Sometimes people say that no universe can contain a perfect simulation of itself, since the universe would need a simulation of the simulation, and a simulation of that, and so on, leading to an infinite stack of simulations. Now, such a stack is not obviously impossible. Perhaps an infinite universe could devote a small fraction of its resources to running a (still infinite) simulation of itself. The resulting stack of simulations would be no problem for an infinite universe. Even a finite but expanding universe could run an ongoing simulation of the past that lags a little behind reality.
