Making predictions in the multiverse

Abstract

I describe reasons to think we are living in an eternally inflating multiverse where the observable "constants" of nature vary from place to place. The major obstacle to making predictions in this context is that we must regulate the infinities of eternal inflation. I review a number of proposed regulators, or measures. Recent work has ruled out a number of measures by showing that they conflict with observation, and focused attention on a few proposals. Further, several different measures have been shown to be equivalent. I describe some of the many nontrivial tests these measures will face as we learn more from theory, experiment, and observation.

20 pages, 3 figures; invited review for Classical and Quantum Gravity; v2: references improved

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• Soft de Sitter Effective Theory
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• Inflation after False Vacuum Decay: Observational Prospects after Planck
• Challenges for String Cosmology
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• Self-Organised Localisation
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• Cycles in the Multiverse
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• The Multiverse and Particle Physics
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• Kahler Moduli Stabilization and the Propagation of Decidability
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• A Smooth Landscape: Ending Saddle Point Inflation Requires Features to be Shallow
• How Decoherence Affects the Probability of Slow-Roll Eternal Inflation
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• Bayesian Reasoning in Eternal Inflation: A Solution to the Measure Problem
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• On memory in exponentially expanding spaces
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• Multiverse Predictions for Habitability: Element Abundances
• Black holes and up-tunneling suppress Boltzmann brains
• Gravity waves from cosmic bubble collisions
• Hubble selection of the weak scale from QCD quantum critical point
• Spacetime Average Density (SAD) Cosmological Measures
• New Scale Factor Measure
• Slepian models for Gaussian Random Landscapes
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• The Noonday Argument: Fine-Graining, Indexicals, and the Nature of Copernican Reasoning
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• The WKB approximation and tunneling in theories with non-canonical kinetic terms