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In What Sense Does the Universe Persist Through Time

Elie During

University of Paris-Ouest-Nanterre

What is the time of the evolving universe? There are various ways to define a cosmic time. All observers within a sufficiently large portion of the universe may be said to share a mean proper time. Alternatively, we may consider a frame of reference co-moving with the Hubble flow. In a more geometrical vein, one may look for a cosmic time parameter in the foliation of space-time into hypersurfaces of constant mean curvature. In order to make sense of these different approaches to cosmic time, one must of course refer them to the particular cosmological models which set their scope and limits. But from a philosophical perspective, the mere fact of this diversity already raises general issues. For one thing, the very concept of cosmic time proves difficult to handle under the hypothesis of an open, infinite universe. Relativity teaches that there is in principle no preferred foliation defining a single “world-wide” instant across the universe. In fact, according to general relativity, space-time as a whole may not even lend itself to such global foliation. In that sense, there is no global becoming. Yet, all cosmological models seem to agree on the fact that the universe as a whole persists through time in some sense—unless, of course, one endorses a timeless or eternalist version of the “block universe” view. Meanwhile, the more fundamental issue of persistence is generally left unaddressed, and this is where metaphysics comes in. It is not concerned so much with cosmic time as with the possibility of considering the universe as a single “object” persisting through time, regardless of the warnings sent by Kant against the abuses of the cosmological impulse in his famous antinomies of pure reason. In the terminology fixed by David Lewis, objects endure by being wholly present at each moment of their careers, that is by being multiply located in time; they perdure by being literally spread out across regions of space-time, and more importantly by having so-called “temporal parts”. This way of distinguishing two modes of persistence—roughly stated, a 3D and a 4D view—has fuelled much discussion in recent years, but it can be traced back to Whitehead and C. D. Broad (who first introduced the “growing block” theory in the early twenties), and it keeps recurring in the works of people who—like Quine—spent some effort reflecting on the general issue of change and identity through time. My goal is to show how this ongoing debate can shed some light on the connection between cosmological time and the local time(s) attributed to finite chunks of matter—and more generally, between the time of physics and other determinations of time in relation to life and mind. My main focus, however, will be on the philosophical impact of relativity theory on competing accounts of persistence, with special attention to the way things may be said to coexist in space and time. From a cosmological perspective, the issue of persistence is indeed inseparable from that of coexistence. In fact my contention is that understanding how the universe as such persists comes down to elucidating what it means for things to coexist within it, and ultimately what it means to be located in space-time. A fresh look at the famous twin paradox may prove useful in that respect: we shall treat it as a toy-model for a universe composed of two substances in relative motion.