they multiply rapidly. If destruction is faster, the 

 bacteria quickly die out. 



With inflation, the two competing processes are 

 the decay of the false vacuum and its "reproduc- 

 tion," because the inflating regions are expanding 

 rapidly. Roughly speaking, the false vacuum decays 

 because of random quantum mechanical fluctua- 

 tions in the energy density of the vacuum. So to es- 

 timate which process would win, I analyzed how 

 the effects of quantum fluctuations compared to 

 the rate of inflation. My analysis showed that false- 

 vacuum regions multiply much faster than they 

 decay. In other words, even though the false vac- 

 uum is constantly decaying, inflation proceeds so 

 rapidly that there is no way to stamp out the false 

 vacuum everywhere in the universe at once. 



Many other big bangs went off 

 before ours did, in remote parts 

 of the universe. 



That result has extraordinary implications. In vast 

 reaches of the universe, the ever-expanding wildfire 

 that is inflation will never end; the volume of the 

 inflating regions will keep growing forever, with- 

 out bound. At this very moment, some distant parts 

 of the universe are filled with false vacuum and are 

 undergoing exponential inflationary expansion. 

 Regions like ours, where inflation has ended, are 

 also constantly being produced. They form "island 

 universes" in the inflating sea of false vacuum. Be- 

 cause of inflation, the space between the island uni- 

 verses rapidly expands, making room for more is- 

 land universes to form. Thus inflation is a runaway 

 process that has stopped in our neighborhood but 

 still continues in other parts of the universe, causing 

 them to expand at a furious rate and constantly 

 spawning new island universes like our own. 



Because the decay of the false vacuum and ensu- 

 ing fireball occur repeatedly, the big bang loses its 

 central status as a one-time event in the history of 

 the universe. "Our" big bang gave rise to the stars 

 and the galaxies we can see, as well as to many ob- 

 jects beyond our visible horizon. But many other 

 big bangs went off before ours did, in remote parts 

 of the universe, and countless others will erupt 

 elsewhere in the future. 



If cosmologists could somehow observe the in- 

 flating universe from the outside, just as the sur- 

 face of the Earth can be observed from space, they 

 would see a multitude of island universes (regions 



of true vacuum) scattered in the vast inflating sea. If 

 the entire universe is shaped like the three-dimen- 

 sional analog of the surface of a sphere, the view 

 that would open in front of them might resemble a 

 globe, with continents and archipelagoes sur- 

 rounded by ocean. The "globe" is expanding with- 

 out bound at a staggering rate, the island universes 

 are also growing exceedingly fast, and tiny new is- 

 lands are constantly appearing and immediately 

 starting to expand [see illustration on opposite page]. 



The inhabitants of island universes, like us, see a 

 different picture. They do not perceive their universe 

 as a finite island. For them it appears as a self-con- 

 tained, infinite universe. That dramatic difference in 

 perspective is a consequence of the differences im- 

 posed by the ways of keeping time appropriate to the 

 global and internal views of the island universe. (Ac- 

 cording to Einstein's theory of relativity, time is not 

 fixed, but depends instead on the observer.) 



When cosmologists talk about a "moment in 

 time," they picture a large number of observers, 

 equipped with clocks and scattered throughout the 

 universe. Each observer sees a small patch of the 

 universe, but the whole assembly is needed to de- 

 scribe a large region. 



In the global view, the definition of a "moment 

 of time" is largely arbitrary, because there is no ob- 

 vious way to synchronize the clocks of observers 

 across the false vacuum and in different island uni- 

 verses. The number and the shape of the islands 

 may differ, depending on this choice of definition. 

 By contrast, to describe one specific island universe 

 from the point of view of its inhabitants, there is a 

 natural rather than arbitrary choice for the origin of 

 time. All observers inhabiting the island universe 

 can count time from the big bang at their respective 

 locations. Their big bang is thus set as time zero in 

 their entire island universe. Remarkably, from such 

 an internal viewpoint the island universe is infinite. 



Perhaps the easiest way to see this is to count 

 galaxies. In the global view, new galaxies are contin- 

 ually formed near the expanding boundaries, so as 

 time passes, the number of galaxies grows without 

 bound. In the internal view, all those galaxies exist 

 simultaneously (say, at time 14 billion years). 



The surprising feature of island universes — that 

 they are infinite when viewed from the in- 

 side — led me to what is perhaps the most striking 

 consequence of eternal inflation. The analysis goes 

 like this. Since each island universe is infinite from 

 the viewpoint of its inhabitants, it can be divided 

 into an infinite number of regions having the same 

 size as our own observable region. My collaborator 

 Jaume Garriga of the University of Barcelona and I 



44 



NATURAL history July/August 2006 



