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

TL;DR

This study examines how nonlinear modeling affects constraints on ultralight axions from CMB data, highlighting the importance of accurate nonlinear prescriptions to avoid artificial preferences for certain axion masses.

Contribution

It demonstrates the significant impact of nonlinear modeling choices on ULA constraints from CMB data, emphasizing careful treatment in analyses.

Findings

Constraints vary with the nonlinear model used.

Naive nonlinear models can falsely suggest a subdominant ULA component.

Lensing-like effects can artificially enhance the CMB power spectrum.

Abstract

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.