[simonh]

All that means is that movements of particle A and thus changes in its gravitational field now will never reach particle B, which is so far away that the relative expansion rate is c or more. However the historical gravitational field of Particle A, the part of its field that is within particle B's light cone, will always affect particle B.

[JadeNB]

What if the light cones of A and B don't intersect at all? There are certainly places in the universe so far apart that light not only can't now, but could never, have travelled from one to the other; that is part of why the near-isotropy of background radiation is something that needs to be explained, I thought.

[simonh]

Are there regions of the universe that have never been causally connected? They all originated in the same event, so I'd be surprised if that's true, but I'm no expert. I know cosmic inflation during the big bang was superluminal, but I have to concede I'm not completely familiar with the consequences of that, so good point.

[JadeNB]

Indeed, the fact that they are moving apart at superluminal speeds is exactly why they have never been causally connected. The fact that such non-causally connected islands still "look the same" is called the horizon problem (https://en.wikipedia.org/wiki/Horizon_problem), and it is one of the issues that Guth's inflation might solve (https://en.wikipedia.org/wiki/Inflation_(cosmology)#Horizon_...).