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There are a lot of questions and answers on this site that deal with galaxies receding from us faster then the speed of light like this one:

In summary, Hubble Law: v=H(t)D, where v is recession velocity, D is distance, and H(t) is the Hubble "constant" at a given time, requires that beyond a certain distance velocity is greater than the speed of light.

How Are Galaxies Receding Faster Than Light Visible To Observers?

And this one:

But remote objects may indeed be becoming more remote faster than light.

Some areas of the universe are moving away from us FTL but surely as we get closer to that area, the speed of expansion would gradually slow?

Now these answers commonly only talk about galaxies receding from us, and as seen from us, they might be receding faster then light. None of them talk about whether it is possible, for us, to view two other (other then ours) distant galaxies to recede from each other faster then light.

Question:

  1. Can we observe two galaxies (other then ours) to be receding from each other faster then the speed of light?
Árpád Szendrei
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    What does "receding from each other" mean here? If you just mean that galaxy A would see galaxy B as receding FTL, then doesn't just picking any galaxy receding FTL from us as galaxy A and any galaxy in the opposite direction as B do the job (since A and B are then then farther apart than we and A, A will see B even faster by Hubble's law)? – ACuriousMind Sep 28 '22 at 22:12
  • You also have to keep in mind that those galaxies are seeing each other in a younger state. Let's say galaxies A and B are 5 billion light years from us (and twice as far from each other). So if we are looking at a galaxy A and it looks like it's seven billion years old, then an observer in Galaxy B will see light that was emitted when Galaxy A was merely 2 billion years old. And since we are looking at the past, these objects are already further away, still. Indeed, most of the universe is already out of our reach. What we are seeing is merely an afterimage of delayed light. – FlatterMann Sep 28 '22 at 22:19
  • Keep in mind: https://physics.stackexchange.com/q/400457/180843 – Sten Sep 28 '22 at 23:21
  • Why the downvote? – Árpád Szendrei Sep 29 '22 at 16:07
  • @ACuriousMind I believe that the question could be interpreted as from out viewpoint, measuring the (change in) distance between two other galaxies, and the time elapsed on our clock. Please reverse the downvote if this clarifies it. – Árpád Szendrei Sep 29 '22 at 16:17
  • Neglecting expansion, the rate of change of displacement between two objects, as measured by a third in the middle, has a limit of 2c, not c. Consider pointing lasers in opposite directions. – g s Oct 05 '22 at 18:39

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Yes. Any galaxy with a redshift $z\gtrsim 2$ has a recessional speed relative to us that is larger than $c$ now and at all past and future times. So you could take, e.g., two galaxies with equal redshifts larger than 2 that are separated by 60° in the sky. They form an equilateral triangle with us and all pairwise recessional speeds are about the same. Any angle larger than 60°, or a smaller angle and larger redshift, would also work.

benrg
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