Coupled dark matter-dark energy in light of near Universe observations

TL;DR

This work systematically classifies dark-matter–dark-energy couplings into two families (DEvel and DMvel) and two coupling scalings (Q ∝ ρ_dm or Q ∝ ρ_de), deriving their background and linear perturbation dynamics and assessing stability. It demonstrates that some couplings effectively mimic modified gravity in growth while others preserve GR at the background but modify perturbations, leading to distinct observational signatures. Using near-universe probes—H0, skewness, peculiar velocities, redshift-space distortions, WEP violations, and void abundances—the paper provides current constraints (|ξ|≲0.2) and forecasts how upcoming data will tighten limits, with voids and WEPV emerging as particularly powerful tests. The results clarify how to distinguish dark-coupling effects from intrinsic w(z) dynamics and outline the potential of multi-probe strategies to robustly constrain or rule out notable dark-sector interactions.

Abstract

Cosmological analysis based on currently available observations are unable to rule out a sizeable coupling among the dark energy and dark matter fluids. We explore a variety of coupled dark matter-dark energy models, which satisfy cosmic microwave background constraints, in light of low redshift and near universe observations. We illustrate the phenomenology of different classes of dark coupling models, paying particular attention in distinguishing between effects that appear only on the expansion history and those that appear in the growth of structure. We find that while a broad class of dark coupling models are effectively models where general relativity (GR) is modified --and thus can be probed by a combination of tests for the expansion history and the growth of structure--, there is a class of dark coupling models where gravity is still GR, but the growth of perturbations is, in principle modified. While this effect is small in the specific models we have considered, one should bear in mind that an inconsistency between reconstructed expansion history and growth may not uniquely indicate deviations from GR. Our low redshift constraints arise from cosmic velocities, redshift space distortions and dark matter abundance in galaxy voids. We find that current data constrain the dimensionless coupling to be |xi|<0.2, but prospects from forthcoming data are for a significant improvement. Future, precise measurements of the Hubble constant, combined with high-precision constraints on the growth of structure, could provide the key to rule out dark coupling models which survive other tests. We shall exploit as well weak equivalence principle violation arguments, which have the potential to highly disfavour a broad family of coupled models.

Figures (6)

• Figure I: Present-day value of the dark matter energy density of the universe for the class II DMvel and DEvel models characterized by $Q=\xi {\cal H} \rho_{\rm de}$ as a function of the parameter $\xi$, necessary to fit WMAP 5 year angular diameter distance data Komatsu:2008hk and the physical dark matter and baryon densities at decoupling. The red solid (blue dashed) curve assumes an equation of state of the dark energy component $w=-0.9$ ($w=-0.7$).
• Figure II: The ratio of the amplitude of dark matter peculiar velocities, $\theta_{\rm dm}$, and the lensing signal, $\Omega_{\rm dm} \delta_{\rm dm}$ in coupled models compared to models with standard GR growth with identical expansion histories (labelled with NC for "Not Coupled"). The curves show $\xi = 0, -0.1, ... -0.6$. The upper left panel shows the velocity ratio in the DMvel model and the upper right panel shows the lensing ratio. The two bottom panels show the same but for the DEvel model.
• Figure III: The short (long) dashed curve shows the bulk flow predictions for the DEvel (DMvel) class II interacting model with $Q=\xi {\cal H}\rho_{\rm de}$. The cosmological parameters have been chosen to fit WMAP 5 year angular diameter distance data Komatsu:2008hk. The dashed (dotted) blue lines represent the measured value of $407\pm 81$ km/s, and the circle depicts the $\Lambda$CDM model prediction for the bulk flows $\langle u^2\rangle^{1/2}= 203$ km/s.
• Figure IV: The red crosses denote the ${\tilde{f}}\sigma_8$ values for the model of Eq. (\ref{['eq:maart']}), for $\Gamma/H_0=-0.3$ and cosmological parameters fixed to model 2 of Ref. Valiviita:2009nu. The blue triangles (squares) depict the ${\tilde{f}}\sigma_8$ expectations for a DMvel (DEvel) class II model with $Q=\xi {\cal H} \rho_{\rm de}$ for $\xi=-0.5$. The points in the left panel show current measurements of ${\tilde{f}}\sigma_8$. The middle and right panels show the expected ${\tilde{f}}\sigma_8$ errors for BOSS and EUCLID-like surveys, respectively, assuming a fiducial $\Lambda$CDM cosmology.
• Figure V: Relative dark matter-baryon acceleration $\frac{\dot \theta_{\rm b}- \dot \theta_{\rm dm}}{\dot \theta_{\rm b}}$ for $Q_\nu=\xi {\mathcal{H}} \rho_{\rm de} u_{\nu}^{({\rm de})}/a$ using Eq. (\ref{['eq:dtb']}) and (\ref{['eq:dtdm']}). The cosmological parameters have been chosen to fit WMAP 5 year data, see text for more details. The red solid (blue short dashed) curve assumes an equation of state of the dark energy component $w=-0.9$ ($w=-0.7$).