Multiverses and Cosmology: Philosophical Issues

TL;DR

This paper explores the philosophical and physical issues of multiverses in cosmology, including definitions, measures, emergence of consciousness, and testability, highlighting their implications for understanding our universe.

Contribution

It clarifies the conceptual foundations of multiverse theories, emphasizing the importance of measures, distributions, and physical mechanisms, and discusses their philosophical and scientific challenges.

Findings

Realized multiverses are not unique and require well-defined probability measures.

Multiverse scenarios can be generated by mechanisms like chaotic inflation.

Testability of multiverse hypotheses remains a significant philosophical and scientific issue.

Abstract

The idea of a multiverse -- an ensemble of universes or universe domains -- 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, which are essential for an anthropic argument. We show that such realised multiverses are by no means unique, and in general require the existence of a well-defined and physically motivated distribution function on the space of all possible universes. Furthermore, a proper measure on these spaces is also needed, so that probabilities can be calculated. We then discuss several other major physical and philosophical problems which arise in the context of ensembles of universes, including the emergence and causal effectiveness of self-consciousness, realized infinities, and fine- tuning, or the apparent need for very special initial conditions for our universe -- whether they or generalized generic primordial conditions are more fundamental. 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.