Are galaxies moving away from each other at a constant speed? Im trying to understand what Hubble's law
$$v = H_0  d$$
means. The thing is I keep getting two different explanations.

*

*Galaxies are moving at a constant speed, the ones further away are so because they move faster.

*Galaxies move faster away the further they are from each other, because there is more matter between them that expands.

Which of these two is correct?
 A: The space is expanding between non gravitationally bound systems, such as galaxies. The greater the space between them, the greater the amount of expansion. It is believed there are galaxies so far away that we will never see their light as the space between us and them expands faster than their light travels toward us. However this does not mean all galaxies are moving away from each other. In fact the Andromeda galaxy and our Milky Way galaxy are nearing each other and are expected to collide in 4 to 5 billion years.
A: I find it helpful to separate the observations from the model and interpretation.  The observation was that there was a curious linear trend between distance from Earth and velocity.  This was simply an observation made on galaxies that we could see.  Such a trend would not be expected in random data.  Indeed, it would suggest that we might actually be the center of the universe, as everyone expands out from us.
Of course, that is not the popular interpretation.  We don't like to assume we are that special.  Instead, it was identified that modern relativistic theories permitted space to expand.  If space itself was expanding, then a linear relationship like we see from Earth would be seen from everywhere.  This is the preferred interpretation: $v=H_0d$ is seen as evidence that space itself is expanding (and taking matter along for a ride).
