Gravitational Wave and CMB Probes of Axion Kination

TL;DR

This paper explores how axion field dynamics can lead to a kination era affecting gravitational wave spectra, with implications for dark matter, baryogenesis, and future gravitational wave observations.

Contribution

It introduces a novel cosmological scenario involving axion-induced kination, deriving observational constraints and predicting distinctive gravitational wave signatures.

Findings

The gravitational wave spectrum features a triangular peak.

Future GW observations can probe axion kination parameters.

High-scale inflation enhances detection prospects.

Abstract

Rotations of an axion field in field space provide a natural origin for an era of kination domination, where the energy density is dominated by the kinetic term of the axion field, preceded by an early era of matter domination. Remarkably, no entropy is produced at the end of matter domination and hence these eras of matter and kination domination may occur even after Big Bang Nucleosynthesis. We derive constraints on these eras from both the cosmic microwave background and Big Bang Nucleosynthesis. We investigate how this cosmological scenario affects the spectrum of possible primordial gravitational waves and find that the spectrum features a triangular peak. We discuss how future observations of gravitational waves can probe the viable parameter space, including regions that produce axion dark matter by the kinetic misalignment mechanism or the baryon asymmetry by axiogenesis. For QCD axion dark matter produced by the kinetic misalignment mechanism, a modification to the inflationary gravitational wave spectrum occurs above 0.01 Hz and, for high values of the energy scale of inflation, the prospects for discovery are good. We briefly comment on implications for structure formation of the universe.