Revisiting Isocurvature Bounds on the Minimal QCD Axion

Abstract

The QCD axion has important connections to early universe cosmology. For example, it is often said that isocurvature limits rule out a combination of high axion decay constant, $f_a$, and high inflationary Hubble scale, $H_I$. High scales are theoretically motivated, so it is important to ask how robust this constraint is. We demonstrate that this constraint is naturally evaded when the quartic coupling of the complex $U(1{)}_{\mathrm{PQ}}$-breaking field is small. In this case, $f_a$ changes from a larger value during inflation to a smaller value in the later universe, suppressing isocurvature perturbations. Importantly, we show that in large parts of parameter space this solution is not jeopardised by overproduction of the axion through parametric resonance. The isocurvature bounds are thus dependent on UV physics. We have found that, even for the minimal QCD axion, large parts of UV parameter space at both high $f_a$ and high $H_I$ are in fact allowed, not ruled out by isocurvature constraints.

16 + 7 pages, 7 + 6 figures; v2: minor changes to text and figures, matches the published version

References in corpus (28)

• Power-counting and the Validity of the Classical Approximation During Inflation
• Stochastic and resolvable gravitational waves from ultralight bosons
• Equivalence of the Einstein and Jordan frames
• Einstein and Jordan frames reconciled: a frame-invariant approach to scalar-tensor cosmology
• A first search for a stochastic gravitational-wave background from ultralight bosons
• Gravitational Waves and Dark Photon Dark Matter from Axion Rotations
• Unifying inflation and dark matter with the Peccei-Quinn field: observable axions and observable tensors
• String theoretic QCD axions in the light of PLANCK and BICEP2
• Probing the string axiverse by gravitational waves from Cygnus X-1
• The meso-inflationary QCD axion
• Warming up cold inflation
• Higgs inflation and suppression of axion isocurvature perturbation
• Axionic Mirage Mediation
• Shapes of non-Gaussianity in warm inflation
• Axion Models with High Scale Inflation
• Solving Cosmological Problems of Supersymmetric Axion Models in an Inflationary Universe
• Relaxing Isocurvature Bounds on String Axion Dark Matter
• Warm Inflation with the Standard Model
• Explosive Axion Production from Saxion
• Charge transfer between rotating complex scalar fields
• Cosmological Perturbations of Axion with a Dynamical Decay Constant
• Opening up a window on the postinflationary QCD axion
• Revisiting isocurvature bounds in models unifying the axion with the inflaton
• Minimal solution to the axion isocurvature problem from a nonminimal coupling
• Radiative PQ Breaking and the Higgs Boson Mass
• Acoustic Misalignment Mechanism for Axion Dark Matter
• Periodic Cosmic String Formation and Dynamics
• Early universe dynamics of PQ field with very small self-coupling and its implications for axion dark matter

Cited by in corpus (5)

• Multiple Axions Save High-Scale Inflation
• How light can ALP dark matter be?
• Ultralight Dark Matter from the Edge of Field Space
• One-loop proton decay from Peccei-Quinn symmetry
• Dynamically generated tilt of isocurvature fluctuations