Former President, The Royal Society; Emeritus Professor of Cosmology & Astrophysics, University of Cambridge; Fellow, Trinity College; Author, From Here to Infinity

An astonishing concept has entered mainstream cosmological thought: it deserves to be more widely known. Physical reality could be hugely more extensive than the patch of space and time traditionally called "the universe." Our cosmic environment could be richly textured, but on scales so vast that our astronomical purview is restricted to a tiny fraction: we’re not aware of the "grand design," any more than a plancton whose "universe" was a spoonful of water would be aware of the world’s topography and biosphere. We may inhabit a "multiverse."

However powerful our telescopes are, our vision is bounded by a horizon: a shell around us, delineating the distance light can have travelled since the big bang. But this shell has no more physical significance than the circle that delineates your horizon if you're in the middle of the ocean. There are billions of galaxies within our horizon. But we expect far more galaxies lying beyond. We can’t tell just how many. If space stretched far enough, then all combinatorial possibilities would be repeated. Far beyond the horizon, we could all have avatars. And it may be some consolation that when we make a bad decision, there’s another one of us, far beyond our horizon, who has made a better one.

So the aftermath of "our" big bang could encompass a stupendous volume. But that’s not all. "Our" big bang could be just one island of space-time in an unbounded cosmic archipelago. A challenge for 21st-century physics is to answer two questions. First, are there many "big bangs" rather than just one? Second—and this is even more interesting—if there are many, are they all governed by the same physics? Or is there a huge number of different vacuum states—each the arena for different microphysics, and therefore offering differing propensities for spawning life?

If the answer to this latter question is "yes" there will still be overarching laws governing the multiverse—maybe a version of string theory. But what we’ve traditionally called the laws of nature will be just local bylaws. Even though it makes some physicists foam at the mouth, we then can’t avoid the A-word—“anthropic." Many domains could be still-born or sterile: the laws prevailing in them might not allow any kind of complexity. We therefore wouldn’t expect to find ourselves in a typical universe. Ours would belong to the unusual subset where there was a "lucky draw" of cosmic numbers conducive to the emergence of complexity and consciousness. Its seemingly designed or fine-tuned features wouldn't be surprising.

Some claim that unobservable entities aren’t part of science. But it’s hard to defend that hard-line view. For instance, (unless we are in some special central position and our universe has an "edge" just beyond the present horizon) there will be some galaxies lying beyond our horizon—and if the cosmic acceleration continues they will remain beyond forever. Not even the most conservative astronomer would deny that these never-observable galaxies (which, as I’ve already mentioned, could hugely outnumber those we can see) are part of physical reality. These galaxies are part of the aftermath of our own big bang. But why should they be accorded higher epistemological status than unobservable objects that are the aftermath of other big bangs? So it’s surely a genuine scientific question to ask whether there’s one big bang or many.

Fifty years ago we weren’t sure whether there was a big bang at all (Fred Hoyle and other "steady statesmen" still contested the idea). Now we can confidently describe cosmic history back to the ultra-dense first nanosecond. So in fifty more years, it’s not overoptimistic to hope that we may have a "unified" physical theory, corroborated by experiment and observation in the everyday world, that tells us what happened in the first trillionth of a trillionth of a trillionth of a second, when inflation is postulated to have occurred. If that theory predicts multiple big bangs we should take that prediction seriously even though it can’t be directly verified (just as we take seriously general relativity’s predictions for the unobservable insides of black holes, because the theory has survived many tests in domains we can observe).

Some physicists don’t like the multiverse: they’d be disappointed if some of the key numbers they are trying to explain turn out to be mere environmental contingencies governing our local space-time patch—no more truly "fundamental" than the parameters of the Earth’s orbit round the Sun. But that disappointment would surely be outweighed by the revelation that physical reality was grander and richer than hitherto envisioned. In any case our preferences are irrelevant to the way physical reality actually is—so we should surely be open-minded.

Indeed, there’s an intellectual and aesthetic upside. If we’re in a multiverse, it would imply a fourth and grandest Copernican revolution; we’ve had the Copernican revolution itself, then the realization that there are billions of planetary systems in our galaxy; then that there are billions of galaxies in our observable universe. But now that’s not all. The entire panorama that astronomers can observe could be a tiny part of the aftermath of "our" big bang, which is itself just one bang among a perhaps-infinite ensemble.

We may, by the end of this century, be able to say, with confidence, whether we live in a multiverse, and how much variety its constituent "universes" display. The answer to this question will determine how we should interpret the "bio friendly" universe in which we live.