Axions, Inflation and the Anthropic Principle

TL;DR

This paper discusses the challenges of integrating high-scale axions with inflationary cosmology, highlighting a fine-tuning problem that persists despite anthropic reasoning, thus questioning the viability of certain string-inspired models.

Contribution

It demonstrates that high-$f_a$ axions in inflationary models face a severe fine-tuning problem, which is not resolved by anthropic selection, challenging existing theoretical assumptions.

Findings

High-$f_a$ axions require extreme fine-tuning in inflationary cosmology.

Anthropic arguments do not resolve the fine-tuning problem.

The problem is worse than the original strong-CP problem.

Abstract

The QCD axion is the leading solution to the strong-CP problem, a dark matter candidate, and a possible result of string theory compactifications. However, for axions produced before inflation, symmetry-breaking scales of $f_a \gtrsim 10^{12}$ GeV (which are favored in string-theoretic axion models) are ruled out by cosmological constraints unless both the axion misalignment angle $\theta_0$ and the inflationary Hubble scale $H_I$ are extremely fine-tuned. We show that attempting to accommodate a high-$f_a$ axion in inflationary cosmology leads to a fine-tuning problem that is worse than the strong-CP problem the axion was originally invented to solve. We also show that this problem remains unresolved by anthropic selection arguments commonly applied to the high-$f_a$ axion scenario.