Looking for interactions in the cosmological dark sector

TL;DR

This paper investigates observational signatures of non-gravitational interactions in the dark sector, analyzing a class of models with evolving dark energy and dark matter, using diverse cosmological data to test their viability.

Contribution

It introduces and tests the $ ext{Lambda}$(t)CDM model with interaction parameter $ extalpha$, considering both background and perturbation evolutions, and compares it against observational data.

Findings

No strong preference for interaction parameter $ extalpha$ different from zero.

Data slightly favor negative $ extalpha$, indicating energy transfer from dark energy to dark matter.

Model extensions with extra relativistic degrees of freedom tend to favor negative $ extalpha$.

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 ($\Lambda$(t)CDM), characterised by the parameter $\alpha$, 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-Lema\^itre parameter, and primordial deuterium abundance from Ly$\alpha$ systems to test the observational viability of the model and some of its extensions. We found that there is no preference for values of $\alpha$ different from zero (characterising interaction), even if there are some indications for positive values when the minimal $\Lambda$(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 $\alpha$, which means an energy flux from dark energy to dark matter.