Gravitational probes of ultra-light axions

TL;DR

This paper reviews how ultra-light axions, as dark matter candidates, can be probed through gravitational effects on various cosmological and astrophysical observations, offering tests for their existence and properties.

Contribution

It summarizes the potential of gravitational signatures of ULAs to serve as observational probes across multiple astrophysical phenomena.

Findings

ULAs can suppress small-scale structure formation.

Interference patterns and solitons are distinctive signatures of ULAs.

ULAs affect cosmic microwave background and galaxy clustering predictions.

Abstract

The axion is a hypothetical, well-motivated dark-matter particle whose existence would explain the lack of charge-parity violation in the strong interaction. In addition to this original motivation, an `axiverse' of ultra-light axions (ULAs) with masses $10^{-33}\,{\rm eV}\lesssim m_{\rm a}\lesssim 10^{-10}\,{\rm eV}$ also emerges from string theory. Depending on the mass, such a ULA contributes to the dark-matter density, or alternatively, behaves like dark energy. At these masses, ULAs' classical wave-like properties are astronomically manifested, potentially mitigating observational tensions within the $\Lambda$CDM paradigm on local-group scales. ULAs also provide signatures on small scales such as suppression of structure, interference patterns and solitons to distinguish them from heavier dark matter candidates. Through their gravitational imprint, ULAs in the presently allowed parameter space furnish a host of observational tests to target in the next decade, altering standard predictions for microwave background anisotropies, galaxy clustering, Lyman-$\alpha$ absorption by neutral hydrogen along quasar sightlines, pulsar timing, and the black-hole mass spectrum.