# How fast were distant galaxies moving?

• I
• kymner
kymner
TL;DR Summary
A quick question of relative velocity of galaxies when expansion based velocities are cancelled out.
I'm aware that the expanding universe causes galaxies to move away at a velocity proportional to distance. However, I have some interest in the velocity that the galaxy was moving at the time in the past that we are seeing at present. This must be known about because the time dilation of relative velocity would cause a red shift independent of the expansion of the universe which must be taken into account in order to solve the rate of expansion. But, how fast were galaxies moving at distant times like a billion years ago or 3.26 million years ago? Were velocities much higher such as 0.008 c for 1 megaparsec away and 0.15 c for 1 billion years ago or are the values much smaller? Does anyone have a list of galaxy velocities based on distance once the expanding universe is factored out?

There are two things going on with non-bound galaxies. The first is expansion and the second is the acceleration of that expansion. I can't tell if you really mean expansion, as you say, or if you mean the acceleration, or if you mean both. If you mean both then the question is not meaningful, so which is it?

kymner said:
TL;DR Summary: A quick question of relative velocity of galaxies when expansion based velocities are cancelled out.

I'm aware that the expanding universe causes galaxies to move away at a velocity proportional to distance. However, I have some interest in the velocity that the galaxy was moving at the time in the past that we are seeing at present. This must be known about because the time dilation of relative velocity would cause a red shift independent of the expansion of the universe which must be taken into account in order to solve the rate of expansion. But, how fast were galaxies moving at distant times like a billion years ago or 3.26 million years ago? Were velocities much higher such as 0.008 c for 1 megaparsec away and 0.15 c for 1 billion years ago or are the values much smaller? Does anyone have a list of galaxy velocities based on distance once the expanding universe is factored out?
Within each galaxy cluster, the galaxies are attracted gravitationally and have some local velocity relative to the overall expansion. The redshift from the expansion of the universe is most clearly seen in galaxies a long way away - as the local velocity has a small effect. For galaxy clusters closer, the redshift will vary depending on the direction of the galaxy's local velocity relative to the expansion. For a galaxy moving towards Earth this may cancel out some or all of the redshift. And, in fact, the closest large galaxy (Andromeda), which is part of the local cluster, is blueshifted.

kymner said:
But, how fast were galaxies moving at distant times like a billion years ago or 3.26 million years ago?
Are you asking how the Hubble constant changes with time? I've seen graphs, but would have to track one down. It doesn't vary that much, though, certainly not enough that nearby galaxies have recession speeds anywhere near ##c##.

To be clear, I am not talking about expansion at all but about the local velocities of the galaxies within their local clusters at distant times in the past. When expansion is taken into account there should be an additional velocity that gets larger faster as you go back in time that represents the velocity before gravity slowed it down. This is a velocity in a random direction that could be towards our galaxy but is dependent on the individual galaxy for direction. Galaxies must be moving with respect to the cosmological background radiation at all times even from their own perspective so my question is not about expansion rate but about how fast the galaxy would have appeared to be moving relative to the cosmological background radiation if you were an observer in that galaxy at the time the photons were emitted that you are now seeing billions of years later.

kymner said:
To be clear, I am not talking about expansion at all but about the local velocities of the galaxies within their local clusters at distant times in the past.
with no expansion, the universe would still be an incredibly hot, incredibly dense plasma and there would not BE any galaxies.

kymner said:
To be clear, I am not talking about expansion at all but about the local velocities of the galaxies within their local clusters at distant times in the past. When expansion is taken into account there should be an additional velocity that gets larger faster as you go back in time that represents the velocity before gravity slowed it down. This is a velocity in a random direction that could be towards our galaxy but is dependent on the individual galaxy for direction. Galaxies must be moving with respect to the cosmological background radiation at all times even from their own perspective so my question is not about expansion rate but about how fast the galaxy would have appeared to be moving relative to the cosmological background radiation if you were an observer in that galaxy at the time the photons were emitted that you are now seeing billions of years later.

kymner said:
When expansion is taken into account there should be an additional velocity that gets larger faster as you go back in time that represents the velocity before gravity slowed it down.
I think you are talking about the velocity of a galaxy relative to the local CMB rest frame. This will obviously vary as it interacts with other galaxies, but I would expect that the average speed was lower in the past, not higher. The universe was more uniform in its early history than it is now.

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