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The main question being: At some point in the past (before the big bang, before time, etc. just keep going back) did there exist complete nothingness or has something always existed? If these theories are based on spacetime, is it safe to assume that spacetime has always existed?

At extremes, our understanding tends to break down. You see this for instance with gravity. We have gravity figured out, right? First we had Newton, then Kepler improved on Newton and then Einstein improved on Kepler and now we understand gravity.
Erm ... no. If you apply Einstein's formulas to the center of a black hole, ie. the extreme of gravity, you get nonsense.

Same thing with spacetime. You mention 'before time'. What does that even mean? 'Before time'? You're stepping outside of a concept to use that concept on itself. It really is like North of the north pole.

As an aside at this point, 'complete nothingness' is also an extremely difficult concept to get one's head around. The vacume of space, for instance, is nowhere near 'complete nothingness'.

But, back to spacetime. At this point, we can't look past the Big Bang. Maybe there IS an endless cycle of bang/crunch/bang/crunch, maybe spacetime IS eternal. Or, maybe the universe DID cause itself; or, as is the case with quantum events the universe is causeless.
The point is, the only answer anyone can give you right now is "We don't know." Possibly with a "Come back in a century or two, maybe then we'll know" added.
Anyone who says that they do KNOW, is making stuff up, like the William Lane Craigs of this world.

Like at some point does the expansion reverse and become compression?

While I mentioned the bang/crunch scenario, it seems that the crunch is never going to happen in this universe. In stead, things will just continue to drift appart and decay until there's nothing left. Again, depending on what you understand as 'nothing'.
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Ataraxia Well explained January 13, 2014, 05:47:46 AM