I know it wasn't originally yours, but you are the one arguing in favor of it in this thread. Let me try again: what does it mean for two regions of spacetime to not be causally connected? It means that anything that's happening in one region doesn't (cannot) affect the other region. There is no causal link. We can only learn about something by it having some influence on us. If something doesn't affect us, we cannot detect it. A star's light reaches us, and its photons cause electrical signals in our eyes; it affects us, therefore we can (in principle) determine that the star exists. There is a causal link. Let's say a hundred years from now, a machine is made that makes a noise. That noise cannot affect us: it's in our future (I'm assuming time-travel doesn't happen). There is no way for us to gain any information about that noise, and we cannot actually determine that that noise really will happen. There is no causal link. If there was something before the BB, and there's a causal link between pre-BB and post-BB, we can (in principle) learn about it. If there was something before the BB, but there's no causal link between pre-BB and post-BB, we cannot ever learn anything about it. There literally is no way for information from the pre-BB period to reach us in that case. We can never know anything about that period, so we might as well assume it doesn't exist (Occam's Razor).