Beyond the Big Banj 



A new cosmic worldview holds that countless replicas of Earth, 

 inhabited by our clones, are scattered throughout the cosmos. 



By Alex Vilenkin 



We all live in the aftermath of a great ex- 

 plosion. This awesome event, some- 

 what frivolously called the big bang, 

 took place some 14 billion years ago. We can actu- 

 ally see some of the cosmic history unfolding be- 

 fore us since that moment — light from remote 

 galaxies takes billions of years to reach our tele- 

 scopes 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 maxi- 

 mum 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 dis- 

 tant parts of the universe differ dramatically from 

 our cosmic neighborhood? Does the universe ex- 

 tend 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 infer- 

 ences 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 an- 

 swers 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 

 m 1980 by Alan Guth, a physicist at the Massachu- 

 setts Institute of Technology. Guth suggested that the 

 early universe contained some highly unusual mate- 



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NATURAL HISTORY July/August 2006 



