# 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.

1. Galaxies are moving at a constant speed, the ones further away are so because they move faster.
2. 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?

• "galaxies move faster away the further they are from each other, because there is more matter between them that expands." This is mostly correct. It is the space itself that expand not the matter. Feb 27, 2021 at 0:49

• I'd like to point out that for a galaxy with superluminal recession velocity, it does not follow that we won't ever receive light from them. In a non-inflationary universe, light from such galaxies will eventually reach earth because $\dot{v}_{rec} < 0$. Even though an emitted photon is initially moving away from earth, it is slowing down in that direction, will eventually have $v_{tot} = v_{rec}+v_{pec} = 0$, and then will begin traveling towards earth, reaching our telescopes with $v_{tot} = v_{pec} = c$. Mar 10, 2021 at 21:11
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).