I am no physicist, JL, but I do like to play with my concepts, and the paradox of a beginning of the universe is too big to go unnoticed.
The way I think of it is that the Big Bang happened when we managed to extend our senses to it. When we managed to apply our human consciousness to information from those distance reaches of space, we gave them reality, in the sense that we evoked our three+1 dimensional reality on these far away phenomena.
Another way of saying it is perhaps that "physical reality" didn't happen until there were creatures around that had any use for the distinction "physical".
"What was before that then", we might ask, and my answer is that before that, there was whatever is there still, whatever we perceive as physical matter and energy. Beginning, ending, before and after are all human concepts with contextual meanings, and when they are applied to processes that extend beyond our conceptual scope, they stop making sense. But that is not an attribute of the universe itself, as much as an attribute of our capacity and methods for understanding. The timeline of the universe might be something we attribute to it by observing it and forcing it into out human parameters, which require linear time to categorize experience.