Gravitational probes of ultra-light axions

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 $10^{-33}\mathrm{eV}\lesssim m_{\mathrm{a}}\lesssim 10^{-10}\mathrm{eV}$ 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.

5 pages, 1 figure, Astro2020 Decadal Survey science white paper

Cited by in corpus (6)

• Ultra-light axions and the $S_8$ tension: joint constraints from the cosmic microwave background and galaxy clustering
• Cosmological structure formation in scalar field dark matter with repulsive self-interaction: The Incredible Shrinking Jeans Mass
• ALP Dark Matter Mini-Clusters from Kinetic Fragmentation
• Accurate effective fluid approximation for ultralight axions
• Induced cosmological anisotropy by a gauge-gravity interaction
• Ultralight axion or axion-like particle dark matter and 21-cm absorption signals in new physics