# Multiverse

Published work on the multiverse, each with its source.

## 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. Source: [Tegmark 2003, Parallel Universes](https://arxiv.org/abs/astro-ph/0302131)
- **Level II**: Other bubble universes born from inflation, each of which can have different physical constants, dimensions and particles. Source: [Tegmark 2003, Parallel Universes](https://arxiv.org/abs/astro-ph/0302131)
- **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. Source: [Everett 1957, Reviews of Modern Physics](https://doi.org/10.1103/RevModPhys.29.454)
- **Level IV**: Every consistent mathematical structure exists as a universe, with its own fundamental laws of physics. Source: [Tegmark 2003, Parallel Universes](https://arxiv.org/abs/astro-ph/0302131)

## How universes multiply
- **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. Source: [Guth 2007, Journal of Physics A](https://arxiv.org/abs/hep-th/0702178)
- **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. Source: [Martel, Shapiro & Weinberg 1998, The Astrophysical Journal; Riess et al. 1998, The Astronomical Journal](https://arxiv.org/abs/astro-ph/9701099)

## The numbers
- **10^(10^29) metres**: the distance to an identical copy of you, by Tegmark’s crude estimate. He notes the real copy is likely much closer. Source: [Tegmark 2003, Parallel Universes](https://arxiv.org/abs/astro-ph/0302131)
- **10^(10^115) metres**: the distance to an identical copy of our entire observable universe. Source: [Tegmark 2003, Parallel Universes](https://arxiv.org/abs/astro-ph/0302131)
- **10^500+**: consistent string-theory vacua, each a possible universe with its own laws: “perhaps greater than 10⁵⁰⁰”. Source: [Douglas & Kachru 2007, Reviews of Modern Physics](https://arxiv.org/abs/hep-th/0610102)
- **10^272,000**: flux vacua from a single F-theory geometry, the one with the most. Source: [Taylor & Wang 2015, Journal of High Energy Physics](https://arxiv.org/abs/1511.03209)
- **10^(10^(10^7))**: 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. Source: [Linde & Vanchurin 2010, Physical Review D](https://arxiv.org/abs/0910.1589)

## Written out
- 10^11 stars in the Milky Way: 1 followed by 11 zeros (NASA)
- 10^20 Earth-like planets in the observable universe: 1 followed by 20 zeros (Behroozi & Peeples 2015)
- 10^500 string-theory vacua: 1 followed by 500 zeros (Douglas & Kachru 2007)
- 10^272,000 flux vacua in one F-theory geometry: 1 followed by 272,000 zeros (Taylor & Wang 2015)
- 10^(10^29) metres 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^(10^115) metres 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^(10^7)) distinguishable universes: 1 followed by a number of zeros that is itself a 1 followed by ten million zeros. (Linde & Vanchurin 2010)