Dark interactions beyond the Lambda-CDM model

MSc (Astrophysical Sciences), North-West University, Potchefstroom Campus

In this study, cosmological models are considered, where dark matter and dark energy are coupled and may exchange energy through non-gravitational interactions with one other. These interacting dark energy (IDE) models are introduced to address problems with the standard ΛCDM model of cosmology, in which dark energy is assumed to be a cosmological constant. The central problem addressed in this study is the cosmic coincidence problem (regarding the presently measured coin- cidental ratio of dark matter to dark energy). Assuming two different linear dark energy couplings, Q1 = δHρdm and Q2 = δHρde, we find that interacting dark energy models may alleviate and even solve the cosmic coincidence problem by stabilising the ratio of dark matter to dark energy in both the past and future. Furthermore, we examine how these dark interactions affect crucial events in the expansion history of the universe. These events include the big bang and cosmic acceleration, as well as the radiation-matter and matter-dark energy equality. Besides studying the cosmological consequences of an interaction between the dark sectors, we also investigate the viability of IDE models on both theoretical and observational grounds. For both models considered, we find that negative energy densities are inevitable if energy flows from dark matter to dark energy and that consequently we should only seriously consider models where energy flows from dark energy to dark matter. To additionally ensure that these models are free from early time instabilities, we need to require that dark energy is in the ‘phantom’ (ω < −1) regime. This has the consequence that model Q1 = δHρdm will end with a future big rip singularity, while Q2 = δHρde may avoid this fate with the right choice of cosmological parameters. Cosmo- logical parameters for these models are obtained from type-Ia supernovae data using a previously developed Markov Chain Monte-Carlo (MCMC) simulation. The predicted expansion history from these models are then statistically compared to the supernovae data and the ΛCDM model, where we find that Q1 = δHρdm is statistically rejected, while Q2 = δHρde may be considered viable.

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