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This question seems too simple to not have been asked, but I cannot seem to find an explanation. So apologies if this is a duplicate.

  1. The current explanation for the expansion of the universe as I understand it, is that generally, the farther you look into space, the more redshifted galaxies tend to be, which indicate larger velocities far away from us.

  2. As most people know, light has a finite velocity, which means the light of farther galaxies take longer to reach us than closer galaxies. So a galaxy 10 billion light years away is also looking at that galaxy 10 billion years ago.

Edited:

  1. If you would plot the redshift of galaxies (y axis), and the time the light was emitted in billions of years since the big bang on the x axis (e.g., the light from a galaxy 10 billion light year away was emitted 10 billion years ago, therefor on the plot it would be at 13.7 - 10 = 3.7 ), you get a decreasing graph. Meaning galaxies were moving away faster in the distant past.

Point 3. is where I believe my confusion comes from.

Questions:

A) Are 1. and 2. correct?

B) The part giving me the most trouble visualizing the evolution of the universe is the time aspect. In the example with two galaxies A and B, with A at distance d and B at distance 2d, all we know is that galaxy B was moving away from us at twice the speed as A twice as long ago. So the hypothetical galaxy 10 billion light years away I mentioned, how do we know it's moving away from us faster than closer galaxies now? How is this calculated to adjust for the time differences at all distances?

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  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Jan 10, 2023 at 15:30
  • $\begingroup$ What plot have you made or seen? Please provide it, or a reference. $\endgroup$
    – ProfRob
    Jan 10, 2023 at 20:00
  • $\begingroup$ Also physics.stackexchange.com/questions/108609/… $\endgroup$
    – ProfRob
    Jan 10, 2023 at 20:08
  • $\begingroup$ and physics.stackexchange.com/questions/170449/… if these aren't what you are asking about then please attempt to clarify the question. $\endgroup$
    – ProfRob
    Jan 10, 2023 at 20:13
  • $\begingroup$ If you can clarify your question (not in comments) then I could look at re-opening it. $\endgroup$
    – ProfRob
    Jan 23, 2023 at 18:19

1 Answer 1

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Cosmic expansion means everything is moving away from everything else in a uniform way. Pick a galaxy. It's moving away from us. Now pick another galaxy behind the first galaxy at twice the distance. The second galaxy is moving away from the first galaxy at the same relative speed that the first galaxy is moving away from us. Thus, the second galaxy is moving away from us at twice the speed.

This is why the cosmic expansion rate is measured not as a velocity but as a velocity per distance. The universe expands at about 70 km/s/Mpc, meaning objects 1 Mpc (megaparsec) away recede at about 70 km/s, objects 2 Mpc away recede at 140 km/s, and so on. This holds for any observer, not just ourselves.

Note that none of this says anything about whether the expansion is accelerating or decelerating. It's a description of the local behavior of cosmic expansion only.

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  • $\begingroup$ This isn't what the OP is asking about or misunderstands. It also isn't true when you compare the observed velocity with the estimated distance when you get to Gpc scales. $\endgroup$
    – ProfRob
    Jan 10, 2023 at 20:01
  • $\begingroup$ The OP talks about galaxies 10 billion light years away. $\endgroup$
    – ProfRob
    Jan 10, 2023 at 20:11
  • $\begingroup$ @ProfRob Sorry, I deleted the comment after rereading the OP confused me, but after rerereading it I still think I addressed the OP's confusion exactly. Yes, they refer to galaxies 10 billion light years away, but their confusion is exactly about the local behavior of cosmic expansion, regardless of the (arbitrary) distance that they specified. $\endgroup$
    – Sten
    Jan 10, 2023 at 20:13
  • $\begingroup$ Fair enough. I can't really tell and we await clarification from the OP. $\endgroup$
    – ProfRob
    Jan 10, 2023 at 20:16
  • $\begingroup$ @ProfRob The part giving me the most trouble visualizing the evolution of the universe is the time aspect. In the example with two galaxies A and B, with A at distance d and B at distance 2d, all we know is that galaxy B was moving away from us at twice the speed as A twice as long ago. So the hypothetical galaxy 10 billion light years away I mentioned, how do we know it's moving away from us faster than closer galaxies now? How is this calculated to adjust for the time differences at all distances? $\endgroup$
    – Zach
    Jan 23, 2023 at 17:43

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