"Once defined as everything that exists, the term "universe" now often refers to just one of an infinite number of space-time bubbles.

"What we've all along been calling the universe," says Arizona State University cosmologist Paul Davies, may be "just an infinitesimal fragment in a much larger, more elaborate system for which want of a better word we call the multiverse."

A generation ago, such multiple universes existed only in science fiction, not science textbooks. Nowadays, the multi-verse is a hot topic at real-world scientific conferences, including a recent symposium on "Origins" at Arizona State, in Tempe. There Davies and other experts explored the anthropic implications of a multiplicity of universes, which owe their newfound importance to a popular astrophysical theory called inflation.

Among the Origins symposium's speakers was Alan Guth of MIT, who invented the inflation idea in 1980. It explained several mysteries about the Big Bang, the cosmic explosion 13.7 billion years ago marking the birth of today's one known universe.

For a tiny fraction of a second, Guth proposed, the universe expanded exponentially, explaining why the visible cosmos is now so uniform in temperature and structure. That exponential inflation would have stretched spacetime enough to eliminate all but the tiniest lumps in the original amalgamation of matter and energy, resulting in smooth skies today. Inflation would also have provided the impetus for the universe to grow to its current size from its minute origin.

"Inflation explains how the universe got to be so big, which is something we might take for granted, but there isn't really any other theory I know of which comes close to actually explaining it," Guth said at the Arizona conference.

Inflation is driven, Guth explains, by a repulsive form of gravity, generated by an energy field residing in space. As spacetime inflates, some of that field loses its strength — so a local region can expand more gradually, allowing stars and galaxies to develop and stick together. But at the same time, other regions of the inflating field continue to grow exponentially. There is always more inflating material available to spawn new spacetime bubbles — Guth calls them "pocket universes" — and no way for that process to ever stop.

"So once started, inflation goes on literally forever, with pieces of the inflating region breaking off and producing these pocket universes," Guth says. "And if this is right, we would be living in one of these infinity of pocket universes."

Goldilocks bubbles

Most experts today believe that inflation is the best explanation available for the visible universe's appearance and contents. And if it's the right explanation for the one known universe, there must be an infinite number of others.

"The question arises as to whether all these other universes are going to be like ours," says Davies, "or whether they may have different laws and the laws in our universe are in some sense special."

Arguments based on string theory, a favorite candidate (although unsubstantiated by experiment) for explaining all of physical law, suggest that the multiverse encompasses bubbles hosting various sorts of physics. Andrei Linde of Stanford University, another pioneer of inflation theory, noted at the Arizona symposium that string theory predicts the existence of an enormous number of different "vacuum states," or spacetime bubbles with different properties, such as physical constants or particle masses. Of an infinite number of bubbles, Linde says, there could be 10500 different varieties. And though any underlying basic law of physics would remain the same, the bubbles could nonetheless exhibit vast physical diversity. "It is the same fundamental law of physics, but different realizations," Linde says.

Some of those bubbles would not have lasted long enough for life, inflating but then shrinking before any interesting chemistry commenced. Others would expand forever, as seems the case with the bubble that humans occupy. In some, the local laws of physics would have welcomed living things; others would have permitted none of the particles and forces that conspire to build atoms, molecules and metabolic mechanisms. It seems that universes come in all sizes and flavors, with the human bubble being the Goldilocks version, just right for life.

It's not possible, or at least it's very unlikely (SN: 6/7/08, p. 22), for any of those other universes to make its presence physically known. So at first glance there is no obvious way to prove that they exist apart from inflation's equations. But in fact, Guth and others argue, applying anthropic reasoning to the multiverse allows calculations of some observable properties of the known universe, otherwise inexplicable. Success in such calculations would validate the assumption that the multiverse is real.

"Whether you like it or not, we may be living in a multiverse —the question is whether or not it will be possible to tell one way or the other," says Alex Vilenkin of Tufts University in Medford, Mass. "Some people complain that this theory is completely untestable. I think it can be tested."..."

From this much longer and more thorough article on infinity, in Science News.