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A non-compact QCD axion

Georgios K. Karananas, Mikhail Shaposhnikov, Sebastian Zell

Abstract

We investigate the cosmology of an axion that is fundamentally non-compact. During inflation, fluctuations of the effectively massless field populate many QCD vacua, thereby evading conventional isocurvature constraints while generating domain walls -- without accompanying cosmic strings. A small non-QCD contribution to the axion potential is required to trigger the timely collapse of domain walls; as a consequence, a residual amount of CP violation in the strong sector must exist, potentially within reach of planned experiments. Non-compact axions can account for the entirety of the dark matter abundance, and the collapse of domain walls sources a stochastic gravitational-wave background at nanohertz frequencies. Such axion dynamics can be embedded in top-down constructions -- such as Weyl-invariant Einstein-Cartan gravity -- where the tilting of the axion potential arises automatically.

A non-compact QCD axion

Abstract

We investigate the cosmology of an axion that is fundamentally non-compact. During inflation, fluctuations of the effectively massless field populate many QCD vacua, thereby evading conventional isocurvature constraints while generating domain walls -- without accompanying cosmic strings. A small non-QCD contribution to the axion potential is required to trigger the timely collapse of domain walls; as a consequence, a residual amount of CP violation in the strong sector must exist, potentially within reach of planned experiments. Non-compact axions can account for the entirety of the dark matter abundance, and the collapse of domain walls sources a stochastic gravitational-wave background at nanohertz frequencies. Such axion dynamics can be embedded in top-down constructions -- such as Weyl-invariant Einstein-Cartan gravity -- where the tilting of the axion potential arises automatically.
Paper Structure (7 sections, 92 equations, 3 figures)

This paper contains 7 sections, 92 equations, 3 figures.

Table of Contents

  1. Self-consistency of the effective theory
  2. Field-dependent noncanonical kinetic term
  3. Cutoff
  4. Onset of axion oscillations
  5. Non-compact axion from Einstein-Cartan gravity
  6. Einstein-Cartan basics
  7. Weyl-invariant Einstein-Cartan gravity

Figures (3)

  • Figure 1: The inflationary fluctuations $\delta\theta_I$ are taken to span a plethora of vacua.
  • Figure 2: The tilted potential, due to the presence of a small, non-QCD related, mass for the axion.
  • Figure :