Cosmology of Gravi-Axions

TL;DR

The paper addresses the massless nature of the gravi-axion in standard dCS gravity by showing that Euclidean wormholes can non-perturbatively break the shift symmetry and generate a mass, yielding a cosinusoidal potential and a nonzero $m_\varphi$ that depends exponentially on the wormhole action. By analyzing GS and KLLS wormholes, the authors demonstrate a wide range of cosmologically relevant masses, enabling gravi-axions to serve as dark matter via misalignment or gravitational particle production, or as dark energy in ultralight regimes, while predicting a decay channel to gravitons that could imprint a high-frequency gravitational-wave background. The work also discusses when the late-time dynamics reduce to minimally coupled scalar behavior and how the gravitational axion’s coupling to gravity remains suppressed in certain regimes, offering observational windows through gravitational waves and cosmological evolution. Overall, the paper connects non-perturbative quantum gravity effects to tangible cosmological phenomena, providing testable predictions and directions for future exploration of gravi-axions as DM or DE.

Abstract

We show how a mass term for gravitational axions (''gravi-axions'') with a Chern-Simons coupling to gravity can naturally arise due to non-perturbative contributions from Euclidean wormholes, breaking the continuous shift symmetry of the standard theory. The induced mass can be generated in a cosmologically relevant range to be the dark matter or dark energy of the universe for a reasonable and well-motivated range of the symmetry breaking scale. Upon generating the gravi-axion mass term, we discuss the cosmology of the theory. We find that the gravi-axion can be produced to be the dominant dark matter component via misalignment or gravitational particle production, and that in a different regime, the gravi-axion can act as dynamical dark energy. We discuss gravi-axion decay into gravitons as a potential observational window for this theory. Finally, we find that, for late-time, astrophysical compact objects, cosmologically relevant gravi-axions behave as minimally-coupled, massive scalar fields.

Figures (5)

• Figure 1: Mass of the gravi-axion in terms of the PQ scale, $\mu$, assuming the Giddings-Strominger wormhole solution. We denote several benchmark mass values for cosmologically relevant quantities. The horizontal gray shaded region indicates where $m_\varphi > M_P$, while the vertical gray shaded region denotes where the EFT condition, $\mu \ll M_p$ is no longer valid. We denote the compton wavelength, $\lambda$, of the gravi-axion on the right y-axis.
• Figure 2: Mass of the gravi-axion in terms of the PQ scale, $\mu$, assuming the wormhole solution and stringy suppression discussed in Kallosh:1995hi. The vertical gray shaded region denotes where the EFT condition, $\mu \ll M_p$ is no longer valid. We denote the compton wavelength, $\lambda$, of the gravi-axion on the right y-axis.
• Figure 3: Gravi-axion abundance due to misalignment in a pre-inflationary symmetry breaking scenario, in terms of the initial misalignment angle, $\theta_i$ and symmetry breaking scale, $\mu$.
• Figure 4: Abundance of gravi-axions produced via gravitational particle production for a range of masses and inflationary scales.
• Figure 5: Gravitational-wave spectrum from gravi-axion decay for three fiducial examples of a gravi-axion population produced via GPP.