I know there’s been a rash of these the last few months. I’ve got a decent layman’s handle on it. I do have a question for you physicists, though.
Is the expansion only of space, or is it of spacetime? I’m trying to imagine the consequences of expanding time; not an easy visualization (would we never then actually reach any theoretical end of the universe?) - I suspect, however, that the universe has kindly made it easy for me and it’s only space which expands (in the three dimensions we’re used to) given that I’ve not heard or read any popular science references to expanding time other than the usual relativistic effects.
What’s the straight dope on the expansion re. space or spacetime?
This is one is easy, it’s space that is expanding (I don’t think expanding spacetime would have any meaning), though the expansion of space is a property of the spacetime.
Imagine an expannding 1-D closed universe expanding at a constant rate from a big bang singularity. Each spatial slice is a circle and the circle’s circumference is getting larger with time. The shape of the (2-D) spacetime is analogous to (the surface of) a cone with the big bang singualrity being analogus to the tip of the cone.
So in our 2-D example the expansion is of the conic sections, thoguh the fact that conic sections expand is due to the properties of the ‘cone spacetime’, our choice of conic sections and our choice of parametrisation of those conic sections.
Of course this is just an analogy as spacetime is 4-D rather than 2-D and is equipped with a Lorentzian metric and space can be infinite. But the basic idea is the same: i.e. it’s space that’s expanding and the expansion of space is a property of spacetime, our choice of spatial slices and our parameterisation of those spatial slices.