Correspondingly, if, as is the case, we know only what has happened since the big bang, we cannot determine what happened beforehand.
As far as we are concerned, events before the big bang can have no consequences and so should not form part of a scientific model of the universe.
We should therefore cut them out of the model and say the big bang was the beginning of time.
This means that questions such as who set up the conditions for the big bang are not questions that science addresses.

— Stephen Hawking [A Briefer History of Time]