I don't think it's for certain, but anyone who has ever implemented Runge-Kutta on an n-body problem knows that simulating a "simple" system can be very not simple. Accurately simulating a chaotic system over a time t requires \Omega(t) memory because of Lyapunov doubling. As such I think any long-term simulation of the Universe would require new physics or be subject to error. The errors might be impossible to detect from within the system but they're still "there" in a formal sense. New physics could include a "pixellated universe" which has the same errors, or something else.