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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?

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That happened during slow roll inflation, but then it ended. What we see in cosmic microwave background radiation coming from different directions used to come from temporarily causally disconnected regions during inflation.

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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.

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    $\begingroup$ I'm not arguing with your definition of "causal contact," but I think the OP probably had in mind "intersection of past light cones" rather than "intersection of any light cones" as you seem to be talking about. $\endgroup$
    – user10851
    Oct 2, 2012 at 0:58

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