Prediction and typicality in multiverse cosmology

TL;DR

This paper examines how assumptions about typicality influence the predictive power of anthropic reasoning in multiverse cosmology, emphasizing the importance of explicitly including measures of typicality in such frameworks.

Contribution

It demonstrates that assumptions about typicality significantly impact predictions in multiverse cosmology and advocates for explicitly incorporating typicality measures in anthropic reasoning.

Findings

Typicality assumptions can dramatically alter dark matter predictions.

Errors in typicality reasoning lead to errors in assessing predictive power.

Explicit measures of typicality are crucial for reliable anthropic predictions.

Abstract

In the absence of a fundamental theory that precisely predicts values for observable parameters, anthropic reasoning attempts to constrain probability distributions over those parameters in order to facilitate the extraction of testable predictions. The utility of this approach has been vigorously debated of late, particularly in light of theories that claim we live in a multiverse, where parameters may take differing values in regions lying outside our observable horizon. Within this cosmological framework, we investigate the efficacy of top-down anthropic reasoning based on the weak anthropic principle. We argue contrary to recent claims that it is not clear one can either dispense with notions of typicality altogether or presume typicality, in comparing resulting probability distributions with observations. We show in a concrete, top-down setting related to dark matter, that assumptions about typicality can dramatically affect predictions, thereby providing a guide to how errors in reasoning regarding typicality translate to errors in the assessment of predictive power. We conjecture that this dependence on typicality is an integral feature of anthropic reasoning in broader cosmological contexts, and argue in favour of the explicit inclusion of measures of typicality in schemes invoking anthropic reasoning, with a view to extracting predictions from multiverse scenarios.