The Multiverse Hierarchy

Understand

I survey physics theories involving parallel universes, arguing that they form a natural four-level hierarchy of multiverses allowing progressively greater diversity.

• Level I: A generic prediction of inflation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions -- including an identical copy of you about 10^(10^29)m away.
• Level II: In chaotic inflation, other thermalized regions may have different physical constants, dimensionality and particle content.
• Level III: In unitary quantum mechanics, other branches of the wavefunction add nothing qualitatively new, which is ironic given that this level has historically been the most controversial.