Testing the interaction between dark energy and dark matter via latest observations

TL;DR

The paper investigates whether a non-gravitational interaction between dark energy (DE) and dark matter (DM) can exist and be constrained by observations. It develops a covariant, gauge-invariant perturbation formalism for a general coupling vector ${Q_{(\lambda)}}^{\nu}$ and analyzes how background degeneracies between the coupling and the DE EoS $w$ can be broken in the perturbative regime. A joint fit to WMAP7, BAO, SNIa (Constitution), and $H_0$ data finds tight constraints for DM- or total-density couplings, with $\xi$ of order $10^{-3}$ and positive, while DE-density couplings leave some degeneracy with $w$; Model IV additionally exhibits an attractor behavior for the DM/DE density ratio, alleviating the cosmological coincidence problem. Overall, the perturbation-based approach enables breaking key degeneracies and shows that a small positive DE–DM coupling is compatible with current observations and may address fundamental cosmological questions.

Abstract

Cosmological analysis based on currently available observations are unable to rule out a sizeable coupling between dark energy and dark matter. However, the signature of the coupling is not easy to grasp, since the coupling is degenerate with other cosmological parameters, such as the dark energy equation of state and the dark matter abundance. We discuss the possible ways to break such degeneracy. Based on the perturbation formalism, we carry out the global fitting by using latest observational data and get a tight constraint on the interaction between dark sectors. We find that the appropriate interaction can alleviate the coincidence problem.

Figures (5)

• Figure 1: The CMB TT power spectrum for different interaction models between DE and DM.
• Figure 2: The dependence of CMB angular power spectrum on physical cosmological parameter $\omega_c = \Omega_c h^2$
• Figure 3: The likelihood of cold dark matter abundance $\Omega_ch^2$, dark energy EoS $w$ and couplings $\xi$ for different cosmological models. The black lines denote the results obtained from WMAP 7 year data set alone and the red lines denote the results obtained from the combination of $\rm{WMAP+SN+BAO+H_0}$
• Figure 4: Cosmological coincidence problem
• Figure 5: $\rho_c^0$ is the critical energy density today. The attractor solutions of $r$ does not depend on the initial conditions at the early time of the universe. The purple lines represent the density evolution of cosmological model with different initial conditions. Noted by points, the density contrast $r$ today are different for different initial conditions but they are bounded in two attractor solutions $r_1\sim\xi,r_2\sim1/\xi$ in $\rho_c-\rho_d$ plane.