The Multiverse Hierarchy

TL;DR

This paper surveys the concept of multiverses across four hierarchical levels, discussing their theoretical basis, implications, and the challenges in testing their existence within modern physics.

Contribution

It introduces a four-level hierarchy of multiverses, clarifies their theoretical foundations, and discusses the falsifiability and measure problem associated with these models.

Findings

Level I: Infinite ergodic universe with identical copies of observers.

Level II: Different physical constants and properties in separate regions.

Level III: Quantum branches add no new physical realities.

Abstract

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. Level IV: Other mathematical structures give different fundamental equations of physics. The key question is not whether parallel universes exist (Level I is the uncontroversial cosmological concordance model), but how many levels there are. I discuss how multiverse models can be falsified and argue that there is a severe "measure problem" that must be solved to make testable predictions at levels II-IV.