Astro Cat wrote: ↑Thu Sep 01, 2022 5:06 am
This is probably what the article you were mentioning is talking about. It's not that galaxies out there aren't moving: they're moving in all sorts of directions, even towards us. It's that the expansion so vastly outstrips their peculiar motion that it's utterly negligible.
I wanted to clarify this further maybe using the balloon analogy, because when I say "even towards us," I don't want to give the wrong impression.
Imagine drawing a big dot on a balloon and a smaller one orbiting it. Then draw a dot some distance away (this second dot is us).
As the balloon expands, the big dot is moving away from our dot. The dot orbiting the big dot, though, has peculiar ("true") motion: it's actually moving on the skin of the balloon rather than just being swept along with the skin of the balloon.
Put yourself in our dot's point of view and look at the dot/orbiting dot that's heading away from us as the balloon expands. Sometimes, the orbiting dot will be moving away from us faster than the big dot (when its orbit has it moving a direction away from our dot), and sometimes the orbiting dot will be moving away from us slower than the big dot (when it swings back around and seems to be moving towards our dot momentarily).
At all times, the orbiting dot is still moving away from us ultimately, so it will be redshifted. But, in terms of "true" motion, during parts of its orbit it's moving "towards" us.
That make more sense?
Yes, and it's definitely going into my comedy routine tonight. I haven't got any balloons so will just settle for a condom.
