Researchers have used gravitational lensing data from the cosmic microwave background (CMB) to set the tightest limits to date on the abundance of ultralight axions (ULAs) within a specific mass range. ULAs are promising dark matter candidates that arise in various extensions of the Standard Model of particle physics. This study combines recent measurements from the Planck, Atacama Cosmology Telescope (ACT), and South Pole Telescope (SPT-3G) with a nonlinear clustering model calibrated by state-of-the-art simulations for ULAs.

Ultralight axions with masses $m_\mathrm{a} \lesssim 10^{-27}$ eV were already strongly constrained by previous CMB temperature and polarization observations. This new analysis focuses on the mass range $10^{-26}\,\mathrm{eV}\leq m_\mathrm{a}\leq 10^{-24.5}\,\mathrm{eV}$, where ULAs could alleviate observed tensions in matter clustering inference if they constituted a small percentage of the universe's total dark matter. The results show that ULAs with a mass of $10^{-26}$ eV account for less than 1.5% of dark matter, while those with $10^{-25}$ eV constitute less than 9%, both at a 95% confidence level.

Although a slight preference for a non-zero axion density at $10^{-24.5}$ eV with a significance of $2.1\sigma$ was identified, the authors note that this signal is primarily driven by a few data points. Therefore, further investigation into the nonlinear physics of ULAs is required to definitively confirm or rule out this possible signal. These findings are crucial for refining dark matter models and guiding future searches for these elusive particles.