Interacting quintessence solution to the coincidence problem

TL;DR

This work introduces an interacting quintessence model where a scalar field exchanges energy with matter, augmented by a bulk dissipative pressure in the matter fluid. The framework yields a dynamical, stationary density-ratio attractor $r_s^-$ in a spatially flat universe, driving late-time acceleration without a cosmological constant. Analytical solutions for the density ratio, scalar potential, and Hubble rate are derived, and the model's viability is tested against Type Ia supernovae and primordial nucleosynthesis, yielding constraints such as $\beta \approx 0.40$ and $c^2 < 0.063$, with evidence that viscous effects are important. While compatible with current observations and addressing the coincidence problem, the model remains degenerate with $\Lambda$CDM, highlighting the need for future data to distinguish between these scenarios and to explore perturbations and structure formation within this interacting framework.

Abstract

We show that a suitable interaction between a scalar field and a matter fluid in a spatially homogeneous and isotropic spacetime can drive the transition from a matter dominated era to an accelerated expansion phase and simultaneously solve the coincidence problem of our present Universe. For this purpose we study the evolution of the energy density ratio of these two components. We demonstrate that a stationary attractor solution is compatible with an accelerated expansion of the Universe. We extend this study to account for dissipation effects due to interactions in the dark matter fluid. Finally, Type Ia supernovae and primordial nucleosynthesis data are used to constrain the parameters of the model.