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The Theory of Multiple Universes

If space is now expanding, then at ever earlier times the universe must have been ever smaller. At some moment in the distant past, everything we now see—the ingredients responsible for every planet, every star, every galaxy, even space itself—must have been compressed to an infinitesimal speck that then swelled outward, evolving into the universe as we know it.

The big-bang theory was born…Yet scientists were aware that the big-bang theory suffered from a significant shortcoming. Of all things, it leaves out the bang. Einstein’s equations do a wonderful job of describing how the universe evolved from a split second after the bang, but the equations break down (similar to the error message returned by a calculator when you try to divide 1 by 0) when applied to the extreme environment of the universe’s earliest moment. The big bang thus provides no insight into what might have powered the bang itself.

Just as the pull of earth’s gravity slows the ascent of a ball tossed upward, the gravitational pull of each galaxy on every other must be slowing the expansion of space… [But] far from slowing down, the expansion of space went into overdrive about 7 billion years ago and has been speeding up ever since. That’s like gently tossing a ball upward, having it slow down initially, but then rocket upward ever more quickly.

We’re all used to gravity being a force that does only one thing: pull objects toward each other. But in Einstein’s theory of relativity, gravity can also push things apart. If space contains an invisible energy, sort of like an invisible mist that’s uniformly spread through space, then the gravity exerted by the energy mist would be repulsive.

When the astronomers deduced how much dark energy would have to permeate every nook and cranny of space to account for the observed cosmic speedup, they found a number that no one has been able to explain:

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In universes with larger amounts of dark energy, whenever matter tries to clump into galaxies, the repulsive push of the dark energy is so strong that the clump gets blown apart, thwarting galactic formation. In universes whose dark-energy value is much smaller, the repulsive push changes to an attractive pull, causing those universes to collapse back on themselves so quickly that again galaxies wouldn’t form. And without galaxies, there are no stars, no planets, and so in those universes there’s no chance for our form of life to exist.

We find ourselves in this universe and not another for much the same reason we find ourselves on earth and not on Neptune—we find ourselves where conditions are ripe for our form of life.

Just as it takes a well-stocked shoe store to guarantee you’ll find your size, only a well-stocked multiverse can guarantee that our universe, with its peculiar amount of dark energy, will be represented. On its own, inflationary cosmology falls short of the mark. While its never-ending series of big bangs would yield an immense collection of universes, many would have similar features, like a shoe store with stacks and stacks of sizes 5 and 13, but nothing in the size you seek.

By combining inflationary cosmology and string theory, the stock room of universes overflows: in the hands of inflation, string theory’s enormously diverse collection of possible universes become actual universes, brought to life by one big bang after another. Our universe is then virtually guaranteed to be among them. And because of the special features necessary for our form of life, that’s the universe we inhabit.