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Anthropic reasoning and typicality in multiverse cosmology and string theory*

Steven Weinstein1,2,3

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Anthropic arguments in multiverse cosmology and string theory rely on the weak anthropic principle (WAP). We show that the principle is fundamentally ambiguous. It can be formulated in one of two ways, which we refer to as WAP1 and WAP2. We show that WAP2, the version most commonly used in anthropic reasoning, makes no physical predictions unless supplemented by a further assumption of 'typicality', and we argue that this assumption is both misguided and unjustified. WAP1, however, requires no such supplementation; it directly implies that any theory that assigns a non-zero probability to our universe predicts that we will observe our universe with probability one. We argue, therefore, that WAP1 is preferable, and note that it has the benefit of avoiding the inductive overreach characteristic of much anthropic reasoning.


Footnote
*  Thanks to Yuri Balashov, Gordon Belot, Rob Caldwell, Marcelo Gleiser, Brad Monton, Ken Olum, Jim Peebles, Lee Smolin and Alex Vilenkin for helpful discussions and comments on an earlier draft.
PACS

11.27.+d Extended classical solutions; cosmic strings, domain walls, texture

95.30.Cq Elementary particle processes

11.25.-w Strings and branes

98.80.Cq Particle-theory and field-theory models of the early Universe (including cosmic pancakes, cosmic strings, chaotic phenomena, inflationary universe, etc.)

98.80.Es Observational cosmology (including Hubble constant, distance scale, cosmological constant, early Universe, etc)

MSC

83E30 String and superstring theories (See also 81T30)

81T30 String and superstring theories; other extended objects (e.g., branes) (See also 83E30)

83F05 Cosmology

85A40 Cosmology (For relativistic cosmology, see 83F05)

Subjects

Gravitation and cosmology

Particle physics and field theory

Astrophysics and astroparticles

Dates

Issue 12 (21 June 2006)

Received 10 April 2006

Published 1 June 2006



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