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PHYSICAL REVIEW D 81, 083525 (2010)
How many universes are in the multiverse?
Andrei Linde* and Vitaly Vanchurin†
Department of Physics, Stanford University, Stanford, California 94305, USA
(Received 8 December 2009; published 20 April 2010)
We argue that the total number of distinguishable locally Friedmann ‘‘universes’’ generated by eternal
inflation is proportional to the exponent of the entropy of inflationary perturbations and is limited by ee3N,
where N is the number of e-folds of slow-roll posteternal inflation. For simplest models of chaotic
inflation, N is approximately equal to de Sitter entropy at the end of eternal inflation; it can be
exponentially large. However, not all of these universes can be observed by a local observer. In the
presence of a cosmological constant the number of distinguishable universes is bounded by ejj3=4.
In the context of the string theory landscape, the overall number of different universes is expected to be
exponentially greater than the total number of vacua in the landscape. We discuss the possibility that the
strongest constraint on the number of distinguishable universes may be related not to the properties of the
multiverse but to the properties of observers.
DOI: 10.1103/PhysRevD.81.083525 PACS numbers: 98.80.Cq