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arXiv · 0911.0421

Axions, Inflation and the Anthropic Principle

Katherine J. Mack
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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 $θ_0$ 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.

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Katherine J. Mack. 2009-11-03. Axions, Inflation and the Anthropic Principle. https://doi.org/10.1088/1475-7516%2F2011%2F07%2F021

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