Axions, Inflation and the Anthropic Principle

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.

19 pages, 3 figures. Submitted to Phys Rev D

References in corpus (9)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• Monodromy in the CMB: Gravity Waves and String Inflation
• Axion Cosmology Revisited
• A Microscopic Limit on Gravitational Waves from D-brane Inflation
• A Delicate Universe
• Why the cosmological constant is small and positive
• Evidence for the Multiverse in the Standard Model and Beyond
• Axion Inflation and Gravity Waves in String Theory
• Cosmological Problems with Multiple Axion-like Fields

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• Cosmological window onto the string axiverse and the supersymmetry breaking scale
• Life at the Interface of Particle Physics and String Theory
• Probing the Early Universe with Axion Physics and Gravitational Waves
• Tensor Detection Severely Constrains Axion Dark Matter
• Looking at the NANOGrav Signal Through the Anthropic Window of Axion-Like Particles
• Axion Miniclusters in Modified Cosmological Histories
• Light axion-like dark matter must be present during inflation
• Ultra-light Axions: Degeneracies with Massive Neutrinos and Forecasts for Future Cosmological Observations
• Axion Isocurvature and Magnetic Monopoles
• String theoretic QCD axions in the light of PLANCK and BICEP2
• Axiverse cosmology and the energy scale of inflation
• Axion global fits with Peccei-Quinn symmetry breaking before inflation using GAMBIT
• Effect of the chiral phase transition on axion mass and self-coupling
• Cosmology of Axions and Moduli: A Dynamical Systems Approach
• Ultimate light-shining-through-a-wall experiments to establish QCD axions as the dominant form of dark matter