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The Mirror universe is considered a stable parallel to the prime universe. The mirror "Enterprise" crew didn't find the Enterprise from TOS, they discovered the Defiant. As for the dimensional shift, my guess is 'a fracture in space-time.'Which one are you referring to? Neither Constitution class Enterprise from either dimension went back in time to the mid-22nd century. It was the USS Defiant in the 2260's from our universe that ended up in the mid-22nd century in the mirror universe, due to the Tholians in the mirror universe using tri-cobalt explosives near a dead star to create an interphasic rift into our universe and the 23rd century.

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