There is no big bang singularity.
Let’s start with the Lorentzian path integral for quantum gravity.
“This is precisely the point of view adopted by Vilenkin in his early papers, although he never performed any path integral calculations. Instead, he imposed “outgoing” boundary con- ditions on solutions of the Wheeler-DeWitt equation, a prescription which is, however, incomplete when perturbations are considered. Our Lorentzian path integral formulation in contrast allows us to simultaneously handle both the background and perturbations with no ambiguities.” (PAGE 3)