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 ${\mathrm{\Omega }}_a/{\mathrm{\Omega }}_d$ and obtain a new upper bound of ${\mathrm{\Omega }}_a/{\mathrm{\Omega }}_d$. Future CMB and BAO measurements will achieve unprecedented precision and will be crucial for understanding the nature of the ULAs.

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• Axions
• Baryon acoustic oscillations
• Cosmic microwave background
• Cosmological parameters
• Cosmology
• Dark energy
• Dark matter
• Evolution of the Universe
• Grand unified models
• Large scale structure of the Universe
• Spontaneous symmetry breaking