Partially Acoustic Dark Matter Cosmology and Cosmological Constraints

TL;DR

This work investigates Partially Acoustic Dark Matter (PAcDM), a model introducing a tightly coupled Dark Radiation (DR) and Acoustic Dark Matter (AcDM) to alleviate tensions between Planck CMB inferences and local measurements of $H_0$ as well as weak-lensing amplitudes. The framework is parameterized by $f_{ m DR}$ and $f_{ m AcDM}$, where DR shifts the expansion history and AcDM suppresses growth below the dark sound horizon, with a growth-suppression metric $p_8$ bridging DR/AcDM to late-time observables. The study finds two principal posterior branches for PAcDM, one favoring higher $H_0$ with modest growth suppression and another with stronger suppression and minimal $H_0$ change; neither can reconcile all tensions simultaneously without conflicting with Planck’s acoustic-peak structure, and Bayesian model comparison shows ΛCDM with neutrinos remains competitive or favored when all data are combined. The results suggest that distinguishing PAcDM from ΛCDM and neutrino extensions will require precise measurements of small-scale CMB damping tails and complementary large-scale structure data from upcoming surveys.

Abstract

Observations of the cosmic microwave background (CMB) together with weak lensing measurements of the clustering of large scale cosmological structures and local measurements of the Hubble constant pose a challenge to the standard $Λ$CDM cosmological model. On one side CMB observations imply a Hubble constant that is lower than local measurements and an amplitude of the lensing signal that is higher than direct measurements from weak lensing surveys. We investigate a way of relieving these tensions by adding dark radiation tightly coupled to an acoustic part of the dark matter sector and compare it to massive neutrino solutions. While these models offer a way of separately relieving the Hubble and weak lensing tensions they are prevented from fully accommodating both at the same time since the CMB requires additional cold dark matter when adding acoustic dark matter or massive neutrinos to preserve the same sharpness of the acoustic peaks which counteracts the desired growth suppression.

Figures (6)

• Figure 1: The recombination spectrum in $k$-space in units of amplitude of primordial comoving curvature perturbation. Different lines correspond to different physical effects and models, as shown in figure and legend. The vertical dashed and dotted lines show the comoving horizon at recombination ($z_*$) for the $\Lambda$CDM and DR models. The dot-dashed line in Panel (b) shows the comoving dark sound horizon at recombination. The $\Lambda$CDM model is defined by $\Omega_b=0.05$, $\Omega_c=0.26$, $h=0.67$, the DR model has $f_{\rm DR}=0.6$ and the DR+AcDM model further adds $f_{\rm AcDM}=0.6$ while leaving unchanged all other parameters.
• Figure 2: The level contours of the mapping between $p_8$ and $f_{\rm AcDM}$. Different colors correspond to different values of $f_{\rm AcDM}$ at fixed CDM physical density $\Omega_c h^2=0.1199$.
• Figure 3: The marginalized joint posterior for the parameters defining the three extended models that we consider. In all panels $N_{\rm eff}$ denotes the effective number of relativistic species, while $p_8$ indicates the growth suppression in the PAcDM case, $m_{\nu, {\rm sterile}}^{\rm eff}$ stands for the effective mass of sterile neutrinos and $\sum_{\nu} m_{\nu}$ denotes the sum of neutrino masses. In all panels the circled points represent the best fit parameter solution for a given data set combination. In all three panels different colors correspond to different combination of cosmological probes, as shown in legend. The darker and lighter shades correspond respectively to the $68\%$ C.L. and the $95\%$ C.L. The black area in Panel b) shows the Dodelson-Widrow cut on the sterile neutrino effective mass.
• Figure 4: Residuals to Planck best fit $\Lambda$CDM temperature spectrum in units of cosmic variance per multipole. Different lines correspond to different models and parameters, as shown in legend, whose parameters are given in Tab. \ref{['Table:Parameters']}. Vertical continuous and dashed lines correspond, respectively, to the location of peaks and troughs in the best fit $\Lambda$CDM model. Points with error bars show the residual of Planck temperature measurements.
• Figure 5: The marginalized joint posterior for the weak lensing inferred amplitude of scalar perturbations $S_8\equiv \sigma_8\Omega_m^{0.5}$ and the present day value of the Hubble constant $H_0$ in units of km/s/Mpc for the different models that we consider. In all panels the circled points represent the best fit parameter solution for a given data set combination. In all three panels different colors correspond to different combination of cosmological probes, as shown in legend. The darker and lighter shades correspond respectively to the $68\%$ C.L. and the $95\%$ C.L.. The continuous black line shows the best fit $H_0$ and $S_8$ values, as obtained by fitting respectively the H and WL data set alone, within the $\Lambda$CDM model. The black dashed lines and shaded area indicate the $68\%$ C.L. region.