Wasn't it Denis Taylor who wrote:
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!

Simply applying Special Relativity to the speeds would give a number a
little higher than 0.98c, but that would lead to the apparently absurd
conclusion that an observer in galaxy A would be able to see galaxy B,
because the galaxies are receding from each other at less than c. An
observer in galaxy A sees a visible universe twice the size of what we
see. The process can be repeated for galaxies on the edge of A's visible
universe, and it would seem that observers there can see twice as far
again, and so on.

One way out of this absurdity is to notice the fact that light has taken
about 15 billion years to get here from galaxy B, so it will take
another 15 billion years for that light to fly past us and reach the
location of galaxy A. When it arrives at galaxy A, the universe will be
about 30 billion years old and could well be twice the size it is now.

Things get more complicated if we notice that those remote galaxies have
moved considerably since the light we now see was emitted from them. Are
we trying to ascertain their relative velocity at the time when the
light we now see was emitted from them, or their relative velocity now
(under the assumption that they're still there)? The answer is very
different in the two cases.

--
Mike Williams
Gentleman of Leisure