In order to ask this question, I have to imagine the Universe in “real time;” that is as if we could observe everything without having to wait for the light to arrive. I think this might appear as a ball expanding at the speed of light. The center might be hollow – all matter having been expelled by the big bang.
My question: If the Universe is subject to collapse in a “big crunch,” would all of the photons at the frontier slow, stop, then reverse direction? Would all of the massy objects contract until they caused a massive singularity that attracted all of the photons? Would the matter and energy in the Universe reverse direction at simultaneously?
The speculation prompting my question: If the Universe were in a contraction would we necessarily know it; given that most of what we observe is millions / billions of light years away – and to our eyes would still be expanding?
I think you’re operating on the assumption that the big bang was an explosion in space. It wasn’t; it was an explosion of space. The normal analogy is to think of our universe as the surface of a balloon, and the expansionary process as the balloon being blown up.
In the event of a big crunch, we would see every other visible object moving toward us rather than away as we do now.
Thanks, ultrafilter. But I’m not sure that answers or invalidates my entire question.
If the entire balloon surface stops expanding and contracts, wouldn’t we still see far away galaxies still receeding until the (blue shifted) light generated when they start to move towards us reaches us, in millions or billions of years? So how would we know that the balloon surface was contracting?
The closer galaxies would also be blue-shifted, but I’m not sure if it would be by enough that our instruments could pick it up.