We don't know, because everything that we know about spacetime breaks down under the conditions of the Big Bang. It's similar to a black hole--you get a singularity where our laws no longer apply. This is one reason people are searching for a unifying theory that puts all of the information together and is capable of explaining what happens at a singularity.
Ok, how do we know it "breaks" down, and what the conditions were of the BB?
I'm not following...unless you are talking about dimensions yet discovered?