To simulate a universe you'd need to be able to solve, or at least approximate with great accuracy, the n-body problem.
And we can't even do that with n >= 3 (without restrictions).
To simulate a small universe you're talking about one hell of a large "n".
Some physics are fundamentally not workable with logic and processing power due to exponential growth requirements for each additional n (and it makes no difference how large or advanced that other universe is)...
Those same physics require something much more fundamental, the fabric of space-time itself with some energy/motion introduced into it.
Hence the only way to "simulate" a universe is to create one... But then it's not a simulation at all.
And we can't even do that with n >= 3 (without restrictions).
To simulate a small universe you're talking about one hell of a large "n".
Some physics are fundamentally not workable with logic and processing power due to exponential growth requirements for each additional n (and it makes no difference how large or advanced that other universe is)...
Those same physics require something much more fundamental, the fabric of space-time itself with some energy/motion introduced into it.
Hence the only way to "simulate" a universe is to create one... But then it's not a simulation at all.