Ultra-light axions and the S ₈ tension: joint constraints from the cosmic microwave background and galaxy clustering

Abstract

We search for ultra-light axions as dark matter (DM) and dark energy particle candidates, for axion masses 10-32 eV ≤ m a ≤ 10-24 eV, by a joint analysis of cosmic microwave background (CMB) and galaxy clustering data — and consider if axions can resolve the tension in inferred values of the matter clustering parameter S 8. We give legacy constraints from Planck 2018 CMB data, improving 2015 limits on the axion density Ωa h 2 by up to a factor of three; CMB data from the Atacama Cosmology Telescope and the South Pole Telescope marginally weaken Planck bounds at m a = 10-25 eV, owing to lower (and theoretically-consistent) gravitational lensing signals. We jointly infer, from Planck CMB and full-shape galaxy power spectrum and bispectrum data from the Baryon Oscillation Spectroscopic Survey (BOSS), that axions are, today, < 10% of the DM for m a ≤ 10-26 eV and < 1% for 10-30 eV ≤ m a ≤ 10-28 eV. BOSS data strengthen limits, in particular at higher m a by probing high-wavenumber modes (k < 0.4h Mpc-1). BOSS alone finds a preference for axions at 2.7σ, for m a = 10-26 eV, but Planck disfavours this result. Nonetheless, axions in a window 10-28 eV ≤ m a ≤ 10-25 eV can improve consistency between CMB and galaxy clustering data, e.g., reducing the S 8 discrepancy from 2.7σ to 1.6σ, since these axions suppress structure growth at the 8h -1 Mpc scales to which S 8 is sensitive. We expect improved constraints with upcoming high-resolution CMB and galaxy lensing and future galaxy clustering data, where we will further assess if axions can restore cosmic concordance.

Cosmic birefringence from the Axiverse

“…Requiring Ω ϕ ≤ 0.003 (1% of the DM abundance in this mass range [49]), we find a constraint for the decay constant f a ≤ 2.2 × 10 16 GeV. It is worth comparing this to the constraint on the coupling g ϕγ ≳ 10 −20 GeV derived in ref.…”

“…[48] and more recently in ref. [49]. In particular, axionsare constrained to comprise about 1% of the total dark matter abundance, Ω ϕ ≤ 0.003, for 10 −33 eV < m < 10 −29 eV, which from eq.…”

“…2 We thus explore the implications of having β ∼ 0.3 deg in the presence of many dynamical axions or ALPs. Previous work has explored the case of a single field [27,28] where constraints on the axion-photon coupling g ϕγ are inferred by inverting the relation β = − 1 2 g ϕγ [ϕ(t LS ) − ϕ(t 0 )], where t 0 and t LS correspond to the present and "last scattering" time and the initial condition is fixed by the maximum axion JCAP11(2023)017 abundance in the relevant mass range [48,49]. In this work, we follow a different approach: we consider the field's initial value and the axion decay constant as random variables drawn from theoretically motivated distributions.…”

“…(4.11), does not exceed the current cosmological upper bound on the axion abundance reported for example in refs. [49] and [69]. In figure . 1 we show the projected abundance and the corresponding upper limit computed as the linear interpolation, in decade of mass, between the following values:…”

The Atacama Cosmology Telescope: DR6 Gravitational Lensing Map and Cosmological Parameters

“…Different probes of the late universe access different redshifts or cosmic epochs (see Figure 1) and are also sensitive to different scales. Consequently, differences among the inferred values of σ 8 from various late-universe probes or with the early-universe prediction based on CMB anisotropies can hint at possibilities such as (1) nonstandard redshift evolution of the growth of structure, possibly due to modifications of general relativity (e.g., Pogosian et al 2022;Nguyen et al 2023); (2) a nonstandard power spectrum of matter fluctuations, e.g., due to axion dark matter (e.g., Rogers et al 2023) or dark matter-baryon scattering (e.g., He et al 2023); (3) incorrect modeling of small-scale fluctuations, e.g., due to nonlinear biasing (for galaxy observables) or baryonic feedback (for lensing observables; e.g., Amon & Efstathiou 2022); or (4) unaccounted-for systematic effects in one or more of these measurements. By providing a measurement of σ 8 with CMB lensing, we probe mainly linear scales with information from a broad range of redshifts z ∼ 0.5-5, which peaks around z = 2 as shown in Figure 1.…”