Lauren Gaughan | Searcharxiv

arXiv2026

Ultra-light axion constraints from Planck and ACT: the role of nonlinear modelling

We study how constraints on the abundance of ultralight axions (ULAs) from cosmic microwave background (CMB) data depend on their nonlinear modelling. We focus on the axion mass range $10^{-25} \leq m/\rm{eV} \leq 10^{-23}$, where the axion Jeans scale falls in the quasi-linear regime probed by CMB lensing, making constraints highly sensitive to the choice of nonlinear prescription. We show that the inferred constraints depend significantly on the choice of nonlinear model, which must therefore be treated carefully. Performing Markov Chain Monte Carlo (MCMC) analyses with \Planck\, 2018, ACT DR6 and DESI DR2 BAO data, we find naive nonlinear modelling of non-cold matter can produce an artificial preference for a subdominant ULA dark matter component with mass $m \approx 10^{-24}\,$eV. This arises from a lensing-like enhancement of the CMB power spectrum.

astro-ph.CO↗

arXiv2025

A Fast and Accurate Implementation of the Effective Fluid Approximation for Ultralight Axions

We present a numerically efficient and accurate implementation of the Passaglia-Hu effective fluid approximation for ultralight axions (ULAs) within the Boltzmann code CAMB. 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 CAMB, 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 sub-percent accuracy in the CMB power spectrum across a broad range of axion masses, from $10^{-28}\,\mathrm{eV}$ to $10^{-24}\,\mathrm{eV}$. We perform Markov Chain Monte Carlo (MCMC) analyses incorporating our implementation, and find improved constraints on the axion mass and abundance compared to previous, simpler fluid-based approximations. For example, using \Planck\ PR4 and DESI BAO data, we find $2σ$ upper limits on the axion fraction $f_{\rm ax} < 0.0082$ and physical density $Ω_{\rm ax}h^2 < 0.0010$ for $m=10^{-28}$ eV. The code is publicly available at \url{https://github.com/adammoss/AxiCAMB}.

astro-ph.CO↗