When did time begin?

17 Jun 2002

When did time begin? Easy: When eternal inflation ended and ordinary inflation began in our corner of the cosmos, says cosmologist Joel Primack of the University of California, Santa Cruz.

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, Primack, a professor of physics, explained to his audience at the American Association for the Advancement of Science this week.

"That the universe began in a hot dense state and then expanded is very well established," Primack said. "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, argues Primack, "Inflation created the proper initial conditions for the Big Bang."

"Ordinary inflation is a one-way street. It can only end," he said. 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."