Three Aspects of Typicality in Multiverse Cosmology

TL;DR

This paper discusses the complexities of assuming typicality in multiverse cosmology, highlighting how such assumptions impact predictions and the challenges in confirming theories based on observational likelihoods.

Contribution

It presents three key lessons on typicality in multiverse theories, challenging the common principle of mediocrity and its role in cosmological predictions.

Findings

Typicality assumptions significantly influence observational predictions.

Different assumptions can lead to coincident predictions, complicating theory confirmation.

Considering multiple theories, typicality may not maximize likelihoods for observations.

Abstract

Extracting predictions from cosmological theories that describe a multiverse, for what we are likely to observe in our domain, is crucial to establishing the validity of these theories. One way to extract such predictions is from theory-generated probability distributions that allow for selection effects---generally expressed in terms of assumptions about anthropic conditionalization and how typical we are. In this paper, I urge three lessons about typicality in multiverse settings. (i) Because it is difficult to characterize our observational situation in the multiverse, we cannot assume that we are typical (as in the 'principle of mediocrity'): nor can we ignore the issue of typicality, for it has a measurable impact on predictions for our observations. (ii) There are spectra of assumptions about both conditionalization and typicality, which lead to coincident predictions for our observations, leading to problems of confirmation in multiverse cosmology. And moreover, (iii) when one has the freedom to consider competing theories of the multiverse, the assumption of typicality may not lead to the highest likelihoods for our observations. These three entwined aspects of typicality imply that positive assertions about our typicality, such as the 'principle of mediocrity', are more questionable than has been recently claimed.