The dark degeneracy and interacting cosmic components

TL;DR

The paper analyzes the dark degeneracy—the indistinguishability of different dark-sector decompositions—by modeling a zero-sound-speed dark fluid that can simultaneously mimic dark matter and dark energy at the background level, yielding a ΛCDM-like expansion for suitable parameters. It extends the analysis to linear and nonlinear perturbations, showing that the degeneracy can persist under specific conditions, and explores a multifluid dark sector with internal interactions. It then studies couplings of the dark sector to baryons, parameterized by two cross-section–like terms, and uses CMB and supernova data to constrain them, finding that nonzero couplings are allowed and can affect inferred baryon density and spectral index, though degeneracies can remain with ΛCDM plus interactions. Overall, the work argues that many cosmologies share identical observational signatures unless the dark sector communicates with the standard model, and presents a framework to test such couplings and their implications for cosmology.

Abstract

We study some properties of the dark degeneracy, which is the fact that what we measure in gravitational experiments is the energy momentum tensor of the total dark sector, and any split into components (as in dark matter and dark energy) is arbitrary. In fact, just one dark fluid is necessary to obtain exactly the same cosmological and astrophysical phenomenology as the LCDM model. We work explicitly the first order perturbation theory and show that beyond the linear order the dark degeneracy is preserved under some general assumptions. Then, we construct the dark fluid from a collection of interacting fluids. Finally, we try to break the degeneracy with a general class of couplings to baryonic matter. Nonetheless, we show that these interactions can also be understood in the context of the LCDM model as between dark matter and baryons. For this last investigation we choose two independent parameterizations for the interactions, one inspired in electromagnetism and the other in Chameleon theories. Then, we constrain them with a joint analysis of CMB and Supernovae observational data.

Figures (9)

• Figure 1: Evolution of perturbation variables for a mode $k = 0.05\,\text{Mpc}^{-1}$. Solid curves are obtained from the dark fluid model for different values of the parameter $\alpha$ in the initial conditions $\delta_d(\tau_i) = \alpha \rho_{\text{DM}}(\tau_i) \delta_{\text{DM}}(\tau_i) / \rho_d(\tau_i)$. $\alpha$ takes values from 0.8 to 1.2. Dashed curves are for the $\Lambda$CDM model variables. The panels show: (a) Gravitational potential $\Phi$. (b) Baryonic density contrast $\delta_b$. (c) Dark fluid (solid lines) and dark matter (dashed line) density contrasts, $\delta_{d}$ and $\delta_{\text{DM}}$. (d) Dark fluid (solid lines) and dark matter (dashed line) velocities, $\theta_d$ and $\theta_{\text{DM}}$. The solutions for the case $\alpha =1$ are depicted with the thick (red) lines, which for panels (a), (b), and (d) coincide with the dashed lines.
• Figure 2: The CMB power spectrum considering different values of the interaction parameter $\Sigma_{I}$ in units of $10^{-6} \sigma_T$. The other parameters are fixed.
• Figure 3: The CMB power spectrum considering different values of the interaction parameter $\Sigma_{II}$ in units of $\sigma_T$. The other parameters are fixed.
• Figure 4: (color online) Marginalized probability for the complete set of parameters. Solid lines (red) are for Model A, dotted lines (blue) for Model B, and double-dotted (black) lines for Model C. The data used are the WMAP seven-year results, Union 2 data set supernovae compilation and a prior of HST on the Hubble constant.
• Figure 5: Contour confidence intervals for $\Sigma_I$ vs $\Sigma_{II}$ at $68 \%$ and $95 \%$ c.l. The shading shows the mean likelihood of the samples