Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Multiverse Hierarchy   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.

22
The categories of this paper are: chao-dyn
21
The categories of this paper are: adap-org
14
The categories of this paper are: math-ph
25
The categories of this paper are: patt-sol
35
The categories of this paper are: mtrl-th
16
The categories of this paper are: stat
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom
5
The categories of this paper are: gr-qc
19
The categories of this paper are: eess
9
The categories of this paper are: nlin
1
The categories of this paper are: cs
33
The categories of this paper are: plasm-ph
15
The categories of this paper are: nucl-ex
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th