Cosmological Problems with Multiple Axion-like Fields

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 $\mathcal{F}$ 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.

17 pages, 2 figures. Submitted to Phys Rev D

References in corpus (12)

• Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• Three Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Polarization Analysis
• 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
• Fitting the Gamma-Ray Spectrum from Dark Matter with DMFIT: GLAST and the Galactic Center Region
• Evidence for the Multiverse in the Standard Model and Beyond
• Axion Inflation and Gravity Waves in String Theory
• Cosmological Constant and Axions in String Theory

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• Looking at the NANOGrav Signal Through the Anthropic Window of Axion-Like Particles
• Ultra-light Axions: Degeneracies with Massive Neutrinos and Forecasts for Future Cosmological Observations
• The Spectrum of the Axion Dark Sector
• The Axiverse Extended: Vacuum Destabilisation, Early Dark Energy and Cosmological Collapse
• Phenomenological Constraints on Axion Models of Dynamical Dark Matter
• String theoretic QCD axions in the light of PLANCK and BICEP2
• Axion global fits with Peccei-Quinn symmetry breaking before inflation using GAMBIT
• Axions, Inflation and the Anthropic Principle
• Cosmology of Axions and Moduli: A Dynamical Systems Approach
• CMB probes on the correlated axion isocurvature perturbation
• Axion inflation with cross-correlated axion isocurvature perturbations
• Non-perturbative Treatments of the Bosonic String and the Axion with Cosmological Implications