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

Physical Reality Could Be Hugely More Extensive Than the Patch of Space and Time Traditionally Called 'The Universe'

An astonishing concept has entered the mainstream of cosmological thought: physical reality could be hugely more extensive than the patch of space and time traditionally called 'the universe'. A further Copernican demotion may loom ahead. We've learnt that we live in just one planetary system among billions, in one galaxy among billions. 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.

Our cosmic environment could be richly textured, but on scales so vast that our purview is restricted to a tiny fragment; we're not aware of the 'big picture', any more than a plankton whose 'universe' was a liter of water would be aware of the world's topography and biosphere. It is obviously sensible for cosmologists to start off by exploring the simplest models. But there is no more reason to expect simplicity on the grandest scale than in the terrestrial environment—where intricate complexity prevails.

Moreover, string theorists suspect—for reasons quite independent of cosmology—that there may be an immense variety of 'vacuum states'. Were this correct, different 'universes' could be governed by different physics. Some of what we call 'laws of nature' may in this grander perspective be local bylaws, consistent with some overarching theory governing the ensemble, but not uniquely fixed by that theory. More specifically, some aspects may be arbitrary and others not. As an analogy (which I owe to Paul Davies) consider the form of snowflakes. Their ubiquitous six-fold symmetry is a direct consequence of the properties and shape of water molecules. But snowflakes display an immense variety of patterns because each is molded by its distinctive history and micro-environment: how each flake grows is sensitive to the fortuitous temperature and humidity changes during its growth.

If physicists achieved a fundamental theory, it would tell us which aspects of nature were direct consequences of the bedrock theory (just as the symmetrical template of snowflakes is due to the basic structure of a water molecule) and which cosmic numbers are (like the distinctive pattern of a particular snowflake) the outcome of environmental contingencies. .

Our domain wouldn't then be just a random one. It 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. 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, I think, determine crucially how we should interpret the 'biofriendly' universe in which we live (and which we share with any aliens who we might one day make contact with).

It may disappoint some physicists if some of the key numbers they are trying to explain turn out to be mere environmental contingencies—no more '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.