this post was submitted on 20 Sep 2025
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ELI5?
If something is red shifting, it's accelerating away from you. If something is blue shifting, it's accelerating towards you. An entire galaxy accelerating towards you is somewhat concerning.
Redshift comes from relative velocity (and other effects), not acceleration. Andromeda's light is blueshifted as it's moving towards us, but it's not accelerating.
I was going to argue with this too, but the meme says blueshifting, not blueshifted. They're right, but mostly because the meme is probably written poorly. Maybe they meant it's accelerating toward us though. Idk.
On top of that, we've found that basically everything is redshifting as the universe expands. So to see a blueshifting galaxy would mean something potentially unnatural.
Right, but I don't get why this thread is full of people talking about collisions. Even if it was moving at the speed of light (it's not) it's still billions of years away.
It would just prove our theory of universe inflation to be incorrect.
You know how the sound of things moving from you changes? Towards you the pitch is higher, away from you it is lower. The same happens with light. We know how some things should look like, so if they are more toward red or blue, we know their speed relative to us. Blue = towards us = "we will collide" (you also do not collide with every car with a siren where you hear that effect).
Great explanation, thanks!