Multiverse · Ross Dahl

Four levels

Level I
Space goes on beyond what we can see, and far enough out every arrangement of matter repeats. Tegmark calls this level uncontroversial: it follows from the standard model of cosmology.
Tegmark 2003, Parallel Universes
Level II
Other bubble universes born from inflation, each of which can have different physical constants, dimensions and particles.
Tegmark 2003, Parallel Universes
Level III
Every outcome of every quantum event happens, each in its own branch. This is the many-worlds interpretation, first set out by Hugh Everett in 1957.
Everett 1957, Reviews of Modern Physics
Level IV
Every consistent mathematical structure exists as a universe, with its own fundamental laws of physics.
Tegmark 2003, Parallel Universes

Forever
Eternal inflation: the space that inflates expands much faster than it decays, so it never runs out, and keeps budding off pocket universes, each with its own big bang.
Guth 2007, Journal of Physics A
1997 → 1998
In a 1997 preprint, reasoning across a multiverse of possible values, Martel, Shapiro and Weinberg found that a small positive cosmological constant was a likely outcome. In 1998 supernova observations found one.
Martel, Shapiro & Weinberg 1998, The Astrophysical Journal; Riess et al. 1998, The Astronomical Journal

10^10²⁹ metres
the distance to an identical copy of you, by Tegmark’s crude estimate. He notes the real copy is likely much closer.
Tegmark 2003, Parallel Universes
10^10¹¹⁵ metres
the distance to an identical copy of our entire observable universe.
Tegmark 2003, Parallel Universes
10⁵⁰⁰+
consistent string-theory vacua, each a possible universe with its own laws: “perhaps greater than 10⁵⁰⁰”.
Douglas & Kachru 2007, Reviews of Modern Physics
10^272,000
flux vacua from a single F-theory geometry, the one with the most.
Taylor & Wang 2015, Journal of High Energy Physics
10^{10^10⁷}
distinguishable universes from eternal inflation in a simple model. In the string landscape the number of universes is expected to be far greater than the number of vacua.
Linde & Vanchurin 2010, Physical Review D

10¹¹stars in the Milky Way

100000000000

11 zeros · NASA
10²⁰Earth-like planets in the observable universe

100000000000000000000

20 zeros · Behroozi & Peeples 2015
10⁵⁰⁰string-theory vacua

100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

500 zeros · Douglas & Kachru 2007
10^272,000flux vacua in one F-theory geometry

272,000 zeros, scroll · Taylor & Wang 2015
10^10²⁹metres to an identical you

1 followed by 100,000,000,000,000,000,000,000,000,000 zeros. Too many to print.

Tegmark 2003
10^10¹¹⁵metres to an identical observable universe

1 followed by a number of zeros that is itself a 1 followed by 115 zeros.

Tegmark 2003
10^{10^10⁷}distinguishable universes

1 followed by a number of zeros that is itself a 1 followed by ten million zeros.

Linde & Vanchurin 2010