The science of cosmology has achieved wonders in recent centuries. It has enlarged the world we can see and think about by ontological orders of magnitude. Cosmology wrenched the Earth from the centre of the Universe, and heaved it, like a discus, into its whirling orbit around one unremarkable star among the billions that speed around the black-hole centre of our galaxy, a galaxy that floats in deep space with billions of others, all of them colliding and combining, before they fly apart from each other for all eternity. Art, literature, religion and philosophy ignore cosmology at their peril.
But cosmology’s hot streak has stalled. Cosmologists have looked deep into time, almost all the way back to the Big Bang itself, but they don’t know what came before it. They don’t know whether the Big Bang was the beginning, or merely one of many beginnings. Something entirely unimaginable might have preceded it. Cosmologists don’t know if the world we see around us is spatially infinite, or if there are other kinds of worlds beyond our horizon, or in other dimensions. And then the big mystery, the one that keeps the priests and the physicists up at night: no cosmologist has a clue why there is something rather than nothing.
To solve these mysteries, cosmologists must make guesses about events that are absurdly remote from us. Guth’s theory of inflation is one such guess. It tells us that our Universe expanded, exponentially, a trillionth of a trillionth of a trillionth of a second after the Big Bang. In most models of this process, inflation’s expansive kick is eternal. It might cease in particular parts of the cosmos, as it did in our region, after only a fraction of a second, when inflation’s energy transformed into ordinary matter and radiation, which time would sculpt into galaxies. But somewhere outside our region, inflation continued, generating an infinite number of new regions, including those that are roaring into existence at this very moment.
Not all these regions are alike. Quantum mechanics puts a slot-machine spin on the cosmic conditions of every region, so that each has its own physical peculiarities. Some contain galaxies, stars, planets, and maybe even people. Others are entirely devoid of complex structures. Many are too alien to imagine. The slice of space and time we can see from Earth is 90 billion light years across. Today’s inflationary models tell us that this enormous expanse is only one small section of one tiny bubble that floats along in a frothy sea whose proportions defy comprehension. This vision of the world is wondrous, in its vastness and variety, in the sheer range of possibilities it suggests to the mind. But could it ever be proved?