In message , Darren
writes
A question has recently been put to me. I thought I'd share it with the
group to see if there is a knowledgeable person able to answer it.

Consider three galaxies. Our own Milky Way and two other galaxies (called A
and B for ease) Galaxies A and B are on exactly opposite sides of the Milky
way at exactly the same distance, right at the edge of the observable
Universe (i.e. where recessional velocity is equal to the speed of light)
For argument's sake, let's say they are 98% of the distance to the edge and
their velocity as seen from our galaxy is 0.98c.

Obviously, if you were in Galaxy A, we would be on the edge of your
observable Universe and Galaxy B would be far beyond it and undetectable.

So, the question is: What is the recessional velocity of Galaxy B from
Galaxy A?

Is it 1.96c?

Is it c?

Or is it something else entirely?

I hope this tickles the grey cells.

I have my own answer, but don't know if there's some weird cosmological
thing that makes my answer wrong.

Have fun,

Darren


Just so nobody shouts at me, I'm new to astronomy and somewhat unsure of
my facts, but I think your answer is 0.98c. I've been reading Roy and
Clarkes' Astronomy, Structure of the Universe, but there are numerous
treatments of special relativity.
Hopefully someone will put me right politely if I'm wrong!

Denis
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Denis Taylor
two ears, one mouth, think first.
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