Composite Models of Dynamical Dark Energy: ΛXCDM and ωXCDM

Abstract

Although the Standard ΛCDM Model works well, it suffers from serious theoretical problems (eg. the Cosmological Constant Problem and the Cosmic Coincidence Problem) as well as phenomenological problems, such as the cosmological tensions (Hubble Tension and growth tension). In this work, we analyze the ΛXCDM model, a composite Dark Energy (DE) model involving a variable cosmological term (Running Vacuum) and an additional dark energy field X, which can play the role of ”phantom matter” (different from phantom DE) [4, 8]. Through the analytical resolution of the cosmological equations and the study of the density evolution and the effective equation of state (ωe), we demonstrate that the model can alleviate the Coincidence Problem thanks to the existence of a local maximum in the density ratio r = ρD/ρm, without need for fine-tuning, and leading the Universe towards a Turning Point of expansion instead of a Big Rip singularity. Finally, we show that the model’s dynamics exhibits a type of crossing of the phantom divide which is fully compatible with the recent releases of DESI 2024 data [7, 10, 11].