Accurate effective fluid approximation for ultralight axions

Abstract

Ultralight axions are theoretically interesting and phenomenologically rich dark sector candidates, but they are difficult to track across cosmological timescales because of their fast oscillations. We resolve this problem by developing a novel method to evolve them efficiently and accurately. We first construct an exact effective fluid which at late times matches the axion but which evolves in a simple way. We then approximate this evolution with a carefully chosen equation of state and sound speed. With our scheme we find that we can obtain subpercent accuracy for the linear theory suppression of axion density fluctuations relative to that of cold dark matter without tracking even a single complete oscillation of the axion field. We use our technique to test other approximation schemes and to provide a fitting formula for the transfer function for the matter power spectrum in linear theory in axion models. Implementing our approach in existing cosmological axion codes is straightforward and will help unleash the potential of high-precision next-generation experiments.

18 pages, 15 figures; matches published version

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• Cosmological constraints on the Multi Scalar Field Dark Matter model
• Warm and Fuzzy Dark Matter: Free Streaming of Wave Dark Matter
• COZMIC. I. Cosmological Zoom-in Simulations with Initial Conditions Beyond Cold Dark Matter
• Starlight from JWST: Implications for star formation and dark matter models
• Implications of multi-axion dark matter on structure formation
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• Hydrodynamics of ultralight complex scalar field dark matter and its impact on the growth of structure
• The Effective Field Theory of Large Scale Structure for Mixed Dark Matter Scenarios
• The SRG/eROSITA all-sky survey: Constraints on Ultra-light Axion Dark Matter through Galaxy Cluster Number Counts
• Cosmological effect of coherent oscillation of ultralight scalar fields in a multicomponent universe
• Ultralight scalar dark matter versus nonadiabatic perfect fluid dark matter in pulsar timing