Fast and accurate implementation of the effective fluid approximation for ultralight axions

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Abstract
We present a numerically efficient and accurate implementation of the Passaglia-Hu effective fluid approximation for ultralight axions within the Boltzmann code . This method is specifically designed to evolve the axion field accurately across cosmological timescales, mitigating the challenges associated with its rapid oscillations. Our implementation is based on the latest version of , ensuring compatibility with other cosmological codes., e.g., for calculating cosmological parameter constraints. Compared to exact solutions of the Klein-Gordon equation, our method achieves subpercent accuracy in the cosmic microwave background power spectrum across a broad range of axion masses, from 10 − 28 eV to 10 − 24 eV . We perform Markov chain Monte Carlo analyses incorporating our implementation, and find improved constraints on the axion mass and abundance compared to previous, simpler fluid-based approximations. For example, using Public Release 4 and Dark Energy Spectroscopic Instrument baryon acoustic oscillation data, we find 2 σ upper limits on the axion fraction f ax < 0.0082 and physical density Ω ax h 2 < 0.0010 for m = 10 − 28 eV . Published by the American Physical Society 2025
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  • Science and Technology Facilities Council (ST/X000672/1, ST/X508639/1)

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