Interactions in the Dark Sector

The evolution of any broadly applied model is accompanied by multiple generalizations that aim to resolve conceptual problems, as well as to explain the ever-increasing array of observations. In the case of Standard Cosmological Model one of the most promising directions of generalization is the replacement of the cosmological constant with a more complicated, dynamic form of dark energy, as well as the inclusion of interaction between the dark components. Typically, dark energy (DE) models are based on the scalar fields minimally coupled to gravity, and do not implement the explicit coupling of the field to the background dark matter (DM). However there is no fundamental reason for this assumption in the absence of an underlying symmetry which would suppress the coupling. Given that we do not know the true nature of either DE or DM, one cannot exclude the possibility that there exists a coupling between them. Whereas new forces between DE and normal matter particles are heavily constrained by observations (e.g. in the solar system and gravitational experiments on Earth), this is not the case for DM particles. In other words, it is possible that the dark components interact with each other, while not being coupled to standard model particles. Therefore, the possibility of DE-DM interaction must be looked at with the utmost seriousness.