As Stanford University physicist Andrei Linde explained in a 2008 interview, if the physical world operated by even slightly different rules, life wouldn't be able to exist. If protons were just 0.2 percent more massive than they actually are, for example, they'd be so unstable that they would break up into simpler particles, and atoms wouldn't be possible. And if gravity was just a bit more powerful, the result would be woeful. Stars such as our sun would be compressed tightly enough that they would burn through their fuel in a few million years, long before life on a planet such as Earth had a chance to evolve. This is known as the "fine-tuning problem."
Some see that precise balance of conditions as evidence of the deliberate hand of a supreme being who created everything, which would put atheists in a serious bind. But the possibility of a multiverse, in which this might simply be the particular realm in which all of these life-giving factors are present, gives them an out.
As Linde put it in an interview with Discover magazine: "For me the reality of many universes is a logical possibility. You might say, 'Maybe this is some mysterious coincidence. Maybe God created the universe for our benefit.' Well, I don't know about God, but the universe itself might reproduce itself eternally in all its possible manifestations" [source: Folger].