This is a good topic, in the sense that it is an issue that is sometimes raised.
I rather like the way
Admin rephrased the issues, so I will comment on them in that form.
Could the physical laws of the universe have been different in the past?
I'm assuming that this has to do with how the universe works, rather than with how physicists say that it works. How scientists say that it works as changed, as science has learned more.
Yes, sure, in principle it is possible that things could have changed. In practice, this seems very unlikely. I look at the Bible and at other ancient literature. I see reports of people living similarly to the way that primitive peoples still live today. I see reports of animals (cattle, sheep, etc). To me it seems highly implausible that things could have changed enough that the YEC version of creationism could be true, yet that they have changed so little that the same biological organisms (humans, cattle, sheep, doves, etc) could have been well enough adapted to that world to be able to survive.
Could there be such a thing as an alternate reality that is undetectable?
Well that's always possible. Maybe there's an alternative universe with elves, gnomes, tooth fairies and the like. But if it is undetectable, then it has no important effects on the world of our experience, so it has no relevance to us.
Jesus was a liberal hippie