Physics & Math / Cosmology

Image courtesy of NASA

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.