The Big Bang (one more time)
Just when it SEEMS the broad outline of the universe's origin is as safely in hand as money in the bank, along come two physicists who could prove to be the Butch Cassidy and Sundance Kid of cosmology.
For 20 years, Paul Steinhardt has played a key role in helping to write and refine the inflationary "big bang" origin of the universe.
But over the past few years, the Princeton University physicist and some of his colleagues have struggled with a vexing question. "Even if our story seems to describe what we see, how do we know it's the right story?" Dr. Steinhardt asks.
He decided to see if he could come up with a plausible alternative to the prevailing notion. The inflationary big-bang model posits that the universe began as a random fluctuation in empty space, grew with extraordinary speed through an "inflationary" period, then slowed, cooled, and formed all the matter and energy astronomers see and infer today. Steinhardt wanted to be able to describe the universe with as much precision as the inflation theory does but without some of its "baggage," including the need for an inflationary period itself. The result: He and Cambridge University physicist Neil Turok have unveiled a model in which the universe has no beginning or end, but replenishes itself in a cycle of expansion and contraction. Each expansion is triggered by its own big bang.
Steinhardt adds that, among its other selling points, the new model naturally accounts for recent observations that the universe is entering an epoch of accelerated expansion, a feature he says is not directly predicted by the inflationary big bang.
The model, published late last month in the journal Science, is a work in progress. Some researchers, such as Princeton astrophysicist Jeremiah Ostriker, have hailed it as "extraordinarily exciting and ... the first new big idea in cosmology in over two decades." Others, such as Stanford University's Andrei Linde, another inflation pioneer, are intensely skeptical.
The duo's rebellion against the reigning explanation for the universe's origin may seem out of character for what many people view as the staid world of science. "People have the idea that scientists and cosmologists believe something like it was dogma," says Rocky Kolb, a cosmologist at the Fermi National Accelerator Laboratory in Batavia, Ill. "We seem like a conservative lot, but ... everybody wants to turn everything upside down."
But new ideas have to pass "sniff tests." And for some researchers, ideas posed in prose, with no math for backup, are sure-fire candidates for the circular file.
"Really beautiful ideas have experimental and observational consequences," although decades may pass before predicted results emerge, Kolb says.
He also points out that "a lot of ideas come from people in different fields or people you may not know. In 1980, not many cosmologists had heard of Alan Guth, who came up with inflation."
Drawing on quantum physics and Einstein's theories of relativity, the inflationary big bang begins 11 billion to 15 billion years ago, when a random change in an astonishingly small bit of all-encompassing vacuum grew very rapidly.
For a tiny fraction of a second, this burgeoning fluctuation in the vacuum expanded faster than light can travel. When the burst of inflation abruptly ended, the proto-universe reached enormous energies, temperatures, and pressures. As this roiling universe continued to expand at a more sedate rate and cooled, its matter, energy, and structure emerged.
The inflationary model has fended off all comers; it matches many observations and has described features of the early universe that astronomers searched for and found, Steinhardt says.
For Dr. Guth, a physics professor at the Massachusetts Institute of Technology, inflation was a solution to a particle-physics conundrum. The problem emerged from efforts in the late 1970s to develop grand unified theories to describe the emergence of three of the four forces of nature and the associated subatomic particles from the big bang.
At the time, Guth and collaborator Henry Tye determined that grand unified theories predicted and sometimes required formation of magnetic monopoles. These "outrageously heavy" particles "had always been consistent with the laws of physics, but no one had ever seen one and there was no real reason to believe they existed," Guth says.
Prodded by Dr. Tye, Guth says he reluctantly calculated the number of these particles that would have been created during the big bang. They discovered that monopoles would have been as ubiquitous as protons leading to a universe that would look much different from the one we inhabit.
Guth's calculations led him to conclude that a brief period of inflation stifled monopole formation. As a bonus, inflation also appeared to solve problems cosmologists had in squaring conventional big-bang thinking with astronomers' observations of the universe.
Particle-physics theorists were the most receptive to this view, Guth says. By contrast, he says, astronomers and cosmologists "took a wait-and-see attitude."
This same interplay between the world of the very tiny and the world of the very large weaves its way through Steinhardt's and Dr. Turok's cyclical universe. They hold that much of their model works well in a four-dimensional universe of height, depth, width, and space-time. They add that it finds its true home in the nine to 10 dimensions of string theory, which tries to explain how the four forces of nature emerged from one unified force early on.
One variation, known as M theory, holds that the universe consists of two parallel sheets, or membranes. The two membranes are separated by a "fifth" dimension a tiny fraction of a centimeter wide.
Steinhardt and Turok's calculations describe the membranes meeting in a slap, triggering the big bang. On the membrane humans inhabit, the bang yields the particles, energy, and forces familiar to scientists. The other contains "we know not what," Steinhardt says. The duo posits the second one may be home to "dark matter."
Over trillions of years, the membranes expand, growing darker, colder, and less dense, until the logic of Steinhardt's equations brings them back together in another cosmic slap. The membranes resume expanding, even as they drift apart, only to repeat the cycle.
Steinhardt says this model yields all the features of the inflationary model, without inflation. What his calculations don't show is what happens when membranes "bounce."
As always, nature will provide the ultimate reality check on his model. "There are many beautiful, insightful ideas that turn out not to be the way nature works. The cyclical universe may be one of them," Kolb says. "I don't think this is the way nature works. Maybe there'll be an application for it someday. Or some of the ideas may be used in some other way."
Steinhardt hasn't tossed in the towel on inflation, either: "I'm just hedging my bets."