Beyond the big bang: a new cosmic worldview holds that countless replicas of Earth, inhabited by our clones, are scattered throughout the cosmos

Natural History, July-August, 2006 by Alex Vilenkin

We all live in the aftermath of a great explosion. This awesome event, somewhat frivolously called the big bang, took place some 14 billion years ago. We can actually see some of the cosmic history unfolding before us since that moment--light from remote galaxies takes billions of years to reach our telescopes on earth, so we can see galaxies as they were in their youth. But there is a limit to how far we can see into space. Our horizon is set by the maximum distance light could have traveled since the big bang. Sources more distant than that horizon cannot be observed, simply because their light has not yet had time to reach Earth.

But if there are parts of the universe we cannot detect, who can resist wondering what they look like? Do they simply harbor more stars, more galaxies, more of the same--or could it be that distant parts of the universe differ dramatically from our cosmic neighborhood? Does the universe extend to infinity, or does it close in on itself, like the surface of the Earth?

As they address these provocative yet fundamental questions, cosmologists can rely only on indirect, circumstantial evidence, using measurements made in the accessible part of the universe to make inferences about the places that cannot be observed. That limitation makes it much harder to prove one's case "beyond a reasonable doubt." But because of remarkable recent developments in cosmology, some of the ultimate cosmic questions now have answers that we have some reason to believe.

The emerging cosmic worldview combines, in surprising ways, some seemingly contradictory features: the universe is both infinite and finite, evolving and stationary. That view of the universe also holds that in some remote regions there are planets exactly like our Earth, with continents of the same outline and terrain, inhabited by identical creatures, some of them holding copies of this magazine in their hands.

The core of the new cosmological paradigm is "eternal inflation" a subject of my research that grew out of the theory of inflation first put forward in 1980 by Alan Guth, a physicist at the Massachusetts Institute of Technology. Guth suggested that the early universe contained some highly unusual material that created a strong repulsive gravitational force. That special material is known as the false vacuum, and according to Guth, it blew the universe up.

A vacuum is empty space-space devoid of all material particles. It is often regarded as synonymous with nothing. But according to modern theories of elementary particles, a vacuum is a physical object; it can be charged with energy and can come in different states. We live in the lowest-energy vacuum, the so-called true vacuum (familiar empty space). High-energy vacuums are called false because, unlike the true vacuum, they are unstable. The most remarkable property of a false vacuum is its repulsive gravity. According to Einstein, if a vacuum has energy, it should also have tension, which has a repulsive gravitational effect. The repulsion due to vacuum tension turns out to be three times stronger than the attractive gravity of the vacuum energy (which is related to mass via Einstein's formula E=[mc.sup.2]). The net effect is a strong repulsive force.

Guth considered what would happen if, at some early epoch, the entire universe were in a false-vacuum state. He found that the repulsive gravity of the vacuum would cause the universe to expand exponentially--or, in other words, by a constant factor for each constant interval of time. Exponential growth can be characterized by the doubling time, or the time it takes for a given quantity to double in size. (The doubling time for $100 invested at 6 percent annual interest, for instance, is about twelve years, so that at the end of twenty-four years the $100 investment is worth about $400.)

For the expansion of a universe permeated by a false vacuum, the doubling time is unbelievably short. It depends on the energy density (measured in units of energy per cubic centimeter) of the particular kind of false vacuum, but it never exceeds one ten-billionth of a second. A straightforward calculation shows that the universe would expand by a factor of a googol ([10.sup.100]) in less than one-thirtieth of a microsecond.

Since a false vacuum is unstable, it eventually decays, turning into the true vacuum. In so doing, its prodigious energy ignites a hot fireball of elementary particles. That event signals the end of inflation and the starting point of the usual cosmological evolution. It plays the role of the big bang in Guth's cosmology. Thus an enormous, hot, expanding universe emerges from a tiny initial seed.

The theory of inflation was little more than a speculative hypothesis when Guth proposed it, but it was soon enhanced and developed by the work of many physicists, most notably Andrei Linde of Stanford University. Moreover, in the late 1990s, observations of distant supernovae and of the cosmic microwave background radiation--a faint afterglow of the big bang--gave the theory an enormous boost of corroborating observational evidence. So today, inflation is well on its way to becoming one of the cornerstones of modern cosmology.