Can causally disconnected regions join up again? For example the universe is expanding faster than light creating cosmological horizon, but what if something causes the expansion to slow down and reverse, will everything just join up again or are things beyond the horizon forever disconnected no matter what?
The important point here is that in general relativity, causal (dis)connection has only a global meaning, that is, over all of spacetime. It isn't something defined for a given instant in time, but over the whole history. Likewise, a horizon doesn't exist at a given moment; events that are on opposite sides of a hoizon were always, and will always be, disconnected causally. If the expansion slows down and reverses, bringing currently spatially distant points back together, then they were never causally disconnected in the first place.