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Bit of an awkward way to phrase it, but basically:

Time in relativity is just one of four dimensions of space-time. Nothing really special about it. Yet the universe was once smaller than it is now and has evolved, changed, from a singularity till its size today.

But this implies that the whole of space-time (since you can't separate time from space) is evolving over time. Which time, then, is it evolving over? There's not supposed to be a global time for the universe, only time as perceived for an observer at a particular location.

So it seems very strange to me that the volume of the entire universe's space-time could change over time when time is part of the thing that's changing. It sounds like we're talking about two different kinds of time...

John
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1 Answers1

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One talks about the size of the universe in the context of a model where spacetime is foliated by three-dimensional spacelike leaves. The "size of the universe" means the size of one of those leaves, not of all spacetime.

For example: If you imagine spacetime to be filled with galaxies, the worldlines of those galaxies give a preferred global time coordinate, and you can take your three-dimensional leaves to be the three-manifolds whose tangent spaces are everywhere orthogonal to the tangents to those worldlines. You can then talk about the volume of a leaf as a function of the global time coordinate to get the rate at which the universe is expanding.

WillO
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