Cosmological Problems with Multiple Axion-like Fields

TL;DR

This paper investigates the extreme fine-tuning required for models with multiple axion-like fields to satisfy cosmological constraints, highlighting challenges for string theory and inflationary models.

Contribution

It introduces a figure of merit to quantify tuning in multi-axion models and demonstrates the exponential increase in fine-tuning with more fields.

Findings

Fine-tuning becomes exponentially more severe with multiple axion-like fields.

Anthropic arguments cannot fully explain the required tuning due to observational constraints.

Compatibility of string models with light axions and inflation is challenged by these tuning issues.

Abstract

Incorporating the QCD axion and simultaneously satisfying current constraints on the dark matter density and isocurvature fluctuations requires non-minimal fine-tuning of inflationary parameters or the axion misalignment angle (or both) for Peccei-Quinn symmetry-breaking scales $f_a > 10^{12}$ GeV. To gauge the degree of tuning in models with many axion-like fields at similar symmetry-breaking scales and masses, as may occur in string theoretic models that include a QCD axion, we introduce a figure of merit ${\cal F}$ that measures the fractional volume of allowed parameter space: the product of the slow roll parameter $\epsilon$ and each of the axion misalignment angles, $\theta_0$. For a single axion, $\mathcal{F} \lesssim 10^{-11}$ is needed to avoid conflict with observations. We show that the fine tuning of $\mathcal{F}$ becomes exponentially more extreme in the case of numerous axion-like fields. Anthropic arguments are insufficient to explain the fine tuning because the bulk of the anthropically allowed parameter space is observationally ruled out by limits on the cosmic microwave background isocurvature modes. Therefore, this tuning presents a challenge to the compatibility of string-theoretic models with light axions and inflationary cosmology.