Actually the problem is quite a bit simpler. You only have to simulate the observable universe, not the part that is outside our light cone, which is probably

Actually the problem is quite a bit simpler. You only have to simulate the observable universe, not the part that is outside our light cone, which is probably most of it. And the observable part makes it simpler still. You dont need to simulate planets more than a few hundred light years away, because they are not observable. It gives you time to upgrade your hardware before the next round of astronomical satellites goes up.