Looking for interactions in the cosmological dark sector

TL;DR

The paper investigates non-gravitational interactions in the dark sector via a Λ(t)CDM framework parameterized by α, linking dark-energy dynamics to dark-matter creation and drawing connections to a non-adiabatic generalized Chaplygin gas. By analyzing background evolution and primordial perturbations against Planck CMB, SNe Ia (JLA), local H0 measurements, and Lyα deuterium data, the authors assess observational viability and degeneracies with extra relativistic degrees of freedom and lensing amplitude. They find α is generally consistent with zero at 1σ, with mild data-driven shifts depending on extensions such as A_l and N_eff, and show that including these extensions can alleviate tensions between early- and late-Universe probes, particularly the H0–σ8 discrepancy. Overall, Λ(t)CDM remains a viable framework for probing dark-sector interactions and their implications for cosmological parameter inferences.

Abstract

We study observational signatures of non-gravitational interactions between the dark components of the cosmic fluid, which can be either due to creation of dark particles from the expanding vacuum or an effect of the clustering of a dynamical dark energy. In particular, we analyse a class of interacting models ($Λ$(t)CDM), characterised by the parameter $α$, that behaves at background level like cold matter at early times and tends to a cosmological constant in the asymptotic future. In our analysis we consider both background and primordial perturbations evolutions of the model. We use Cosmic Microwave Background (CMB) data together with late time observations, such as the Joint Light-curve Analysis (JLA) supernovae data, the Hubble Space Telescope (HST) measurement of the local value of the Hubble-Lemaître parameter, and primordial deuterium abundance from Ly$α$ systems to test the observational viability of the model and some of its extensions. We found that there is no preference for values of $α$ different from zero (characterising interaction), even if there are some indications for positive values when the minimal $Λ$(t)CDM model is analysed. When extra degrees of freedom in the relativistic component of the cosmic fluid are considered, the data favour negative values of $α$, which means an energy flux from dark energy to dark matter.

Figures (3)

• Figure 1: Left panel: Evolution of the matter contrast in the $\Lambda$CDM model (black curve), and of the total matter (blue) and baryonic (red) contrasts for $\alpha = -1/2$Velten. Right panel: Evolution of the baryonic $f_{rsd} \sigma_8$ for $\Lambda$CDM (black) and for $\alpha = -1/2$ (red) CB2.
• Figure 2: Comparison between the constraints on the $\Lambda$CDM and $\Lambda$(t)CDM models using TT+lowP+JLA+ priors (Riess+Cooke). The confidence intervals are shown in Table \ref{['tab:Tabel_LtCDM']}.
• Figure 3: Samples in the $H_0$ - $\alpha$ plane for the $\Lambda$CDM + $\mathcal{A}_l$ + $N_{\text{eff}}$ model using TT+lowP+JLA+ priors (Riess+Cooke), coloured by the value of the $\sigma_8$ parameter. The confidence intervals are given in the last column of Table \ref{['tab:Tabel_LtCDM']}.