Conformally and Disformally Coupled Vector Field Models of Dark Energy
Thu, Feb. 16th 2023, 11:30
Salle Claude Itzykson, Bât. 774, Orme des Merisiers
Scalar fields coupled to dark matter by conformal or disformal transformations give rise to a general class of scalar-tensor theories which leads to a rich phenomenology in a cosmological setting. While this possibility has been studied comprehensively in the literature for scalar fields, the vector case has been hardly treated. In this seminar, I will present a general prescription of how to build models based on vector fields conformally and disformally coupled to dark matter independent of the gravity theory. Thus, the resulting theory belongs to a class of interacting dark energy models. I will also discuss general conditions to avoid ghost instabilities in the theory. For concreteness, the standard Proca theory with a vector exponential potential is taken to describe the vector-tensor sector, and some specific coupling functions are assumed to study the cosmological background dynamics by dynamical system techniques. Despite choosing such a minimalist form for the underlying theory, the parameter space is considerably enriched compared to the uncoupled case due to the novel interactions, leading to new branches of solutions for the vector equation of motion. Thus, different trajectories can exist in phase space depending on the coupling parameters associated to the conformal and disformal functions. From here, new emerging vector-dark matter scaling solutions, and renewed stable attractor points are found to drive the late-time accelerated expansion of the universe. Numerical calculations are performed as well to investigate more quantitatively the impact of the conformal and disformal couplings on the cosmological background evolution which can significantly affect the cosmological dynamics during different stages of the evolution of the universe.