My question is, expanding into where, exactly?
I thought the universe, by definition, included EVERYTHING EVERYWHERE. So you can imagine my surprise to learn that there may be some places that everything everywhere does not include.
It may be just a matter of semantics. Apparently universe has different definitions, one or two of which are of fairly recent invention.
For instance, it has been proposed that the universe should only include those parts of intergalactic space with which we have at least the potential of interacting. For these purposes, potential is limited only by the speed of light and interacting includes any form of perception. By this definition, the universe would have a radius of about 46 billion light years. Beyond that distance, objects are moving away so fast that light from them would never reach us.
Now that's all well and good for some astrophysicist stuck in some lab or observatory, but I'm a traveling man, and I throw a pretty wide loop. If it's out there, whether I can see it or not, it's in my universe.
Then there is the definition which finds our universe to be just one of a potentially infinite number of universes which are unable to interact. This is the multiverse theory. Well, I say hah! to that, and see above.
One of the reasons that the multiverses cannot interact is due to their probable operation on entirely different and incompatible physical laws. Our laws of physics now envision a continuum of space and time; these two concepts are just different ways of describing the same thing. (That almost makes it sound like I understand that, doesn't it?) Notably, this would mean that space-time is contained within our universe; outside of our universe, space-time may not exist.
Neat. I can see the billboards just beyond the edge of our expanding universe:
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