Axiverse Lampposts

TL;DR

The paper models a string axiverse with many coupled axions, revealing how their collective properties affect cosmological abundances, detection prospects, and observational signals, especially highlighting the roles of the QCD axion and heavy axions.

Contribution

It introduces a simplified model of the axiverse with hierarchical axion masses, analyzing how collective effects influence field ranges, interactions, and detection prospects.

Findings

Axion field ranges decrease as 1/√N with increasing axion number.

Heavy axions and the QCD axion are less suppressed and more detectable.

Coupled axion dark matter relaxes initial condition tuning compared to independent axions.

Abstract

The string axiverse predicts a unique connection between the high scales approachable only through theory and the low energies within reach of experimental verification: a multitude of light, feebly interacting axions. In order to capture the collective effects of such an axion ensemble, we model the string axiverse by $N$ coupled axions with a simple assumption: hierarchical axion masses that arise from hierarchical instantons with statistically distributed axion couplings. In this limit, we find that axion field ranges, which determine late-time cosmological abundances, shrink as $1/\sqrt{N}$ as the number of axions grows. Moreover, the heaviest modes tend to align with the smallest kinetic eigenvalues, further reducing their field ranges. Interactions with the Standard Model (SM) are largely set by the kinetic structure and do not grow with $N$, thus suppressing detection prospects relative to the individual-axion expectation. The exceptions are the ensemble's lightest and heaviest states as well as the Quantum Chromodynamics (QCD) axion, which incur no such suppression. We further find that coupled axiverse dark matter has parametrically relaxed tuning on initial conditions when produced via long, low-scale inflation relative to independent axions and high-scale inflation. Taken together, these results sharpen the observational outlook: the most accessible signals typically come from the QCD axion and from heavy axions that make up small dark matter subcomponents. An anthropic plateau of comparable energy density states produces subdominant signals; meanwhile, if light axions have SM interactions independent of QCD, they can also be within reach of future direct-detection experiments.