As I mentioned, hopefully correctly, in a previous question, space-time has a definite boundary at the Big Bang because this is where all world lines 'terminate' in their pasts, no matter where the particle may be in space. However, because in an infinite universe, it has always been infinite in spatial volume even at the Big Bang, the boundary of the spatial part of the universe is not well defined at the Big Bang.
When cosmologists refer to the 'void' that MAY have pre-existed before the Big Bang, they really do not know what this 'condition' may have been. We like to imagine it as some kind of pre-existing space, but we have to remember that the 3-dimensional space we always think in terms of only appeared AFTER the Big Bang according to current cosmological models.
If you follow the general relativistic equations that describe space-time back to the Big Bang, they show that 3-d space and time 'came into existence' at the Big Bang, and the equations themselves do not let us peak back beyond the singularity of this creation epoch UNLESS we add some extension to general relativity theory such as quantum gravity theory. The void then becomes a curious type of dynamic 'froth' with an ill-defined dimensionality, and the Big Bang event is seen as the growth and separation of an independent 'sub-manifold' of some undefined, larger 'thing'. Like a soap bubble being formed, the Big Bang represents the earliest instant in our history when our space-time became a topologically independent entity. Any boundary you might think of gets lost in the many ambiguities that arise in the quantum mechanical picture of how our space-time became an independent entity.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.