A Model of Anthropic Reasoning, Addressing the Dark to Ordinary Matter Coincidence

TL;DR

The paper argues that anthropic reasoning can address the dark-to-baryon matter ratio $r$ within axion cosmologies, focusing on post-PQ-inflation scenarios where the initial misalignment $\ theta$ is fixed across our observable universe. In this unconventional axion cosmology, $r$ becomes a contingent constant across the Multiverse and can be made compatible with observations even for large decay constants $F$ if $\ theta$ is sufficiently small, with the predicted $r$ depending on $F$ and $\ theta$. The author analyzes two measures—volume $V$ and observer-based $A$—to compute probabilities for observed $r$, finding that $V$ disfavors large $F$ while $A$ allows a broad range of $r$, including near unity, thus supporting anthropic arguments. The work discusses implications for axion detectability, supersymmetry phenomenology, isocurvature fluctuations, and potential links to a varying cosmological term, suggesting observable consequences and cross-connections to broader anthropic cosmology frameworks.

Abstract

If inflation occurs after the breaking of Peccei-Quinn symmetry then large values of the breaking scale $F$, which are forbidden in conventional axion cosmology, are permitted, provided that we inhabit a region of the Multiverse where the initial misalignment is small. Regions having approximately this initial misalignment may occupy a small volume of the Multiverse, but they contain a large fraction of potential observers. This scenario has many consequences, including a possible explanation of the approximate equality of dark and baryon matter densities.