crispybits wrote:Sure thing then, you said "the fastest signal is the speed of light as far as we know",
I figured I might get into trouble with that comment. A more precise thing to say is that the speed of light is
the fastest possible signal speed, according to general relativity. Information
cannot be propagated faster than this speed, according to conventional wisdom. Now, the whole issue with the Big Bang is that general relativity starts to break down at very high densities, so we can no longer really say whether this limit still holds at the earliest times in our universe, or whatever happened before the universe. But what's interesting is that we have already seen that quantum mechanics allows for an "action at a distance," of a sort, where in some sense you can act on a particle in one location and instantaneously affect what happens to a particle somewhere else, without any communication between the two particles. This is the type of causality that probably still holds in the earliest times of the universe -- particles are connected because their quantum wave-functions overlap. It's not the same thing as logically acausal, where there is simply no relationship governing what nearby particles do.
So, what Carroll and Ostlie meant in that sentence you underlined is that there are currently parts of the universe that
cannot ever have exchanged information with each other, due to the finite speed of light. This is true today, and it was true in the Big Bang too. The universe is nevertheless locally causal, even if distant parts of it have never interacted with each other in the classical sense.