one. In place of all the conflicting notions about the Big Bang and what came before, he offers a way out. He proposes letting go of the past — of the whole idea of the past — and living fully, happily, in the Now.
In one model, each round of existence stretches a trillion years. By that reckoning, our universe is still in its infancy.
