Some physicists subscribe to many worlds theory. It's a paradigm that states at every instant a new universe is created to house the infinitely many choices at any moment. This is a consequence of treating the universe as the system, rather than picking a smaller, more manageable environment. In doing so, the universe must be a closed system. This is not something that can be directly determined. The necessity of a closed universe is related to adiabatic functions that must be conserved. In a forest of nearly touching universes, conservation cannot be verified. For instance, if particles were permitted to leak and interact, that would permit energy not accounted for. As it relates to the Cat problem, the many worlds are virtual within the confines of our universe. As another example, Richard Feynman developed a graphical means of describing particle interaction. The curious thing about the diagrams are they represent all the different paths possible, except when being looked at. This is exactly what Schrodinger was trying to convey. The quantum universe behaves chaotically until observed. Then, it's more like only third chaotic once a measurement is taken. Bottom line, many worlds theory is not supported by evidence, Cat problem is consistent whether or not in a many worlds environment, and virtual systems are allowable provided they pay back what gets borrowed before its surroundings are made aware. If all that sounded confusing, I'll try once more. Parallel worlds are fantasy.