Exactly how many universes are out there in the vast expanse of space? Are we the only one? The idea of multiple universes, possibly with their own set of natural laws, coalesced into a giant multiverse has been gaining ground recently-- only it's not something that at this time is more than just speculation. Cosmologists Andrei Linde and Vitaly Vanchurin at Stanford University in California calculate that the number of universes dwarfs the 10⁵⁰⁰ universes postulated in string theory, and raise the provocative notion that the answer may depend on the human brain.

From New Scientist:

The idea that there is more than one universe, each with its own laws of physics, arises out of several different theories, including string theory and cosmic inflation. This concept of a "multiverse" could explain a puzzling mystery - why dark energy, the furtive force that is accelerating the expansion of space, appears improbably fine-tuned for life. With a large number of universes, there is bound to be one that has a dark energy value like ours.

Calculating the probability of observing this value - and other features of the cosmos - depends on how many universes of various kinds populate the multiverse. String theory describes 10⁵⁰⁰ universes, but that just counts different vacuum states, which are like the blank canvases upon which universes are painted. The features of each canvas determine what the overall painting will look like - such as the laws of physics in that universe - but not the details.

Thanks to the randomness of quantum mechanics, two identical vacuum states can end up as very different universes. Small quantum fluctuations in the very early universe are stretched to astronomical scales by inflation, the period of faster-than-light expansion just after the big bang. These fluctuations lay down a gravitational blueprint that eventually determines the placement of stars and galaxies across the sky. Small differences in the form of these fluctuations can produce a universe in which the Milky Way is slightly bigger, or closer to its neighbours.

So just how many of these different universes can inflation's quantum fluctuations produce? According to Linde and Vanchurin, the total is about 10^1010,000,000 - that's a 10 raised to a number ending with 10 million zeros . Suddenly string theory's multiverse of 10⁵⁰⁰ universes is looking rather claustrophobic.