Multiverses and physical cosmology

Abstract

The idea of a multiverse -- an ensemble of universes -- has received increasing attention in cosmology, both as the outcome of the originating process that generated our own universe, and as an explanation for why our universe appears to be fine-tuned for life and consciousness. Here we carefully consider how multiverses should be defined, stressing the distinction between the collection of all possible universes, and ensembles of really existing universes that are essential for an anthropic argument. We show that such realised multiverses are by no means unique. A proper measure on the space of all really existing universes or universe domains is needed, so that probabilities can be calculated, and major problems arise in terms of realised infinities. As an illustration we examine these issues in the case of the set of Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) universes. Then we briefly summarise scenarios like chaotic inflation, which suggest how ensembles of universe domains may be generated, and point out that the regularities which must underlie any systematic description of truly disjoint multiverses must imply some kind of common generating mechanism. Finally, we discuss the issue of testability, which underlies the question of whether multiverse proposals are really scientific propositions.

32 pages, 2 figures Completely revised text addressing physics issues more fully and omitting philosophical discussions previously included

References in corpus (12)

• From the Big Bang Theory to the Theory of a Stationary Universe
• Cosmic Topology
• Predictions from Quantum Cosmology
• Is "the theory of everything'' merely the ultimate ensemble theory?
• The Covariant Approach to LRS Perfect Fluid Spacetime Geometries
• Stationary Universe
• On likely values of the cosmological constant
• A Priori Probability Distribution of the Cosmological Constant
• The Cold Big-Bang Cosmology as a Counter-example to Several Anthropic Arguments
• Asymptotic dynamics of the exceptional Bianchi cosmologies
• Many worlds in one
• Late-time asymptotic dynamics of Bianchi VIII cosmologies

Cited by in corpus (19)

• Varying constants, Gravitation and Cosmology
• The Mathematical Universe
• The Fine-Tuning of the Universe for Intelligent Life
• The Beginning and the End: The Meaning of Life in a Cosmological Perspective
• Multiple universes, cosmic coincidences, and other dark matters
• The Degree of Fine-Tuning in our Universe -- and Others
• Screams for Explanation: Finetuning and Naturalness in the Foundations of Physics
• The Born Rule Dies
• Cosmological Measures without Volume Weighting
• On the Onset of Inflation in Loop Quantum Cosmology
• Drake Equation for the Multiverse: From the String Landscape to Complex Life
• Sensitivity of Carbon and Oxygen Yields to the Triple-Alpha Resonance in Massive Stars
• Cosmological zoo -- accelerating models with dark energy
• Agnesi Weighting for the Measure Problem of Cosmology
• Nuclear Processes in Other Universes: Varying the Strength of the Weak Force
• Effects of Bound Diprotons and Enhanced Nuclear Reaction Rates on Stellar Evolution
• Anthropic Selection of Physical Constants, Quantum Entanglement, and the Multiverse Falsifiability
• Paths of Friedmann-Robertson-Walker brane models
• Is there paradox with infinite space?