Can you provide a link that demonstrates that GR proves there was no space before the big bang ?
I know it doesn't .
You quite clearly haven't a clue about phyiscs if you have not heard of zero point energy etc .
Zero point energy is a quantum effect. General relativity is a classical theory. In general relativity, there is no time or space before the Big Bang. If you want a reference, look at page 99 of Wald's book on General Relativity. I quote:
"Note that the nature of this singularity is that resulting from a homogeneous contraction of space down to 'zero size'. The Big Bang does not represent an explosion of matter concentrated at a point of pre-existing, non-singular spacetime, as it is sometimes depicted and as its name suggests. Since spacetime structure is itself singular at the big bang, it does not make sense, either physically or mathematically, to ask about the state of the universe 'before' the big bang: there is no natural way to extend the spacetime manifold and metric beyond the big bang singularity. Thus, general relativity leads to the viewpoint that the universe began at the big bang. "