this post was submitted on 20 Sep 2025
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Might be a dumb question, but if it's blue-shifting surely we wouldn't know it's far away in the first place? I thought the amount of redshift is broadly how we determine cosmic distances?
Armchair expert here
From my understanding blue and red shifting is mostly related to movement. Like when a firetruck run past you with sirens on, you can hear change in pitch when compared it moving towards you vs away from you.
It's a similar effect with galaxies, red shifting means that after the light was emitted the space between us has increased and the light kind of stretched out to longer wave.
Now anyone with more knowledge on the subject, please correct me.
You're missing the point that the universe is expanding uniformly. That means two points acceleration away from each other is dependent on their distance apart. The further they are from each other the faster they accelerate from each other.
So GP is right. We measure red shift and infer distance.
Yes, but everything red shifts "naturally" as well as the light travels because of the expansion of the universe. So something traveling towards us will still red shift, just slightly less so. To determine distance you have to use something called the cosmic distance ladder. It consists of known properties of stellar objects that we can measure to determine the distance of objects.
Now imagine if it did blue shift.
https://www.youtube.com/watch?v=opvuEVpl_PM&list=RDopvuEVpl_PM&start_radio=1
Yeah, I'm pretty sure red shift is our best method for getting distances on billions of light year distant objects, no idea of it's the only one at that range