There are two problems -- storing the state of your emulated universe (storage space) and the operating speed. You can undoubtably run it at less than realtime

There are two problems -- storing the state of your emulated universe (storage space) and the operating speed. You can undoubtably run it at less than realtime, and might even be able to find a way to scrub fwd/rev through time to the interesting bits. The bigger issue is how to store the data. If the universe is compressible we might be able to store it and operate on it. If it's not sanely compressible then we either need to use a large chunk, or all of, the universe's matter/energy to store the state of our emulator and any inhabitants of our universe would need to file written complaints in advance at Alpha Centauri if they did not consent to being subsumed into the Giant Grey Blob(tm).