From the beginning there were several serious problems with the standard Big Bang theory, thus the reason Inflation theory was concocted. Plenty of problems and uncertainties with that as well.
Top Ten Scientific Flaws In The Big Bang Theory
https://thetechreader.com/top-ten/top-ten-scientific-flaws-in-the-big-bang-theory/
From the beginning there were several serious problems with the standard Big Bang>>>>>>>>>>>>>>
The Assumption was that time is > 0
The proper assumption according to this theory is that time = 0