ΛXCDM: a cosmon model solution to the cosmological coincidence problem?

We consider the possibility that the total dark energy (DE) of the Universe is made out of two dynamical components of different nature: a variable cosmological term, Λ, and a dynamical “cosmon”, X, possibly interacting with Λ but not with matter – which remains conserved. We call this scenario the ΛXCDM model. One possibility for X would be a scalar field χ, but it is not the only one. The overall equation of state (EOS) of the ΛXCDM model can effectively appear as quintessence or phantom energy depending on the mixture of the two components. Both the dynamics of Λ and of X could be linked to high energy effects near the Planck scale. In the case of Λ it may be related to the running of this parameter under quantum effects, whereas X might be identified with some fundamental field (say, a dilaton) left over as a low-energy “relic” by e.g. string theory. We find that the dynamics of the ΛXCDM model can trigger a future stopping of the Universe expansion and can keep the ratio ρD/ρm (DE density to matter-radiation density) bounded and of order 1. Therefore, the model could explain the so-called “cosmological coincidence problem”. This is in part related to the possibility that the present value of the cosmological term can be Λ0 < 0 in this framework (the current total DE density nevertheless being positive). However, a cosmic halt could occur even if Λ0 > 0 because of the peculiar behavior of X as “Phantom Matter”. We describe various cosmological scenarios made possible by the composite and dynamical nature of ΛXCDM, and discuss in detail their impact on the cosmological coincidence problem.