Cosmology of Gravi-Axions

TL;DR

This paper explores how gravitational axions acquire mass through non-perturbative effects, leading to potential roles as dark matter or dark energy, with implications for cosmology and observational signatures.

Contribution

It introduces a mechanism for gravi-axion mass generation via Euclidean wormholes and analyzes their cosmological roles as dark matter or dark energy.

Findings

Gravi-axions can be produced as dark matter via misalignment or gravitational particle production.

They can act as dynamical dark energy in certain regimes.

Decay into gravitons offers potential observational signatures.

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.