February 17, 1997

When did time begin?

Easy: When eternal inflation ended and ordinary inflation began in our corner of the cosmos

By Robert Irion

Time and the heaven came into being at the same instant in order that, having been created together, if ever there was to be a dissolution of them, they might be dissolved together.
--Plato, Timaeus

Many questions in cosmology appear to straddle the fuzzy fence between science and philosophy. "When did time begin?" is one of them.

One approach to answering that question falls firmly on the side of science, says UCSC cosmologist Joel Primack. Admittedly, it calls upon some seemingly fantastic scenarios for the origins of the universe itself. But these scenarios are grounded in equations and physical principles, not merely in intellectual parlor discourse.

"The equations come first," says Primack, a professor of physics. "We take them seriously, and then we push them as far as our imaginations lead us, including to before the beginning of time."

It may seem fruitless to raise this topic when most people can't grasp that the next millennium begins on January 1, 2001, not on January 1, 2000. But Primack is going to try, in a session titled "The Nature of Time" today (February 17) at a meeting of the American Association for the Advancement of Science in Seattle.

Here's his short answer: Time began precisely at the boundary between "eternal inflation," an everlasting cauldron of chaotic expansion in which our entire visible universe is just one tiny bubble, and "inflation," a more orderly but still extraordinarily rapid expansion that set the stage for the Big Bang. Not to worry, Primack will oblige those who seek a longer answer with some background on each of those concepts.

First, the Big Bang, now embedded in public consciousness as the moment of our universe's birth. Many independent lines of evidence support the Big Bang. However, the theory has manifold shortcomings, Primack notes. "That the universe began in a hot dense state and then expanded is very well established," he says. "The Big Bang is almost certainly true. However, it's terribly unsatisfying because the Big Bang itself requires such special conditions to turn into a universe like the one we observe. So, we need to figure out what came before."

In the early 1980s several theorists, notably Alan Guth of MIT, devised the theory of cosmic inflation to address those issues. Inflation holds that the universe grew exponentially--doubling in size, then doubling again, and so on--for an extremely small fraction of a second before the Big Bang (perhaps as little as ten billion-billion-billion-billionths of a second). This made the primordial universe extremely uniform. However, inflation also caused minuscule quantum fluctuations within the fabric of this burst, forming seeds for the vast clusters of galaxies we see today, cosmologists believe. In essence, says Primack, "Inflation created the proper initial conditions for the Big Bang."

Remarkably, it's possible to test this hypothesis by observing the most distant reaches of our universe. Thus far inflation has withstood scrutiny, but a new generation of astrophysical satellites and ground-based observations will put it to a true test over the next decade. In the meantime, theorists have no qualms speculating what might have preceded the brief inflationary era. One such proposal, by Stanford University physicist Andrei Linde, is a wild notion indeed.

Linde devised his scenario, "eternal inflation," by extrapolating the equations of inflation back in time. The tiny quantum fluctuations at the end of inflation (about one part in 100,000) would have dominated the equations as inflation began, he found. These random fluctuations are big enough, Linde maintains, to drive an eternal chain reaction. If the fluctuations die down sufficiently in one spot, a "bubble" of ordinary inflation forms, ending quickly in a part of the universe in which galaxies, stars, and life might arise. Linde envisions bubbles spawning bubbles spawning bubbles in an endless self-reproducing fractal universe, in which our visible cosmos is but a mote in a chaotically seething sea. (See Linde's article in Scientific American, November 1994, p. 48.)

Primack studies this notion and finds intriguing consequences for our concept of time. "Ordinary inflation is a one-way street. It can only end," he says. But during eternal inflation, quantum effects are so overwhelming that time, according to the bizarre consequences of quantum mechanics, would flutter randomly both forward and backward. "It is reasonable," Primack deduces, "to say that time did not really exist until our bubble of the universe exited eternal inflation. At that instant, time began to flow in one direction."

Primack knows of no scientific observations that could test Linde's scenario. In the near future, however, subtle imprints and wisps of clues may emerge as researchers push their sensitive observations back to the moment of the Big Bang itself.

Time doesn't make sense without the existence of a physical universe, philosophers such as St. Augustine have stated. "In a sense, that is what we are saying too," Primack says. "But we are bridging the gap, for the first time, with equations that lead us back to the era of eternal inflation. The Big Bang is real, inflation is speculation, and eternal inflation is speculation upon speculation. But it's the best way I know to approach the issue of the beginning of time."

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