A search for ultra-light axions using precision cosmological data

TL;DR

The paper conducts a comprehensive cosmological search for ultralight axions (ULAs) across masses from 10^{-33} to 10^{-22} eV, combining a self-consistent Boltzmann treatment with a Bayesian, nested-sampling analysis of Planck, WMAP, ACT, SPT, and WiggleZ data. ULAs are modeled as an effective fluid whose background and perturbations are evolved alongside standard components, enabling robust constraints on the ULA density fraction as a function of mass. The main result is a tight bound on the ULA contribution to the dark-matter density, with Ω_a/Ω_d < 0.048 (95% CL) for 10^{-32} eV ≤ m_a ≤ 10^{-25.5} eV, and Ω_a h^2 < 0.0058; ULAs outside this constrained window can mimic dark energy (lower masses) or cold dark matter (higher masses). The study demonstrates the efficacy of precision cosmology in probing fundamental particle properties and illustrates avenues for future improvements via CMB lensing, isocurvature constraints, and extended parameter spaces.

Abstract

Ultra-light axions (ULAs) with masses in the range 10^{-33} eV <m <10^{-20} eV are motivated by string theory and might contribute to either the dark-matter or dark-energy density of the Universe. ULAs could suppress the growth of structure on small scales, or lead to an enhanced integrated Sachs-Wolfe effect on large-scale cosmic microwave-background (CMB) anisotropies. In this work, cosmological observables over the full ULA mass range are computed, and then used to search for evidence of ULAs using CMB data from the Wilkinson Microwave Anisotropy Probe (WMAP), Planck satellite, Atacama Cosmology Telescope, and South Pole Telescope, as well as galaxy clustering data from the WiggleZ galaxy-redshift survey. In the mass range 10^{-32} eV < m <10^{-25.5} eV, the axion relic-density Ω_{a} (relative to the total dark-matter relic density Ω_{d}) must obey the constraints Ω_{a}/Ω_{d} < 0.05 and Ω_{a}h^{2} < 0.006 at 95%-confidence. For m> 10^{-24} eV, ULAs are indistinguishable from standard cold dark matter on the length scales probed, and are thus allowed by these data. For m < 10^{-32} eV, ULAs are allowed to compose a significant fraction of the dark energy.

Figures (17)

• Figure 1: Marginalized $2$ and $3\sigma$ contours show limits to the ultralight axion (ULA) mass fraction $\Omega_{a}/\Omega_{d}$ as a function of ULA mass $m_{a}$, where $\Omega_{a}$ is the axion relic-density parameter today and $\Omega_{d}$ is the total dark-matter energy density parameter. The vertical lines denote our three sampling regions, discussed below. The mass fraction in the middle region is constrained to be $\Omega_{a}/\Omega_{d}\mathrel{\hbox{$\sim$ $<$}} 0.05$ at $95\%$ confidence. Red regions show CMB-only constraints, while grey regions include large-scale structure data.
• Figure 2: Adiabatic matter power-spectra generated with the modified camb described in Sec. \ref{['axion_fluid_pert']}, with varying axion mass and energy-density fraction $\Omega_a/\Omega_d$ at fixed total dark-matter density fraction $\Omega_{d}$. Power is suppressed for modes that enter the horizon when the axion sound speed $c_{s}\sim 1$.
• Figure 3: Evolution of the fractional DM density-perturbation $\delta$ when $\Omega_{a}/\Omega_{d}=1$ (solid), for a ULA mass of $m_{a}=10^{-26}~{\rm eV}$ and a series of wave numbers $k$ (as shown in the figure), compared to standard CDM (dashed). The overall normalization of the mode amplitude is arbitrary here. The range of k-values encompasses different behaviors, with suppression of growth relative to CDM when $k\sim k_J(a)$, oscillation when $k>k_J(a)$ and growth as CDM when $k<k_J(a)$. This leads to an overall suppression of power for large-$k$ modes.
• Figure 4: Evolution of the fractional dark-matter density perturbation with wave number $k=10^{-4}h~{\rm Mpc}^{-1}$ for the $3$ different ULA masses indicated compared to the standard CDM case (dashed). For these ULA masses, $k<k_m$ always, and so soon after $a>a_{\rm osc}$, the mode behaves just as CDM.
• Figure 5: Evolution of the integrated Sachs-Wolfe (ISW) source term camb for a mode with $k=10^{-4}h~{\rm Mpc}^{-1}$. The overall amplitude is arbitrary. Dark colored curves are generated using the modified camb described in the text. Lighter curves are generated using direct numerical integration of scalar-field perturbation EOMs. Green curves show the effect of choosing $\Omega_{a}/\Omega_{d}=0.1$ (with all other parameters set to $\Lambda$CDM values) with $m_{a}=10^{-32}~{\rm eV}$. Blue curves are obtained assuming $\Omega_{\Lambda}=0$, $\Omega_{m}=1$ and $\Omega_{a}/\Omega_{d}=0.1$ with $m_{a}=5\times 10^{-32}~{\rm eV}$.