CHAPTER 6: Unified Dark Fluid Models of Interacting Dark Energy

Abstract

Interacting dark energy (IDE) models in which non-gravitational interactions occur between dark matter and dark energy are popular candidates for addressing various cosmological tensions, but these models may suffer from negative energy densities and divergences in the effective equations of state near phantom-divide crossing. Here, we present an equivalent Unified Dark Fluid (UDF) description of a group of eight interacting dark energy models, which can avoid these pathologies. We present analytical solutions for the total UDF density ρD and the effective UDF equation of state weff D , and show that both negative energies and divergences are avoided, which normally arise in the two-fluid decomposition. As a case study, we show that one of the IDE models is equivalent, at the background level, to a Generalized Chaplygin Gas (GCG) fluid under a new transformation. We then use recent observational constraints on the IDE models to infer implications for their unified dark fluid description. This study demonstrates that certain pathologies of interacting dark energy models arise from the two-fluid decomposition rather than from the background cosmology itself.