Four rather different claims travel under one name. Max Tegmark's taxonomy ranks them by how speculative they are: Level I is merely space continuing past the observable edge, which the flatness measured in the microwave background hints at; Level II is the bubbles of eternal inflation; Level III is the branches of the many-worlds interpretation; Level IV holds that every mathematical structure exists. The last three lie beyond the causal horizon or beyond experimental reach, so it is hard to say how they could be shown false — which is exactly why their scientific standing is disputed.
Four rather different claims travel under one name. Max Tegmark's taxonomy ranks them by how speculative they are: Level I is merely space continuing past the observable edge, which the flatness measured in the microwave background hints at; Level II is the bubbles of eternal inflation; Level III is the branches of the many-worlds interpretation; Level IV holds that every mathematical structure exists. The last three lie beyond the causal horizon or beyond experimental reach, so it is hard to say how they could be shown false — which is exactly why their scientific standing is disputed.
Related topics in Frontiers of Knowledge