Nonminimal Couplings and the Forgotten Field of Axion Inflation

Abstract

We study the multifield dynamics of axion models nonminimally coupled to gravity. As usual, we consider a canonical $U(1)$ symmetry-breaking model in which the axion is the phase of a complex scalar field. If the complex scalar field has a nonminimal coupling to gravity, then the (oft-forgotten) radial component can drive a phase of inflation prior to an inflationary phase driven by the axion field. In this setup, the mass of the axion field is dependent on the radial field because of the nonminimal coupling, and the axion remains extremely light during the phase of radial inflation. As the radial field approaches the minimum of its potential, there is a transition to a second phase of inflation during which the axion field contributes a substantial fraction of the system's total energy density, yielding a phase of "natural inflation." This system exhibits ultra-light isocurvature perturbations, which are converted to adiabatic perturbations by a turning field-space trajectory. For models wherein the CMB pivot scale exited the horizon well before the turn, this acts to suppress the tensor-to-scalar ratio $r$, without generating CMB non-Gaussianity or observable isocurvature perturbations. Finally, we note that for certain parameters the interaction strength between axion and gauge fields can be suppressed during the first phase of inflation relative to its value during the second phase by several orders of magnitude. This decouples the constraints on the inflationary production of gauge fields (e.g., from primordial black holes) from the constraints on their production during (p)reheating.

17 pages, 12 figures. v2: 19 pages, 13 figures. Updated and extended to include new parameter examples and dynamics

References in corpus (12)

• The Standard Model Higgs boson as the inflaton
• Monodromy in the CMB: Gravity Waves and String Inflation
• Primordial Non-Gaussianities from Inflation Models
• The Unity of Cosmological Attractors
• Perturbations in generalized multi-field inflation
• Generalised constraints on the curvature perturbation from primordial black holes
• Conformal Transformations with Multiple Scalar Fields
• Primordial Bispectrum from Multifield Inflation with Nonminimal Couplings
• Standard Clock in Primordial Density Perturbations and Cosmic Microwave Background
• A Direct Probe of the Evolutionary History of the Primordial Universe
• Testing Two-Field Inflation
• Spectral Distortions of the CMB as a Probe of Inflation, Recombination, Structure Formation and Particle Physics

Cited by in corpus (1)

• Natural-Scalaron Inflation