(October 16, 2020 at 5:43 am)FlatAssembler Wrote: As far as I understand it, mainstream physics considers there to have been no time before the Big Bang, so talking about "universe before ours" contradicts that.
It's more accurate to say that the simplest model, based on general relativity, does not allow for the extension of the time coordinate to before the Big Bang.
But, when quantum effects are modeled as well, that singularity at t=0 is 'smoothed out', which allows for time to make sense prior to the BB.
Unfortunately, we do not know which of several quantum gravity models is accurate (if any of the current ones are), so saying anything more is pure speculation.
Penrose was working on aspects of quantum gravity, so it is reasonable to talk about 'before the Big Bang' in that context.