Accurate method for ultralight axion CMB and matter power spectra

Abstract: Ultralight axions (ULAs) with masses $10^{-33} \lesssim m/{\rm eV} \lesssim 10^{-12}$ are well motivated in string-inspired models and can be part or all of the dark energy or the dark matter in this range. Since the ULA field oscillates at a frequency $m$ that can be much larger than the expansion rate $H$, accurate and efficient calculation of cosmological observables requires an effective time averaged treatment. While these are well established for $m\gg 10 H_{\rm eq}$, the Hubble rate at matter radiation equality, here we extend and develop these techniques to cover the mass range $10^{-33} \lesssim m/{\rm eV} \lesssim 10^{-18}$. We implement this technique in a full cosmological Boltzmann code ($\mathrm{AxiECAMB}$) with numerical precision sufficiently accurate for current and next-generation cosmic microwave background as well as large-scale structure data analysis. New effects including the time averaging of metric perturbations and hydrostatic equilibrium of the effective fluid result in many orders of magnitude improvements for power spectra accuracy over some previous treatments such as $\mathrm{AxionCAMB}$ in some extreme regions of parameter space and order unity changes of the ULA effects near $\Lambda$CDM models. These improvements may impact the model parameters that resolve various tensions in $\Lambda$CDM at a comparable level.