Interacting Dark Energy -- constraints and degeneracies

TL;DR

The paper investigates dark-sector interactions where the energy transfer is proportional to the dark energy density, examining two covariant momentum-transfer frames ($Q^\mu_x=Q_x u_c^\mu$ and $Q^\mu_x=Q_x u_x^\mu$). Using a combined CMB, BAO, and SNIa dataset within a CosmoMC/CAMB framework, it finds that background evolution tolerates sizable interaction rates while perturbations reveal strong growth effects that are partially degenerate with galaxy bias, and the ISW signal can be suppressed by nonstandard background evolution. The study highlights a residual degeneracy with modified gravity and emphasizes that breaking this degeneracy requires probes of anisotropic stress, the DM density evolution, and the DM–baryon bias. Overall, ΛCDM remains an excellent fit, but the IDE parameter space with $Q_x=\Gamma\bar{\rho}_x$ and the two frames provides a viable extension whose signatures in growth and ISW motivate future, more precise measurements to disentangle from MG scenarios.

Abstract

In standard cosmologies, dark energy interacts only gravitationally with dark matter. There could be a non-gravitational interaction in the dark sector, leading to changes in the effective DE equation of state, in the redshift dependence of the DM density and in structure formation. We use CMB, BAO and SNIa data to constrain a model where the energy transfer in the dark sector is proportional to the DE density. There are two subclasses, defined by the vanishing of momentum transfer either in the DM or the DE frame. We conduct a Markov-Chain Monte-Carlo analysis to obtain best-fit parameters. The background evolution allows large interaction strengths, and the constraints from CMB anisotropies are weak. The growth of DM density perturbations is much more sensitive to the interaction, and can deviate strongly from the standard case. However, the deviations are degenerate with galaxy bias and thus more difficult to constrain. Interestingly, the ISW signature is suppressed since the non-standard background evolution can compensate for high growth rates. We also discuss the partial degeneracy between interacting DE and modified gravity, and how this can be broken.

Figures (8)

• Figure 1: CMB and total matter power spectra from the modified CAMB code for 3 $Q\| u_c~$ models with different values of $\Gamma$ but identical values of their remaining parameters (see $\Gamma w$CDM A,B,C in Table \ref{['models']}).
• Figure 2: Smoothed 68% and 95% contours of the marginalised probability distribution for IDE model with $Q\| u_c~$ (left) and $Q\| u_x~$ (right) in the range of stability, $w>-1$ and $\Gamma\ge0$. Crosses identify models chosen to be analyzed in more detail (see Table I).
• Figure 3: CMB and total matter power spectra from the modified CAMB code for the WMAP7 $w$CDM best-fit values and the $\Gamma w$CDM 1a,1b,2a,2b models chosen from the 95% confidence range for further analysis (see Table \ref{['models']}). The best-fit values of standard cosmological parameters were found using CosmoMC. Models 1a,1b have $\Gamma=0.4H_0$ and $Q\| u_c~$ while 2a,2b have $\Gamma=0.7H_0$ and $Q\| u_x~$.
• Figure 4: Left: Normalized growth rates for $\Lambda$CDM and the same best-fit models as in Fig. \ref{['best']}. Right: The same models but showing a normalized combination of $a^2\bar{\rho_c}\delta_c$ which is important for the ISW effect.
• Figure 5: Deviations from $\Lambda$CDM of the effective Hubble parameter (left) and effective Newton constant for $\delta_c$ (right), for the same best-fit models as in Fig. \ref{['best']}.