How sound are our ultralight axion approximations?

Abstract

Ultra-light axions (ULAs) are a promising dark-matter candidate. ULAs may have implications for small-scale challenges to the ΛCDM model, and arise in string scenarios. ULAs are already constrained by cosmic microwave background (CMB) experiments and large-scale structure surveys, and will be probed with much greater sensitivity by future efforts. It is challenging to compute observables in ULA scenarios with sufficient speed and accuracy for cosmological data analysis because the ULA field oscillates rapidly. In past work, an effective fluid approximation has been used to make these computations feasible. Here this approximation is tested against an exact solution of the ULA equations, comparing the induced error of CMB observables with the sensitivity of current and future experiments. In the most constrained mass range for a ULA dark matter component (10 −27 eV ≤ max ≤ 10 −25 eV), the induced bias on the allowed ULA fraction of dark matter from Planck data is less than 1σ. In the cosmic-variance limit (including temperature and polarization data), the bias is 2σ for primary CMB anisotropies, with more severe biases (as high as ∼ 4σ) resulting for less reliable versions of the effective fluid approximation. If all of the standard cosmological parameters are fixed by other measurements, the expected bias rises to 4 − 20σ (well beyond the validity of the Fisher approximation), though the required level of degeneracy breaking will not be achieved by any planned surveys.PACS numbers: 14.80. Mz,90.70.Vc,95.35.+d,98.80.Cq

The landscape of QCD axion models

“…33 Due to this early dark energy-like behaviour, pseudo-scalars are often invoked when building models of quintessence [174][175][176] and inflation [177,178]. 34 The code is available as a Mathematica package at the GIT repository https://github.com/lucavisinelli.…”

Beyond Schrödinger-Poisson: nonrelativistic effective field theory for scalar dark matter

“…On the more formal side, it would be interesting to consider an effective fluid description of our corrections to the SP system using the Madelung-like transforms [24,[94][95][96]. Finally, repeating our work for more general matter content (higher-spin fields, such as vector dark matter [97,98]), or different relativistic theories of gravity [99], could change the nature of the corrections to the SP system (or even the SP system itself).…”

Accurate effective fluid approximation for ultralight axions

“…This approximation inevitably induces errors when making predictions for observables because the axion field is initially effectively frozen and the approximate equations of state used to evolve the effective fluid efficiently may then be insufficiently accurate. Even for axions in the range 10 −27 eV m 10 −24 eV where the clustering suppression in linear theory is testable with CMB measurements and the timescale hierarchy is less extreme, these errors already been shown to be significant at the 1 ∼ 2σ level [24] for upcoming experiments like CMB-S4 [25]. Other similar averaging approches such as the one proposed by Ref.…”