You live in a universe that is contracting back to the big crunch. No worries yet, plenty of time before thw universe becomes too small and hot to live.
As an astonomer you look out to distant galaxies. Instead of being red shifted, they are?purple shifted? meaning they are moving toward you.
As you look out at greater and greater distances, galaxies are ‘moving’ toward us at faster than light. Well, not really, but the space between us and them is contracting so fast that they are getting closer at faster than the speed of light.
So, my question…what does the astonomer see when he looks at one of these galaxies? I can’t seem to wrap my mind around this mind experiment.
One thought I had was that he would see the galaxy moving away but ‘purple-shifted’ because the light when it was further away takes longer to get to us and light when it is closer gets to us more quickly so we would see the galaxy ‘moving away’ from us…but I think I am missing something.
I don’t think you’d see a reverse motion. The light that’s moving out in front of the galaxy is still going to be ahead of the rest of the light because it is moving through space. The contraction might squeeze that light together by squeezing space, but the leading edge of the light remains the leading edge.
I guess we just need to look at the rubber sheet analogy again. If the light is made up of ants crawling along, letting an expanded sheet contract will only cluster the ants closer together. The first ant will always be the first and the last will always be the last.
So the blue shift is probably the only effect of note.
Exactly! That light will be ahead of the previous light meaning it reaches us first!..therefore looks like it is moving away. The rubber sheet analogy is the same as if space was expanding except for the blueshift.
Arrrggg. I think you are right, just can’t wrap my mind around it. Should be simple.
dracoi’s good explanation said that light will arrive in the order it was sent: light from when the galaxy was most distant will arrive at you first and light sent from lesser distances will arrive later.
I cannot fathom how you got from that to somehow thinking that will appear as if the object is receding. The appearance of receding would require closer-sent light arriving first & more distant-sent light arrived later.
Your astronomer would see just what we’d all expect. Blue shifted light & galaxies appearing bigger over time as they got closer. Nothing weird or exciting or paradoxical at all.
Nevermind, mods, just close this thread. I think I have wrapped my mind around it…galaxies that fit my OP have little/no lateral motion to the observer so the question is irrelevant.
In a contracting universe, distant galaxies will slow down as they get closer. So even if a galaxy is moving toward us faster than the speed of light at some point, by the time we see it, it will be moving slower.
What we see exactly depends a lot on the state of the universe when it starts contracting.
