We won't have the technology to do that in the next 100 years—or probably even in the next billion years. Nevertheless, a sufficiently advanced civilization might be able to master the intricacies of creating baby universes—maybe even selling kits to do it in science fairs. Unfortunately, the new space-time such a universe inhabits will be forever cut off from our cosmos by the black-hole bottleneck (which destroys everything that passes through it), and thus will be just as undetectable as those in quantum theory's many-worlds interpretation.

One more type of alternate universe remains, however, and in this case there's a chance of detection—albeit a highly circumstantial one.

Since 1965, astronomers have had powerful evidence that the cosmos began with a Big Bang and that everything has been expanding outward ever since. But in the 1970s and early '80s, U.S. and Russian physicists (including Guth) realized that powerful energy fields dominating the cosmos when it was a fraction of a second old could have turbocharged the expansion, forcing the universe to fly apart—or "inflate"—at a rate many times faster than the speed of light. (The light barrier can't be broken by things moving through space, but space itself is exempt from this universal speed limit.)

So far, we're still talking about one universe—though one vastly larger than the tiny patch, a mere 30 billion light-years across, that we can see. But then scientists, including the Russian emigre Andrei Linde, realized that this inflation was more flexible than anyone had thought. Energy fields of early-universe intensity could arise purely by chance in subatomic-size regions of even a normal cosmos.

Our universe could thus be the result of an inflationary bubble that formed in a pre-existing universe—an arena better described as a metauniverse, or metaverse. Other, parallel bubbles could have formed just as easily. (If two expanding bubbles somehow met, the result would be a wall of fiery energy spanning one side of the cosmos. No evidence of that to date.)

But if bubbles of inflation could percolate in a pre-existing metaverse, they might also spring forth from our cosmos. New universes could be sprouting from ours all the time, in fact spewing out into unimaginable dimensions to evolve along their own paths. These universes might have laws of physics dramatically different from our own—gravity so powerful that they collapse almost instantly, or so weak that stars can never form, to give just two examples. And they could give birth in turn to other universes, creating a metaverse in which universes bud off of universes in an endlessly self-reproducing fashion.

It gets even more bizarre. According to Princeton astrophysicist J. Richard Gott, the flow of time loses its meaning when you hop from one universe to another. In the timeless metaverse, in fact, a baby cosmos could beget a baby that would beget a baby that might ultimately give birth to the universe that started it all. "It's quite possible," says Gott, "that the universe could end up being its own great-grandmother."

In many ways, the physics of these budding cosmoses are like the baby universes Hawking and Guth hatch inside black holes. The difference is that unlike the details of what transpires in black holes, the evidence for or against inflation could be settled within just a few years. If the universe did inflate, that brief period of breakneck expansion should have left a telltale pattern imprinted on the radiation left over from the Big Bang, which still echoes around the universe in the form of electromagnetic microwaves. Two satellites set to be launched later this year are sensitive enough to detect such a pattern.

The detection would be a powerful boost to alternate-universe theories. "Confirming inflation," says Princeton physicist Paul Steinhardt, "would give us a lot more confidence about some of its implications." And that includes ideas that have lived until now only in the parallel universe of fantasy.

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