If you knew enough about the big bang to simulate it (which we dont), you wouldnt have to know the state of the universe now. You could start at the big bang which had... hm.... probably the starting condition "0", and calculate from then to now.
But we couldnt do it anyway...
There is still a lot about subatomic physics which we dont understand, and constants that are impossible to have at 100% precision, Pi for example. If we were to simulate the univers, I assume you cant do it without having 100% precise constants, which are impossible to have for those constants.
Apart from that, the computer that simulates the universe would have to be bigger than the universe.
Where would we get the material to do that?
Even if we managed to build a computer like that from... uhm... that dimension that Morgan Freeman killed the Nihilanth in... its impossible that our computer will have no impact on our universe/dimension, cause after all... we are the ones that built it right? So they cant be independant.
If they cant be independant, the computer would also have to simulate the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that simulates the computer that